Chapter 1: Chemical Equilibrium Part 8 SABIS Grade 11 (Level M) Chemistry

Altering the State of Equilibrium

What conditions might alter the equilibrium state? Concentration and temperature are factors that affect the rate of reaction. Equilibrium is attained when the rates of opposing reactions become equal. Any condition that changes the rate of one of the reactions involved in the equilibrium may affect the conditions at equilibrium.

1.5.1 Concentration

Consider the reaction you encountered in the laboratory—that between iron (III) ion (Fe3+) and thiocyanate ion (SCN–):

Fe3+(aq) + SCN–(aq) ⇌ FeSCN2+(aq)

Again we have visual evidence of concentration at equilibrium since the intensity of the color is fixed by the concentration of the FeSCN2+ ion. The addition of either more iron (III) ion (by adding a soluble salt such as Iron (III) nitrate, Fe(NO3)3) or more thiocyanate ion (by adding, say, sodium thiocyanate) changes the concentration of one of the reactants. Immediately the color of the solution darkens, showing that there is an increase in the amount of the colored ion, FeSCN2+.

Generalization: The equilibrium concentrations are affected if the concentrations of reactants (or products) are altered.

1.5.2 Temperature

We have already considered an example of the change of equilibrium concentrations as the temperature is altered. The relative amounts of NO2 and N2O4 are readily and obviously affected by a temperature change.

Generalization: The equilibrium concentrations are affected if the temperature is altered.

1.5.3 Catalysts

Catalysts increase the rate of reactions. It is found experimentally that addition of a catalyst to a system at equilibrium does not alter the equilibrium state. Hence it must be true that any catalyst has the same effect on the rates of the forward and reverse reactions. You will recall that the effect of a catalyst on reaction rates can be discussed in terms of lowering the activation energy. This lowering is effective in increasing the rate in both directions, forward and reverse. Thus, a catalyst produces no net change in the equilibrium concentrations even though the system may reach equilibrium much more rapidly than it did without the catalyst. 

1.5.4 Attainment Of Equilibrium

The equilibrium state is not always attained in chemical reactions. Consider: 

H2O(g) ⇌ H2(g) + 1⁄2O2(g)           ΔH = + 242 kJ

A large amount of heat is absorbed in this reaction, 242 kJ/mole of water decomposed. If the temperature is lowered, the state of equilibrium is even more favorable to the production of water at room temperature than it is at 2273 K. Yet a mixture of hydrogen and oxygen can remain at room temperature for a long period without an apparent reaction.

Equilibrium is not attained in this system because the rate of the reaction between hydrogen and oxygen at room temperature is too low.This explanation is easily verified by speeding up the reaction slightly. If a mixture of H2 and O2 is disturbed with a small spark, reaction begins and it enthusiastically (and explosively) continues until most of the gases have been converted to water.

This distinction between the conditions in a chemical system at equilibrium and the rate at which these conditions are attained is very important in chemistry. As a result of arguments that we shall consider, a chemist can decide with confidence whether equilibrium favors reactants, or products, or neither. He cannot predict, however, how rapidly the system will approach the equilibrium conditions. That is a matter of reaction rates, and the chemist must perform separate experiments to learn whether a given rate is rapid or not.

The following chemical equation represents the reaction between hydrogen and chlorine to form hydrogen chloride:

H2(g) + Cl2(g)   ⇌   2HCl(g) + 184 kJ

Which of the following information are conveyed by this equation?

A catalyst is used.

 Heat is released during the reaction.

 1 mole H2 reacts with 1 mole Cl2 to produce 2 moles HCl.

Temperature at which the reaction occurs.

The reaction is fast.

 It is a reversible reaction.

 Reactants and products are in the gaseous state.

 The concentrations of reactants and products.

answer :

Heat is released during the reaction.

Reactants and products are in the gaseous state.

It is a reversible reaction.

1 mole H2 reacts with 1 mole Cl2 to produce 2 moles HCl. Fill in the blank. A(n)  does not alter the equilibrium state. It only increases the rate of the forward and backward reactions.answer : catalyst

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Chapter 1: Chemical Equilibrium  Part 9 SABIS Grade 11 (Level M) Chemistry