Equillibria 1

Question 1

What is an equilibrium mixture

Answer 1

The mixture of reactants and products achieved when a reaction is at equilibrium.
The proportion of each compound remains constant-this is the equilibrium mixture.

Question 2

What is dynamic equilibrium

Answer 2

The point where the rates of the forwards and backwards reactions are equal.

Question 3

What four conditions describe an equilibrium

Answer 3

1) Equilibrium can only be reached in a closed system (one where the reactants and products can’t escape). The system does not have to be sealed. Eg a beaker is fine as long as none of the reactants/products could evaporate.
2) Equilibrium can be approached from either direction, starting with either reactants or products, and the final equilibrium position will be the same (as long as conditions remain constant)
3) Equilibrium is a dynamic process. It is reached when the rates of the forwards and backwards reaction are equal.
4) You know that equilibrium has been reached when the macroscopic properties of the system do not change with time. These are properties such as density, concentration, colour and pressure.

Question 4

What are macroscopic properties (dynamic equilibrium)

Answer 4

Properties that do not depend on the total quantity of matter

Question 5

What are some examples of macroscopic properties

Answer 5

Density, concentration, colour and pressure

Question 6

What is Le Chateliers principle

Answer 6

If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.

Question 7

What happens to a reversible reaction at equilibrium if the concentration of a reactant is increased

Answer 7

The position of equilibrium shifts to the right (forwards/product direction) in order to counteract the change.

Question 8

What happens to a reversible reaction at equilibrium if the concentration of a product is increased

Answer 8

The position of the equilibrium moves to the left (backwards/reactant) direction in order to counteract the change

Question 9

What happens to a reversible reaction at dynamic equilibrium when pressure is increased

Answer 9

• Increasing pressure causes the equilibrium to move to decrease pressure
• This means it shifts towards the side of the reaction with fewest moles of gas.
• This is because gas molecules exert pressure, so the less there are, the lower the pressure.
• This counteracts the change in conditions.

Question 10

What happens to a reversible reaction at equilibrium when pressure is decreased

Answer 10

• The system tries to oppose the decrease in pressure by increasing it.
• It does this by shifting towards the side of the reaction with the most moles of gas.
• This is because gas molecules exert pressure, so the more there are, the higher the pressure.
• This counteracts the change in conditions.

Question 11

When will changing the pressure have no effect on the position of equilibrium

Answer 11

When none of the products or reactants are gases and when there are the same number of moles of gas either side of the reaction.

Question 12

What will happen to the position of equilibrium if temperature is increased

Answer 12

• The system works to counteract the increase in temperature by decreasing it.
• So it shifts in the endothermic direction

Question 13

What will happen to the position of equilibrium if temperature is decreased

Answer 13

• The system works to counteract the decrease in temperature by increasing it.
• So it shifts in the exothermic direction.

Question 14

How does the use of a catalyst affect the position of equilibrium

Answer 14

• Catalysts have no affect on the position of equilibrium
• They lower the activation energy required for the reaction to occur, but this affects the forwards and backwards reactions equally.
• They do, however, allow an equilibrium to be reached more quickly.

Question 15

What does Kc represent

Answer 15

The equilibrium constant Kc

Question 16

List the key facts about the calculation of Kc

Answer 16

• it is calculated from the molar concentrations of the species involved
• it is calculated when the system is at constant temperature
• it tells us whether there are more products or reactants and equilibrium.

Question 17

What does it mean if Kc> 10^2

Answer 17

There are more products than reactants at equilibrium.

Question 18

What does it mean if Kc<10^-2

Answer 18

There are more reactants than products at equilibrium

Question 19

What does it mean if Kc is 1

Answer 19

The equilibrium lies exactly halfway between the reactants and products.

Question 20

In words, what is the equation for calculating Kc

Answer 20

Kc= concentrations of the products raised to the power of their stochiometric co-efficients/ concentration of the reactants raised tot he power of their stochiometric co-efficients.

Question 21

Describe how to calculate Kc

Answer 21

1) write a balanced chemical equation
2) write an expression for Kc
3) calculate the concentrations of each product and reactant
4) Input the values into the equation and raise to the power of the compounds stochiometric co-efficient
4) calculate the units

Question 22

How do you work out the units for Kc

Answer 22

• the units for concentration used are moldm^-3
• replace the product/reactant concentrations in the expression for Kc with moldm^-3 keeping all of the powers
• cancel out the moldm^-3 on the fraction to get your unit
• Kc can have no units

Question 23

What has happened to the position of equilibrium if the value for Kc increases

Answer 23

The position of equilibrium has moved to the right (forwards product direction)

Question 24

What has happened to the position of equilibrium if the value for Kc has decreased

Answer 24

The position of the equilibrium has shifted to the left (the backwards, reactant direction-less product being made)

Question 25

What affect does increasing temperature have on Kc in exothermic and endothermic reactions

Answer 25

• For an exothermic reaction (-Ve) increasing temperature decreases the equilibrium constant Kc
• For an endothermic reaction (+Ve) increasing temperature increases the equilibrium constant Kc

Question 26

What is required for the equilibrium constant Kc to change

Answer 26

There needs to be a change in temperature.

Question 27

What happens when the equilibrium constant is greater than 10^10

Answer 27

The reaction is regarded as having gone to completion

Question 28

What happens when the equilibrium constant is less than 10^-10

Answer 28

The reaction is regarded as not having taken place at all.

Question 29

How do catalysts affect the value of Kc

Answer 29

They have no affect on the value of Kc and therefore the position of equilibrium

Question 30

What affect does changing concentration have on the value of Kc when the temperature remains constant

Answer 30

• it has no affect on the value of Kc
• the system works to counteract the change by increasing the rate of one of the reactions to increase the concentration of reactants or products. (Whichever is needed to be increased)
• the result is that the new values for concentration, when inputted into the formula for Kc, end up calculating the same number as before.
• temperature must change for the value of Kc to change