Enthalpy Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Enthalpy.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A thermodynamic state function $H = U + pV$ ; at constant pressure the enthalpy change equals the heat exchanged, $\Delta H = q_p$ , which is negative for exothermic and positive for endothermic processes.

Enthalpy change tells you how much heat a reaction releases or absorbs at constant pressure: $\Delta H < 0$ means heat is released (exothermic, the surroundings warm up) and $\Delta H > 0$ means heat is absorbed (endothermic).

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How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Enthalpy starts by naming reactants and products, then checks conservation with a balanced equation.

Common stuck point: Students often know a formula related to enthalpy but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?

Worked Examples

Example 1

medium

Given

\Delta H_f^\circ

values:

NO(g) = +90

NO_2(g) = +33

kJ/mol. Find

\Delta H

for

2NO + O_2 \rightarrow 2NO_2

Answer

\Delta H = -114 \text{ kJ}

First step

\Delta H = \sum \Delta H_f^\circ \text{(products)} - \sum \Delta H_f^\circ \text{(reactants)}

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Example 2

medium

Use bond enthalpies (kJ/mol):

H{-}H = 436

Cl{-}Cl = 243

H{-}Cl = 431

. Estimate

\Delta H

for

H_2 + Cl_2 \rightarrow 2HCl

Example 3

hard

Use Hess's law: (1)

2H_2 + O_2 \rightarrow 2H_2O

\Delta H = -572

kJ; (2)

C + O_2 \rightarrow CO_2

\Delta H = -394

kJ; (3)

C + 2H_2 \rightarrow CH_4

\Delta H = -75

kJ. Find

\Delta H

for

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

Example 4

hard

For combustion of glucose

C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O

\Delta H = -2803

kJ. Find heat released by combustion of

9.0

g of glucose (180 g/mol).

Example 5

hard

For

4Fe + 3O_2 \rightarrow 2Fe_2O_3

\Delta H = -1648

kJ. Find heat released when

11.2

g of

Fe

(55.8 g/mol) reacts completely.

Example 6

challenge

Use bond enthalpies (kJ/mol):

C{-}H = 414

O{=}O = 498

C{=}O = 799

O{-}H = 463

. Estimate

\Delta H

for

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

If a reaction releases heat, is

\Delta H

positive or negative?

Exothermic reaction — what is the sign of ΔH?

Example 2

easy

If a reaction absorbs heat, is it endothermic or exothermic?

Is this reaction endothermic or exothermic?

Example 3

easy

Write the definition of

\Delta H

in terms of product and reactant enthalpies.

Example 4

easy

A reaction has

\Delta H = -890

kJ. Is it exothermic or endothermic?

Energy profile for a reaction with ΔH = −890 kJ — classify it

Example 5

easy

If the forward reaction has

\Delta H = +200

kJ, what is

\Delta H

for the reverse?

Forward reaction ΔH = +200 kJ (endothermic) — what is ΔH for the reverse?

Example 6

easy

\Delta H

the same thing as temperature?

Example 7

easy

When you double all coefficients of a reaction, what happens to

\Delta H

Example 8

easy

For

C + O_2 \rightarrow CO_2

with

\Delta H = -394

kJ, how much heat is released per mole of carbon?

C + O₂ → CO₂ with ΔH = −394 kJ — heat released per mole of carbon?

Example 9

medium

Combustion of methane releases 890 kJ per mole. How much heat is released burning 2.0 mol

CH_4

Example 10

medium

Reaction

A \rightarrow B

has

\Delta H = +50

kJ;

B \rightarrow C

has

\Delta H = -30

kJ. Find

\Delta H

for

A \rightarrow C

A → B (ΔH = +50 kJ) then B → C (ΔH = −30 kJ): net ΔH for A → C?

Example 11

medium

For

2H_2 + O_2 \rightarrow 2H_2O

\Delta H = -572

kJ. Find

\Delta H

for

H_2 + \frac{1}{2}O_2 \rightarrow H_2O

Example 12

medium

Reaction

X \rightarrow Y

has

\Delta H = -80

kJ. Find

\Delta H

for

2Y \rightarrow 2X

Example 13

medium

Given

\Delta H_f

CO_2 = -394

H_2O = -286

kJ/mol, and

CH_4 = -75

kJ/mol, set up

\Delta H

for

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O

Example 14

medium

A reaction's

\Delta H = -100

kJ for 1 mol. If only 0.25 mol reacts, how much heat is released?

Example 15

medium

Is bond breaking endothermic or exothermic, and how does that affect

\Delta H

of a reaction?

Example 16

medium

Combustion of propane releases 2220 kJ per mole. How much heat is released burning 0.50 mol?

Example 17

medium

Reaction

P \rightarrow Q

has

\Delta H = -120

kJ. Find

\Delta H

for

3P \rightarrow 3Q

Example 18

challenge

Use Hess's law:

C + O_2 \rightarrow CO_2

(

-394

kJ) and

CO + \frac{1}{2}O_2 \rightarrow CO_2

(

-283

kJ). Find

\Delta H

for

C + \frac{1}{2}O_2 \rightarrow CO

Example 19

challenge

Combustion of 4.0 g of

CH_4

(16 g/mol) releases how much heat if

\Delta H = -890

kJ/mol?

Example 20

challenge

Given

N_2 + 3H_2 \rightarrow 2NH_3

\Delta H = -92

kJ. Find

\Delta H

for

\frac{1}{2}N_2 + \frac{3}{2}H_2 \rightarrow NH_3

and for

2NH_3 \rightarrow N_2 + 3H_2

Example 21

easy

What is the standard unit of

\Delta H

in chemistry?

Example 22

easy

State Hess's law in one sentence.

Example 23

easy

If a reaction has

\Delta H = +120

kJ, classify it as exothermic or endothermic.

Example 24

easy

For

H_2(g) + Cl_2(g) \rightarrow 2HCl(g)

\Delta H = -184

kJ. How much heat is released when 1 mol of

H_2

reacts?

Example 25

easy

What is the value of

\Delta H_f^\circ

for an element in its standard state?

Example 26

medium

Combustion of ethanol releases

1367

kJ/mol. How much heat is released burning

0.20

mol of ethanol?

Example 27

medium

Reaction:

S + O_2 \rightarrow SO_2

\Delta H = -297

kJ. Find

\Delta H

for

SO_2 \rightarrow S + O_2

Example 28

medium

Burning 1 mol

C_2H_2

(acetylene) releases

1300

kJ. Find

\Delta H

for

2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O

Example 29

medium

If 50.0 g of

H_2O

(

c = 4.18

J/(g·K)) warms from

20.0

°C to

80.0

°C, how much heat (kJ) was absorbed?

Example 30

medium

For

2A \rightarrow B

\Delta H = -80

kJ. Find

\Delta H

for

A \rightarrow \tfrac{1}{2}B

Example 31

medium

A reaction has

\Delta H = -240

kJ per mol of

A

. How much heat is released using

0.75

mol of

A

Example 32

medium

Reaction

R_1

A \rightarrow B

\Delta H_1 = +30

kJ.

R_2

B \rightarrow C

\Delta H_2 = -50

kJ. Find

\Delta H

for

C \rightarrow A

Example 33

medium

Standard enthalpy of formation of

H_2O(l)

-286

kJ/mol. Find

\Delta H

for

H_2(g) + \tfrac{1}{2}O_2(g) \rightarrow H_2O(l)

Example 34

medium

A coffee-cup calorimeter (constant pressure) records

q = -2.5

kJ for a reaction with 0.10 mol of limiting reagent. Find

\Delta H

per mole.

Example 35

hard

10.0

g of

NaOH

(40 g/mol) dissolves in

100

g of water and raises the temperature by

11.9

°C (assume

c = 4.18

J/(g·K) for solution, mass ≈ 110 g). Find

\Delta H

of solution in kJ/mol.

Example 36

hard

Given

\Delta H_f^\circ

(kJ/mol):

CH_4 = -75

H_2O(l) = -286

CO_2 = -394

. Find

\Delta H

for

CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O(l)

Example 37

hard

For

CaCO_3(s) \rightarrow CaO(s) + CO_2(g)

\Delta H_f^\circ

values (kJ/mol) are

CaCO_3 = -1207

CaO = -635

CO_2 = -394

. Find

\Delta H

Example 38

hard

A reaction's

\Delta H = +85

kJ. At constant pressure,

1.00

mol reacts in a system that does

5.0

kJ of work on the surroundings. Find

\Delta U

for this process.

Example 39

challenge

Given that the standard enthalpy of vaporization of water at 100°C is

+40.7

kJ/mol and

\Delta H_f^\circ[H_2O(l)] = -285.8

kJ/mol, estimate

\Delta H_f^\circ[H_2O(g)]

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Related Concepts

Exothermic Reaction Endothermic Reaction Activation Energy

Background Knowledge

These ideas may be useful before you work through the harder examples.

exothermicendothermicactivation energy