Chemistry, Third edition

KEY POINTS ABOUT ENTHALPY CHANGES

2H 2 (g)O 2 (g) 2H 2 O(l)

(in which 2 mol of liquidwater is made) is571.66 kJ mol^1. The difference in the

standard enthalpy changes for these reactions,

571.66(483.64) 88.02 kJ mol^1

is the energy released when 2 mol of H 2 O(g) condense to form 2 mol of H 2 O(l).

3.Reverse reactions possess H values which are equal in size, but opposite in

sign, to the forward reaction.

What is H—^ for the following reaction?

2H 2 O(g) 2H 2 (g)O 2 (g)

The answer is simply that since the reaction is exactlythe reverse of the equation

2H 2 (g)O 2 (g) 2H 2 O(g)

then the enthalpy change is equal in size but opposite in sign, i.e. 483.64 kJ mol^1.

We summarize this in the following rule: reactions which are exothermic in the

forward direction are endothermic in the reverse direction.

This rule applies to both physical and chemical changes. For example, heat is

absorbed in vaporization because liquid molecules require energy to escape from

their neighbours into the gas phase. Therefore, vaporization is always endothermic.

Thestandard enthalpy of vaporizationof acetone at 25 °C is 29.1 kJ per mol of

acetone:

CH 3 COCH 3 (l) CH 3 COCH 3 (g) H—^ vap=29.1 kJ mol^1

It follows that 29.1 kJ of heat energy is given out when 1 mol of acetone condenses

into acetone liquid:

CH 3 COCH 3 (g) CH 3 COCH 3 (l) H—^  29.1 kJ mol^1

We can also look at these relationships at a molecular level. Suppose that we were

able magically to video the collision of two molecules to produce a new molecule in

an exothermic reaction. If we were to play back the video in reverse, we would see that

heat is absorbed just before the new molecule began breaking apart. The principle

that if reactions are reversed the energy changes are equal in size but opposite in sign

is important because if this were not so, it would be possible to destroy or create

energy, and the Law of Conservation of Energy would be broken.

219

Enthalpy changes

of back reactions

The formation of

magnesium oxide from its

elements at 298 K obeys

the thermochemical

equation

2Mg(s)O 2 (g) 2MgO(s)

H—^  601.7 kJ mol^1

How much energy would be

required to decompose one

mol of magnesium oxide

into oxygen gas and

magnesium metal?

Exercise 13C

Thermochemical equations

(i)Explain why the statement ‘for the reaction of sodium with chlorine, H^ — 411.15 kJ mol^1 ’

is ambiguous. What extra information would make sense of this statement?

(ii)Given that

C(s)O 2 (g) CO 2 (g) H^ — 393.15 kJ mol^1

calculate the amount of heat given out when 120 g of carbon is completely burned in excess air.

Exercise 13B