Equlibrium

Question 1

what does equilibrium mean

Answer 1

a state of balance

Question 2

what is a reversible reaction

Answer 2

the forward and backwards reaction take place at the same time

Question 3

what is dynamic equilibrium

Answer 3

the forward and backward reaction occur at the same rate and the concentrations of the reactants and product do not change

Question 4

what are the conditions of eq

Answer 4

-the eq can only be reached in a closed system - where products and reactants cant escape
-eq can be approached from either direction and the final eq positon will be the same
-dynamic process - reached when the rates of two opposing processes which go on at the same time and are the same
-conc, pressure, density and colour do not change

Question 5

where is the positon of eq if there is more reactants than products

Answer 5

to the left

Question 6

where is the positon of eq if there is less reactants than products

Answer 6

to the right

Question 7

what is Le Chateliers principle

Answer 7

the position of eq of a system changes to minimise the effect of any imposed change on the conditions - at dynamic eq

Question 8

how does equilibrium shift temperature

Answer 8

increase
-favour endothermic direction and shifts towards this - positive enthalpy
decrease
-favours exothermic directions and shifts in this direction - negative enthalpy

Question 9

how does equilibrium shift pressure

Answer 9

increase
-favour the side with fewer molecules/moles
decrease
-favour the side with more molecules/ moles

Question 10

how does equilibrium shift conc

Answer 10

increase
-shift right to reduce effect of increase of reactants
decrease
-shift left to reduce effect of decrease of reactant/increase in products

Question 11

effect of catalyst on equilibrium

Answer 11

no effect

Question 12

effect of surface area on eq

Answer 12

no effect

Question 13

how to describe change in eq

Answer 13

eq shifts to counteract/oppose increase/decrease in temp,pressure or conc

Question 14

what is the eq constant Kc

Answer 14

The equilibrium expression links the equilibrium constant, Kc, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account

Question 15

what is an overall expression for Kc

Answer 15

aA + bB = cC+ dD

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

Question 16

examples of Kc

Answer 16

Deduce the equilibrium expression for the following reactions:

Ag+ (aq) + Fe2+ (aq) ⇌ Ag (s) + Fe3+ (aq)

[Fe3+]/[Ag+] [Fe2+]
Ag (s) not included as is a solid

Question 17

how to work out units using Kc

Answer 17

all molecules are moldm-3
then look at how many of each molecule and see if cancel
if not then add remaining moldm-3 together
if all cancel out then write no units

Question 18

if the values are in mols and u are given a volume what do you do for Kc equations

Answer 18

use conc =mol/vol
then put in Kc expression
solve and work out units

Question 19

what changes do not affect Kc

Answer 19

conc, pressure , catalyst and surface area

Question 20

how does temp affect Kc

Answer 20

for an increase in temp
forward reaction is endo
-then products increase and reactants decrease causing kc to increase
forward exo
-products decrease and reactants increase meaning kc decreases

Question 21

how is le Chatelier’s principle used to maximise the yield of ammonia in the haber process

Answer 21

Haber process
The Haber process involves the synthesis of ammonia according to:
N­2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔHr = -92 kJ mol-1

Le Chatelier’s principle is used to get the best yield of ammonia
Maximising the ammonia yield

Pressure
An increase in pressure will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure
In this case, the equilibrium shifts towards the right so the yield of ammonia increases
An increase in pressure will cause the particles to be closer together and therefore increasing the number of successful collisions leading to an increased reaction rate
Very high pressures are expensive to produce therefore a compromise pressure of 200 atm is chosen

Temperature
To get the maximum yield of ammonia the position of equilibrium should be shifted as far as possible to the right as possible
Since the Haber process is an exothermic reaction, according to Le Chatelier’s principle the equilibrium will shift to the right if the temperature is lowered
A decrease in temperature will decrease the energy of the surroundings so the reaction will go in the direction in which energy is released to counteract this
Since the reaction is exothermic, the equilibrium shifts to the right
However, at a low temperature the gases won’t have enough kinetic energy to collide and react and therefore equilibrium would not be reached therefore a compromise temperature of 400-450oC is used in the Haber process
A heat exchanger warms the incoming gas mixture to give molecules more kinetic energy such that the gas molecules collide more frequently increasing the likelihood of a reaction

Removing ammonia
Removing ammonia by condensing it to a liquid causes the equilibrium position to shift to the right to replace the ammonia causing more ammonia to be formed from hydrogen and nitrogen
The recovered ammonia is stored at very low temperatures and there is no catalyst present with the stored ammonia so the decomposition reaction of ammonia to decompose back into hydrogen and nitrogen will be too slow to be a problem

Catalysts

In the absence of a catalyst the reaction is so slow that hardly anything happens in a reasonable time!
Adding an iron catalyst speeds up the rate of reaction

Question 22

how is le Chatelier’s principle used in the contact process

Answer 22

Contact process
The Contact process involves the synthesis of sulfuric acid according to:
2SO­2(g) + O2(g) ⇌ 2SO3(g) ΔHr = -197 kJ mol-1

SO3 + H2SO4 → H2S2O7

H2S2O7 + H2O → 2H2SO4

Le Chatelier’s principle is used to get the best yield of sulfuric acid
Maximising the sulfuric acid yield

Pressure
An increase in pressure will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure
In this case, the equilibrium shifts towards the right so the yield of sulfur trioxide increases
In practice, the reaction is carried out at only 1 atm
This is because Kc for this reaction is already very high meaning that the position of the equilibrium is already far over to the right
Higher pressures than 1 atm will be unnecessary and expensive

Temperature
The same principle applies to increasing the temperature in the Contact process as in the Haber process
A compromise temperature of 450 oC is used

Removing sulfuric acid
In practice, SO3 is removed by absorbing it in 98% sulfuric acid
This is because adding SO3 directly to water produces an uncontrollable and hazardous exothermic reaction
The SO3 reacts with the sulfuric acid to make oleum, H2S2O7, which is then carefully diluted to make more H2SO4
Catalysts

The Contact process uses vanadium(V) oxide as a catalyst to increase the rate of reaction

Question 23

how to find eq moles when only given starting moles

Answer 23

ICE

intial moles = the ones given in equation - if none given for the products assume 0
eq moles= the one that will be given in the equations is used
change = work out the change in the moles by looking at start and final eq moles

if products are +ve then reactants will be same val but -ve
and vice versa

Question 24

what is homogenous eq

Answer 24

all reactants and products in the same state

Question 25

what is heterogenous eq

Answer 25

reactants and products in different states