Equilibria and kc

Define dynamic equilibrium

The forward and backward reactions occur at the same rate so the concentrations of all the reactants and products remain constant

Give an essential condition for an equilibrium mixture

Equilibrium only occurs in a closed system

State Le Chatelier’s principle

If a reaction at equilibrium is subjected to a change in conditions, the position of the equilibrium will shift to counteract the change

What effect do catalysts have on the position of equilibrium? Why?

No effect

They lower the activation energy for the forward and backward reaction equally

What effect does increasing temperature have on equilibrium?

• Equilibrium shifts to decrease temperature

• So it will shift in the endothermic direction

What effect does decreasing temperature have on equilibrium?

• Equilibrium shifts to increase temperature

• So it will shift in the exothermic direction

What effect does decreasing pressure have on equilibrium?

• Equilibrium will shift to increase the pressure

• So it will shift to the side with the most moles of gas

What effect does increasing pressure have on equilibrium?

• Equilibrium will shift to decrease the pressure

• So it will shift to the side with the fewest moles of gas

What effect would increasing the concentration of [X] have on equilibrium?

• Equilibrium shifts to decrease [X]

• So equilibrium shifts to the side without [X]

What effect would decreasing the concentration of [X] have on equilibrium?

• Equilibrium shifts to increase [X]

• So equilibrium will shift to the side with [X]

For methanol synthesis

2H2 (g) + CO (g) → CH3OH (g) H = -100kJmol-1

Give the conditions and explain why they’re used

50-100atm - high pressure will make equilibrium favour the side with lower pressure (right hand side) to produce more methanol

250C - low temp would be optimal as this would make equilibrium shift to the exothermic side (right hand side) HOWEVER a compromise temperature is used as it needs to be high enough for the rate of reaction

Catalyst of copper, zinc oxide and aluminium oxide - speeds up the reaction and makes up for the compromise (lets us use as a low a temperature as we can)

For ethanol synthesis

C2H4 (g) + H2O (g) → CH3CH3OH (g) H=-50kJmol-1

Give the conditions and explain why they’re used

60-70 atm - high pressure to shift equilibrium to the right BUT not too high as equipment for this is expensive to build and run and so ethene doesn’t polymerise

300C - low temperature to shift equilibrium to exothermic side (right hand side) BUT not too low for rate of reaction

Phosphoric acid catalyst - to speed up reaction because of compromise on pressure on temperature

Why are compromise conditions required in industrial reactions?

To balance yield and rate

What is Kc?

The equilibrium constant which indicates the proportion of reactants to products

What does Kc being greater than or less than 1 suggest for the position of equilibrium?

Greater than 1 = products are predominate so equilibrium lies to the right

Less than 1 = reactants are predominate so equilibrium lies to the left

What does Kc being 1 suggest and how does this affect the position of equilibrium?

Products and reactants are present in similar amounts so equilibrium lies roughly in the middle

What condition affects the value of Kc?

Temperature only

Deduce the Kc expression for this reaction:

2A + B → C + D

Kc = [C][D] / [A]²[B]

Deduce the Kc expression for this reaction:

2A + 3B +C → D + 4E

Kc = [D][E]^4 / [A]²[B]³[C]

What are the units for Kc?

Mol-1dm³

But depends on moles of products and reactants

When would Kc have no units?

If coefficients on left hand side = coefficients on right hand side

Give units of Kc for this reaction:

A + B → C + 2D

Moldm-3

Give units of Kc for this reaction:

P + 2Q → 3R + 2S

Mol2dm-6

Give units of Kc for this reaction:

2A → B

Mol-1dm3

Give units of Kc for this reaction:

3C + 2D → E + 2F

Mol-2dm6

For this reaction:

PCl5 → PCl3 + Cl2 total volume=2dm³

start mol (PCl5) = 6 equilibrium mol (PCl5) = 4

Show how you would calculate the value for Kc and its units

1. Find the change in moles between start moles and equilibrium moles for PCl5: 6-4=2

2. Starting moles for all products = 0

3. Find equilibrium moles for products using the change in moles for PCl5: 0+2=2 for PCl3 and 0+2=2 for Cl2

4. Use equilibrium moles to find equilibrium concentrations: PCl5 (4/2=2), PCl3 (2/2=1), Cl2 (2/2=1)

5. Substitute these values into the Kc expression: (1.0)(1.0) / (2.0) = 0.50

6. Find units by combining concentration units: (moldm-3)(moldm-3) / (moldm-3) = moldm-3

7. So Kc = 0.50moldm-3

How would decreasing volume affect the position of equilibrium and the value of Kc?

• It would increase the pressure so equilibrium would shift to the side with fewer gas molecules

• It would have no effect on Kc

What effect does decreasing the temperature in an endothermic reaction have on Kc?

Kc decrease as equilibrium shifts to the left so reactants are predominate

What effect does increasing the temperature in an endothermic reaction have on Kc?

Kc increases as equilibrium shifts to the right so products are predominate

What effect does decreasing the temperature in an exothermic reaction have on Kc?

Kc increases as equilibrium shifts to the right so products are predominate

What effect does increasing the temperature in an exothermic reaction have on Kc?

Kc decreases as equilibrium shifts to the left so reactants are predominate