10.4 - Dynamic equilibrium & le Chatelier's principle Flashcards
What is a reversible reaction?
Reactions that take place in both a ‘forward’ and ‘reverse’ directions.
Give an example of a reversible reaction
the Haber process for manufacturing ammonia
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
What does the ⇌ symbol indicate?
The symbol indicates that the reversible reaction is in equilibrium.
Describe an equilibrium system
In an equilibrium system :
- The rate of the forward reaction is equal to the rate of the reverse reaction
- The concentrations of reactants and products do not change
Why do the concentrations of the reactants and products remain unchanged in equilibrium?
The forward and reverse reactions are still taking place, but as fast as the reactants are becoming products, the products are becoming reactants.
Why is a closed system needed for a reaction to remain in equilibrium?
A closed system is isolated from its surroundings, so the temperature, pressure and concentrations of reactants and products are unaffected by outside influences.
State le Chatelier’s principle
When a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change.
What happens to a system in equilibrium if more reactant molecules are added?
More products are made than reactants until a new equilibrium is established.
Which direction has equilibrium shifted if more products are formed?
Equilibrium has shifted to the right.
Which direction has equilibrium shifted if more reactants are formed?
Equilibrium has shifted to the left.
How can you investigate changes to the position of equilibrium with concentration using colours?
If reactants and products have different colours, change in position of equilibrium is easy to see.
e.g. chromate ions are yellow and dichromate ions are orange.
Reaction is sensitive to changes in acid concentration.
Which side does the equilibrium shift when you increase the pressure of a system?
Increasing the pressure of the system shifts the position of equilibrium to the side of the reaction with the fewest moles of gas, to lower the pressure.
What happens if concentration of reactants increases?
If you increase the concentration of reactants or decrease the concentration of products, the position of equilibrium will shift to the product side.
Reactants are then consumed and the concentration of product increases, (more product made) so the imposed change is countered.
Describe the effects on position of equilibrium when increasing/lowering the temperature
If a reversible reaction is exothermic (energy-releasing) in one direction, it will be endothermic (energy-absorbing) in the other. Increasing the temperature will shift the position of equilibrium to favour the endothermic reaction. Lowering the temperature favours the exothermic reaction.
