Unit 2) 1) Equilibria , Factors Affecting Equilibrium And The Equilibrium Constant

Question 1

What does it mean by “ a reaction is in equilibrium”

Answer 1

Some reactions never go to completion but are in a state of dynamic equilibrium
A chemical reaction is said to be in equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction : so the forward reaction and E verse reaction do not stop

Question 2

At equilibrium the concentration of reactants and products will be ________ but not ________

Answer 2

At equilibrium the concentration of reactants and products will be constant but not equal

Question 3

What is a closed system

In a closed system heat is able to be ________ to or from surroundings

Answer 3

For an equilibrium to establish the reaction must take place in a closed system : this is where the chemicals are trapped and nothing is added or removed , but heat is allowed to be transferred to or from the surroundings

Question 4

What did Henri le chatelier say about equilibrium

Answer 4

“If a system is at equilibrium and a change is made in any of the conditions ,then the system responds to counteract the change as much as possible”

• in other words if you make a change to a reaction at equilibrium , the system will alter the concentrations to minimise the effect until equilibrium is re established.

Question 5

How can the position of equilibrium be altered

Answer 5

• altering concentration of reactant or product
• altering pressure (increasing or decreasing )
• altering temperature (increasing or decreasing )

Question 6

Effect of concentration on equilibrium position

Answer 6

Increasing concentration of reactant - equilibrium shifts right
Increasing equilibrium of product - equilibrium shifts left

Question 7

Effect of temperature on equilibrium position

Answer 7

Increasing temperature - equilibrium shifts in direction of the endothermic reaction
Decreasing temperature- equilibrium shifts in direction of the exothermic reaction

Question 8

Effect of pressure on equilibrium position

Answer 8

Increase in pressure - equilibrium shifts to side with least moles of gas
Decrease in pressure - equilibrium shifts to side with most moles of gas

Question 9

Composition of equilibrium mixtures

Answer 9

Look at notes for examples and explanation

Question 10

What is equilibrium constant

Answer 10

The equilibrium constant is a mathematical relationship that shows how the concentration of the products vary with the concentration of the reactants

Question 11

What symbol does the equilibrium constant have , what symbol does it have when in terms of concentration

Answer 11

Equilibrium constant is - K

In terms of constant Kc

Question 12

Equilibrium constant of general equation aA + bB —— > < cC + dD

Answer 12

Kc = [C]^c x [D]^d / [A]^a x [B]^b
- where the brackets indicate the concentration of species present at equilibrium , this is then raised to the power of the number of moles of that species from the balanced equation

(Look in notes it makes more sense)

Question 13

What is a homogenous equilibrium

Answer 13

In a homogenous equilibrium , all the species present at equilibrium are all in the same state

Question 14

Worked out examples of equilibrium constant expressions

Answer 14

Look in notes ! Important

Question 15

Does K have units

Answer 15

No , K does not have units

Question 16

What happens when a pure solid / pure liquid is present at equilibrium
Example

Answer 16

When a pure solid / liquid is present at equilibrium it’s concentration takes a value of 1
This is NOT true for aqueous solutions
Example in notes

Question 17

What is a heterogenous equilibria

- example

Answer 17

• in a heterogenous equilibria , not all the species present are in the same state
• an example of a heterogenous equilibrium is heating calcium carbonate in a closed system so that the carbon dioxide gas produced cannot escape and equilibrium is established : look in notes for details

Question 18

What can affect the position of equilibrium , but not the equilibrium constant

Answer 18

Changing the concentration or pressure may affect the position of equilibrium, but it does not affect th equilibrium constant ,K.

Question 19

What happens to equilibrium when concentration or pressure is changed (in terms of constant )

Answer 19

When the concentration (or pressure in a chemical equilibrium involving gases) is changed , the position of equilibrium also changes - until the ratio of products and reactants return to original value (position of equilibrium changes until numerical value of K is de established )

Question 20

What effect does a catalyst have on position of equilibrium and value of K

Answer 20

• presence of a catalyst has no effect on the position of equilibrium
• therefore a catalyst has no effect on the numerical value of the equilibrium constant K

Question 21

Effect of temperature on equilibrium constant K

Answer 21

The value of equilibrium constant , K , is dependent on temperature

Question 22

Increase in temperature effect on K

• in endothermic reactions
• in exothermic reactions

Answer 22

• in an endothermic reaction , increase in temp favours products , so ratio of products / reactants increases so K increases
• in endothermic reactions increase in temp causes increase in yield of products so value of K increases
• in an exothermic reaction , an increase in temp favours the reactants so ratio of products /reactants decreases and therefore K decreases.
• in exothermic reactions an increase in temp causes a decrease in the yield of the product and the value of K decreases

Question 23

What does the equilibrium constant value give indication of

Answer 23

Since the equilibrium constant is the ratio of concentration of products divided by the concentration of reactants, it’s actual value gives guidance to the extent of a reaction once it has reached equilibrium

Question 24

The greater the value of Kc the ________ the concentration of products compared to reactant

Answer 24

Greater

- in other words the further the reaction has gone to completion

Question 25

Value of Kc an extent of reaction

Answer 25

Much less than 1 (or 10-3 ) - position of equilibrium is to the left (effectively no reaction )
Approximately 1- neither side is favoured , significant quantities of reactants and products at equilibrium
Much greater than 1 - to the right , reaction is effectively complete