Chemical equilibria, Le Chatelier’s principle and Kc

Question 1

Reversible reactions

Answer 1

at dynamic equilibrium
- forward+reverse reactions=same rate
- closed system
- conc of reactants+products=constant
- all reactants+products are present

Question 2

Le Chatelier’s Principle

Answer 2

• if a change is made to the system in dynamic equilibrium, the position of equilibrium moves to oppose the change
• used to predict changes to position of equilibrium when changes to temp, pressure and conc happen

Question 3

How does conc affect homogeneous equilibrium?

Answer 3

• remove a reactant/products =equilibrium moves to make more
• add a reactant/product=equilibrium moves to remove it

Question 4

How does temp affect homogeneous equilibrium?

Answer 4

• up temp=equilibrium moves to endothermic reaction
• down temp=equilibrium moves to exothermic reaction

Question 5

How does pressure affect homogeneous equilibrium?

Answer 5

• only affects gases
• up pressure=equilibrium shifts to side w the smaller no of molecules
• down pressure=equilibrium moves to side larger no of molecules

Question 6

How does a catalyst affect homogeneous equilibrium?

Answer 6

• no change=increases rates of both reaction equally
• only help reach equilibrium faster

Question 7

Haber process

Answer 7

• N2 (g) + 3H2 (g) <—> 2NH3(g)
• Air provides nitrogen gas
• water and natural gas (CH4) provide hydrogen gas
  CH4(g) + H2O (g) → CO (g) + 3H2 (g)
• Nitrogen and hydrogen are fed into a converter in a 1:3 atio and passed over an iron catalyst (pea-sized lumps)
• Flow continously over catalyst - 15% conversion=cannot reach equilibrium
• ammonia condenses into a liquid
• conditions= pressure (200 atm), temp (450 C), iron catalyst
• 400-450°C is a compromise temp producing a reasonably high proportion of NH3 in a short time
• 200 atm is a compromise pressure chosen on economic grounds=very high pressure=cost to generate ammonia exceeds price of the extra NH3 u get+safety concerns

Question 8

Uses of ammonia and its compounds

Answer 8

• making fertilisers=80% goes to this eg ammonium nitrate (nitram), ammonium sulphate
• making nitric acid=oxidising NH3=make fertilisers, explosives, dyes etc

Question 9

Expression for Kc

Answer 9

Kc=[products]/[reactants]
- used to calculate units+value for Kc

Question 10

Calculating Kc

Answer 10

• construct balanced equation inc. state symbols
• find no moles of each species at equilibrium (using ICE)
• use vol in dm3, if no vol use V
• start by writing expression for Kc

Question 11

How does temp affect Kc?

Answer 11

• forward reaction=exothermic=up temp=equilibrium shifts to oppose it=lower value for Kc
• forward reaction=endothermic=down temp= equilibrium shifts to oppose it=higher value for Kc
• Kc is constant at a given temp