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Sections
Internal Energy
Calorimetry
Thermochemical Equation
Hess's Law
Enthalpy of Formation
End of Chapter 6 Problems
Additional Practice
Units of Energy
Endothermic & Exothermic Reactions
Additional Guides
Enthalpy
GuideLearnAdditional Practice
Next SectionEnthalpy of Formation
Jules Bruno

Many chemical reactions require multiple steps in order to create their final products. Hess’s Law states that determining the enthalpy or heat of reaction (under constant pressure) is independent of the pathway taken to make those final products. 

Understanding Hess’s Law

Hess’s Law states that the enthalpy of reaction (ΔH­Rxn) is the sum of the enthalpy changes of its individual thermochemical steps and represents a state function. 


For example, the oxidation of acetylene is represented by the following equation: 

Thermochemical-Equation-ReactionThermochemical Equation (Oxidation of Acetylene)

In order to determine its enthalpy of reaction we use the enthalpies of its individual thermochemical steps. 

Multiple-stages-Germain-Hess-Constant-Heat-Summation-Diagram Thermochemical Steps (Oxidation of Acetylene)

Since each thermochemical step balances out to give the final equation we can add together the enthalpy values of ΔH1, ΔH2 and ΔH3 to obtain the ΔHRxn. 

Total-enthalpy-changeTotal Enthalpy Change

Application of Hess’s Law

There will be occasions when the individual thermochemical steps will not balance out to give the final overall equation. In these cases we must manipulate and alter them. 


PRACTICE 1: Find the ΔHRxn for the given overall chemical equation: 

Definition-Hess-Law-ProblemsThermochemical Step (Xenon Tetrafluoride)

The given thermochemical steps and ΔH values are

Final-State-Initial-StateThermochemical Steps (Xenon Tetrafluoride)

STEP 1: Start with XeF­2 from the overall chemical equation and locate where it is in the given thermochemical steps. If XeF2 does not match the XeF2­ in the overall chemical equation then we will need to manipulate the thermochemical step. 

Application-Hess-LawApplication of Hess's Law (XeF2)

In our thermochemical step XeF2 is listed as a product, but in our overall chemical equation XeF2 is a reactant. This means we must reverse the thermochemical step so that XeF2 will now be a reactant. 

Reversing-Thermochemical-stepReversing a thermochemical step

By reversing the thermochemical step we have XeF2 as a reactant just like in the overall chemical equation. Also notice that reversing a thermochemical step causes a reversal of the sign for its ΔH value. 


STEP 2: Continue to the next compound in the overall chemical equation, F2, and locate where it is in the given thermochemical steps. As we did before, if F2

does not match the F2­ in the overall chemical equation then we will need to manipulate the thermochemical step. 

Thermochemical Steps (F2)

We find that F2 is found in both thermochemical steps. Whenever a compound is found in more than one step then we ignore it for now and move onto our next compound in the overall chemical equation. 


STEP 3: Next we look at XeF4 from the overall chemical equation and locate where it is in the given thermochemical steps. If necessary, manipulate the thermochemical step. 

Thermochemical-step-XeF4Thermochemical Step (XeF4)

The XeF4­ ­in the thermochemical step matches with the XeF4 found in the overall chemical equation. This means we don’t have to do anything in this step. 


STEP 4: Now bring down all the thermochemical steps and cancel out the reaction intermediates to obtain the overall chemical equation. 

Cancel-Reaction-IntermediatesCanceling Reaction Intermediates

STEP 5: Canceling out the intermediates gives us the overall chemical equation and by adding the two ΔHo values of the thermochemical steps you can isolate the ΔHRxn. 

Enthalpy-Reaction-Chemical-EnergyCalculation of Enthalpy of Reaction

Now test your skills with a more complex Hess’s Law problem. 


PRACTICE 2: Find the ΔHRxn for the given overall chemical reaction: 

Thermochemical-Equation-CO2Thermochemical Equation (Carbon Dioxide)

The given thermochemical steps and ΔH values are

Thermochemical-Steps-CO2Thermochemical Steps (Carbon Dioxide)

STEP 1: Start with CO­2 from the overall chemical equation and locate where it is in the given thermochemical steps. If CO2 does not match the CO2­ in the overall chemical equation then we will need to manipulate the thermochemical step. 

Thermochemical-Step-CO2Thermochemical Step (Carbon Dioxide)In our thermochemical step CO2 is listed as a reactant like we need, but there are two moles of it. In the overall chemical equation we have only 1 mole of CO2 so we must divide the thermochemical equation by 2. 

Reversal-Thermochemical-StepReversal of a Thermochemical Step

Realize that dividing the thermochemical step by 2 also means you divide your original ΔH value of 3511.1 kJ by 2. Dividing 3511.1 kJ by 2 gives a new ΔH value of 1755.55 kJ. 

STEP 2: Continue to the next compound in the overall chemical equation, C (s), and locate where it is in the given thermochemical steps. If necessary, manipulate the thermochemical step. 

Thermochemical-Step-Carbon-solidThermochemical Step (Carbon solid)

In our thermochemical step we have two moles of C (s) listed products, but we need only 1 mole. So we divide the thermochemical step by 2.

Thermochemical-Step-Dividing-DivisionThermochemical Step (Dividing by 2)

Dividing by 2 means we also divide the ΔH value by 2. 

STEP 3: Next we search for O2 in the given thermochemical steps. If necessary, manipulate the thermochemical step. 

Thermochemical Steps (Oxygen Gas)We find O2 in more than one thermochemical step so we can ignore it for it. 

STEP 4: Now bring down all the thermochemical steps and cancel out the reaction intermediates to obtain the overall chemical equation.

Thermochemical-Steps-carbon-dioxide-CO2Thermochemical Steps (Carbon Dioxide)We must cancel out H2O and H2 because they are not found in the overall chemical equation. In order to cancel out the H2 and H­2O molecules we will need to reverse and multiple the first thermochemical step by 1.5 or 3/2. 

Thermochemical Step (Reversal & Multiplication)

Reversing the thermochemical step reverses the sign for the ΔH value and multiplying the thermochemical step by 1.5, which is the same as 3/2, means we multiply the ΔH value by 1.5. Now we bring down all the thermochemical steps again and finally cancel out all reaction intermediates. Canceling Reaction Intermediates (Carbon Dioxide)

STEP 5: Canceling out the intermediates gives us the overall chemical equation and by adding the three ΔHo values of the thermochemical steps you can isolate the ΔHRxn. 

Overall-Enthalpy-ChangeOverall Enthalpy Change

Hess’s Law serves as one component in our understanding of the branch of chemistry dealing with thermal and chemical energy called Thermochemistry. Under Thermochemistry, we examine the concepts of calorimetry, the conservation of energy, internal energy, formation equations, thermochemical equations and stoichiometry. 

Jules Bruno

Jules felt a void in his life after his English degree from Duke, so he started tutoring in 2007 and got a B.S. in Chemistry from FIU. He’s exceptionally skilled at making concepts dead simple and helping students in covalent bonds of knowledge.

Previous SectionThermochemical Equation
Next SectionEnthalpy of Formation
Additional Problems
Calculate the enthalpy of the following reaction:
Using Hess’s Law, determine the ΔH of the following reaction: N2(g) +  2O2(g)  →  2NO2(g) Given the following equations N2(g)  +  3H2(g)  →  2NH3(g)                                       ΔH = -115 kJ 2NH3(g)  +  4H2O(l)  →  2NO2(g)  +  7H2(g)                ΔH = -142.5 kJ H2O(l)  →  H2(g)  +  1/2O2(g)                                      ΔH = -43.7 kJ        
Use the standard reaction enthalpies given below to determine ΔH°rxn for the following reaction: P4(g) + 10Cl2(g) → 4PCl5(s)                ΔH°rxn = ? Given PCl5(s) → PCl3(g) + Cl2(g)                   ΔH°rxn = +157kJ P4(g) + 6Cl2(g) → 4PCl3(g)                  ΔH°rxn = -1207kJ A) -1835 kJ B) - 1364 kJ C) -1050 kJ D) -1786kJ E) -2100 kJ
Given the chemical reactions: 2 NO2(g) → N2(g) + 2 O2(g)                  ΔH = −68 kJ N2O4(g) → N2(g) + 2 O2(g)                   ΔH = −10 kJ Determine the ΔH for the following reaction: N2O4(g) → 2 NO2(g)   A. −78 kJ B. −58 kJ C. 48 kJ D. 58 kJ E. 78 kJ
The values for the enthalpy of reaction are given for two reactions below Mg(s) + Na 2CO3(s) → 2 Na(s) + MgCO 3(s)           ΔH rxn = 317.1 kJ   Mg(s) + 2 NaF(s) → 2 Na(s) + MgF 2(s)                ΔH  rxn = 35.6 kJ   Based on this information, what is the value for ΔH rxn for the reaction MgCO3(s) + 2 NaF(s) → Na 2CO3(s) + MgF2(s) a. + 352.7 kJ b. + 281.5 kJ c. + 245.9 kJ d. - 281.5 kJ e. - 352.7 kJ
Given these values of ΔH° : CS2 (l) + 3O2(g) → CO2(g) + 2SO2(g)            Δ H° = -1077 kJ H2 (g) + O2 → H2O2(l)                                    Δ   H° = -188 kJ                 H2 (g) 1/2 O2(g) → H2O(l)                              Δ   H° =  -286 kJ What is the value of  ΔH° for this reacion? CS2(l) + 6H2O2(l) → CO2(g) + 6H2O (l) + 2SO2(g) a) -1175 kJ•mol-1 b) -1151 kJ•mol-1 c) -1665 kJ•mol-1 d) -3921 kJ•mol-1
Given the following reactions Fe2O3 (s) + 3CO (s) → 2Fe (s) + 3CO2 (g) ∆H = -28.0 kJ 3Fe (s) + 4CO2 (s) → 4CO (g) + Fe 3O4 (s) ∆H = +12.5 kJ the enthalpy of the reaction of Fe 2O3 with CO 3Fe2O3 (s) + CO (g) → CO2 (g) + 2Fe3O4 (s) is ________ kJ.   a) 40.5 b) +109 c) -15.5 d) -109 e) -59.0
Given the following reactions N2 (g) + 2O2 (g) → 2NO2 (g)   ΔH = 66.4 kJ 2NO (g) + O2 (g) → 2NO2 (g)  ΔH = -114.2 kJ the enthalpy of the reaction of the nitrogen to produce nitric oxide is ________ kJ.   a) -47.8 b) 47.8 c) 180.6 d) -180.6 e) 90.3
Calculate ΔH° (in kJ) for the reaction. 2S (s) + 3O2 (g) → 2SO3 (g)  ∆H = -790 kJ S (s) + O2 (g) → SO2 (g)        ∆H = -297 kJ the enthalpy of the reaction in which sulfur dioxide is oxidized to sulfur trioxide is ________ kJ.   a) -196 b) -543 c) 1087 d) 196 d) -1384
S(s) + O2(g) → SO2 (g)      ∆H = ‐296.8 kJ 2SO2 + O2 → 2SO3             ∆H = ‐198.4 kJ Write the equation for the formation of sulfur trioxide gas from sulfur solid and oxygen. What is the ∆H of that equation?    
The United States gets 95% of its hydrogen gas by "steam reforming" methane, CH4(g). Another option is the “partial oxidation” of hydrocarbons to make carbon monoxide and hydrogen gas. For octane, that reaction would be  C8H18(l) + 4O2(g) → 8CO(g) + 9H2(g)   We know ΔH° for the combustion of octane  C8H18(l) + 25/2 O2(g) → 8CO2(g) + 9H2O(l)      ΔH° = -5500 kJ/mol  Assume the arbitrary values below for the combustion reactions of carbon monoxide and hydrogen: CO(g) + 1/2 O2(g) → CO2(g)             ΔH° = “-290 kJ/mol”  H2(g) + 1 /2O2(g) → H2O(l)               ΔH° = “-285 kJ/mol” A. Do you expect the magnitude of ΔH° for the partial oxidation of octane, to be greater or less than the magnitude of ΔH° for the combustion of octane? Explain.     B. Now, determine   ΔH°  for the partial oxidation of octane with those assigned values. Choose from the choices below         C. Do you think the hydrogen produced provides more or less energy than you would get from the combustion of octane? Explain. 
Researchers at Purdue University have exploited the reactivity of a reaction to develop a novel type of rocket fuel consisting of aluminum nanoparticles dispersed in H2O(s). They nicknamed this propellant “ALICE” (get it? Al-ice) and noted that in addition to being environmentally friendly, ALICE could be produced on any extraterrestrial body that contains water. The recent discovery of water on Mars could draw new attention to this intriguing fuel. (International Journal of Aerospace Engineering, 2012) One can speculate that the reaction that causes ALICE to act as a propellant is: 2Al (s) + 3H2O(s) → Al2O3(s) + 3H2(g)   where the hydrogen gas is subsequently burned.  A. Given the molar enthalpies for the following reactions,  2Al(s) + 3/2 O2 (g) → Al2O3 (s)        ΔH° = -1669.8 kJ/mol   H2 (g) + 1/2 O2 (g) →  H2O (l)         ΔH° =    -285.8 kJ/mol H2O (s) → H2O (l)                           ΔH° =       6.01 kJ/mol determine the ΔH° for the ALICE reaction 2Al (s) + 3H2O (s) → Al2O3 (s) + 3H2 (g)       Is the ΔH° you calculated consistent with Al-ice serving as a fuel? ________________ Give one reason to support your answer.    
Given the following data: 3CO2(g) + 4H2O(l) ⇌ C3H8(g) + 5O2(g) ∆H = 1, 110 kJ · mol−1 H2O(l) ⇌ H2(g) + 1/2O2(g) ∆H = 142.5 kJ · mol−1 3Cgraphite + 4H2(g) ⇌ C3H8(g) ∆H = −52 kJ · mol−1 calculate ∆H for the reaction Cgraphite + O2(g) ⇌ CO2(g) 1. 1202 kJ · mol−1 2. −592 kJ · mol−1 3. 543 kJ · mol−1 4. −197 kJ · mol−1 5. −543 kJ · mol−1  
Use the following equations C (s) + O2 (g) → CO2 (g)                                                  ΔH     o = 393.5 kJ H2 (g) + 1/2 O2 (g) → H2O (l)                                           ΔH     o = 285.8 kJ 2 C4H10 (g) + 13 O2 (g) → 8 CO2 (g) + 10 H2O (l)           ΔH o = 5754.6 kJ to calculate the heat formation, ΔH of, for butane. 4 C (s) + 5 H2 (g)  →  C4H10 (g) A.  125.7 kJ B. -5880.3 kJ C. -5862.3 kJ D. -5075.3 kJ E. -251.4 kJ    
Determine the answer for the question below.
Enthalpy diagram illustrating Hesss law. The net reaction is the same as in Figure 5.21 in the textbook, but here we imagine different reactions in our two-step version. As long as we can write a series of equations that add up to the equation we need, and as long as we know a value for H for all intermediate reactions, we can calculate the overall H.Suppose the overall reaction were modified to produce 2H2O(g) rather than 2H2O(l). Would any of the values of H in the diagram stay the same?
The heat of solution is LiCl is –37.1 kJ/mol, and the lattice energy of LiCl(s) is 828 kJ/mol. Calculate the total heat of hydration of 1 mol of gas phase Li+ ions and Cl - ions.1) 791 kJ2) 865 kJ3) -865 kJ4) -791 kJ5) None of these
Consider the following set of reactions:CO + 1/2O2 → CO2              ΔH = −283 kJ/molC + O2 → CO2                    ΔH = −393 kJ/molThe equations given in the problem can be added together to give the following reaction:overall: C + 1/2O2 → CO Which one of them must be reversed?
Calculate the enthalpy of the reaction4B(s) + 3O2(g) → 2B2O3(s)given the following pertinent information:B2O3(s) + 3H2O(g) → 3O2(g) + B2H6(g),      ΔH∘A=+2035 kJ2B(s) + 3H2(g) → B2H6(g),                            ΔH∘B=+36 kJH2(g) + 1/2 O2(g) → H2O(l),                            Δ  H∘C=−285 kJH2O(l) → H2O(g),                                          ΔH∘D=+44 kJExpress your answer with the appropriate units.
Using the enthalpy of reaction for the 2 reactions with ozone, determine the enthalpy of reaction for the reaction of chlorine with ozone. ClO(g) + O3(g) → Cl(g) + 2O2(g)       ΔH°rxn =  -122.8 kJ2O3(g) → 3O2(g)                               ΔH°rxn =  -285.3 kJO3(g) + Cl(g) → ClO(g) + O2(g)         ΔH°rxn = ?
Consider these reactions, where M represents a generic metal1. 2M(s) + 6HCl(aq) → 2MCl3(aq) + 3H2(g) ΔH1 = -749.0 KJ2. HCl(g) → HCl(aq) ΔH2 = -74.8 KJ3. H2(g)+Cl2(g) → 2HCl(g) ΔH3 = -1845.0 KJ4. MCl3(s) → MCl3(aq) ΔH4 = -298.0 KJUse the information above to determine the enthalpy of the following reaction.2M(s) + 3Cl2(g) → 2MCl3(s) ΔH = 
Calculate ΔHrxn for the following reaction: Express the enthalpy in kilojoules to one decimal place.CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)given these reactions and their ΔH values:C(s) + 2H2(g) → CH4(g)    ΔH = −74.6 kJC(s) + 2Cl2(g) →CCl4(g)   ΔH = −95.7 kJH2(g) + Cl2(g) →2HCl(g)  ΔH = −184.6 kJ
Consider these reactions, where M represents a generic metal. Use the information below to determine the enthalpy of the following reaction. 2M(s) + 3Cl2(g) → 2MCl3(s) 
Using Hess' Law, calculate the enthalpy of formation for C 3H8(g) using the following thermodynamic data.C(s) + O2(g) → CO2(g) ΔH = -393.5 kj/molH2(g) + 1/2O2(g) → H2O(I) ΔH = -285.8 kj/molC3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(I) ΔH = 2199.0 kJ/mol
Which of the following is a statement of Hess's law?a. if a reaction carried out in a series of steps the, ΔH for the reaction will equal the sum of the enthalpy changes for the individual steps.b. if a reaction carried out in a series of steps the, ΔH for the reaction will equal the product of the enthalpy changes for the individual steps.c. the ΔH for a process in the forward direction is equal to the ΔH for the process in the reverse direction.d. the ΔH for a process in the forward direction is equal in magnitude and opposite in sign to the ΔH for a process in the reverse direction.e. the ΔH of a reaction depend on the physical state of the reactants and products.
H2(g) + F2(g) → 2HF (g)           ΔH rxn = -546.6 kJ 2H2(g) + O2(g) → 2H2O (l)       ΔH rxn = -571.6 kJ Calculate the value of  ΔHrxn for: 2F2(g) + 2H2O(l) → 4HF(g) + O2(g)
Calculate the enthalpy of the reaction2NO(g) + O2(g) → 2NO2(g)given the following reactions and enthalpies of formation:12N2(g) + O2(g) → NO2(g),   ΔH∘A = 33.2 kJ12N2(g) + 1/2 O2(g) → NO(g),  ΔH∘B = 90.2 kJ
Given the following data2 O3 (g) → 3 O2 (g)            ΔH = -427 kJO2 (g) → 2O (g)                  ΔH = 495 kJNO (g) + O3 (g) → NO2 (g) + O2 (g)   ΔH = -199 kJcalculate ΔH for the reactionNO (g) + O (g) → NO2 (g) 
Calculate the enthalpy of the reaction4B(s) + 3O2(g) → 2B2O3(s)given the following pertinent information:A. B2O3(s) + 3H2O(g) → 3O2(g) + B2H6(g),       ΔH∘A = +2035 kJB. 2B(s) + 3H2(g) → B2H6(g),   ΔH∘B = +36 kJC. H2(g) + 1/2O2(g) → H2O(l),    ΔH∘C = −285 kJD. H2O(l) → H2O(g),     ΔH∘D = +44 kJ
Calculate ΔH°rxn for the reaction 2 Ni(s) + 2 S(s) + 3 O2(g) → 2 NiSO3(s) from the following information: (1) NiSO3(s) → NiO(s) + SO2(g)       ΔH°rxn = 156 kJ   (2) S(s) + O2(g) → SO2(g)                 ΔH°rxn = -297 kJ    3) Ni(s) + 1/2 O2(g) → NiO(s)           ΔH°rxn = -241 kJ 
Coal gasification can be represented by the equation:2 C(s) + 2 H2O(g) → CH4(g) + CO2(g)          ΔH = ?Use the following information to find ΔH for the reaction above.CO(g) + H2(g) → C(s) + H2O(g)                    ΔH = −131 kJCO(g) + H2O(g) → CO2(g) + H2(g)               ΔH = −41 kJCO(g) + 3 H2(g) → CH4(g) + H2O(g)            ΔH = −206 kJ      A) −15 kJ            B) 15 kJ     C) −372 kJ       D) 116 kJ          E) −116 kJ 
Calculate ΔH for the reaction C4H4(g) + 2H2(g) → C4H8(g), using the following information:ΔHcombustion for C4H4(g)= -2341 kJ/molΔHcombustion for C4H8(g)= -3071 kJ/molΔHcombustion for H2(g) = -286 kJ/mola. -128 kJb. -316 kJc. +158 kJd. -700 kJe. -4810 kJ
At 1 atm pressure, the heat of sublimation of gallium is 277 kJ/mol and the heat of vaporization is 271 kJ/mol. To the correct number of significant figures, how much heat is required to melt 4.50 mol of gallium at 1 atm pressure?a. 250 kJb. 6 kJc. 274 kJd. 27 kJ
Calculate the enthalpy change for the reactionP4O6 (s) + 2 O2 (g) → P4O10 (s)given the following enthalpies of reaction:P4 (s) + 3 O2 (g) → P4O6 (s)          ΔH = -1640.1 kJP4 (s) + 5 O2 (g) → P4O10 (s)        ΔH = -2940.1 kJ 
Now consider the following set of reactions:2NO2 → 2NO + O2, H=109 kJ/molN2O4 → 2NO + O2, H=172 kJ/molThe equations given in the problem introduction can be added together to give the following reaction:overall: N2O4 → 2NO2 However, one of them must be reversed. Which one:a. 2NO2 → 2NO + O2b. N2O4 → 2NO + O2
From the enthalpies of reaction2 H2(g) + O2(g) → 2 H2O(g)          Δ = -483.6 kJ             3 O2(g) → 2 O3(g)            Δ = +284.6 kJcalculate the heat of the reaction3 H2(g) + O3(g) → 3 H2O(g)
From the enthalpies of reactionH2 (g) + F2 (g) → 2 HF (g)          ΔH = -537 kJC (s) + 2 F2 (g) → CF4 (g)          ΔH = -680 kJ2 C (s) + 2 H2 (g) → C2H4 (g)     ΔH = +52.3 kJcalculate ΔH for the reaction of ethylene with F 2:C2H4 (g) + 6 F2 (g) → 2 CF4 (g) + 4 HF (g) 
Given the dataN2 (g) + O2 (g) → 2 NO (g)            ΔH = +180.7 kJ2 NO (g) + O2 (g) → 2 NO2 (g)      ΔH = -113.1 kJ2 N2O (g) → 2 N2 (g) + O2 (g)       ΔH = -163.2 kJuse Hess’s law to calculate ΔH for the reactionN2O (g) + NO2 (g) → 3 NO (g)
Diamond and graphite are two crystalline forms of carbon. At 1 atm and 25°C, diamond changes to graphite so slowly that the enthalpy change of the process must be obtained indirectly. Using equations from the list below, determine ΔH  for C(diamond) ⟶ C(graphite)(1) C(diamond) + O2(g) ⟶ CO2(g)          ΔH = −395.4 kJ(2) 2CO2(g) ⟶ 2CO(g) + O2(g)              Δ H = 566.0 kJ(3) C(graphite) + O2(g) ⟶ CO2(g)          Δ H = −393.5 kJ(4) 2CO(g) ⟶ C(graphite) + CO2(g)      ΔH = −172.5 kJ
Consider the following generic reaction: A+2B→C+3D, with ΔH = 103 kJ . Determine the value of ΔH for each of the following related reactions. a. 3A + 6B → 3C + 9D b. C + 3D → A + 2B c. 12C + 32D → 12A + B
Calculate the ΔHrnx for the following reaction CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)Use the following reactions and given ΔH'sC(s) + 2H2(g) → CH4(g) Δh = -74.6 kJC(s) + 2Cl2(g) → CCl4(g) Δh = 95.7 kJH2(g) + Cl2(g) → 2HCl(g) Δh = -184.6 kJ
Consider the following set of reactions: N2 + 2O2 → N2O4 ,ΔH=−8 kJ/mol N2 + O2 → 2NO ,ΔH=180 kJ/mol The equations given in the problem introduction can be added together to give the following reaction: overall: N2O4 → 2NO + O2 However, one of them must be reversed, reaction 1: N2 + 2O2 → N2O4 What is the enthalpy for reaction 1 reversed? reaction 1 reversed: N2O4 → N2 + 2O2 Express your answer numerically in kilojoules per mole. What is the enthalpy for reaction 2?
Given these reactions: X(s) +1/2 O2(g) → XO(s) H=-639.7 XCO3(s) → XO(s)+CO2(g) H=+349.7   What is H(rxn) for this reaction? X(s)+1/2 O2(g)+CO2(g) --> XCO3(s)  
Now consider the following set of reactions: Reaction 1: N2 + 2O2 → N2O4 ,ΔH=?8 kJ/mol Reaction 2: N2 + O2 → 2NO ,   ΔH=180 kJ/mol a. The equations given in the problem introduction can be added together to give the following reaction: N2O4 → 2NO + O2 However, one of them must be reversed. Which one? b. What is the enthalpy for reaction 1 reversed? Reaction 1 reversed: N2O4 → N2 + 2O2 Express your answer numerically in kilojoules per mole.        
Calculate ΔHrxn for the following reaction 2 NOCl (g) → N2 (g) + 02 (g) + Cl2 (g) Given the following set of reactions; 1/2 N2 (g) + 1/2 02 (g) → NO (g)          ΔH= 90.3KJ NO (g) + 1/2 Cl2 (g) → NOCl (g)          ΔH = - 38.6 KJ
Calculate the ΔHrnx for the following reaction C(s) + H2O(g) → CO(g) + H2(g)Use the following reactions and given ΔH'sC(s) + O2(g) → CO2(g) Δh = -393.5 kJ2CO(s) + O2(g) → 2CO2(g) Δh = -566.0 kJ2H2(g) + O2(g) → 2H2O(g) Δh = -483.6 kJ
Even though so much energy is required to form a metal cation with a 2+ charge, the alkaline earth metals form halides with the general formula MX2, rather than MX.(c) Use Hess’s law to calculate ΔH° for the conversion of MgCl to MgCl2 and Mg (ΔH° f of MgCl2 = −641.6 kJ/mol).
Find ΔHrxn for the following reaction: N2O(g) + NO2(g) → 3 NO(g)Use the following reactions with known ΔH values:2 NO(g) + O2(g) → 2 NO2(g); ΔH = –113.1 kJN2(g) + O2(g) → 2 NO(g); ΔH = +182.6 kJ2 N2O(g) → 2 N2(g) + O2(g); ΔH = –163.2 kJ
Calculate ΔH for the process Hg 2Cl2(s) ⟶ 2Hg(l) + Cl2(g) from the following information:Hg(l) + Cl2(g) ⟶ HgCl2(s) ΔH = −224 kJHg(l) + HgCl2(s) ⟶ Hg2Cl2(s) ΔH = −41.2 kJ
Explain how the value of ΔH for a reaction changes upon multiplying the reaction by a factor.
Explain how the value of ΔH for a reaction changes upon reversing the reaction.
What is Hess' law?
Find ΔHrxn for the following reaction: 3 H2(g) + O3(g) → 3H2O(g)Use the following reactions with known ΔH values:2 H2(g) + O2(g) → 2 H2O(g); ΔH = –483.6 kJ3 O2(g) → 2 O3(g); ΔH = +285.4 kJ
Calculate ΔH°298 for the process Co3O4(s) ⟶ 3Co(s) + 2O2(g) from the following information:Co(s) + 1/2O2(g) ⟶ CoO(s)  ΔH°298 = −237.9kJ 3CoO(s) +1/2O2(g) ⟶ Co3O4(s)  ΔH°298 = −177.5kJ
Calculate the standard molar enthalpy of formation of NO(g) from the following data:N2(g) + 2O2 ⟶ 2NO2(g) ΔH°298 = 66.4 kJ2NO(g) + O2 ⟶ 2NO2(g) ΔH°298 = −114.1 kJ
Calculate ΔH°298 for the process Sb(s) + 5/2Cl2(g) ⟶ SbCl5(s)from the following information:Sb(s) + 3/2Cl2(g) ⟶ SbCl3(s)             ΔH°298 = −314 kJSbCl3(s) + Cl2(g) ⟶ SbCl5(g)            ΔH°298 = −80 kJ
Calculate ΔH°298 for the process Zn(s) + S(s) + 2O 2(g) ⟶ ZnSO4(s)from the following information:Zn(s) + S(s) ⟶ ZnS(s)                  ΔH° 298 = −206.0 kJZnS(s) + 2O2(g) ⟶ ZnSO4(s)     ΔH°298 = −776.8 kJ
Given the following dataFe2O3 (s) + 3 CO (g) → 2 Fe (s) + 3 CO2 (g)         ΔH° = -23 kJ3Fe2O3 (s) + CO (g) → 2 Fe3O4 (s) + CO2 (g)     ΔH° = -39 kJFe2O3 (s) + CO (g) → 3FeO (s) + CO2 (g)            ΔH° = 18 kJcalculate ΔH° for the reactionFeO (s) + CO (g) → Fe(s) + CO2 (g)
For each generic reaction, determine the value of ΔH2 in terms of ΔH1.A + B → 2C; ΔH12C → A + B; ΔH2 = ?
For each generic reaction, determine the value of ΔH2 in terms of ΔH1.A + 1/2B → C; ΔH12A + B → 2C; ΔH2 = ?
For each generic reaction, determine the value of ΔH2 in terms of ΔH1.A → B + 2C; ΔH11/2B + C → 1/2A; ΔH2 = ?
Calculate ΔHrxn for the following reaction: Fe2O3(s) + 3 CO(g) → 2 Fe(s) + 3 CO2(g)Use the following reactions and given ΔH's.2 Fe(s) + 3/2 O2(g) → Fe2O3(s), ΔH = –824.2  kJCO(g) + 1/2 O2(g) → CO2(g), ΔH = –282.7  kJ
Calculate ΔHrxn for the following reaction: 5 C(s) + 6 H2(g) → C5H12(l)Use the following reactions and given ΔH's.C5H12(l) + 8 O2(g) → 5 CO2(g) + 6 H2O(g), ΔH = –3244.8 kJC(s) + O2(g) → CO2(g), ΔH = –393.5 kJ2 H2(g) + O2(g) → 2 H2O(g), ΔH = –483.5 kJ
Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2 (g) → CO2(g) + 2 H2O(l), ΔH = –726.5kJBalance the forward reaction with whole-number coefficients. What is ΔH for the reaction represented by this equation?
The chemistry of nitrogen oxides is very versatile. Given the following reactions and their standard enthalpy changes,(1) NO(g) + NO2(g) ⟶ N2O3(g)                        ΔH° rxn = −39.8 kJ(2) NO(g) + NO2(g) + O2(g) ⟶ N2O5(g)          ΔH° rxn = −112.5 kJ(3) 2NO2(g) ⟶ N2O4(g)                                   ΔH° rxn = −57.2 kJ(4) 2NO(g) + O2(g) ⟶ 2NO2(g)                       ΔH° rxn = −114.2 kJ(5) N2O5(s) ⟶ N2O5(g)                                  ΔH° rxn = 54.1 kJcalculate the standard enthalpy of reaction forN2O3(g) + N2O5(s) ⟶ 2N2O4(g)
Given the following data:2ClF (g) + O2 (g) → Cl2O (g)+ F2O (g)              ΔH = 167.4 kJ2ClF3 (g) + 2O2 (g) → Cl2O (g) + 3F2O (g)       ΔH = 341.4 kJ2F2 (g) + O2 (g) → 2F2O (g)                               ΔH = -43.4 kJcalculate ΔH for the reactionClF (g) + F2 (g) → ClF3 (g) 
The energy changes for many unusual reactions can be determined using Hess’s law. Calculate ΔE for the conversion of F−(g) into F+(g).
The energy changes for many unusual reactions can be determined using Hess’s law. Calculate ΔE for the conversion of Na+(g) into Na−(g).
Calculate ΔH for the reaction2NH3 (g) + 1/2 O2 (g) → N2H4 (l) + H2O (l) given the following data:2NH3 (g) + 3N2O (g) → 4N2 (g) + 3H2O (l)  ΔH = -1010 kJN2O (g) + 3H2 (g) → N2H4 (l) + H2O (l)      ΔH = -317 kJN2H4(l) + O2 (g) → N2(g) + 2H2O (l)      ΔH = -623 kJH2(g) + 1/2O2 (g) → H2O (l)          ΔH = -286 kJ
Calculate ΔH for the reaction N2H4 (l) + O2 (g) → N2 (g) + 2H2O (l)given the following data2 NH3 (g) + 3N2O (g) → 4N2(g) + 3H2O (l)      ΔH = -1010 kJN2O (g) + 3 H2 (g) → N2H4 (l) + H2O (l)           ΔH = -317 kJ2NH3 (g) + 1/2 O2 (g) → N2H4 (l) + H2O (l)       ΔH = -143 kJH2 (g) + 1/2 O2 (g) → H2O (l)                            ΔH  = -286 kJ
Consider the reactions:A → 2 B; ΔH1A → 3 C; ΔH2What is ΔH for the reaction 2 B → 3 C?
Given the following dataP4 (s) + 6Cl2 (g) → 4PCl3 (g)      ΔH = -1225.6 kJP4 (s) + 5O2 (g) → P4O10 (s)      ΔH = -2967.3 kJPCl3 (g) + Cl2 (g) → PCl5 (g)      ΔH = -84.2 kJPCl3 (g) + 1/2 O2 (g) → Cl3PO (g)    ΔH = -285.7 kJcalculate ΔH for the reactionP4O10 (s) + 6PCl5 (g) → 10Cl3PO (g)
You may want to reference (Pages 184 - 186) Section 5.6 while completing this problem.Consider the following hypothetical reactions:A  →  B, ΔH = +21kJB  →  C, ΔH = +61kJUse Hess's law to calculate the enthalpy change for the reaction A → C.
Calculate the enthalpy change for the reaction: P4O6(s) + 2 O2(g) → P4O10(s), given the following enthalpies of reaction:P4(s) + 3 O2(g) → P4O6(s), ΔH = –1640.1 kJP4(s) + 5O2(g) → P4O10(s), ΔH = –2940.1 kJ
From the enthalpies of reaction:H2(g) + F2(g) → 2 HF(g), ΔH = –537 kJC(s) + 2F2(g) → CF4(g), ΔH = –680 kJ2 C(s) + 2 H2(g) → C2H4(g), ΔH = +52.3 kJcalculate ΔH for the reaction of ethylene with F2: C2H4 (g) + 6 F2(g) → 2 CF4(g) + 4 HF(g)
Calculate ΔH forCa (s) + 1/2O2 (g) + CO2 (g) ⟶ CaCO3 (s)given the following reactions:Ca (s) + 1/2O2 (g) ⟶ CaO (s)             Δ H = −635.1 kJCaCO3 (s) ⟶ CaO (s) + CO2 (g)         ΔH = 178.3 kJ
Calculate ΔH for2NOCl (g) ⟶ N2 (g) + O2 (g) + Cl2 (g)given the following reactions:1/2N2 (g) + 1/2O2 (g) ⟶ NO (g)         Δ H = 90.3 kJNO (g) + 1/2Cl2 (g) ⟶ NOCl (g)        Δ H = −38.6 kJ
Write the balanced overall equation (equation 3) for the following process, calculate ΔHoverall, and match the number of each equation with the letter of the appropriate arrow in Figure P6.72:(1) N2 (g) + O2 (g) ⟶ 2NO (g)                Δ  H = 180.6 kJ(2) 2NO (g) + O2 (g) ⟶ 2NO2 (g)           Δ H = −114.2 kJ--------------------------------------------------------------------------------------------(3)                                                 ΔH     overall = ?
Write the balanced overall equation (equation 3) for the following process, calculate ΔHoverall, and match the number of each equation with the letter of the appropriate arrow in Figure P6.73:(1) P4 (s) + 6Cl2 (g) ⟶ 4PCl3 (g)                Δ H = −1148 kJ(2) 4PCl3 (g) + 4Cl2 (g) ⟶ 4PCl5 (g)         Δ H = −460 kJ----------------------------------------------------------------------------------(3)                                                    ΔH     overall = ?
At a given set of conditions, 241.8 kJ of heat is released when 1 mol of H  2O(g) forms from its elements. Under the same conditions, 285.8 kJ is released when 1 mol of H2O(l) forms from its elements. Find ΔH for the vaporization of water at these conditions.
Consider the following generic reaction: A + 2B → C + 3D, with ΔH = 148 kJ . Determine the value of ΔH for the following related reaction: 3A + 6B → 3C + 9D
Consider the following generic reaction: A + 2B → C + 3D, with ΔH = 148 kJ. Determine the value of ΔH for the following related reaction: C + 3D → A + 2B
When 1 mol of CS2(l) forms from its elements at 1 atm and 25°C, 89.7 kJ of heat is absorbed, and it takes 27.7 kJ to vaporize 1 mol of the liquid. How much heat is absorbed when 1 mol of CS2(g) forms from its elements at these conditions?
Calculate ΔHrxn for the following reaction: CaO(s) + CO2(g) → CaCO3(s)Use the following reactions and given ΔH’s:Ca(s) + CO2(g) + 1/2 O2(g) → CaCO3(s); ΔH = –812.8 kJ2 Ca(s) + O2(g) → 2 CaO(s); ΔH = –1269.8 kJ
Calculate ΔHrxn Delta  H_{ m rxn} for the following reaction: CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)Given these reactions and their ΔH values:C(s) + 2 H2(g) → CH4(g); ΔH = –74.6 kJC(s) + 2 Cl2(g) → CCl4(g); ΔH = –95.7 kJH2(g) + Cl2(g) → 2 HCl(g); ΔH = –92.3 kJ
The ΔHf˚ of TiI3(s) is –328 kJ/mol and the ΔH˚ for the reaction Ti(s) + 3 I 2(g) → 2 TiI3(s) for the reaction is –839 kJ{ m kJ}. Calculate the ΔH of sublimation of I2(s), which is a solid at 25˚C.
Consider the following balanced thermochemical equation for the decomposition of the mineral magnesite:MgCO3 (s) ⟶ MgO (s) + CO2 (g)                         ΔH = 117.3 kJWhat is ΔH for the reverse reaction?
Given the following thermochemical data:                                                                                ∆H°f (kJ)I. P4(s) + 6 Cl2(g) ⟶ 4 PCl3(g)                        –1225.6II. P4(s) + 5 O2(g) ⟶ P4O10(s)                         –2967.3III. PCl3(g) + Cl2(g) ⟶ PCl5(g)                         –84.2IV. PCl3(g) + 1/2 O2(g) ⟶ POCl3(g)                –285.7Calculate the value of ∆H° for the following reaction:P4O10(s) + 6 PCl5(g) ⟶ 10 POCl3(g)A. –110.5 kJ B. –610.1 kJ C. –2682.2 kJ D. –4562.8 kJE. –7555.0 kJ
(i) Consider this reactionC2H5OH + 3O2 → 2CO2 + 3H2O Change in Heat = -1370 kJ/molWhat is the enthalpy for this reaction reversed?Reaction reversed: 2CO2 + 3H2O → C2H5OH + 3O2Change in Heat = ? kJ/mol(ii) Calculate the enthalpy of the reaction2NO(g) + O2(g) → 2NO2(g)Given the following reactions and enthalpies of formation:½ N2(g) + O2(g) → NO2(g) Change in Heat a = 33.2 KJ½ N2(g) + ½ O2 (g) → NO(g) Change in Heat b = 90.2 KJExpress your answer numerically in kilojoules.Change in Heat = ? kJ/molPlease show all your steps and explain why you did what.
Given:1. A → 2B, ΔH1 2. B → C + D, ΔH2 3. E → 2D, ΔH3ΔH of A → 2C + E is:A. ΔH1 + 2ΔH2 – ΔH3 B. ΔH2 - ΔH2 – ΔH3 C. ΔH3 – ΔH3 D. ΔH1 + 2ΔH1 – ΔH2 E. None of the above
Calculate the ΔH (in kJ) for the reaction:3 Fe2O3 (s) + CO (g) → CO2 (g) + 2 Fe3O4 (s)             ∆Hrxn = ?Given the following reactions:Fe2O3 (s) + 3 CO (g) → 2 Fe (s) + 3 CO2 (g)                 ∆H1 = −28. 0 kJ3 Fe (s) + 4 CO2 (g) → 4 CO (g) + Fe3O4 (s)                 ∆H2 = +12. 5 kJA. -59.0B. +40.5C. -15.5D. -109E. +109
Given the following reactionsH2O (l) →H2O (g)                    ΔH = 44.01 kJ2H2 (g) + O2 (g) → 2H2O (g)  ΔH = -483.64 kJthe enthalpy for the decomposition of liquid water into gaseous hydrogeand oxygen2H2O (l) → 2H2 (g) + O2 (g) is __________ kJ.A) -395.62B) -527.65C) 439.63D) 571.66E) 527.65
From the enthalpies of reaction2C(s) + O2 (g) → 2CO(g)                          ΔH   rxn = -221.0 kJ2C(s) + O2 (g) + 4H2 (g) → 2CH3OH(g)    ΔHrxn = -402.4 kJcalculate ΔH for the reaction:CO(g) + 2H2 (g) → CH3OH(g)(a) -623.4 kJ (b) 623.4 kJ (c) 181.4 kJ (d) -90.7 kJ (e) -181.4 kJ
Consider these reactions, where M represents a generic metal.1.  2M (s) + 6HCl (aq)  →  2MCl3 (aq) + 3H2 (g)                 ΔH1 = -579.0 kJ2. HCl (g) → HCl (aq)                                                         ΔH2 = -74.8 kJ3. H2 (g) + Cl2 (g) → 2HCl (g)                                            ΔH3 = -1845.0 kJ4. MCl3 (s) → MCl3 (aq)                                                     ΔH4 = -141.0 kJUse the following information above to determine the enthalpy (kJ) of the following reaction.2 M(s) +  3Cl2 (g) → 2 MCl3 (s)
Given the following dataCa(s) + 2C (graphite) → CaC2 (s)                      ΔH = -62.8 kJCa(s) + 1/2 O2 (g) → CaO (s)                              ΔH = -635.5 kJCaO (s) + H2O (l) → Ca(OH)2 (aq)                     ΔH = -653.1 kJC2H2 (g) + 5/2O2 (g) → 2CO2 (g) + H2O (l)      ΔH = -1300. kJC(graphite) + O2 (g) → CO2 (g)                        ΔH = -393.5 kJCalculate ΔH for the reactionCaC2 (s) + 2H2O (l) → Ca(OH)2 (aq) + C2H2 (g)
Using the following data, calculate the standard heat of formation of ICl(g) in kJ/mol:Cl2 (g) → 2Cl (g)          ΔH° = 242.3 kJI2 (g) → 2I (g)               ΔH° = 151.0 kJICl (g) → I (g) + Cl (g)    ΔH° = 211.3 kJI2 (s) → I2 (g)               ΔH° = 62.8 kJ
The following sequence of reactions occurs in the commercial production of aqueous nitric acid:4NH3(g) + 5O2(g) ⟶ 4NO(g) + 6H2O(l)           ΔH = −907 kJ2NO(g) + O2(g) ⟶ 2NO2(g)                             ΔH = −113 kJ3NO2 + H2O(l) ⟶ 2HNO3(aq) + NO(g)            ΔH = −139 kJDetermine the total energy change for the production of one mole of aqueous nitric acid by this process.
Calculate ΔHrxn for the reaction CaO(s) + CO2(g) → CaCO3(s) given these reactions and their ΔH values: Express the enthalpy in kilojoules to one decimal place.Ca(s) + CO2(g) + 1/2O2 → CaCO3 (s)   ΔH = −814.1 kJ2Ca(s) + O2(g) → 2CaO(s)                    ΔH = −1269.8 kJ
Use the standard reaction enthalpies given below to determine ΔH°rxn for the following reaction: 2S(s) + 3O2(g) → 2SO3(g)     ΔH°rxn = ? Given: SO2(g) → S(s) + O2(g)       ΔH°rxn = +296.8 kJ 2SO2(g) + O2(g) → 2SO3(g)       ΔH°rxn = -197.8 kJ a. -494.6 kJ b. -692.4 kJ c. -791.4 kJ d. -98.8 kJ e. -293.0 kJ
Reaction of gaseous ClF with F 2 yields liquid ClF3, an important fluorinating agent. Use the following thermochemical equations to calculate ΔH°rxn for this reaction:(1) 2CIF(g) + O2(g) ⟶ Cl2O(g) + OF2(g)          ΔH°rxn = 167.5 kJ(2) 2F2(g) + O2(g) ⟶ 2OF2(g)                         ΔH°rxn = −43.5 kJ(3) 2ClF3(l) + 2O2(g) ⟶ Cl2O(g) + 3OF2(g)    ΔH°rxn = 394.1 kJ2ClF  + 2F2 ⟶  2ClF3
A chemist measures the enthalpy change ΔH during the following reaction: 2H2O (l) + O2 (g) → 2H2O2 (l)            ΔH = 196. kJ Use this information to calculate ΔH of each reaction. Round each of your answers to the nearest kJ/mol.
Given the following data:Calculate ΔH for the reaction On the basis of the enthalpy change, is this a useful reaction for the synthesis of ammonia?  
Calculate the enthalpy change of2A + E -->3Cgiven the following:A + 1/2B --> C + D Delta H = -490.0 kJC + B --> 2D + E Delta H = -160.0 kJ
Use the standard reaction enthalpies given below to determine ∆Hrxn for the following reactionP4 (g) + 10 Cl2(g) ---> 4PCI5(s)  ∆Hrxn = ?GivenPCI5(s) --> PCl5(s) + Cl2(g) ∆Hrxn = +157 kJP4(g) + 6 Cl2(g) --> 4 PCl3(g) ∆Hrxn = -1207 kJ-2100. kJ-1050. kJ-1786 kJ-1364 kJ-1835 kJ
Given the following reactions:(1) 2NO --> N2 + O2            ∆H = -180 kJ(2) 2NO + O2 --> 2NO2      ∆H = -112 kJthe enthalpy of the reaction of nitrogen with oxygen to produce nitrogen dioxide2N2 + 4O2 --> 4NO2is _________.a. -136 kJb. 68kJc. 136 kJd. 292kJe. -292 kJ
Consider these reactions, where M represents a generic metal2M(s) + 6HCl(aq) → 2MCl3(aq) + 3H2(g)                H=-748.0 kJHCl(g) → HCl(aq)                                                   H=-74.8H2(g) + Cl2(g) → 2HCl(g)                                       H=-1845MCl3(s) → MCl3(aq)                                               H=-317.0Using the information above to determine the enthalpy of the following reaction2M(s) + 3Cl2(g) → 2MCl3(s)
Calculate the enthalpy of formation of SO2 (g) from the standard enthalpy changes of the following reactions: 2SO2 (g) + O2 (g) → 2SO3(g)             ΔH°rxn = -196 kJ 2S (s) + 3O2 (g) → 2SO3 (g)              ΔH°rxn = -790 kJ S (s) + O2 (g) → SO2 (g)                    ΔH°rxn = ?
A chemist measures the enthalpy change ΔH during the following reaction: CS2 (g) + 3Cl2 (g) → CCl4 (l) + S2Cl2 (l)     ΔH = -304. kJ Use this information to complete the table below. Round each of your answers to the nearest kJ/mol.
Use the following data to determine the Delta H for the conversion of diamond into graphite: Cdiamond (s) + O2 (g) → CO2 (g)    ΔH° = -395.4 kJ 2CO2 (g) → 2CO (g) + O2 (g)     ΔH° = 566.0 kJ 2CO (g) → Cgraphite (s) + CO2 (g)     ΔH° = -172.5 kJ Cdiamond (s) → Cgraphite (s)     ΔH° = ?
Consider the equations below. (1) Ca (s) + CO2 (g) + 1/2O2 (g) → CaCO3 (s) (2) 2Ca (s) + O2 (g) → 2CaO (s) How should you manipulate these equations so that they produce the equation below when added: Check all that apply. CaO (s) + CO2 (g) → CaCO3 (s) (A) reverse the direction of equation (2) (B) multiply equation (1) by 3 (C) multiply equation (2) by 1/2
Given these reactions, X (s) + 1/2O2 (g) → XO (s)     ΔH = -993.7 kJ XCO3 (s) → XO (s) + CO2 (g)     ΔH = +424.5 kJ What is ΔHrxn for this reaction? X (s) + 1/2O2 (g) + CO2 (g) → XCO3 (s) 
How can the first two reactions be combined to obtain the third reaction? Include phases in the balanced chemical equation. 
Calculate Δ Hrxn for the following reaction:CaO(s) + CO2 (g) ---> CaCO3 (s)Use the following reactions and given Δ H values:Ca(s) + CO2(g) + 1/2 O2(g) --> CaCO3 (s), ΔH = -812.8 kJ2Ca(s) + O2 (g) --> 2CaO(s),  ΔH = -1269.8 kJExpress your answer using four significant figures.
Nitroglycerine is a powerful explosive that forms four different gases when detonated:2 C3H5(NO3)3(l) → 3 N2(g) + 1/2 O2(g) + 6 CO2(g) + 5 H2O(g)Calculate the enthalpy change that occurs when 15.0 g of nitroglycerine a detonated. The standard enthalpies of formation are shown below.
Calculate the enthalpy of the reaction 4B (s) + 3O2 (g) → 2B2O3 (s) given the following pertinent information: 1. B2O3 (s) + 3H2O (g)  → 3O2 (g) + B2H6 (g), ΔH°A = +2035 kJ 2. 2B (s) + 3H2 (g) → B2H6 (g), ΔH°B = +36kJ 3. H2 (g) + 1/2O2 (g) → H2O (l), ΔH°C = -285 kJ 4. H2O (l) → H2O (g), ΔH°D = +44 kJExpress your answer with the appropriate units.
Given the standard enthalpy changes for the following two reactions: 4C (s) + 5H2 (g) → C4H10 (g)      ΔH° = -125.6 kJ C2H4 (g) → 2C (s) + 2H2 (g)       ΔH° = -52.3 kJ what is the standard enthalpy change for the reaction: 2C2H4 (g) + H2 (g) → C4H10 (g)  ΔH° = ?
A chemist measures the enthalpy change ΔH during the following reaction: P4 (s) + 6Cl2 (g) → 4 PCl3 (g)       ΔH = -1148.kJ Use this information to complete the table below. Round each of your answers to the nearest kJ/mol.
Research is being carried out on cellulose as a source of chemicals for the production of fibers, coatings and plastics. Cellulose consists of long chains of glucose molecules (C6H12O6), so for the purposes of modeling the reaction we can consider the conversion of glucose to formaldehyde (H2CO). Calculate the heat of reaction for the conversion of 1 mole of glucose into formaldehyde, given the following thermochemical data:H2CO (g) + O2 (g) → CO2 (g) + H2O (g)     ΔH°comb = -572.9 kJ/mol 6C (s) + 6H2 (g) + 3O2 (g) → C6H12O6 (s)     ΔH°f = -1274.4 kJ/mol C (s) + O2 (g) → CO2 (g)     ΔH°f = -393.5 kJ/mol H2 (g) + 1/2O2 (g) → H2O (g)     ΔH°f = -285.8 kJ/mol C6H12O6 (s) → 6H2CO (g)     ΔH°rxn = ?
Find ΔH° for BaCO3 (s) → BaO (s) + CO2 (g) given that 2Ba (s) + O2 (g) → 2BaO (s)     ΔH° = -1107.0 kJBa (s) + CO2 (g) + 1/2O2 (g)→ BaCO3 (s)     ΔH° = - 822.5 kJ a. -1929.5 kJ b. -1376.0 kJ c. -284.5 kJ d. 269.0 kJ e. 537 kJ
Enter your answer in the provided box.From these data, S (rhombic) + O2 (g) → SO2 (g)            ΔH°rxn = -296.06 kJ/mol S (monoclinic) + O2 (g) → SO2 (g)       ΔH°rxn = - 296.36 kJ/mol calculate the enthalpy change for the transformation S (rhombic) → S (monoclinic) (Monoclinic and rhombic are different allotropic forms of elemental sulfur.)
From the enthalpies of reaction 2C(s) + O2(g) → 2CO(g)                             ΔH = -221.0 kJ 2C(s) + O2(g) + 4H2(g) → 2CH3OH(g)       ΔH = -402.4kJ calculate ΔH for the reaction:CO(g) + 2H2 → CH3OH(g) Express your answer with the appropriate units.
Calculate the ΔH° for the following reaction using the information below it. C2H5OH (l) + O2 (g) → CH3CO2H (l) + H2O (l): ΔH° = ? C2H5OH (l) + 1/2O2 (g) → CH3CHO(l) + H2O(l): ΔH° = -175 kJ CH3CHO (l) + 1/2O2 (g) → CH3CO2H (l): ΔH° = -318 kJ (A) 493 kJ (B) 247 kJ (C) 143 kJ (D) -143 kJ (E) -493 kJ
Using data from the following reactions and applying Hess's law, calculate the heat change for the slow reaction of zinc with water: Zn (s) + 2H2O (l) → Zn2+ (aq) + 2OH- (aq) + H2 (g)    ΔH°rxn = ? H+ (aq) + OH- (aq) → H2O (l)     ΔH°rxn, 1 = -56 kJ/mol H2O Zn (s) → Zn2+ (aq)    ΔH°rxn, 2 = -153.9 kJ 1/2 H2 (g) → H+ (aq)     ΔH°rxn, 3 = 0.0 kJ
Given the following reactions: 2S (s) + 3O2 (g) → 2SO3 (g)  H = -840.0 kJ S (s) + O2 (g) → SO2 (g)        H = -300.0 kJCalculate the enthalpy of the following reaction in kJ 2SO2 (g) + O2 (g) → 2SO3 (g)
Given that H2 (g) + F2 (g) → 2HF (g)     ΔH°rxn = - 546.6 kJ 2H2 (g) + O2 (g) → 2H2O (l)     ΔH°rxn = - 571.6 kJ calculate the value of ΔH°rxn for 2F2 (g) + 2H2O (l) → 4HF (g) + O2 (g) 
Calculate the enthalpy of the reaction 2NO (g) + O2 (g) → 2NO2 (g) given the following reactions and enthalpies of formation: 1. 1/2N2 (g) + O2 (g) → NO2 (g), ΔH°A  = 33.2 kJ 2. 1/2N2 (g) + 1/2O2 (g) → NO (g), ΔH°B = 90.2 kJ Calculate the enthalpy of the reaction.Express your answer with the appropriate units.
The industrial process for making sulfuric acid has three steps. Using the data given, calculate the enthalpy change for the overall reaction for the process (the equation is given below).2S(s) + 3O2(g) rightarrow 2H2SO4(l) Data:S(s) + O2(g) → SO(g) Δ Hrxn = -296.83 kJ2SO3(g) → O2(g) + 2SO2(g) ΔHrxn = - 198.2 kJSO3(g) + H2O(l) → H2SO4(l) ΔHrxn = - 227.72 kJSelect one:•-849.9 kJ•-722.8 kJ•-1246.3 kJ•-326.4 kJ
Calculate ΔH°rxn for the reaction 2Ni (s) + 2S (s) + 3O2 (g) → 2NiSO3 (s) from the following information: (1) NiSO3 (s) → NiO (s) + SO3 (g)     ΔH°rxn = 156 kJ (2) S (s) + O2 (g) → SO2 (g)     ΔH°rxn = - 297 kJ (3) Ni (s) + 1/2O2 (g) → NiO (s)     ΔH°rxn = -241 kJ
The combustion reaction of butane is as follows. C4H10(g) + 13/2 O2(g) → 4 CO2(g) + 5 H2O(l) Using Hess's law and the reaction enthalpies given below, find the change in enthalpy for this reaction. reaction (1):    C(s) + O2(g) → CO2(g)                               ΔH = -393.5 kJ/mol reaction (2):   H2(g) + 1/2 O2(g) → H2O(I)                        ΔH = -285.8 kJ/mol reaction (3):   4C(s) + 5H2(g) → C4H10 (g)                       ΔH = -125.7 kJ/mol
Given the following data: N2(g) + O2(g) → 2NO(g),          ΔH = +180.7 kJ 2NO(g) + O2(g) → 2NO2(g),     ΔH = -113.1 kJ 2N2O(g) → 2N2(g) + O2(g),      ΔH = -163.2 kJ Use Hess's law to calculate ΔH for the following reaction: N2O(g) + NO2(g) → 3NO(g) Express the enthalpy in kilojoules to four significant digits. 
Enter your answer in the provided box.You are given the following data: H2 (g) → 2H (g) ΔH° = -436.4 kJ/mol Br2 (g) → 2HBr (g) ΔH° = 192.5 kJ/mol H2 (g) + Br2 (g) → 2HBr (g) ΔH° = -72.4 kJ/mol Calculate ΔH° for the reaction H (g) + Br (g) → HBr (g) 
Given the following data (1) 2ClF (g) + O2 (g) → Cl2O (g) + OF2 (g)         ΔH°rxn = 167.5 kJ (2) 2F2 (g) + O2 (g) → 2OF2 (g)                          ΔH°rxn = -43.5kJ (3) 2ClF3 (l) + 2O2 (g) → Cl2O (g) + 3OF2 (g)    ΔH°rxn = 394.1 kJ Calculate ΔHrxn for the reaction. Show your work! ClF (g) + F2 (g) → ClF3 (l)
Consider the following chemical reaction. NH3(g) + 2 O2(g) → HNO3(aq) + H2O(l) Calculate the change in enthalpy (ΔH) for this reaction, using Hess' law and the enthalpy changes for the reactions given below.
Consider the following hypothetical reactions:A → B          ΔH = +20kJ B → C         ΔH = +66kJ Use Hess's law to calculate the enthalpy change for the reaction A → C. Express your answer using two significant figures. 
The enthalpy of decomposition of NO2Cl is -114 kJ. Use the following data to calculate the heat of formation of NO2Cl from N2, O2, and Cl2: NO2Cl (g) → NO2 (g) + 1/2Cl2(g)    ΔH°rxn = -114 kJ 1/2 N2 (g) + O2 (g) → NO2 (g)    ΔH°f = 33.2 kJ 1/2 N2 (g) + O2 (g) + 1/2 Cl2 (g) → NO2Cl (g)    ΔH°f  = ?
Use the following data to calculate the standard heat (enthalpy) of formation, ΔH °f, of manganese(IV) oxide, MnO2 (s). 2MnO2 (s) → 2MnO (s) + O2 (g)          ΔH = 264 kJ MnO2 (s) + Mn(s) → 2MnO (s)           ΔH = -240 kJ Select one:(a) -504 kJ (b) -372 kJ (c) 24 kJ (d) -24 kJ (e) 504 kJ
Calculate ΔHrxn for the following reaction:CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)Use the following reactions and given ΔH′s.C(s) + 2H2(g) → CH4(g)     ΔH = −74.6kJC(s) + 2Cl2(g) → CCl4(g)    ΔH = −95.7kJH2(g) + Cl2(g) → 2HCl(g)   ΔH = −184.6kJ
From the enthalpies of reaction2 C(s) + O2(g) → 2CO(g)                               ΔH = -221.0 kJ2C(s) + O2(g) + 4H2(g) → 2CH3OH(g)       ΔH = -402.4 kJcalculate ΔH for the reaction:CH3OH(g) → CO(g) + 2H2(g) a) -90.7 kJ      b) 90.7 kJ       c) -623.4 kJ    d) 180.7 kJ     e) 623.4 kJ