DOC BROWN'S Science-CHEMISTRY HOMEPAGE KS3 SCIENCE QUIZZES and WORKSHEETS (~US grades 6-8)
GCSE SCIENCE help links GCSE ADDITIONAL SCIENCE help links
KS3 BIOLOGY Quizzes KS3 CHEMISTRY Quizzes & Worksheets KS3 PHYSICS Quizzes
KS4 Science GCSE/IGCSE CHEMISTRY NOTES (~US grades 8-10) KS4 Science GCSE/IGCSE CHEMISTRY QUIZZES and WORKSHEETS (~US grades 8-10) ADVANCED LEVEL CHEMISTRY QUIZZES and WORKSHEETS (~US grades 11-12)
Custom Search

(c) doc bExtra ΔH QUESTIONS for Advanced Level Chemistry

Enthalpy Calculation Revision Questions

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.

Recently, 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

topQuestion 1

(a) Define the standard enthalpy of (i) combustion, ΔHθc, and (ii) formation, ΔHθf

(b) Given the following standard enthalpies of combustion, ΔHθc (298K, 1 atm)

C(s) -393 kJmol-1; H2(g) -285.6 kJmol-1; C2H6(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, ΔHθf(C2H6(g))

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

bond ΔHBE: 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?

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 2

Given the following standard enthalpies of combustion ΔHθc (298K, 1 atm)

C(s) -393 kJmol-1; H2(g) -285.6 kJmol-1; C8H18(l) -5512 kJmol-1

Calculate the standard enthalpy of formation of octane, ΔHθf(C8H18(l))

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 3

(a) Given the following standard enthalpies of combustion ΔHθc(298K, 1 atm)

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

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

Calculate the standard enthalpy of combustion of cyclohexane, ΔHθc(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.

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

(c) doc bQuestion 4

(a) Given the following standard enthalpies of combustion ΔHθc (298K, 1 atm)

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

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

Calculate the standard enthalpy of combustion of ethanoic acid, ΔHθc(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 ΔHcomb(ethanoic acid) calculated in parts (a), (b) and (c).

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 5

Given the following standard enthalpies of formation ΔHθf(298K, 1 atm)

NH3(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, ΔHθr

NH4Cl(s) ====> NH3(g) + HCl(g)

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 6

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

CH4(g) -74.9; CH3Br(l) -36.0; HBr(g) -36.2

Calculate the standard enthalpy change for the reaction, ΔHθr

Br2(l) + CH4(g) ====> CH3Br(l) + HBr(g)

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 7

(a) Define the term 'average bond enthalpy'.

(b) Given the following standard enthalpies of formation, ΔHθf in kJmol-1 (298K, 1 atm)

C3H8(g)   -104 ;      C3H7Cl(g)  -105 ;     HCl(g)  -92

Calculate the standard enthalpy change for the reaction, ΔHθr

  Cl2(g)   +   C3H8(g)    ====>    C3H7Cl(g)    +   HCl(g)

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

      bond ΔHBE: C-H  412 ;       Cl-Cl  242 ;       C-Cl  338 ;     H-Cl  431

      Calculate the enthalpy change for the same reaction as in (b)

(d) Explain which of the calculations in (b) or (c) will be the most accurate and why?

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 8

(a) Given the following standard enthalpies of formation, ΔHθf in kJ mol-1 at 298K, 1 atm

         C2H6(g)   -84.7 ;      C2H5I(l)  -39.1 ;     HI(g)  +25.9

      Without using a Hess's Law Cycle, calculate the standard enthalpy change, ΔHθr for the reaction:

     C2H6(g)    +    I2(s)   ====>    C2H5I(l)    +   HI(g)

(b) Given the following average bond energies in kJ mol-1

      bond ΔHBE: C-H  412 ;     C-I   228 ;     H-I  299,

      and the enthalpy of vapourisation of iodoethane is 32.0 kJmol-1,

and the enthalpy of atomisation of iodine is 107 kJmol-1,

calculate the enthalpy change for the same reaction in (a) by any method you choose!

(c) How do your answers to (a) and (b) compare?

topANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes

 

Question 9

Given the bond enthalpies (ΔHBE) in kJ mol-1: C-H 412, C-C 348 and H-H 436 (∆Hatom = 218 kJ mol-1).

Calculate the enthalpy of formation of propane if the enthalpy of atomisation of carbon (graphite) is 715kJ mol-1.

ANSWERS to the ΔH enthalpy Q's  *  Thermochemistry and Enthalpy Notes


A level Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) Cambridge University Chemistry courses Oxford University Chemistry courses Durham University Chemistry courses St Andrews University Chemistry courses Bristol University Chemistry courses Imperial College London University Chemistry courses York University Chemistry courses Warwick University Chemistry courses Edinburgh University Chemistry courses Manchester University Chemistry courses Southampton University Chemistry courses Sheffield University Chemistry courses University College London University Chemistry courses Nottingham University Chemistry courses Surrey University Chemistry courses Heriot-Watt University Chemistry courses Glasgow University Chemistry courses Leeds University Chemistry courses Strathclyde University Chemistry courses Bath University Chemistry courses Liverpool University Chemistry courses Birmingham University Chemistry courses East Anglia University Chemistry courses Sussex University Chemistry courses Cardiff University Chemistry courses Hull University Chemistry courses Leicester University Chemistry courses Keele University Chemistry courses Newcastle University Chemistry courses Queen's, Belfast University Chemistry courses Reading University Chemistry courses Loughborough University Chemistry courses Huddersfield University Chemistry courses Bangor University Chemistry courses Aberdeen University Chemistry courses Plymouth University Chemistry courses Central Lancashire University Chemistry courses Aston University Chemistry courses Robert Gordon University Chemistry courses Queen Mary University Chemistry courses Kent University Chemistry courses Sheffield Hallam University Chemistry courses Liverpool John Moores University Chemistry courses Bradford University Chemistry courses Teesside University Chemistry courses Glamorgan University Chemistry courses Nottingham Trent University Chemistry courses Brighton University Chemistry courses Greenwich University Chemistry courses Northumbria University Chemistry courses Kingston University Chemistry courses Coventry University Chemistry courses West of Scotland University Chemistry courses Manchester Metropolitan University Chemistry courses London Metropolitan University Chemistry courses Abertay Dundee University Chemistry courses  A Level AQA Edexcel OCR Salters Chemistry revision Advanced Level chemistry courses University Chemistry courses


Advanced Chemistry Page Index and LinksWebsite content copyright © Dr Phil Brown 2000–2014 All rights reserved on revision notes, images, puzzles, quizzes, worksheets, x–words etc. * Copying of website material is not permitted * chemhelp@tiscali.co.uk

Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

BIG SITE HELP SEARCH Enter several specific words/formula etc.