TYPES OF ENTHALPY CHANGES ASSIGNMENTS

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?

ANSWERS to the ΔH enthalpy Q’s *

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))

ANSWERS to the ΔH enthalpy Q’s *

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.

ANSWERS to the ΔH enthalpy Q’s *

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).

ANSWERS to the ΔH enthalpy Q’s *

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)

ANSWERS to the ΔH enthalpy Q’s *

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)

ANSWERS to the ΔH enthalpy Q’s *

Question 7

(a) Define the term ‘average bond enthalpy’.

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

C₃H_8(g)   -104 ;      C₃H₇Cl_(g)  -105 ;     HCl_(g)  -92.3

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

Cl_2(g)   +   C₃H_8(g)    ====>    C₃H₇Cl_(g)    +   HCl_(g)

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

bond ΔH_BE: 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?

ANSWERS to the ΔH enthalpy Q’s *

Question 8

(a) Given the following standard enthalpies of formation, ΔH^θ_f 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, ΔH^θ_r for the reaction:

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

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

bond ΔH_BE: 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?

ANSWERS to the ΔH enthalpy Q’s  *

Question 9

Given the bond enthalpies (ΔH_BE) in kJ mol^-1: C-H 412, C-C 348 and H-H 436 (∆H_atom = 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