Chemical Equilibria

Question 1

Define dynamic equilibria

Answer 1

The rate of the forward reaction is equal to the rate of the reverse reaction

Question 2

How do you know when you have reached equilibrium?

Answer 2

When the macroscopic properties don’t change. Properties like density, colour, concentration, pressure

Question 3

What are the conditions for equilibria?

Answer 3

•Equilibria can only be reached using a closed system
•Equilibrium can be approached from either direction
•Equilibrium is a dynamic process

Question 4

What does Le Chatelier’s principle state?

Answer 4

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

Question 5

What effect does increasing temperature have on equilibrium?

Answer 5

Increasing temperature favours the endothermic reaction as the excess heat needs to be removed from the system to lessen the effect of the initial increase
It will increase the yield of the endothermic products.

Question 6

What effect does decreasing temperature have on equilibrium?

Answer 6

It favours the exothermic reaction as heat needs to be gained and the yield of the exothermic products will be increased

Question 7

What effect does increasing pressure have on equilibrium?

Answer 7

Favours the side of the reaction with fewer moles as this will help to release the build up in pressure
It will increase the yield of the products on this side of the reaction

Question 8

What effect does decreasing pressure have on equilibrium?

Answer 8

Favours the side of the reaction with more moles as pressure has been lost
The yield of the products on this side of the reaction will be increased

Question 9

What effect does increasing concentration have on equilibrium?

Answer 9

Favour the reaction that produces the products as more molecules are available to react
It will increase the yield of the products

Question 10

What effect does decreasing concentration have on equilibrium?

Answer 10

Decreasing concentrations of products will favour the reaction that reforms the reaction

Question 11

What effect does catalysts have on equilibrium?

Answer 11

Catalysts appear not to affect the equilibrium position as they affect the forward and backward reaction equally
However, it does allow equilibrium to be reached faster

Question 12

What is the symbol for the equilibrium constant?

Answer 12

Kc

Question 13

What does the equilibrium constant, Kc indicate?

Answer 13

The position of equilibrium for a reaction at a certain temperature

Kc for a reaction always has the same value unless the reaction conditions and therefore the position of equilibrium is changed

Question 14

How you find the value of Kc?

Answer 14

The concentration of substances in the reaction at the equilibrium position needs to be known
Its equal to the concentration of the products÷concentration of reactants

Question 15

What does a large value of Kc mean?

Answer 15

That products predominate as the equilibrium is towards the right

Question 16

What does a small value of Kc mean?

Answer 16

That reactants predominate as the equilibrium is towards the left

Question 17

When has a reaction gone to completion?

Answer 17

Reactions where Kc is greater than 10¹⁰ are said to have gone to completion

Question 18

When does a reaction not take place?

Answer 18

Reactions where Kc is less than 10-¹⁰ are said to not to take place

Question 19

What is the symbol for gaseous equilibria constant?

Answer 19

Kp

Question 20

How are the concentrations in gaseous equilibria expressed?

Answer 20

Gaseous equilibria also obey the equilibrium law but their concentrations are expressed using partial pressures