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Untitled - Kelly Walsh High School

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Chemical Equilibria 207<br />

14-3 Le Châtelier’s Principle<br />

Don’t Forget!<br />

Le Châtelier discovered that if a chemical system at equilibrium is stressed (disturbed)<br />

it will reestablish equilibrium by shifting the rates of the reactions involved.<br />

This means that the amounts of the reactants and products will change, but the ratio<br />

will remain the same. One can stress the equilibrium in a number of ways: changes<br />

in concentration, pressure, and temperature. However, a catalyst will have no effect<br />

on the equilibrium amounts since it affects both the forward and reverse reactions<br />

equally. It will simply allow the reaction to reach equilibrium faster.<br />

Changes in Concentration<br />

If the stress to the equilibrium system is a change in concentration of one of the<br />

reactants or products, then the equilibrium will react to remove that stress. If, for<br />

example, we decrease the concentration of a chemical species, the equilibrium<br />

will shift to produce more of it. In doing so, the concentration of chemical species<br />

on the other side of the reaction arrows will be decreased. If the concentration of<br />

a chemical species is increased, the equilibrium will shift to consume it, increasing<br />

the concentration of chemical species on the other side of the reaction arrows.<br />

For example, again consider the Haber process: N 2(g) 3 H 2(g) L 2 NH 3(g).<br />

If one increases the concentration of hydrogen gas then the equilibrium shifts<br />

to the right in order to consume some of the added hydrogen. In doing so, the<br />

concentration of ammonia (NH 3) will increase and the concentration of nitrogen<br />

gas will decrease.<br />

The concentrations may change, but the value of K c or K p would remain the<br />

same.<br />

Changes in Pressure<br />

Changes in pressure are only significant if there are gases involved. The pressure<br />

may be changed by changing the volume of the container or by changing<br />

the concentration of a gaseous species. If the container becomes smaller, the<br />

pressure increases because there are an increased number of collisions on the<br />

inside walls of the container. This stresses the equilibrium system and it will<br />

shift in order to reduce the pressure. A shift towards the side of the equation<br />

that has the least number of moles of gas will accomplish this. If the container

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