CHEMICAL EQUILIBRIUM

Qualitative Aspects

H2O(g) H2(g) + 1/2 O2(g)

1. The double arrow represents an equilibrium reaction.

2. The equation for the equilibrium reaction does not give any information about the actual concentration of the reactants or products.

3. The equation for the equilibrium reaction does not tell us the extent to which the reaction occurred to reach equilibrium.

As reactants are used up, the rate of the forward reaction decreases.

As products accumulate, the rate of the reverse reaction increases.

What is Equilibrium?

As the reaction continues, eventually the rate of the forwardreaction will equal the rate of the reverse reaction.

At this point, EQUILIBRIUMis established.

What is Equilibrium?

The equality between the rate of the forward and reverse reactions gives the CONSTANT properties to the system.

Constant Properties

Recognizing Equilibrium

Reactions proceed to a point where no more observable changes occur.

When this happens, the system is said to be in a state of equilibrium.

In terms of the KINETIC MOLECULAR THEORY, equilibrium is reached when there is a dynamic balance between opposing forces.

AT EQUILIBRIUM

Microscopic processes continue but in a balance that yields no Macroscopic changes.

The system is dynamic. (The forward and reverse reactions continue to proceed but at equal rates.)

The RATE of the FORWARD reaction equals the RATE of the REVERSE reaction.

Factors which are common to all equilibrium systems:

1. Equilibrium occurs only when a uniform temperature is maintained.

2. After the reaction starts, no substances may be added to or removed from the system.

3. Some easily measured property, such as pressure or color, at first changes and then reaches a constant value.

GENERALIZATION:

For a closed system at a uniform temperature, EQUILIBRIUM is recognized by the constant properties of that system.