Extra 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. EMAIL

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!

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, 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?

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))

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.

Question 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).

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)

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)

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)  -92.3

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?

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!

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.

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

using enthalpy of combustion in calculation questions, using enthalpy of formation in calculation questions how to do A level enthalpy calculations for AQA AS chemistry, how to do A level enthalpy calculations for Edexcel A level AS chemistry, how to do A level enthalpy calculations for A level OCR AS chemistry A, how to do A level enthalpy calculations for OCR Salters AS chemistry B, how to do A level enthalpy calculations for AQA A level chemistry, how to do A level enthalpy calculations for A level Edexcel A level chemistry, how to do A level enthalpy calculations for OCR A level chemistry A, how to do A level enthalpy calculations for A level OCR Salters A level chemistry B how to do A level enthalpy calculations for US Honours grade 11 grade 12 how to do A level enthalpy calculations for pre-university chemistry courses pre-university A level revision notes for how to do A level enthalpy calculations  A level guide notes on how to do A level enthalpy calculations for schools colleges academies science course tutors images pictures diagrams for how to do A level enthalpy calculations A level chemistry revision notes on how to do A level enthalpy calculations for revising module topics notes to help on understanding of how to do A level enthalpy calculations university courses in science careers in science jobs in the industry laboratory assistant apprenticeships technical internships USA US grade 11 grade 11 AQA A level chemistry notes on how to do A level enthalpy calculations Edexcel A level chemistry notes on how to do A level enthalpy calculations for OCR A level chemistry notes WJEC A level chemistry notes on how to do A level enthalpy calculations CCEA/CEA A level chemistry notes on how to do A level enthalpy calculations for university entrance examinations how do you do bond enthalpy calculation questions? how do you do enthalpy calculations? how do you do calculations using enthalpies of formation and enthalpies of combustion? practice enthalpy questions for advanced A level chemistry, how do you calculate an unknown enthalpy change using Hess's law and known enthalpy values? practice bond enthalpy calculation questions

 KS3 SCIENCE QUIZZES ALPHABETICAL INDEX GCSE grade 9-1 & IGCSE CHEMISTRY Doc Brown's Travel Pictures & Notes ADVANCED LEVEL CHEMISTRY [SEARCH BOX] - see below All website content © Dr Phil Brown 2000 onwards. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial. Email doc b: chem55555@hotmail.com

Doc Brown's Chemistry

*