A selection of practice exam calculation questions is presented dealing with enthalpy of formation, enthalpy of combustion, standard enthalpy measurements and data and problem solving using Hess's Law including enthalpy of reaction via bond enthalpy calculations. Enthalpy questions are set out on this page and the logical deductions of the answers are set out on another page. I hope this will help you to be more proficient in solving enthalpy data problem calculations.

Some corrections and re-editing have been applied to questions 1, 3, 4 and 8 concerning bond enthalpy (bond energy) calculations and apologies for still not typing up the original scribbled out answers, though the additional calculations have been typed up neatly on the answer page!

Associated LINKS

1. Thermodynamics Part 1: Thermochemistry - Calorimetry experiments - Enthalpies of reaction, formation, combustion and bond dissociation are explained with exemplar calculations

2.  ANSWERS to the ΔH enthalpy Q's below

3. GCSE/IGCSE/O Level notes Energy Changes, exothermic/endothermic reactions

See also fully worked out examples from calorimeter data and using Hess's Law cycles to solve problems

Question 1

(a) Define the standard enthalpy of (i) combustion, Δ^H_θc, and (ii) formation, ΔH^θ_f

(b) Given the following standard enthalpies of combustion, ΔH^θ_c (298K, 1 atm)

C_(s) -393 kJmol^-1; H_2(g) -285.6 kJmol^-1; C₂H_6(g) -1560 kJmol^-1

Note: the first two enthalpies correspond to the standard enthalpy of formation of carbon dioxide and water respectively

Calculate the standard enthalpy of formation of ethane, ΔH^θ_f(C₂H₆(g))

(c) Given the following bond enthalpies (bond energies) in kJmol^-1

bond ΔH_BE: C-H 412; C-C 347; O-H 464; O=O 498; C=O 805 (for CO₂ only); C-O 358

Note in other questions the C=O bond enthalpy is 743 kJmol^-1 or a similar value in organic molecules such as aldehydes, ketones, esters, carboxylic acids etc. BUT NOT in CO₂ (eg see Question 4)

(i) Calculate the enthalpy of combustion of ethane assuming all the species are gaseous.

(ii) Why in (i) do you not get the value of -1560 kJ mol^-1?

Question 2

Given the following standard enthalpies of combustion ΔH^θ_c (298K, 1 atm)

C_(s) -393 kJmol^-1; H_2(g) -285.6 kJmol^-1; C₈H_18(l) -5512 kJmol^-1

Calculate the standard enthalpy of formation of octane, ΔH^θ_f(C₈H₁₈(l))

Question 3

(a) Given the following standard enthalpies of combustion ΔH^θ_c(298K, 1 atm)

C(s) -393 kJmol^-1; H₂(g) -285.6kJ mol^-1

and the standard enthalpy of formation of liquid cyclohexane, ΔH^θ_f(C₆H₁₂(l)) = -156 kJ mol^-1

Calculate the standard enthalpy of combustion of cyclohexane, ΔH^θ_c(C₆H₁₂(l))

(b) Using the appropriate bond energies from Q1c, calculate the theoretical enthalpy of combustion of cyclohexane, assuming ALL reactants and products are gases.

Remember, you can only calculate enthalpy of reaction changes from bond enthalpies if all the species are gaseous!

(c) If the enthalpy of vapourisation of water is +40.7 kJmol^-1 and the enthalpy of vapourisation of cyclohexane is +30.0 kJmol^-1, from your answer to (b), recalculate the enthalpy of combustion of cyclohexane.

You need to think about which state changes are endothermic and which state changes are exothermic!

(d) Compare and comment on your answers to (a), (b) and (c) and, where appropriate, quote percentage errors.

Question 4

(a) Given the following standard enthalpies of combustion ΔH^θ_c (298K, 1 atm)

C(s) -393 kJmol^-1; H₂(g) -285.6 kJmol^-1;

and the enthalpy of formation of ethanoic acid, ΔH^θ_f(CH₃COOH(l)) = -487 kJ mol^-1

Calculate the standard enthalpy of combustion of ethanoic acid, ΔH^θ_c(CH₃COOH(l))

(b) Using the bond enthalpies listed in Q1(c), and assuming all reactants and products are gases, calculate the enthalpy of combustion of ethanoic acid, noting that the bond enthalpy for C=O is +743 kJmol^-1 when it is NOT in the carbon dioxide molecule.

(c) If the enthalpy of vaporisation of ethanoic acid is 51.6 kJmol^-1 and the enthalpy of vaporisation of water is 40.7 kJmol^-1, use your answer from (b) to calculate the enthalpy of combustion of ethanoic acid under standard conditions, ie a corrected value to take into account the standard states of reactants and products at 298K/1atm.

(d) Compare and comment on the three values of ΔH_comb(ethanoic acid) calculated in parts (a), (b) and (c).

Question 5

Given the following standard enthalpies of formation ΔH^θ_f(298K, 1 atm)

NH_3(g) -46.2 kJmol^-1; HCl_(g) -92.3 kJmol^-1; NH₄Cl_(s) -315.0 kJmol^-1

Calculate the standard enthalpy change for the reaction, ΔH^θ_r

NH₄Cl_(s) ====> NH_3(g) + HCl_(g)

Question 6

Given the following standard enthalpies of formation,  ΔH^θ_f, in kJmol^-1 (298K, 1 atm)

CH_4(g) -74.9; CH₃Br_(l) -36.0; HBr_(g) -36.2

Calculate the standard enthalpy change for the reaction, ΔH^θ_r

Br_2(l) + CH_4(g) ====> CH₃Br_(l) + HBr(g)