Equilibria

Question 1

What is a reversible change?

Answer 1

A change that can take place in both directions

Question 2

What happens when a reaction is in a dynamic equilibrium?

Answer 2

The forward and reverse reactions of reactants and products occur at the same rate and the concentrations are constant

Question 3

Give three examples of reversible reactions and their equations

Answer 3

• Ammonia manufacture: Haber process
3H2 + N2 2NH3

• Co2 reacting in sparkling mineral water
CO2 + H20 HCO3- + H+

• Haemoglobin
Haemoglobin + oxygen oxyhaemoglobin

Question 4

Describe an experiment to monitor forward and reverse reactions

Answer 4

•Hydrogen and iodine reacy in a reversible reaction to produce hydrogen iodide

H2 + I2 2HI
• two experiments are carried out one starting with hydrogen iodide and the other starting with hydrogen and iodine. In both cases the gasses are sealed in a glass bulb and kept at a constant temperature of 445°c and the reaction is stopped at regular intervals by cooling the bulb rapidly and the contents analysed
• Experiment A: 1 mol hydrogen iodide was put in the flask after 84 mins 0.78 mol hydrogen iodide was present and no further change took place
• Experiment B: 0.5 mol hydrogen and 0.5 mol iodine put in the flask. After 80 mins there were 0.79 mol of hydrogen iodide and the number remained the same.

The equilibrium stayed the same in both experiments

Question 5

Why does the forward reaction happen quicker than the reverse reaction initially in a closed system?

Answer 5

Because the reactants are at their highest concentrations

Question 6

What is the equilibrium constant and what’s the symbol for it?

Answer 6

Kc

It is the concentration of all the substances measured if a reversible reaction is allowed to reach equilibrium combined into an expression

Question 7

What is the equation for the equilibrium constant? And explain it

Answer 7

Kc = [C]c [D]d/ [A]a [B]b

Where the letter on the outside of the bracket is the number of moles of the substance

The right hand side of the equation goes on the top line and the left on the bottom

The square brackets indicate concentrations in mol dm-3 in the equilibrium mixture

Question 8

What is a closed system?

Answer 8

Where no substances are exchanged with the surroundings and so no reactants or products were lost

Question 9

What has to be kept constant for an equilibrium to be maintained and why?

Answer 9

The equilibrium concentrations of reactants and products

Because if a reactant is allowed to escape its concentration decreases and the rate of forward reaction decreases do the original state of equilibrium no longer exists

Question 10

What is the difference between a homogenous and heterogeneous equilibria?

Answer 10

In a homogenous equilibria all the substances are in the same phase (e.g. all gases) in a heterogeneous equilibria they are in different phases

Question 11

What would be the equation for the equilibrium constant of the following reaction?

2SO2 + O2 2SO3

Answer 11

Kc = [SO3]2/ [SO2]2 [O2]

Question 12

What is the constant that uses partial pressures rather than concentrations and is used for gaseous reactions?

Answer 12

Kp

Question 13

How are the units of Kc found?

Answer 13

By substituting the concentration units in the equation for Kc and then cancelling

Question 14

Calculate these concentration units:

Mol dm-3)2/ (moldm-3)2 (moldm-3

Answer 14

1/ mol dm-3 = mol -1 dm3

Question 15

What is the acronym in the table you need to draw in order to do calculations involving Kc and what do they stand for?

Answer 15

I= initial moles/ concentration
C = change in moles / concentration 
E = equilibria moles / concentration

Question 16

In a Kc equation what should you look for if you are only given moles and not volume

Answer 16

That there are equal number of moles of reactants and products and then you dont need the volume

Question 17

If you are not given the equilibria concentrations how should you solve the equation?

Answer 17

Algerbaically

Question 18

What does the Kc value tell us?

Answer 18

• If it is 1 or near to one it shows that the concentrations of reactants and products are similar and the position of equilibria lies near the centre
• if the it is greater than 1 the position of equilibria lies towards the products
• if it is less than 1 the position of equilibria lies towards the reactants

Question 19

What does the position of equilibrium in a reaction tell us and in what occasion would it lie to the right?

Answer 19

The position of equilibrium tells you the proportion of products to reactants

If products concentrations at equilibrium are high and reactant concentrations low then the position of equilibrium lies to the right

Question 20

What is the position of equilibrium changed and not changed by?

Answer 20

It can be changed by changes in temperature, pressure and concentration

It is not affected by catalysts

Question 21

What is le Chatelier’s principle?

Answer 21

The position of the equilibrium of a system moves to minimise the effect of any imposed change in conditions

Question 22

How can le Chateliers principle be applied to the haber process?

Answer 22

N2+3H2 2NH3
Delta H = -92 KJ mol -1

The reaction is exothermic in the forward direction so under heating the reaction moves to the equilibroum position to the left maximising the yield of the reactants. This happens because raising the temperature shifts the equilibrium in the direction of the endothermic change so that the system can absorb the energy.

However in the Haber process we want the products to be made but we cant lower the temperature because although that would increase the yield it would decrease the rate of reaction

Question 23

What is the effect of inceeasing pressure on equilibria with the following reaction and why?

N2 + 3H2 2NH3

Answer 23

It shifts the equilibria to the right because there are fewer miles of gaseous product on the right than the left and in a gas the pressure depends on the number of molecules colliding and exerting a force. When pressure is increased the equilibrium shifted to the right as the reaction opposed the change imposed on it.

Question 24

What is the effect of concentration on equilibria?

Answer 24

If the concentration is increased the equilibria will shift to the side with fewer moles to oppose the change

Question 25

What is the effect of a catalyst?

Answer 25

• it increases the rate of reaction by providing an alternative route with a lower activation energy
• it does not alter the position of the equilibrium because the concentrations of the products and reactants remain exactly the same and the value of Kc does not change but equilibrium is reached more quickly

Question 26

What catalyst is used in the haber process?

Answer 26

Finely divided iron

Question 27

Why cant high pressures of temperatures be used in the haber process and what is used instead?

Answer 27

• They are both expensive
• a high temperature decreases ammonia yield
• a compromise temperature of 700 K is used
• a compromise pressure between 5000 and 20 000 kpa is used