Chemistry, Third edition

13 · ENERGY CHANGES IN CHEMICAL REACTIONS

13.4.50 cm^3 of 0.050 mol dm^3 Ag(aq) and 50 cm^3 of

0.050 mol dm^3 Cl(aq) were rapidly mixed in a simple

calorimeter. The temperature rose by 0.41 °C. Estimate the

enthalpy change of the reaction

Ag(aq)Cl(aq) AgCl(s)

13.5.H—f^ for methanoic acid,

H 2 (g)O 2 (g)C(s) HCOOH(l) H—f^ ?

cannot be determined directly. However, since methanoic

acid readily burns in air, its enthalpy of combustion can be

measured accurately. It is found that Hc—^  277.2 kJ mol^1.

Using this value of H^ —c(and the H—f^ data for CO 2 and H 2 O

contained in Table 13.2), calculate H—f^ (HCOOH(l)).

13.6.Glucose (C 6 H 12 O 6 (s)) is an important ‘fuel’ in the body.

(i)Write a balanced equation for the complete combustion of

glucose in air to make liquid water and carbon dioxide gas.

(ii)Calculate the amount of energy generated in the body

(assume that only 40% of the enthalpy of combustion of the

fuel is utilised) from the metabolism of 10 g of glucose (H—f^

(C 6 H 12 O 6 (s)) 2816 kJ mol^1 ).

13.7.(i)The third standard ionization energy of aluminium is

2751 kJ mol^1. Write a thermochemical equation to which this

enthalpy term applies.

(ii)The first, second and third standard ionization energies

of aluminium are 584, 1823 and 2751 kJ mol^1 , respectively.

The standard enthalpy of atomization of aluminium is

326 kJ mol^1. Calculate the enthalpy change for the process

Al(s) Al^3 (g)3e

13.8.The following standard enthalpy data (at 298 K) are pro-

vided:

(a)enthalpy change of atomization of calcium metal, H—atom^

(Ca(s)),193 kJ mol^1

(b)first ionization energy of calcium, H—ion^ (Ca(g)) 

590 kJ mol^1

(c)second ionization energy of calcium, Hion—^ (Ca(g))

1150 kJ mol^1

(d)bond dissociation enthalpy of O 2 (g), H^ —(O=O) 

497 kJ mol^1

(e)enthalpy of double electron gain of the oxygen atom (form-

ing O^2 (g)),H^ —eg,703 kJ mol^1.

(f) lattice enthalpy of CaO, H^ —L3513 kJ mol^1.

(i)Write down thermochemical equations that apply to the

above enthalpy terms.

(ii)Use the data to calculate the standard enthalpy change of

formation (H—f^ ) of calcium oxide (CaO).

13.9.The standard enthalpy change for the formation of sul-

fur tetrafluoride (SF 4 ) from the gaseous elements at 298 K fol-

lows the thermochemical equation

2F 2 (g)S(g) SF 4 (g) H^ — 994 kJ mol^1

Given that H—F–F^ 158 kJ mol^1 , estimate HS–F^ — in SF 4.

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