In a formation equation one mole of product is formed from the standard or elemental forms of each element.
Concept: Understanding a Formation Equation2m
Welcome back, guys. In this new video, we're going to take a look at the standard heats of formation.
We're going to say in a formation equation, one mole of a compound forms from its elemental forms. We're going say that delta F represents our heat of formation. And it's just the enthalpy change for the chemical reaction when all the substances are in their standard states.
Basically, a formation equation will be the one I provide here. The natural state of carbon in our environment is C graphite. The natural form of hydrogen in our environment is H2. Remember, hydrogen is a diatomic molecule. We're going to say that they combine together to give us one mole of this compound. CH4 is known as methane. This would be the formation equation for one mole of methane compound. This represents a formation equation. We have the elements in their natural states as reactants combining together to give us one mole of some type of product.
Concept: Understanding Heats of Formation2m
Now what we should remember, when it comes to the heats of formation, there's certain things that we need to remember. We're going to say one, an element in its elemental form or standard state has a delta H of formation of zero. When I mean natural state or standard state, that means that the element is either by itself, like sodium solid, or it's connected to copies of itself like P4 or Cl2 or S8. These guys are connected to copies of themselves. If an element is in its standard state, it's delta H value is zero.
We're going to say most compounds have a negative delta H of formation. Because, remember, we said when you're forming bonds, that's an exothermic process. We said earlier, exothermic reactions have a delta H value that is negative.
Finally, we can use these heats of formations in order to find the enthalpy of our reaction. To find the enthalpy of our reaction, we just simply do products minus reactants.
The enthalpy value associated with an element or compound can be used to find the enthalpy of a reaction.
Example: The oxidation of ammonia is given by the following reaction:
4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2O (g)
Calculate the Horxn if the Hof value for NH3 , NO and H2O are -45.9 kJ/mol, 90.3 kJ/mol and -241.8 kJ/mol respectively.3m
The enthalpy of a reaction can be determined if we are given the enthalpy of formation value for each compound.
Problem: Ibuprofen is used as an anti-inflammatory agent used to deal with pain and bring down fevers. If it has a molecular formula of C13H18O2 determine the balanced chemical equation that would give you directly the enthalpy of formation for ibuprofen.2m
Example: Use the following bond strength values (kJ/mol):
C–H 412 C–O 360 C=O 743
C–C 348 H–H 436 C=C 611
C≡C 837 C≡O 1072 O–H 464
Calculate the enthalpy of the reaction shown in the formula below:7m
When given bond energies then calculating the enthalpy of the reaction requires a different equation.
Which of the following equations represents a reaction that provides the enthalpy of formation of NaClO3?
A. NaClO(s) + O2(g) → NaClO3(s)
B. NaClO2(s) + 1/2 O2(g) → NaClO3(s)
C. NaClO2(s) + O(g) → NaClO3(s)
D. Na(s) + Cl(g) + 3/2 O2(g) → NaClO3(s)
E. Na(s) + 1/2 Cl2(g) + 3/2 O2(g) → NaClO3(s)
Calculate ΔH for the reaction C4H4(g) + 2H2(g) → C4H8(g), using the following information:
ΔHcombustion for C4H4(g)= -2341 kJ/mol
ΔHcombustion for C4H8(g)= -3071 kJ/mol
ΔHcombustion for H2(g) = -286 kJ/mol
a. -128 kJ
b. -316 kJ
c. -158 kJ
d. -700 kJ
e. -4810 kJ
Calculate ΔH for the following reaction using the given enthalpies of formation.
2NO(g) + 5O2(g) → 2NO2(g)
ΔHfNO(g) = 90.25 kJ mol-1
ΔHfNO2(g) = 33.18 kj mol-1
a. +57.07 kJ
b. -57.07 kJ
c. +114.14 kJ
d. -114.14 kJ
e. +66.36 kJ
What is the standard enthalpy of reaction ΔH orxn for the following reaction using the standard enthalpies of formation provided ?
SiCl4(l) + 2H2(g) + O2(g) → SiO2 (s) + 4HCl(g) ΔHorxn = ?
ΔH°f(SiCl4) = -640.1 kJ/mol
ΔH°f(HCl) = -92.3 kJ/mol
ΔH°f(SiO2) = -910.9 kJ/mol
The thermite reaction, in which powdered aluminum reacts with iron oxide, is highly exothermic. Use the standard enthalpies of formation given to find the ∆H°rxn for the thermite reaction.
2 Al + Fe2O3 → Al2O3 + 2 Fe
∆H Fe2O3 = -824.2 kJ/mol
∆H Al2O3 = -1675.7 kJ/mol
Determine the enthalpy of reaction for
4 NH3(g) + 7 O2(g) → 4 NO2(g) + 6 H2O(l)
1. −1397.6 kJ/mol rxn
2. +2034.4 kJ/mol rxn
3. −1899 kJ/mol rxn
4. −298.7 kJ/mol rxn
5. −1766 kJ/mol rxn
The value of ∆H0 for which of the following reactions is referred to as the standard molar enthalpy of formation of carbon monoxide?
1. C(g) + 12 O2(g) → CO(g)
2. C(graphite) + 1/2 O2(g) → CO(g)
3. C(graphite) + O(g) → CO(g)
4. C(g) + O(g) → CO(g)
Calculate ΔH°rxn for the following reaction and indicate whether it is exothermic or endothermic.
2H2 O2 (l) →2H2 O(l) + O2 (g)
Given that ΔH°f[H2 O(l)] = -285.8 kJ/mol and ΔH°f[H2 O2 (l)] = -187.6 kJ/mol.
Given the data in the table below, what is ΔH°rxn ?
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l)
A) -1172 kJ
B) -150 kJ
C) - 1540 kJ
D) -1892 kJ
Consider 0.0110 moles of HCl consumed in the chemical reaction at standard temperature and pressure:
2 HCl (aq) + Mg (s) → MgCl2 (aq) + H2 (g)
ΔHf° (HCl) = −167.2 kJ; ΔH f° (MgCl2) = −801.2 kJ
Identify the statements below as either true or false.
Magnesium is undergoing oxidation.
Mg has the most protons of any atom in the reaction.
The reaction will form 0.123 L of H 2 gas.
The reaction will also consume 0.134 g of Mg.
The ΔH of the reaction cannot be calculated.
The energy change from the reaction, q rxn = −2.57 kJ.
For which one of the following reactions is ΔH°rxn equal to the heat of formation of the product?
A) 12C (g) + 11H2 (g) + 11O (g) → C6H22O11 (g)
B) N2 (g) + 3H2 (g) → 2NH3 (g)
C) P (g) + 4H (g) + Br (g) → PH4Br (l)
D) 1/2 N2 (g) + O2 (g) → NO2 (g)
E) 6C (s) + 6H (g) → C6H6 (l)
For which should the standard heat of formation ΔH°f, be zero at 25°C?
D) all the above
Find the ΔH of for propanol, C3H8O(l), given the following information:
C3H8O (I) + 9/2 O 2 (g) → 3 CO2 (g) + 4 H2O (g) ΔH ocomb = 2021 kJ
ΔHof of CO2 (g) = - 393.5 kJ/mol ; ΔH of of H2O (g) = - 241.8 kJ/mol
A. -1642.5 kJ
B. -2021 kJ
C. -4168.7 kJ
D. -2656.3 kJ
E. -126.7 kJ
Find the standard heat of formation (ΔH f) for butanol, C4H10O(l) given the following combustion reaction and information below:
2C4H10O (l) + 12O2 (g) → 8CO2 (g) + 10H2O (l) ΔH = -2676 kJ
H2O (l) ΔH f = -285.8 kJ/mol
CO2 (g) ΔHf = -393.5 kJ/mol
a. + 832.5 kJ
b. - 998.4 kJ
c. - 1665 kJ
d. - 1483 kJ
e. - 378.9 kJ
The combustion of ammonia is represented by the equation.
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) ΔH=-904.8 kJ
What is the enthalpy of formation of NH 3(g)?
Enthalpy of Formation Data
NO(g) +90.4 kJ/mol
H2O(g) -241.8 kJ/mol
A) -449 kJ/mol
B) -46.1 kJ/mol
C) -184 kJ/mol
D) 227 kJ/mol
Calculate the enthalpy change (ΔH°rxn ) for the following reaction at 25°C. The value of ΔH°f in kJ/mol is given below.
2 Fe2O3(s) + 6 CO(g) → 4 Fe(s) + 6 CO2(g)
A. −1380 kJ
B. −49.6 kJ
C. 541 kJ
D. −3350 kJ
E. −24.8 kJ
If the combustion of liquid octane, C 8H18, has a heat of reaction that is equal to –10.94 kJ, calculate the standard heat of formation of CO2.
Consider the reaction of N2O5 at 25°C for which the following data are relevant:
2 N2O5 (g) → 4 NO2 (g) + O2 (g)
Substance ΔH°f S°
N 2O5 11.29 kJ/mol 355.3 J/K mol
NO 2 33.15 kJ/mol 239.9 J/K mol
O 2 ? 204.8 J/K mol
What is ΔH° for the reaction?
a) 110.02 kJ
b) 21.86 kJ
c) -21.86 kJ
d) 155.20 kJ
e) -155.20 kJ
Which of the following does not have a standard enthalpy of formation equal to zero at 25°C and 1.0 atm?
a) F2 (g)
b) Al (s)
c) H2 (g)
d) H2O (l)
e) They all have a standard enthalpy equal to zero.
2C2H2 + 5 O2 → 4CO2 + 2H2O
Use the given standard enthalpies of formation to calculate ∆H for this reaction
C2H2 = 227.4
CO2 = -393.5
H2O = -241.8
Urea, CO(NH2)2(s), is an important molecule. Our bodies convert the nitrogen from metabolizing proteins in urea which is less toxic than ammonia and can be stored in the kidneys until released. (Fish, on the other hand, release ammonia directly into water through their gills.) Urea was the first “biological” compound to be synthesized entirely from inorganic compounds. Before then, biological processes were thought to require a “vital force” that only existed in living systems.
Industrial synthesis of urea generally follows the Bosch–Meiser process. A balanced chemical equation for the overall reaction is given below.
2NH3(l) + CO2(g) → (NH2)2CO(s) + H2O(l)
Determine ΔH°rxn for the above reaction.
Is this reaction endothermic or exothermic?
Consider the partial oxidation of ethane as shown in the following reaction:
C2H6(g) + O2(g) → 2CO(g) + 3H2(g)
Now calculate ΔH° for the partial oxidation of ethane. The enthalpies of formation for C2H6(g) and CO(g) are -84.68 kJ/mol and -110.5 kJ/mol, respectively.
Given the data in the table below, ΔH°rxn for the reaction
Ca(OH)2 + 2H3AsO4 → Ca(H2AsO4)2 + 2H2O
The value of ΔH° for the following reaction is -3351 kJ:
2Al (s) + 3O2 (g) → 2Al2O3 (s)
The value of ΔHf° for Al2O3 (s) is __________ kJ.
The thermochemical reaction for the combustion of 1 mole of methane is given below. From the enthalpy and the bond dissociation enthalpies (given below), determine the bond dissociation enthalpy of the C=O in CO2.
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g) ΔH = -669.68 kJ
[ΔH(C-H) = 418.0 kJ/mol ; ΔH(O=O) = 498.7 kJ/mol ; ΔH(H-O) = 464.0 kJ/mol.]
Consider the following reactions:
1. 2Al(s) + 3/2 O2 (g) → Al2O3 (s) ΔH° = -1669.8 kJ/mol
2. H2 (g) + 1/2 O2 (g) → H2O (l) ΔH° = -285.8 kJ/mol
3. H2O (s) → H2O (l) ΔH° = 6.01 kJ/mol
For which, if any, reactions does ΔH° correspond to a molar enthalpy of formation?
Write the oxidation numbers in the spaces provided.
It is often argued that natural gas (CH4) is preferred to petroleum (which we will approximate as isooctane, C8H18) in generating energy in that it produces less CO 2. The balanced combustion reactions for CH4 and C8H18 are given below.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(g)
Using the following heats of formation, calculate Joules of energy generated per mole of CO2 produced for each combustion. Is methane better than petroleum in lowering CO2 production (in other words, does it produce more CO2 per joules of energy)?
H2O(g) ΔH°f = -241.83 kJ/mol
CO2(g) ΔH°f = -395.51 kJ/mol
CH4(g) ΔH°f = -74.1 kJ/mol
C8H18(g) ΔH°f = -224.1 kJ/mol
Predict the value of ΔHo f is greater than, less than or equal to zero for the following elements at 25oC.
a) Pb (g)
b) S8 (s, rhombic)
Predict the value of ΔHo f is greater than, less than or equal to zero for the following elements at 25oC.
a) Cl2 (s) b) Hg (g)
O2 (g), O2 (ℓ), H2 (g), H2 (ℓ), H2O (g), H2O (ℓ)
have a heat of formation equal to zero?
1. O2(g), O2(ℓ), H2(g), H2(ℓ)
2. O2(g), H2(g)
3. O2(g), O2(ℓ), H2(g), H2(ℓ), H2O(g), H2O(ℓ)
4. All of them, but only at absolute zero
5. O2(g), H2(g), H2O(g)
Determine the answer for the question below.
Use the information provided to determine ΔH°rxn for the following reaction:
CH4(g) + 3 Cl2(g) → CHCl3(l) + 3 HCl(g
A) -151 kJ
B) -335 kJ
C) +662 kJ
D) +117 kJ
E) -217 kJ
ΔHf° is not zero for which one of the following:
For which of the following reaction(s) is the enthalpy change for the reaction equal to ∆Hf° of the product?
I. 2 F (g) → F2 (g)
II. H2 (g) + O2 (g) → H2O2 (l)
III. C (graphite) → C (diamond)
A. Only I
B. Only II
C. Only III
D. I and II
E. II and III
Which of the following has a non-zero ΔHf°?
a. O2 (l)
b. C (graphite)
c. N2 (g)
d. F2 (g)
e. Cl2 (g)