Key Concepts
- Equilibrium reactions are reversible chemical reactions.
- At equilibrium (dynamic equilibrium):
the rate of the forward reaction = the rate of the reverse reaction
- At equilibrium the concentration of reactants does not change with time.
- At equilibrium the concentration of products does not change with time.
- Catalysts do not affect the position of equilibrium.
- Heat increases the rate of both the forward and reverse reactions
- Le Chatelier's Principle predicts the position of the equilibrium after a chemical system has been disturbed.
Disturbances include: change in temperature, concentration, pressure, volume.
- Only closed systems can reach equilibrium.
An open system in which either reactants or products are being removed cannot reach equilibrium.
Identifying Equilibrium in Chemical Reactions
For the chemical reaction: reactants  products
| Time (h) |
[reactants](M) |
[products](M) |
|---|
| 0 |
0.40 |
0 |
| 2 |
0.22 |
0.22 |
| 4 |
0.08 |
0.28 |
| 6 |
0.06 |
0.30 |
| 8 |
0.06 |
0.30 |
| 10 |
0.06 |
0.30 |
|
 |
Initially, at time=0, only reactants are present in the reaction vessel.
After time 0, reactants begin to react to produce products, the concentration of reactants decreases and the concentration of products increases with time.
A little after 4 hours, the concentration of reactants remains steady at 0.06M and the concentration of products remains steady at 0.30M.
Equilibrium is reached a little after 4 hours.
At equilibrium the rate of the forward reaction = the rate of the reverse reaction so the concentration of reactants and products remain constant.
Disturbing the Equilibrium
Affect of a Catalyst
A catalyst speeds up the rate of both the forward and reverse reactions.
The initial slope of the concentration vs time lines will be steeper for both reactants and products.
Reactants disappear faster while products are formed faster.
A catalyst does NOT affect the equilibrium position.
At equilibrium the concentrations of reactants and products for the catalysed reaction are the same as for the uncatalysed reaction.
Affect of a Temperature Change
Increasing the temperature of a reaction mixture speeds up both the forward and reverse reactions.
However, the final equilibrium position is determined by whether the reaction is exothermic or endothermic according to Le Chatelier's Principle.
In an exothermic reaction, heat is a product.
Increasing the temperature of an exothermic reaction favours the formation of reactants.
The intial rate of formation of products and disappearance of reactants is greater at higher temperatures.
The equilibrium concentration of reactants will be higher at higher temperature and the equilibrium concentration of products will be lower at higher temperature.
In an endothermic reaction, heat is a reactant.
Increasing the temperature of an endothermic reaction favours the formation products.
The intial rate of formation of products and disappearance of reactants is higher at higher temperatures.
The equilibrium concentration of products will be higher at higher temperature, and the equilibrium concentration of reactants at higher temperature will be lower.
Affect of a Change in Concentration
If our system of: reactants products
is allowed to come to equilibrium then more reactants are added, ie, increasing the concentration of reactants, then
- at the time reactants are added there would be an instantaneous increase in reactant concentration
- followed by a readjustment as more reactants react to produce products according to Le Chatelier's Principle
- A new equilibrium position is established in which the concentration of reactants added and products produced is higher than before.
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