Chemistry, Third edition

EFFECTS OF CHANGING CONCENTRATION

1. Formation of hydrogen-bonded compounds

The reactions in which hydrogen-bonded compoundsare formed are exothermic and

often possess very low equilibrium constants at room temperature. For example,

Kc(T)for the reaction between hydrogen fluoride and hydrogen cyanide gases,

HF(g)HCN(g)\===\HCN · · · HF(g)

hydrogen-bonded ‘complex’

is only 1.04 mol^1 dm^3 at 298 K. For a starting mixture of 2.7  10 ^3 mol dm^3 HCN

and 2.7  10 ^3 mol dm^3 HF, this corresponds to an equilibrium concentration of

HCN···HF complex of only 7  10 ^6 mol dm^3! For this reason it is beneficial to

study such complexes at lower temperatures when Kc(T)is larger, as the increased

concentration of complex makes them easier to detect.

However, since reaction rates generally decrease with decreasing temperature, it

is important to remember that a lower temperature alwaysmeans that a reaction

takes longer to reach equilibrium. This may not be a severe hindrance in the labora-

tory, but it is frequently a very important economic consideration in the chemical

and metallurgical industries.

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Deducing whether a reaction is exothermic or

endothermic from the effect of temperature

upon its equilibrium constant

The decomposition of dinitrogen tetroxide into nitrogen dioxide

produces a colour change:

N 2 O 4 (g)\===\2NO 2 (g)

colourless gas brown gas

A mixture of the gases was sealed in a steel tube containing a quartz

window through which the colour of the gases could be observed. As the

contents of the tube were heated, the mixture became darker brown. Is

the reaction exothermic or endothermic?

Exercise 15I

2. Metal extraction

The extraction of a metal (M) from its oxide (MO) using carbon as a reducing agent is

generally endothermic. As expected, experiments show that the equilibrium con-

stants of the general reaction,

MO(s)C(g)\===\CO(g)M(s)

increase with increasing temperature. This means that virtually anymetal oxide may

be reduced by carbon if the temperature is high enough. For example, in the reduc-

tion of zinc oxide,

ZnO(s)C(g)\===\CO(g)Zn(s)

the equilibrium yield of the free metal is only high at temperatures above 800 °C. For

aluminium, reduction takes place only above 2000 °C, and it is cheaper to produce

this metal by electrolysis rather than attempt to maintain a furnace at such a high

temperature.