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Question 13.13: Applying Le Châtelier’s Principle to Concentration Changes T......

Applying Le Châtelier’s Principle to Concentration Changes

The reaction of iron(III) oxide with carbon monoxide occurs in a blast furnace when iron ore is reduced to iron metal:

\mathrm{Fe}_2 \mathrm{O}_3(s)+3 \mathrm{CO}(g) \rightleftharpoons 2 \mathrm{Fe}(l)+3 \mathrm{CO}_2(g)

Use Le Châtelier’s principle to predict the direction of the net reaction when an equilibrium mixture is disturbed by:

(a) Adding Fe_2O_3

(b) Removing CO_2

(c) Removing CO; also account for the change using the reaction quotient Q_c.

STRATEGY

To predict the direction of net reaction, recall that a concentration stress is relieved by reaction in the direction that consumes an added substance or replenishes a removed substance. This rule does not apply to pure solids or pure liquids because their concentrations have a constant value equal to 1.

Step-by-Step
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(a) Because Fe_2O_3 is a pure solid, its concentration is equal to 1 and doesn’t change when more Fe_2O_3 is added. Therefore, there is no concentration stress and the original equilibrium is undisturbed.

(b) Le Châtelier’s principle predicts that the concentration stress of removed CO_2 will be relieved by net reaction from reactants to products to replenish the CO_2.

(c) Le Châtelier’s principle predicts that the concentration stress of removed CO will be relieved by net reaction from products to reactants to replenish the CO. The reaction quotient is

Q_c=\frac{\left[\mathrm{CO}_2\right]_t^3}{[\mathrm{CO}]_t{ }^3}

When the equilibrium is disturbed by reducing [CO], Q_c increases, so that Q_c > K_c. For the system to move to a new state of equilibrium, Q_c must decrease—that is, [ CO_2 ] must decrease and [CO] must increase. Therefore, the net reaction goes from products to reactants, as predicted by Le Châtelier’s principle.

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