Chemistry, Third edition

222 13 · ENERGY CHANGES IN CHEMICAL REACTIONS

Determination of H in the laboratory

The following section gives an example of a laboratory determination of the

enthalpy change for a chemical reaction in which the reactants are mixed in water.

After the reaction has taken place the final mixture consists of products, unused

reactants and water. The principles of such measurements are as follows.

● The reaction is carried out in an insulated container which ideally prevents the

reaction mixture from losing heat to (or gaining heat from) the surroundings.

13.3

Application of Hess’s law

Given the following standard enthalpy changes at 298 K,

S(s)O 2 (g) SO 2 (g) H—^  296.8 kJ mol^1 (13.12)

SO 2 (g)^1 ⁄ 2 O 2 (g) SO 3 (g)H—^  98.9 kJ mol^1 (13.13)

calculate the standard enthalpy change for the reaction:

S(s)^3 ⁄ 2 O 2 (g) SO 3 (g) H—^ ? (13.14)

Exercise 13D

Example 13.2 (continued)

Answer

We start by multiplying equation (13.6) by^1 ⁄ 2 , giving

^1 ⁄ 2 H 2 (g) ^1 ⁄ 2 Cl 2 (g)HCl(g)

The arrow in this equation may be likened to the equals sign () in a

mathematical equation, and the chemical equation may be re-arranged in a similar

way to algebraic equations giving:

HCl(g)^1 ⁄ 2 H 2 (g)^1 ⁄ 2 Cl 2 (g) (13.10)

(Compare this to the rearrangement of xzy, to give yxz). The

rearrangement puts HCl(g) on the left-hand side of an equation, as in equation (13.9).

Dividing equation (13.7) throughout by 2 gives

NH 3 (g)^3 ⁄ 2 H 2 (g)^1 ⁄ 2 N 2 (g) (13.11)

which keeps NH 3 (g) on the left-hand side, as in equation (13.9).

Adding equations (13.10) and (13.11) to equation (13.8) gives

HCl(g)NH 3 (g)^1 ⁄ 2 N 2 (g)2H 2 (g)^1 ⁄ 2 Cl 2 (g)

^1 ⁄ 2 H 2 (g)^1 ⁄ 2 Cl 2 (g)^3 ⁄ 2 H 2 (g)^1 ⁄ 2 N 2 (g)NH 4 Cl(s)

Collection and cancellation of like terms gives:

HCl(g)NH 3 (g) NH 4 Cl(s)

which we recognize as equation (13.9). The enthalpy changeis now obtained by

carrying out similar operations to the H valuesas follows:

H—^ (13.9) ^1 ⁄ 2 H—^ (13.6)^1 ⁄ 2 H—^ (13.7)H—^ (13.8)

 ^1 ⁄ 2 (184.6)^1 ⁄ 2 (92.2 ) (314.4) 176.0 kJ mol^1