chemical equillibria kc

Question 1

What are many chemical reactions

Answer 1

Reversible

Question 2

What happens at equilibrium in a reversible reaction

Answer 2

Forward and reverse reactions proceed at equal rates
The concentrations of reactants and products remain constant

Question 3

How is dynamic equilibrium reached (3)

Answer 3

Rate of forward starts fast and decreases because less reactants become products
Rate of backward reaction starts slow and increase because products are formed
Eventually, the rate of forward reaction equals the rate of backward reaction

Question 4

What is a close system

Answer 4

Where dynamic equilibrium can only occur if the reversible reaction is carried out in a sealed container, so that no products can escape

Question 5

What does the position of equilibrium depend on

Answer 5

How much products and reactants (moles) are present at equilibrium

More products than reactants = equilibrium shifts to the right
More reactants than products = equilibrium shifts to the left

Question 6

State Le Chatelier’s principle

Answer 6

The position of equilibrium will shift to oppose a change made to it

E.g if overall concentration decreases (E.g by adding more water) E.g shifts to the side with the more males to oppose decrease in concentration

Question 7

What is the change in concentration in relation to equilibrium

Answer 7

If the concentration of one of the substances at dynamic equilibrium is changed, the equilibrium will shift to oppose the change.

Question 8

What would happen to the equilibrium if the concentration of CO increases

CO + H2O 🔁 CO2 + H2

Answer 8

The equilibrium shifts to the right
To oppose the increase in CO
So the yield of CO2 + H2 increases

Question 9

What to include in a change if concentration at equilibrium question

Answer 9

1. What way equilibrium will shift
2. What it opposes
3. The yield and whether it will increase or decrease

Question 10

What happens if they are equal number of moles of products and reactants , in terms of pressure

Answer 10

Pressure would not affect the position of equilibrium

Question 11

When does pressure change affect equilibrium

Answer 11

Only affect reactions that involve gas

Question 12

What is the change in pressure in relation to equilibrium

Answer 12

If the pressure of one of the substances at dynamic equilibrium is changed, the equilibrium will shift to oppose that change. The more molecule (or moles) the greater the pressure. The fewer molecules the smaller the pressure

Question 13

2 molecules : 1 molecule
2NO2 (g) 🔁 N2O4 (g)

State what would happen with the following :
If the pressure is increased
If the pressure decreased

Answer 13

Increased =
Fewer molecules on the right hand side
Equilibrium shifts right
To oppose the increase in pressure
Yield of N2O4 (product) increases

Decreased =
More molecules on the left hand side
Equilibrium shifts left
To oppose decrease in pressure
Yield of N2O4 decreases

Question 14

CH4 (g) + H2O (g) 🔁 CO (g) + 3H2 (g)

Use Le Chatiler’s principle to explain why increasing the volume of the reaction container increases the equilibrium yield of hydrogen

Answer 14

Increase in volume decreases pressure
More molecules on the right hand side
Equilibrium shifts right
To oppose the decrease in pressure

Question 15

What to include in a change of pressure at equilibrium question

Answer 15

1. More or less molecule
2. Equilibrium shifts
3. Yield, whether it increases or decreases

Question 16

What does not affect the position of equilibrium

Answer 16

A catalyst

Question 17

What is the change in temperature in relation to equilibrium

Answer 17

If the temperature of the equilibrium system is changed, the equilibrium will shift to oppose that change.

Question 18

CH4 (g) + H2O 🔁 CO (g) + 3H2 ΔH = - 206 kj mol-1

State what will happen for the following :
If the temperature increases
If the temperature decreases

Answer 18

CH4 (g) + H2O 🔁 CO (g) + 3H2 ΔH = - 206 kj mol-1
COLD HOT

Increase =
Forward reaction is exothermic
Equilibrium shifts right
To oppose the decrease in temperature

Decrease =
Backward reaction is endothermic
The equilibrium shifts to the left to oppose the increase in temperature
Yield of CH4 & H2O increases

Question 19

What is a catalyst

Answer 19

A substance that speeds up the rate of reaction by providing an alternative reaction pathway with a lower activation energy

Question 20

When a catalyst is added to a reversible reaction

Answer 20

No change in equilibrium position
Increases the rate of forward reaction and backwards reaction equally
Decreases the time taken to reach equilibrium

Question 21

What are the things to consider when compromising conditions

Answer 21

Cost
Yield
Reaction rate

Question 22

Why are compromised conditions necessary

Answer 22

Temperature & pressure
- need high yield and fast rate of reaction
- higher temperature = faster rate
- high temperature is expensive
- higher pressure = requires expensive equipment

Catalysts
- increase rate of reaction/ product produced quicker

Removing product as it forms

Question 23

What does Kc mean (the c)

Answer 23

Concentration

Question 24

What is the Kc equation

Answer 24

Kc - products/ reactants

Question 25

How do you calculate the units for Kc

Answer 25

Add the top powers
Add bottom powers
Take the top powers - bottom powers

Question 26

What happens when the Kc is greater than one

Answer 26

More products than reactants at equilibrium
Equilibrium shifted to the right

Question 27

What happens when Kc is less than 1

Answer 27

More reactants than products
Equilibrium shifted light

Question 28

What happens when Kc is exactly 1

Answer 28

Concentration of product is equal to concentration of reactants

Question 29

How do you work out the Kc from the concentration of reactants and products at equilibrium using the following process :

Answer 29

1. Initial amounts of each product or reactant
2. Change in each reactant
3. Equilibrium number of moles for each product and reactant
4. Equilibrium concentration of each reactant (mol dm-3)
5. Kc expression (and calculation)
6. Units