Enthalpy Examples: 45 Problems with Answers

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, ΔH=qp, 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: ΔH<0 means heat is released (exothermic, the surroundings warm up) and ΔH>0 means heat is absorbed (endothermic).

Read the full concept explanation →

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 ΔHf∘ values: NO(g)=+90, NO2(g)=+33 kJ/mol. Find ΔH for 2NO+O2→2NO2.

Answer

ΔH=−114 kJ

First step

1
ΔH=∑ΔHf∘(products)−∑ΔHf∘(reactants).

See the full worked solution + why-it-works coaching

SetupKey insightWhy it worksCommon pitfallConnection

Unlock answer keys One Family plan — every worked solution, all subjects

Example 2

medium
Use bond enthalpies (kJ/mol): H−H=436, Cl−Cl=243, H−Cl=431. Estimate ΔH for H2+Cl2→2HCl.

Example 3

hard
Use Hess's law: (1) 2H2+O2→2H2O, ΔH=−572 kJ; (2) C+O2→CO2, ΔH=−394 kJ; (3) C+2H2→CH4, ΔH=−75 kJ. Find ΔH for CH4+2O2→CO2+2H2O.

Example 4

hard
For combustion of glucose C6H12O6+6O2→6CO2+6H2O, ΔH=−2803 kJ. Find heat released by combustion of 9.0 g of glucose (180 g/mol).

Example 5

hard
For 4Fe+3O2→2Fe2O3, ΔH=−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 ΔH for CH4+2O2→CO2+2H2O.

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 ΔH positive or negative?

Example 2

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

Example 3

easy
Write the definition of ΔH in terms of product and reactant enthalpies.

Example 4

easy
A reaction has ΔH=−890 kJ. Is it exothermic or endothermic?

Example 5

easy
If the forward reaction has ΔH=+200 kJ, what is ΔH for the reverse?

Example 6

easy
Is ΔH the same thing as temperature?

Example 7

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

Example 8

easy
For C+O2→CO2 with ΔH=−394 kJ, how much heat is 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 CH4?

Example 10

medium
Reaction A→B has ΔH=+50 kJ; B→C has ΔH=−30 kJ. Find ΔH for A→C.

Example 11

medium
For 2H2+O2→2H2O, ΔH=−572 kJ. Find ΔH for H2+12O2→H2O.

Example 12

medium
Reaction X→Y has ΔH=−80 kJ. Find ΔH for 2Y→2X.

Example 13

medium
Given ΔHf: CO2=−394, H2O=−286 kJ/mol, and CH4=−75 kJ/mol, set up ΔH for CH4+2O2→CO2+2H2O.

Example 14

medium
A reaction's ΔH=−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 ΔH 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→Q has ΔH=−120 kJ. Find ΔH for 3P→3Q.

Example 18

challenge
Use Hess's law: C+O2→CO2 (−394 kJ) and CO+12O2→CO2 (−283 kJ). Find ΔH for C+12O2→CO.

Example 19

challenge
Combustion of 4.0 g of CH4 (16 g/mol) releases how much heat if ΔH=−890 kJ/mol?

Example 20

challenge
Given N2+3H2→2NH3, ΔH=−92 kJ. Find ΔH for 12N2+32H2→NH3 and for 2NH3→N2+3H2.

Example 21

easy
What is the standard unit of ΔH in chemistry?

Example 22

easy
State Hess's law in one sentence.

Example 23

easy
If a reaction has ΔH=+120 kJ, classify it as exothermic or endothermic.

Example 24

easy
For H2(g)+Cl2(g)→2HCl(g), ΔH=−184 kJ. How much heat is released when 1 mol of H2 reacts?

Example 25

easy
What is the value of ΔHf∘ 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+O2→SO2, ΔH=−297 kJ. Find ΔH for SO2→S+O2.

Example 28

medium
Burning 1 mol C2H2 (acetylene) releases 1300 kJ. Find ΔH for 2C2H2+5O2→4CO2+2H2O.

Example 29

medium
If 50.0 g of H2O (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→B, ΔH=−80 kJ. Find ΔH for A→12B.

Example 31

medium
A reaction has ΔH=−240 kJ per mol of A. How much heat is released using 0.75 mol of A?

Example 32

medium
Reaction R1: A→B, ΔH1=+30 kJ. R2: B→C, ΔH2=−50 kJ. Find ΔH for C→A.

Example 33

medium
Standard enthalpy of formation of H2O(l) is −286 kJ/mol. Find ΔH for H2(g)+12O2(g)→H2O(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 ΔH 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 ΔH of solution in kJ/mol.

Example 36

hard
Given ΔHf∘ (kJ/mol): CH4=−75, H2O(l)=−286, CO2=−394. Find ΔH for CH4+2O2→CO2+2H2O(l).

Example 37

hard
For CaCO3(s)→CaO(s)+CO2(g), ΔHf∘ values (kJ/mol) are CaCO3=−1207, CaO=−635, CO2=−394. Find ΔH.

Example 38

hard
A reaction's ΔH=+85 kJ. At constant pressure, 1.00 mol reacts in a system that does 5.0 kJ of work on the surroundings. Find ΔU for this process.

Example 39

challenge
Given that the standard enthalpy of vaporization of water at 100°C is +40.7 kJ/mol and ΔHf∘[H2O(l)]=−285.8 kJ/mol, estimate ΔHf∘[H2O(g)].
← Back to Enthalpy Formula Explained Practice Mode

Background Knowledge

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

exothermicendothermicactivation energy