Unit 4a – Introduction to Equilibrium
Collision theory predicts that it is possible for product molecules to collide and revert back into reactants. We can even predict the Ea for the reverse reaction on the PE diagram.
In this case the reverse reaction rate would be much slower than the rate of the forward reaction. (Illustrate exothermic reaction)
When we initially add reactants together there are no products. Immediately the rate of reaction is quite fast and products form rapidly. As time progresses, the amount of product increases and no more reactants will react. The reverse reaction is now favoured to occur. The graph appears as follows:
(reactant decreases and levels off, product increases and levels off below)
Problem :Why does the 1st spoonful of sugar dissolve readily into water, while the 25th spoonful may not dissolve at all?
Explanation :1)The spaces between the molecules of water become
‘saturated’ with sugar molecules and the dissolving process halts.
2)The sugar is still dissolving at the same rate, however the process of crystallization back to the solid state is now becoming more favourable than dissolving
(we have reached the point where the reaction looks stopped, however the rate of dissolving is equal to the rate of crystallization)
The reaction has said to have passed ‘equilibrium’ between the 2 competing reactions.
We demonstrate this equilibrium condition by using an equilibrium arrow
water
Ex:C6H12O6 (s) C6H12O6 (aq)
As the product concentration increases collisions between dissolved sugar molecules increases and the reverse reaction rate will also increase (recall Factors affecting Rate of rxn)
Recognizing a System in Equilibrium
1)The reaction must appear to be stopped (rate = 0) and macroscopic properties (T, P, colour) must be constant
2)It must be a closed system .Nothing can enter nor leave.
In a steady state, reactants are continually being added and products being removed.
3)There must be measurable amounts of all reactants and products. Both the forward and reverse rxns are occuring at equal rates.
4)The conditions above can be obtained by starting with only reactants initially OR with only products initially present in the container.
RATE LAW : think that both sides of the equation will contain ‘reactants’ .