Exambusters Study Cards 15 Equilibrium

Reversible Reaction

A system where the following opposite reactions are taking place:


reactant becoming product

product becoming reactant

The point in a reversible reaction where the forward and reverse reactions are taking place at the same rate.
Are concentrations of product and reactant equal at equilibrium?
No. They are constant but not equal. Their relative concentrations are determined by the value of the equilibrium constant at that temperature.

Write the equilibrium expression:


aA + bB ↔ cC + dD

Keq = [C]c[D]d/[A]a[B]b


Keq = equilibrium constant

Haw are reactant and product concentrations related to the magnitude of Keq?

If Keq is large

[reactant] is small and [product ] is large.


If Keq is small

[reactant] is large and [product] is small.

Le Chatelier’s Principle
If stress is placed on a system at equilibrium, equilibrium shifts in order to counteract the effects of stress and regain equilibrium.
How does a concentration change affect equilibrium?
If one of the substances is added or removed, all concentrations of substances adjust to a new equilibrium with the same Keq.
How does a change in temperature affect equilibrium?
The reaction shifts to a new equilibrium point with a new Keq. If the temperature is raised, the equilibrium is shifted to reaction which absorbs heat.
How does a pressure change affect equilibrium?
(Only in reactions where gases are involved.) The reaction will shift to oppose the pressure change, resulting in fewer moles of gas particles.

Ionization Constant

Write the expression for ionization of acid: HA

For substances in solution that partially ionize, an equilibrium expression may be written with Ki.

Ki = [H+][A]/[HA]

What are the two driving forces which control reactions?

A drive toward increased entropy (disorder).

A drive toward decreased enthalpy (lower heat content).

Second Law of Thermodynamics
The entropy of the universe increases for any spontaneous process.
List three processes where entropy increases.

A gas if formed from a solid or liquid.

A gas is evolved from a solution.

Crystals dissolve in water.

Free energy Change (;G)
A property which reflects a system’s capacity to do useful work.
Write the free energy equation.



G = free energy

S = entropy

H = enthalpy

T = Kelvin

How can ΔG be used to predict if a reaction is spontaneous?



positive: not spontaneous

negative: spontaneous

equals 0: at equilibrium

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