Equillibrium

Question 1

Features of dynamic equilibrium

Answer 1

• Closed system
• Forward reaction = Reverse reaction
• Concentration of Reactants and products are constant.

Question 2

Why is a catalyst finely divided in a reaction?

Answer 2

Chemical reaction takes place at the surface of the catalyst. Greater the SA of catalyst, the greater the chance of successful frequent collisions, increasing the rate.

Question 3

How do you calculate Kc (equilibrium constant)?
What is homogenous/heterogenous equilibria?

Answer 3

Kc = Concentration of products / concentration of reactants

Homogenous contains species that all have the same state.
Heterogenous contains species that have different states (in heterolytic, Kc NEVER includes solids or liquids except water)

Question 4

What is the value of Kc affected by?

Answer 4

• depends on the temperature and is unaffected by changes in concentration, pressure and a catalyst.
• if Kc<1, equillbrium lies towards reactants.
• if kc>1 equillibrium lies towards products.
• if Kc=1 equillibrium in middle.

Question 5

How do you calculate Kc based off initial amounts?

Answer 5

In a table:
- Components
- Ratio
- Initial moles: (products would be 0)
- Equillibrium moles:
- Equilibrium concentration: (divide volume by moles)
Find the units.

Question 6

How is Kc affected by Temperature?

Answer 6

If the forward reaction is EXOTHERMIC, and temp is INCREASED:
- equilibrium shifts left, Kc decreases
If forward reaction is ENDOTHERMIC and temp INCREASES:
- Equilibrium shifts right, Kc increases

Question 7

In terms of kc how does concentration affect it?

Answer 7

• Kc doesn’t change.
• when conc of reactants increases, the bottom value in Kc expression increases so equillbrium shifts right so that top of expression increases and bottom decreases until Kc is restored.
• opposite if decreased

Question 8

Whats Kp and how do you calculate it?

Answer 8

Kp is partial pressure of gases.

In a table:
- Components
- Calculate mole fraction: number of moles of gas/total moles of gas in mixture(includes reactants/products)
- partial pressure: *mole fraction x total pressure. *
- find units: use units in Q don’t have to convert

Question 9

How do you write an expression for Kp?

Answer 9

Kp= p(product) / p(reactant) p(reactant)

• calculate units same as Kc but with pressure units.
• IGNORE other state symboles other than GASES, dont include solids and liquids.

Question 10

What effect to temperature have on Kp?

Answer 10

• On EXOTHERMIC reactions, Kp DECREASES with INCREASING temp as equilibrium shifts LEFT.
  If 2/3marks: - increase in temp shifts equilbrium left, partial pressure products decreases and partial pressure reactants increases.
• ENDOTHERMIC if temperature INCREASES, Kp INCREASES as equilibrium shifts RIGHT.
  If 2/3 marks: increase in temp shifts equilbrium right, partial pressure reactants decreases and partial pressure products increases.

Question 11

How does changes in pressure affect Kp?

Answer 11

if pressure increases and equilibrium shifts left :
- Top of the Kp expression increases more than the bottom (… mol V …mol)
- equillbrium shifts to side with least number moles
- bottom expression increases and top decreases until Kp is restored