Equilibrium

Question 1

What is equilibrium?

Answer 1

Equilibrium

chemical reactions are said to be in equilibrium when:

→ reactants form products and products form reactants at the same rate.

→ so the forward reaction and the backwards reactions proceed at the same rate.

→ the reaction is reversible and this symbol is used.

→ the chemical reaction must occur in a closed system so none of the reactants or products can escape.

Question 2

Constant expression

Answer 2

at equilibrium the ratio of products to reactants is a constant Kc
Reactants Products
Kc = [Products] / [Reactants] = concentration in mol L-1
Consider the following general reversible chemical reaction: aA + bB cC + dD Kc = [C]c[D]d / [A]a[B]b
The value of Kc is a constant at a specific temperature and can be calculated using the concentrations of the reactants and products.
Rules
Do Not include Solids (s) or Liquids (l) in the expression as their concentrations are constant.
So only gases (g) or aqueous solutions (aq) are considered.
Copy the Species as it is written in the chemical equation - you can use molecules, ions or elements.
Kc is a Ratio of Products to reactants -
Kc = [P]2 / [R]2 = 1000 favours the Products
Kc = [R]2 / [P]2 = 1000 favours the reactants
when Kc = 1 the Reactants = Products.

Question 3

Equilibrium concentration

Answer 3

If the change increases the [Reactants] the system shifts right to decrease the [Reactants]
If the change increases the [Products] the system shifts left to decrease the [Products]
If the change decreases the [Products] the system shifts right to increase the [Products]

Question 4

Equilibrium temperature

Answer 4

Temperature
the system shifts in the direction to oppose the change
Increase in temperature of reaction:
If a temperature of the reaction is increased, the system’s response will be to decrease the temperature (Le Chatelier’s principle), the system will shift in the endothermic direction so that heat is absorbed from the surroundings and this will result in a decrease in temperature.
Decrease in temperature of reaction:
If a temperature of the reaction is decreased the system’s response will be to increase the temperature. the system will shift in the exothermic direction so that heat is released to the surroundings resulting in an increase in temperature.
Note you have to answer in context of the question and not a generic response

Question 5

Equilibrium pressure

Answer 5

3. Pulling up increases the volume and decreases the pressurePushing down decreases the volume and increases the pressure-> In the smaller space the particles suffer more collisions with the walls of the container - it’s this that we measure as ‘pressure exerted by the gas’For Level 2 chemistry, Pressure is defined as: Fixed number of moles of a gas in a Set volume - a change in pressure is created by changing the volume that fixed number of moles of the gas are contained in.High Volume = Low PressureLow Volume = High Pressure
4. the System Shifts in the direction to oppose the change.
5. Pressure comes from gas particles bouncing off the walls of containers
6. A smaller volume of container
   increases Pressure (and vice versa)
7. the System Shifts in the direction that minimises change, i.e. if the change increases pressure the System Shifts in the direction of fewer moles or gas particles to decrease the pressure.

Question 6

Equilibrium catalyst

Answer 6

Catalyst
1) a catalyst does not change the position of equilibrium.
2) A catalyst increases the rate of both forward and reverse reactions equally - thus equilibrium is reached quicker, but the catalyst does not change position.

Question 7

Effects on observations

Answer 7

Do not get caught up trying to work out what mixtures of colours will look like. If one side has purple and the other has green - then the change will make the mixture look more green and less purple or more purple and less green.

Colourless gases cannot be seen, more colourless gas is irrelevant to observations (it will not dilute the coloured component) - ignore colourless compounds.

Question 8

Effect on Kc

Answer 8

> a change in concentration does not change the value of Kc
-> a change in pressure does not change the value of Kc
-> a catalyst does not change the value of Kc
-> temperature does change Kc - the change will depend on if the temperature is increasing or decreasing and if the forward reaction is exo or endothermic