Equilibrium

Question 1

What is dynamic equilibrium?

Answer 1

A reversible reaction in which the rate of the forward reaction equals the rate of the reverse reaction, in which the concentration of the reactants and the products do not change

Question 2

How is dynamic equilibrium established?

Answer 2

In the early stages of the reaction, the concentration of reactants is high and products is low, so the rate of forward reaction is much greater than the rate of the reverse reaction, but, as the reaction proceeds, the concentration of reactants decreases and products increases, so the rate of the forward reaction decreases while rate of backwards reaction increases. Eventually, when the reactants and products are reacting at the same rate then dynamic equilibrium will be reached

Question 3

Why is the equilibrium of water and sodium nitrate dynamic rather than static when there is an excess of sodium nitrate?

Answer 3

There is an interchange of some solute between the solution and the solid - some sodium nitrate dissolves and some crystallises

Question 4

What happens to the position of equilibrium when more reactant is added?

Answer 4

Equilibrium moves to the right hand side - more products are able to form

Question 5

What happens to the position of equilibrium when more product is added?

Answer 5

Equilibrium moves to the left hand side

Question 6

What happens to the position of equilibrium when pressure is increased?

Answer 6

An increase in pressure moves the equilibrium in the direction which produces the fewer moles of gas, no gas = no change

Question 7

What happens to the position of equilibrium when temperature is increased?

Answer 7

An increase in temperature moves the equilibrium in the endothermic direction

Question 8

What happens to equilibrium when a condition is changed?

Answer 8

If the conditions of a chemical system initially at equilibrium are changed then the equilibrium will shift in the direction that tends to annul the change, if possible

Question 9

What happens to the position of equilibrium when a catalyst is added?

Answer 9

A catalyst has no effect on the position of equilibrium as it is a reversible reaction. It speeds up the forward and reverse reactions equally, so equilibrium is reached sooner

Question 10

What do square brackets mean ( [X] )?

Answer 10

Concentration of X

Question 11

What is Kc?

Answer 11

Equilibrium constant, expressed in terms of concentration

Question 12

What is Kc affected by?

Answer 12

Only affected by temperature

Question 13

How would Kc for a general reaction xA + yB ⇌ nC + mD be calculated?

Answer 13

[C]^n[D]^m / [A]^x[B]^y

Products over reactants

Question 14

What does a large Kc represent?

Answer 14

A high proportion of products

Question 15

What does a small Kc represent?

Answer 15

A high proportion of reactants

Question 16

What does Kc tend towards if the reaction goes to completion?

Answer 16

Infinity

Question 17

What does Kc tend towards if there is ‘no reaction’?

Answer 17

0

Question 18

How does adding extra reactants or products affect Kc?

Answer 18

Does not change the value of Kc - the relative proportions will change to re-establish the conditions for Kc to be constant

Question 19

What are the units of Kc?

Answer 19

Where the number of particles remain constant, the concentration units cancel, so Kc has no units (e.g. in H2 + Br2 ⇌ 2HBr)
For other reactions, Kc has units which must be stated (e.g in 2SO2 + O2 ⇌ 2SO3 where the concentration units do not cancel, so the units for this reaction are mol^-1 dm3 according to the Kc equation)

Question 20

Can the rate at which equilibrium is reached be seen in the value of Kc?

Answer 20

No.

Question 21

How does an equation with heterogeneous equilibrium change how Kc is calculated?

Answer 21

If there is a solid in the reaction, then they shouldn’t be included when calculating Kc as they have a constant concentration

Question 22

Why are catalysts useful in a reversible reaction in industry?

Answer 22

For some reactions (e.g the Haber process) the conditions of the reaction do not favour high yield and efficient rate. which makes a process uneconomic, particularly if a reaction is reversible and exothermic. Because of this, a catalyst can be used to allow a reaction to take place efficiently at a lower temperature so an acceptable yield can be produced at a reasonable rate. This is referred to as a ‘compromise’ temperature as it balances an economic yield with an economic rate