6. Chemical Equilibria
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What is a reversible reaction?
A reaction that can take place in both directions and is therefore incomplete
Why is the reaction rate fast at the beginning of a reversible reaction?
There are only reactants
How is a dynamic equilibrium reached in a reversible reaction?
• beginning - only reactants so fast rate • reactants produce products so forward reaction slows • products then react to form reactants • continues until equilibrium reached
What happens at dynamic equilibrium?
• rates of forward and reverse reactions are the same * net concentrations of the components of the reaction mixture remain constant
What condition must be true for dynamic equilibrium?
Reaction takes place in a closed system
If the concentration of products at equilibrium is high compared to that of the reactants, where does equilibrium lie?
To the right
If the concentration of reactants at equilibrium is high compared to that of the products, where does equilibrium lie?
To the left
What can change the position of equilibrium?
• temperature • concentration • pressure
What principle can be used to explain which way equilibrium will shift?
Le Chatelier’s
What does Le Chatelier’s principle state?
That if a factor affecting the position of equilibrium is altered, the position of equilibrium shifts to oppose the effect of the change
Which way does equilibrium shift when the temperature of a reaction is increased?
In the endothermic direction (in order to ↓ the temperature)
Which way does equilibrium shift when the temperature of a reaction is decreased?
In the exothermic direction (in order to ↑ the temperature)
The reaction N₂ + 3H₂ ⇌ 2NH₃ has a -ve enthalpy change. What happens when the temperature is increased?
• equilibrium shifts left • in endothermic direction • to take in heat energy to oppose the change and ↓ temperature
Would a high or low temperature increase the yield of ammonia? [N₂ + 3H₂ ⇌ 2NH₃]
Lower - will shift equilibrium to produce more products - but rate will be slower
When will changing pressure have an effect on equilibrium?
When reactions involve gases
Which way does equilibrium shift when the pressure of a reaction is increased?
In the direction with fewer moles of gas to oppose the change (i.e. ↓ pressure)
Which way does equilibrium shift when the pressure of a reaction is decreased?
In the direction with the most moles of gas to oppose the change (i.e. ↑ pressure)
What happens if pressure is increased but the number of moles is the same on both sides of the reaction?
The position of equilibrium remains unaffected, but equilibrium will be reached faster
What is the position of equilibrium for reactions in the liquid/aqueous phase affected by?
Concentration, not pressure
What happens to equilibrium when the concentration of a reactant is increased?
Shift position to the right
What happens to equilibrium when the concentration of a product is increased?
Shift position to the left
In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more acid is added?
Equilibrium will shift right to counteract the increase in HCl
In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more water is added?
Equilibrium will shift left to counteract the increase in H₂O
How does a catalyst increase the rate of reaction?
• lowering activation energy * provide alternate route for the reaction