REACTION YIELDS
Very few reactions proceed unhindered to completion. Some begin reversing as soon as products are present.
Examples: 1)
2)
Chemical reactions that consist of two opposing processes (forward and reverse reactions) will eventually reach an equilibrium. The state of equilibrium is characterised by the forward and reverse reactions proceeding at the same rate ie reactions do not stop ? we have a dynamic situation
1)
2)
3)
4)
Figure 9.3
The Equilibrium Law
It can be shown experimentally that for the general equilibrium
aA + bB ??> cC + dD
[C]c[D]d = constant
[Ala [Blb
This allows to evaluate the concentration fraction at any time. When the system is at equilibrium the concentration fraction is constant ? this called the equilibrium constant (K).
For a particular reaction, K is constant for all equilibrium mixtures (provided temp. remains constant)
Example : (copy examples 1+2, P93)
Information From The Equilibrium Constant
Disturbing Equilibrium
There are 4 major means of disturbing a system at equilibrium
1)
2)
3)
4)
Note: Addition of a catalyst will increase both the rate of the forward and reverse reactions equally it will simply reduce the time taken to reach equilibrium.
Le Chatelier's Principle
"Whenever a change is made to a system at equilibrium, the equilibrium position will shift to partially oppose the change"
(a) Temperature
Reactions can be classified as either
(1)
(2)
eg CH4(g) + 202(g) à CH4(g) + 2H20(g) AH= ?802 kJ mol?'
Temperature is the only change that can be made to a system at equilibrium that will actually change the equilibrium constant (ie K is temperature dependant)
(1)
(2)
Figure 9.8
(b) Adding / Removing Reactant or Product
(1)
(2)
Figure 9.9
(c) Changing Pressure
(1) Changing volume
Example : (a)
(b)
(2) Adding an inert gas
(d) Dilution
When dilution occurs, a net reaction results which produces the greater number
of particles
Example:
Questions:Answer Q 1?10 (p91-98) 11 - 16, 20 - 23, 28,30 (P102 - 104)