Topic 10 - Equilibrium I

Question 1

What is a reversible reaction?

Answer 1

A reaction that can go forward or backwards.

Question 2

How do you show a reversible reaction in an equation?

Answer 2

Use an double arrow that points both ways in the equation.

See pg 118 of revision guide

Question 3

What is dynamic equilibrium?

Answer 3

When the concentrations of reactant and products in a reversible reaction are constant, and the forward and backwards reactions happen at the same rate.

Question 4

In dynamic equilibrium, what can be said about:
• The reaction rates
• The concentrations

Answer 4

• The forward and backwards reactions happen at the same rate
• The concentrations of the products and reactants stay constant (but not necessarily equal to each other)

Question 5

In dynamic equilibrium, when there are more products than reactants, where does the position of the equilibrium lie?

Answer 5

To the right.

Question 6

In dynamic equilibrium, when there are more reactants than products, where does the position of the equilibrium lie?

Answer 6

To the left.

Question 7

Describe how dynamic equilibrium is reached from a system with just the reactants in at first.

Answer 7

• As the reactants get used up, the forward reaction slows down
• As more product is formed, the reverse reation speeds up
• Eventually, the two happen at the same rate and dynamic equilibrium is reached

Question 8

Where can a dynamic equilibrium be reached?

Answer 8

In a closed system.

Question 9

What is the equilibrium constant?

Answer 9

A measure of how far to the left or right the equilibrium is in a particular system.

Question 10

What is the symbol for the equilibrium constant?

Answer 10

Kc

‘c’ is subscript

Question 11

What is a homogeneous system?

Answer 11

One in which everything is in the same physical state,

Question 12

What is a heterogeneous system?

Answer 12

One in which not everything is in the same physical state.

Question 13

In a homogeneous system, which reactants and products are used in the equation for Kc?

Answer 13

All of them.

Question 14

Write an equation for Kc for the reaction:

aA + bB dD + eE

Answer 14

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

Question 15

In the equation for Kc, what goes on the top and bottom?

Answer 15

Top: Products
Bottom: Reactants

Question 16

In the equation for Kc, what do the square brackets and powers mean?

Answer 16

• Square brackets: Concentration of each substance.

* Powers: Number of moles of each substance from equation.

Question 17

In a heterogeneous system, which reactants and products are used in the equation for Kc?

Answer 17

All of them, except solids and pure liquids.

Question 18

Why are solids and pure liquids not included in the equation for Kc in a heterogeneous system?

Answer 18

Their concentrations stay constant throughout the reaction.

Question 19

Write an expression for the equilibrium constant for:

H2 (g) + I2 (g) 2HI (g)

Answer 19

Kc = [HI]^2 / [H2] [I2]

Question 20

Write an expression for the equilibrium constant for:

H2O (g) + C (s) H2 (g) + CO (g)

Answer 20

Kc = [H2] [CO] / [H2O]

Question 21

What are the units for the concentrations used in the eqution for Kc?

Answer 21

mol / dm3

Question 22

What are the units for Kc?

Answer 22

• It depends.

* Divide the units on top of the euqtion by those on the bottom.

Question 23

Remember to practise writing out dynamic equilibrium equations.

Answer 23

Pgs 118-119 of revision guide.

Question 24

Do catalysts affect the equilibirum constant (Kc)?

Answer 24

No, they just speed up the rate at which dynamic equilibrium is reached.

Question 25

If the equilibirum shifts to the left, you get more...

Answer 25

Reactants

Question 26

If the equilibrium shifts to the right, you get more...

Answer 26

Products

Question 27

State Le Chatelier's principle.

Answer 27

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

Question 28

What changes in conditions can cause a shift in the position of an equilibrium

Answer 28

* Temperature * Concentration * Pressure

Question 29

How does increasing the concentration of a reactant affect equilibrium?

Answer 29

* Equilibrium tries to get rid of extra reactant | * Forward reaction is favoured -> So the equilibrium shifts to the right

Question 30

How does decreasing the concentration of a reactant affect equilibrium?

Answer 30

* Equilibrium tries to get produce extra reactant | * Backwards reaction is favoured -> So the equilibrium shifts to the left

Question 31

How does increasing the pressure affect equilibrium?

Answer 31

* Equilibrium tries to reduce the pressure | * Equilibrium shifts to the side with fewer moles of gas

Question 32

How does decreasing the pressure affect equilibrium?

Answer 32

* Equilibrium tries to increase the pressure | * Equilibrium shifts to the side with more moles of gas

Question 33

How does increasing the temperature affect equilibrium?

Answer 33

* Equilibrium tries to reduce the temperature | * Endothermic reaction is favoured -> So equilibrium shifts in that direction

Question 34

How does decreasing the temperature affect equilibrium?

Answer 34

* Equilibrium tries to increase the temperature | * Exothermic reaction is favoured -> So equilibrium shifts in that direction

Question 35

If the foward reaction is exothermic, the reverse reaction is...

Answer 35

Endothermic

Question 36

Does changing the pressure always change the position of equilibrium?

Answer 36

No, only if there are gases involved.

Question 37

When working out how a change in pressure will affect equilibrium, what must you look at?

Answer 37

The total number of MOLES of gas on each side, not just the number of gases on each side. (i.e. Look at the numbers before each gas!)

Question 38

What is the equation for the production of ethanol in industry?

Answer 38

C2H4 (g) + H2O (g) C2H5OH (g)

Question 39

What are the conditions for the production of ethanol in industry?

Answer 39

* 60-70 atm * 300*C * Phosphoric(V) acid catalyst

Question 40

C2H4 (g) + H2O (g) C2H5OH (g) ΔH = -46 kJ/mol The reaction takes place at 60-70 atm, 300*C and with a phosphoric(V) acid catalyst. Explain the temperature used.

Answer 40

* Forward reaction is exothermic, so low temperatures favour the forward reaction -> Better yield * But lower temperatures mean a slow rate of reaction * So 300*C is a compromise between maximum yield and rate of reaction

Question 41

C2H4 (g) + H2O (g) C2H5OH (g) ΔH = -46 kJ/mol The reaction takes place at 60-70 atm, 300*C and with a phosphoric(V) acid catalyst. Explain the pressure used.

Answer 41

* High pressure favours the forward reaction -> Better yield * High pressure also increases the rate of reaction * But high pressure is expensive to maintain * So 60-70 atm is a compromise between maximum yield (+ rate) and expense

Question 42

Remember to revise how to word industry equilibrium consition questions.

Answer 42

Pg 121 of revision guide