Enthalpies of Formation

In a formation equation one mole of product is formed from the standard or elemental forms of each element. 

Formation Equation

Concept: Understanding a Formation Equation

Video Transcript

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 Formation

Video Transcript

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. 

Enthalpy of a Reaction

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.


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.


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

O=O 498

Calculate the enthalpy of the reaction shown in the formula below: 


When given bond energies then calculating the enthalpy of the reaction requires a different equation. 

Enthalpies of Formation Additional Practice Problems

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

Watch Solution

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

Watch Solution

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

Watch Solution

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

Watch Solution

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)

Watch Solution

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.


Watch Solution

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

Watch Solution

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.

Watch Solution

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)



Watch Solution

Find the ΔH of for propanol, C3H8O(l), given the following information:

C3H8O (I) + 9/2 O 2 (g) → 3 CO(g) + 4 H2O (g)     ΔH ocomb = 2021 kJ
ΔHoof CO2 (g) = - 393.5 kJ/mol ; ΔH oof 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



Watch Solution

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


Watch Solution

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

Watch Solution

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

Watch Solution

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

Watch Solution

Consider the reaction of N2O5 at 25°C for which the following data are relevant:

2 N2O(g) → 4 NO(g) + O(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

Watch Solution

Which of the following does not have a standard enthalpy of formation equal to zero at 25°C and 1.0 atm?

a) F(g) 

b) Al (s)

c) H(g)

d) H2O (l) 

e) They all have a standard enthalpy equal to zero.


Watch Solution

2C2H2  +  5 O2  →  4CO2  +  2H2

Use the given standard enthalpies of formation to calculate ∆H for this reaction

C22 = 227.4

CO2 = -393.5

H2O = -241.8

  1. 1256.2 kJ      b. -1256.2 kJ       c. -2512.4 kJ        d. 2512.4 kJ
Watch Solution

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?

Watch Solution

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. 

Watch Solution

Given the data in the table below, ΔH°rxn for the reaction

Ca(OH)2 + 2H3AsO4 → Ca(H2AsO4)2 + 2H2O

a) -4219

b) -130.4

c) -4519

d) -76.4

e) -744.9

Watch Solution

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.


a) -3351

b) -1676

c) +3351

d) -16.43

e) -32.86

Watch Solution

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.

CH(g) + 2 O(g) → CO(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.]

a) 741.54

b) 881.88

c) 1088.51

d) 1117.99

Watch Solution

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.



Watch Solution

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°= -74.1 kJ/mol                   

C8H18(g)   ΔH°f = -224.1 kJ/mol


Watch Solution

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)

Watch Solution

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) 

Watch Solution

Which of

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)

Watch Solution

Determine the answer for the question below.

Watch Solution

Use the information provided to determine ΔH°rxn for the following reaction:

CH4(g) + 3 Cl2(g) → CHCl3(l) + 3 HCl(g

            ΔH°f (kJ/mol)

CH4(g)        -75

CHCl3(l)    -134

HCl(g)         -92

A) -151 kJ

B) -335 kJ

C) +662 kJ

D) +117 kJ

E) -217 kJ

Watch Solution

ΔHf° is not zero for which one of the following:

  1. 2(g)
  2. C (graphite)
  3. F2(s)
  4. H2(g)
  5. Br­2(l)
Watch Solution

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

Watch Solution

Which of the following has a non-zero ΔHf°?


a. O2 (l)

b. C (graphite)

c. N2 (g)

d. F2 (g)

e. Cl2 (g)

Watch Solution