3.1 Physical Chemistry: .6 Equilibria

Question 1

in a reversible reaction at equilibrium the… (two things)

Answer 1

-forward reaction is equal to the rate of the backward reaction -the concentration of reactants and products remain constant

Question 2

what is meant by a dynamic equilibrium?

Answer 2

this is where the forward reaction and backward reaction are continuously occurring. A dynamic equilibrium can only take place in a closed system.

Question 3

what is meant by the statement ‘microscopically changing but macroscopically constant’

Answer 3

when it seems that the reaction is constant from afar but you look close and can see that the reactions are going forward and backward

Question 4

what are the conditions

Answer 4

-equilibrium can only be reached in a closed system (where reactants and products cannot escape.) -equilibrium can be approached from either direction and the final equilibrium position will be the same. -dynamic equilibrium where the reactions are continuously occurring at the same time -macroscopic properties of the system do not change with time e.g density, colour, pressure (theses are properties that do not depend on the total quantity of matter) q

Question 5

what does Le Chatelier’s principle state?

Answer 5

if a system at equilibrium is disturbed, the equilibrium moved in the direction that tends to counteract the disturbance

Question 6

what are the factors that affect equilibrium?

Answer 6

-a change in concentration -a change in temperature -a change in pressure

Question 7

why does a catalyst not affect the position of equilibrium?

Answer 7

it does not increase yield it only speeds up the rate of the backward and forward reaction equally so equilibrium is reached faster

Question 8

This is the equilibrium mixture for the Haber Process:

N₂(g) + 3H₂(g) ↔ 2NH₃(g) ΔH= -92 kJ mol^-1

conditions:

• Pressure= 200 atm/MPa
• Temperature= 450 degrees celcius
• Catalyst= Iron

state what would happen when the pressure of this mixture is increased and how it would affect the rate of reaction.

Answer 8

if you were to increase the pressure:

• the equillibrium shifts to the right to oppose the change and lower the pressure again.
• It does this by moving to in the direction with the least number of moles.
• Therefore increasing the yield.

A high pressure increases the rate of reaction

• as there are more particles per cm³
• and therefore more successful collisions per second

Question 9

This is the equilibrium mixture for the Haber Process:

N2(g) + 3H2(g) ↔ 2NH3(g) ΔH= -92 kJ mol-1

conditions:

• Pressure= 200 atm/MPa
• Temperature= 450 degrees celcius
• Catalyst= Iron

explain the effect on the equiibrium if the temperature is decreased and the effect on the rate of reaction when the temperature is increased.

Answer 9

A decrease in temperature;

• would cause the equillibrium to shift to right to oppose the change and increase the temperature.
• It does this by moving in the exothermic direction so that heat is given out.
• increasing the yield of ammonia

An increase in temperature would increase the rate of reaction;

• because there will be more successful collisions per second
• due to the fact that there are more particles which have more energy/equal energy to the activation energy (E ≥ Ea).

Question 10

This is the equilibrium mixture for the Haber Process:

N2(g) + 3H2(g) ↔ 2NH3(g) ΔH= -92 kJ mol-1

conditions:

• Pressure= 200 atm/MPa
• Temperature= 450 degrees celcius
• Catalyst= Iron

explain why there are compromises used in this process

Answer 10

Pressure: It is too expensive and uses lots of energy to maintain the high pressure needed.

Temperature: 450 degrees C is a compromised to make a reasonable amount of yield in a reasonable amount of time

Question 11

what is meant by Kc?

Answer 11

Kc is the;

• equillibrium constant at a particular temperature
• Kc is the ratio of the product of the product concentrations concentrations compared to the product of the reactant concentrations all raised to the power of the indicies in the balnaced chemical eqaution

Question 12

Here is an equillibrium mixture:

2A + B ↔ 2C

write an equation for the Kc of this reaction

Answer 12

Kc= [C]²/ [A]² x [B]¹

Question 13

Here is an equillibrium mixture:

2A + B ↔ 2C

what are the units for Kc for this reaction

Answer 13

Question 14

what is the only factor that affects Kc?

Answer 14

temperature

Question 15

what will happen to the Kc of reaction if an increase in temperature results in more yield?

Answer 15

Kc will increase

Question 16

what will happen to the Kc of reaction if an increase in temperature results in less yeld?

Answer 16

Kc will decrease

Question 17

answer these exam questions:

Answer 17

answers

Question 18

what are the regular units for Kc?

Answer 18

mol dm^-3

Question 19

what does not affect Kc?

Answer 19

• concentration
• addition of a catalyst