Chemistry, Third edition

REVISION QUESTIONS 239

mean bond enthalpies have been used in for O–H and C–H bonds. Overall, we pre-

dictthat the reaction involves the production of 696 kJ of heat per mol of methane

burned. The reason that heat is given out is simply that, on balance,

the product molecules possess stronger bonds than the reactants. The arithmetic shows

that if the C–H bonds and O=O bonds were stronger, little or no heat would be given

out!

We may generalize our results to all reactions:

●If the sum of the bond enthalpies of the product molecules is greater than

the sum of the bond enthalpies for the reactants, the reaction will be

exothermic.

●If the sum of the bond enthalpies of the product molecules is less than the sum of

the bond enthalpies for the reactants, the reaction will be endothermic.

Whereas the calculated enthalpy change for reaction (13.18) is816 kJ mol^1 ,

the experimentally measured value is Hc—^  802 kJ mol^1. The error would be

much greater if we had used the mean bond energy if C=O in all common com-

pounds (743 kJ mol^1 ) rather than the mean bond energy for CO 2 (816 kJ mol^1 ) –

see Exercise 13O. This example serves to remind us that the choice of bond

enthalpies in such calculations should be carefully reviewed beforehand.

Bond enthalpies 2

Estimate the standard enthalpy change (H^ —) for the following reactions at 298 K:

(i)CH 2 =CH 2 (g)Cl 2 (g)CH 2 Cl–CH 2 Cl(g)

(ii)2H 2 O(l) 2H 2 (g)O 2 (g) (Note: H 2 O(l) H 2 O(g)H^ —44 kJ mol^1 ).

Exercise 13Q

13.1.Which of the following is false?

(i) For exothermic reactions, H—^ is always positive

(ii) H—c^ (H 2 (g))H^ —f(H 2 (g))

(iii)H—c^ (C(s) graphite) H—f^ (CO 2 (g))

(iv)H—^ for the reaction 2S(s) 3O 2 (g) 2SO 3 (g) is the

same as H—f^ (SO 3 (g)).

(v) H—f^ (C(s) graphite)  0

(vi)H—c^ (C(s) graphite) H—c^ (C(s) diamond)

(vii)The O–H bonds in the water molecule are equally strong.

13.2.(i)Define ‘standard enthalpy of combustion’.

(ii)Calculate the standard enthalpy of combustion of methyl

benzene (toluene), C 7 H 8 (l), given that H^ —f(methyl benzene)

12.0 kJ mol^1. (For other data, use Table 13.2.)

(iii)Calculate the energy value of toluene fuel in units of kJ g^1 ,

and kJ cm^3 (the density of methyl benzene at room tempera-

ture is 0.86 g cm^3 ).

13.3.Use the data in Table 13.2 to calculate the standard

enthalpy changes for the following reactions at 298 K:

(i) P(s) (white) P(s) (red)

(ii)the production of ethyl ethanoate from ethanoic acid and

ethanol:

CH 3 COOH(l)C 2 H 5 OH(l) CH 3 COOC 2 H 5 (l)H 2 O(l)

using H—f^ (CH 3 COOH(l)) 485 kJ mol^1 , and H^ —f

(CH 3 COOC 2 H 5 (l))

 481 kJ mol^1.

(iii)6NH 4 ClO 4 (s)  10Al(s) 3N 2 (g)  9H 2 O(g) 

5Al 2 O 3 (s)6HCl(g) (This is the reaction used in the

booster rockets of the space shuttle – the huge billowing

clouds are made of Al 2 O 3 (s).)

Revision questions