Le Chatlier's Principle

Question 1

Changes in equilibrium

Answer 1

A system at equilibrium could experience a change in conditions

If this happens the system will no longer be at equilibrium and the rate of reaction in one of the directions will be higher, until a new equilibrium is established

Question 2

Le Chatliers principle

Answer 2

If a system at equilibrium is subjected to a change in conditions, the system will readjust to re-establish equilibrium in such a way as to partially counteract the imposed change.

Question 3

Changes that can occur and how you approach them

Answer 3

Change in concentration in gases and solutions
Look for that particular substance

Total pressure (change in volume)
Count the particles on each side

Changes in temperature
Is the reaction exothermic or endothermic

Added catalyst
Has no effect, but will reach equilibrium faster

Substances added, but not written in the equation
How will this effect chemicals
Precipitation reactions (using solubility changes)

Question 4

Change in concentration

Answer 4

If the concentration of one of the reactants in an equilibrium mixture is increase the mixture will no longer be at equilibrium
The Kc value will change
According to Le Chatelier’s principle the system will re-establish equilibrium in a way that will decrease the concentration of the substance added. (the substance will be consumed)

Question 5

Change in concentration rate of reaction and concentration graphs

for the equation
2CrO4 2- + 2H+ -><- Cr2O7 -2 + H2O:

Answer 5

see onenote (diagrams) 3 and 4

Question 6

Change in temperature

Answer 6

The effect of a change in temperature on an equilibrium depends on wether the reaction is exothermic or endothermic

As temperature increases

For exothermic reactions
the K value and products decreases
favours the reverse reaction

For endothermic reactions
The k value and products increases
favours the forward reaction

Question 7

Change in temperature rate of reaction and concentration graphs

for the equation
N2O4 -><- 2NO2 delta H = 58kj

Answer 7

see onenote (diagrams) 5 and 6

Question 8

Partial pressures

Answer 8

treated the same as concentration to a liquid
a system reaches equilibrium when these are constant NOT equal

Question 9

Change in partial pressures

Answer 9

increase inpressure = equilibrium shiftstowards the side of the reaction with fewer moles of gas.

decreaseinpressure =equilibriumshiftstowards the side of the reaction with more moles of gas.

e.g.
the pressure has increased for the equation
N2 + 3H2 -><- 2NH3
the equilibrium would shift to the products side as there are only 2 molecules

the pressure has decreased for the equation
N2 + 3H2 -><- 2NH3
the equilibrium would shift to the reactants side as there are 4 molecules

Question 10

Changes in pressure (volume) for solutions

Answer 10

If a solutions volume is increased it will act in the same way as if a gas was under higher pressure
If the system is diluted by adding water the equilibrium will shift to the side of the reaction with the more moles of dissolved ions

Question 11

partial pressure rate of reaction graph

for the equation
2NO2 -><- N2O4

Answer 11

see onenote (diagrams) 7

Question 12

effects of catalysts

Answer 12

When a catalyst is added it lowers the activation energy for BOTH the forward and reverse reaction
This means it increases both of the reactions rate of reaction
They will both achieve equilibrium faster
It will not affect the concentration of the reactants or the products

Question 13

catalyst graph

Answer 13

see onenote (diagrams) 8