Practice Enthalpy in Chemistry

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick 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).

Showing a random 20 of 50 problems.

Example 1

hard
For 4Fe+3O2→2Fe2O3, ΔH=−1648 kJ. Find heat released when 11.2 g of Fe (55.8 g/mol) reacts completely.

Example 2

medium
On an enthalpy diagram, an endothermic reaction shows products at a ____ enthalpy than reactants.

Example 3

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 4

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

Example 5

easy
If forward ΔH=−150 kJ, what is ΔH for the reverse reaction?

Example 6

easy
For C+O2→CO2 with ΔH=−394 kJ, how much heat is released per mole of carbon?

Example 7

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 8

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

Example 9

medium
Is bond breaking endothermic or exothermic, and how does that affect ΔH of a reaction?

Example 10

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

Example 11

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

Example 12

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

Example 13

medium
Reaction: S+O2→SO2, ΔH=−297 kJ. Find ΔH for SO2→S+O2.

Example 14

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

Example 15

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

Example 16

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

Example 17

easy
What is the standard unit of ΔH in chemistry?

Example 18

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 19

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

Example 20

easy
State Hess's law in one sentence.
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