Topic 7 : Equilibrium

Question 1

Reversible reaction

Answer 1

1. Some reactions go to completion where the reactants are used up to form the products and the reaction stops when all of the reactants are used up
2. In reversible reactions the products can react to reform the original reactants
3. To show a reversible reaction, two half arrows are used: ⇌

Question 2

Dynamic equilibrium

Answer 2

1. In a dynamic equilibrium the reactants and products are dynamic (they are constantly moving)
2. In a dynamic equilibrium the rate of the forward reaction is the same as the rate of the backward reaction in a closed system and the concentrations of the reactants and products is constant
3. There is no change in macroscopic properties such as colour and density as they depend on concentration

Question 3

Equilibrium graphs

Answer 3

1. Once equilibrium is reached, both lines will plateau
2. A line will show the increase in concentration of the products formed during the reaction until equilibrium is reached and that concentration remains constant
3. A line will show the decrease in concentration of the reactants during the reaction until equilibrium is established than the concentration will be constant
4. The concentration of the reactants will not go to 0 as there will still be some reactants left

Question 4

Closed System

Answer 4

A closed system is one in which none of the reactants or products escape from the reaction mixture

Question 5

Open System

Answer 5

In an open system some matter is lost to the surroundings

Question 6

Equilibrium in a closed and open system

Answer 6

1. When a reaction takes place entirely in solution, equilibrium can be reached in open flasks
2. If the reaction involves gas, equilibrium can only be reached in a closed system

Question 7

Equilibrium Expression

Answer 7

1. The equilibrium expression is an expression that links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
2. Solids are ignored in equilibrium expressions
3. The Kc of a reaction is specific to a given equation

Question 8

Size of Kc

Answer 8

1. The size of Kc tells us how the equilibrium mixture is made up with respect to reactants and products
2. If Kc > 1, the concentration of products is greater than the concentration of reactants and we say that the equilibrium lies to the right hand side
3. If Kc < 1, then the concentration of reactants is greater than the concentration of products and we say that the equilibrium lies to the left hand side
4. Kc is a constant at a specified temperature
5. Since temperature can affect the position of equilibrium it follows that Kc is dependent on temperature

Question 9

Reaction Quotient

Answer 9

1. The reaction quotient, Q, is the ratio of products and reactants for a reaction that has NOT yet reached equilibrium
2. Q = [products]/[reactants]
3. It is a useful concept because the size of Q can tell us how far a reaction is from equilibrium and in which direction the reaction proceeds

Question 10

Relationship between Q and Kc

Answer 10

1. If Q = Kc then the reaction is at equilibrium, no net reaction occurs
2. If Q < Kc the reaction proceeds to the right in favour of the products
3. If Q > Kc the reaction proceeds to the left in favour of the reactants

Question 11

Position of the equilibrium

Answer 11

1. The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
2. When the position of equilibrium shifts to the left, it means the concentration of reactants increases
3. When the position of equilibrium shifts to the right, it means the concentration of products increases

Question 12

Le Chatelier’s principle

Answer 12

1. Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
2. The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Question 13

Shift in Equilibrium due to change in concentrations

Answer 13

1. An increase in concentration would make the equilibrium shift to the right to reduce the effect of the increase in the concentration of the reactants
2. A decrease in concentration would make the equilibrium shift to the left to reduce the effect of the decrease in the concentration of the reactants (or an increase in the concentration of product)

Question 14

Shift in Equilibrium due to change in pressure

Answer 14

1. An increase in pressure would cause the equilibrium to shift in the direction that produces the smaller number of molecules of gas to decrease the pressure again
2. A decrease in pressure would cause the equilibrium to shift in the direction that produces the greater number of molecules of gas to increase the pressure again

Question 15

Shift in Equilibrium due to change in temperature

Answer 15

1. An increase in temperature will cause the equilibrium to move in the endothermic direction to reverse the change
2. A decrease in temperature will cause equilibrium to move in the exothermic direction to reverse the change

Question 16

Effects of Catalysts on the position of equilibrium

Answer 16

1. A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
2. Catalysts only cause a reaction to reach its equilibrium faster
3. Catalysts therefore have no effect on the position of the equilibrium once this is reached

Question 17

How a change in concentration affects Kc

Answer 17

If all other conditions stay the same, the equilibrium constant Kc is not affected by any changes in concentration of the reactants or products

Question 18

How a change in pressure affects Kc

Answer 18

1. A change in pressure only changes the position of the equilibrium (see Le Chatelier’s principle)
2. If all other conditions stay the same, the equilibrium constant Kc is not affected by any changes in pressure of the reactants and products

Question 19

How a change in temperature affects Kc

Answer 19

Changes in temperature change the equilibrium constant Kc