Chemistry, Third edition

13 · ENERGY CHANGES IN CHEMICAL REACTIONS

ning and end of the reaction is 298 °C, and we shall therefore take H—^ to mean

H^ —(298 K). (In fact, provided there are no changes in phase, the variation of H—^

withsmallchanges in temperature is often negligible, and H—^ may then be con-

sidered as independent of temperature.)

Key points about enthalpy changes

1.H values are meaningless unless we state how many moles of product are

made, or else include a chemical equation to which that enthalpy change applies.

The enthalpy change of a reaction is an extensive property. This means that the

enthalpy change depends upon the amounts of reactants consumed, which, in turn,

control the quantities of product made. In the absence of any other information, we

always assume that the Hvalue for a reaction is produced when the number of

moles of reactants that combine are those indicated by the chemical equation. So,

when we write the thermochemical equation

N 2 (g)3H 2 (g) 2NH 3 (g) H^ — 92.22 kJ mol^1

we immediately understand that the stated enthalpy change arises when threemoles

of hydrogen gas react with onemole of nitrogen gas to make twomoles of ammonia.

If we use up a different number of moles of reactants, the enthalpy change is worked

out by simple proportion, as in the following example.

2.The physical states of the reactants and products must be clearly shown.

The importance of specifying the physical states of the substances involved in a reac-

tion may be illustrated by referring to the reaction of hydrogen and oxygen gases,

with gaseous water as the product:

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

H—^ for this reaction is483.64 kJ mol^1. Experiments show that H^ —for the reac-

tion

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Example 13.1

Calculate the amount of heat produced when 0.32 g of methanol

is completely burned in excess oxygen at 25 °C.

2CH 3 OH(l)3O 2 (g) 2CO 2 (g)4H 2 O(g) H—^  726 kJ mol^1

Answer

0.32 g

amount of methanol (in mol) 

32 g mol^1

0.010 mol

The thermochemical equation shows that two mol of methanol burn to produce

726 kJ of heat. Therefore, the enthalpy change that accompanies the burning of

0.010 mol of methanol is

0.010

 726 3.6 kJ i.e. 3.6 kJ of heat is given out

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