Equilibrium

Question 1

What is dynamic equilibrium?

Answer 1

Where the forward reaction as at the same rate as the backwards reaction

Question 2

What can be said about the concentration of reactants and products for a reaction in dynamic equilibrium?

Answer 2

Because the forwards and backwards reactions are at the same rate, there is no overall change in concentrations of the reactants or products

Question 3

What is the only type of system a dynamic equilibrium can occur?

Answer 3

Closed system at a constant temperature

Question 4

Give an example of a physical system that an equilibria can be set up in

Answer 4

When liquid bromine is shaken in a closed flask, it turns into bromine gas. After a while, equilibrium is reached, where the exact amount of liquid turning into gas is the same as the exact amount of gas turning into liquid

Question 5

Give an example of a chemical system that equilibria can be set up in

Answer 5

Hydrogen gas reacted with iodine gas to form hydrogen iodide

Question 6

What is the equilibrium constant?

Answer 6

A ratio worked out from the concentration of the products and reactants once a reversible reaction has reached its equilibrium

Question 7

For the general equation aA + bB ⇌ dD + eE, what is the equation for the equilibrium constant?

Answer 7

Kc = ([D]^d [E]^e) / ([A]^a [B]^b)

Lowercase letters are the number of moles of each reactant as shown in the balanced equation

Question 8

What are the 2 steps to work out the equilibrium concentrations?

Answer 8

Find out how many moles of each reactant and product there are at equilibrium
Calculate the molar concentrations of each reactant and product by dividing the number of moles by the given volume

Question 9

Why do you use square brackets for working out the equilibrium constant?

Answer 9

Because square brackets denote concentration (you won’t get mark in exam if use round brackets)

Question 10

How can you work out unknown equilibrium concentrations if given the equilibrium constant?

Answer 10

Put all the values you know into the expression for Kc

Rearrange to solve for the unknown concentrations

Question 11

What is a homogeneous reaction?

Answer 11

A reaction where all the reactants and products are in the same state

Question 12

What is a heterogeneous reaction?

Answer 12

A reaction where reactants and products are in different states

Question 13

What is the rule for putting in values for the equilibrium constant expression for a homogeneous reaction?

Answer 13

All the reactants and products are put into the expression for the equilibrium constant

Question 14

What is the rule for putting in values for the equilibrium constant expression for a heterogeneous reaction?

Answer 14

Only gases and aqueous substances go into the expression for the equilibrium constant

Question 15

Why do you not put solids and liquids into the equilibrium constant expression for a heterogeneous reaction?

Answer 15

Because they do not affect the reactant amount at equilibrium in the reaction

Question 16

When you’ve found the equilibrium constant for a reaction, what else do you need to work out?

Answer 16

The units

Question 17

How do you work out the units of the equilibrium constant

Answer 17

Make an equation out of the units for all the concentrations and then simplify

Question 18

What is partial pressure?

Answer 18

The pressure exerted by a particular gas in a mixture of gases

Question 19

How do you quite often see partial pressures written down as?

Answer 19

p(X) is the partial pressure of X

Question 20

What is a mole fraction?

Answer 20

A proportion of a gas mixture that is made up of a particular gas

Question 21

If you have 4 moles of a gas in total and 2 of them are gas A, what is the mole fraction of gas A?

Answer 21

4/2 = 0.5

Question 22

What is the equation for the mole fraction?

Answer 22

Mole fraction = Number of moles of gas / Total number of moles of gas in mixture

Question 23

What is the equation for partial pressure of a gas?

Answer 23

Partial pressure = Mole fraction of gas x Total pressure of mixture

Question 24

What is the difference between Kc and Kp?

Answer 24

Kc is the equilibrium constant when it is found through the use of concentrations, whereas Kp is the equilibrium constant when it is found through the use of partial pressures

Question 25

For the equilibrium aA(g) + bB(g) ⇌ dD(g) + eE(g), what is the equation for the equilibrium constant, Kp?

Answer 25

Kp = ( p(D)^d x p(E)^e) / ( p(A)^a x p(B)^b)

Question 26

When using the equation for the equilibrium constant with partial pressures, what don't you include in the equation?

Answer 26

Solids, Liquids and Aqueous substances

Question 27

Why don't you include solids, liquids and aqueous substances in the equation for Kp?

Answer 27

Because they have no effect on the pressure of the whole system

Question 28

What is a particular value of Kp only valid for?

Answer 28

A specific temperature, as Kp is temperature dependant

Question 29

When writing an expression for Kp for a heterogeneous equilibria, what don't you include in the equation?

Answer 29

Solids or liquids

Question 30

What is Le Chatelier's Principle?

Answer 30

If there's a change in concentration, pressure or temperature, the equilibrium will move to help counteract the change

Question 31

Do you get more product or reactants if the equilibrium shifts left?

Answer 31

More reactants

Question 32

Do you get more product or reactants if the equilibrium shifts right?

Answer 32

More products

Question 33

What does Kc tell you about equilibrium position?

Answer 33

The size of Kc tells you where the equilibrium lies

Question 34

Where is the equilibrium if Kc is a large value?

Answer 34

Further to the right (more product)

Question 35

Where is the equilibrium if Kc is a small value?

Answer 35

Further to the left (more reactant)

Question 36

What happens to the equilibrium position if you increase the temperature?

Answer 36

It'll move in the endothermic (+ve ΔH) direction to absorb the heat

Question 37

What happens to the equilibrium position if you decrease the temperature?

Answer 37

It'll move in the exothermic (-ve ΔH) direction to try and replace the heat

Question 38

For a equilibrium reaction, if the forward reaction is endothermic (+ve ΔH), what is the backwards reaction?

Answer 38

Exothermic (-ve ΔH)

Question 39

What do the ΔH values given for a reversible reaction tell you?

Answer 39

ΔH of the forward reaction

Question 40

If you change the concentration of one of the things in the equilibrium mixture, what do the other compounds do to keep the equilibrium constant the same for a given temperature?

Answer 40

They have to change to balance

Question 41

What state does changing the pressure have an effect on the equilibrium?

Answer 41

For gases

Question 42

What does the equilibrium do if you increase the pressure of the equilibrium mixture?

Answer 42

It moves to the side with the fewest moles of gas (fewest molecules of gas) to reduce the pressure

Question 43

What does the equilibrium do if you decrease the pressure of the equilibrium mixture?

Answer 43

It moves to the side with the most moles of gas (most molecules of gas) to increase the pressure

Question 44

What is the effect of adding a catalyst to the equilibrium position and the value of the equilibrium constant?

Answer 44

Adding a catalyst has no effect on either

Question 45

Why does adding a catalyst have no effect on the equilibrium position and the value of the equilibrium constant?

Answer 45

Because it increases the rate of the forward and backwards reactions by the same amount (meaning equilibrium will be reached faster)

Question 46

Is endothermic +ΔH or -ΔH?

Answer 46

+ΔH, because it takes in energy from its surroundings

Question 47

Is exothermic +ΔH or -ΔH?

Answer 47

-ΔH, because it releases energy to its surroundings

Question 48

What 3 factors have no effect on the equilibrium constants?

Answer 48

Changing pressure Changing concentration Adding a catalyst