TOPIC 10 - Equilibrium I

Question 1

When does a reaction reach equilibrium?

Answer 1

When the rates of both the froward and the backward reaction are equal, making the concentration of each substance constant.

(the AMOUNTS can still be different)

Question 2

What type of system must there be for a reaction to reach dynamic equilibrium?

Answer 2

Closed system

Question 3

What does Le Chatelier’s principle say?

Answer 3

If a reaction at equilibrium is subjected to a change in pressure, temperature or concentration, the position of te equilibrium will move to counteract the change.

Question 4

What happens if in homogeneous equilibra, we increase the concentration of a substance?

Answer 4

The equilibrium will shift to the opposite side.

i.e. if it’s a product, more reactants would be produced etc.

Question 5

What happens if, in homogeneous equilibria, we increase the pressure?

Answer 5

The equilibrium will shift to favour the reaction that produces less gas particles (represented by the stochiometric coefficient in gases)

Question 6

What happens if, in an equilibrium reaction, a catalyst is added?

Answer 6

Has no effect on the position of the equilibrium. Speeds up both rate of reactions equally.

(would speed up the rate at which equilibrium is reached, but has no effect on yield).

Question 7

What happens if, in homogeneous equilibria, the temperature is increased?

Answer 7

Equilibrium will shift to favour the endothermic reaction.

Question 8

Manufacture of ethanol: C₂H_4(g) + H₂O_(g) <> C₂H₅OH_(g)

Delta(H) = -46kJmol^-1

Why is a temperature of 300ºC used?

Answer 8

Forward reaction is exothermic so the lower the temperature, the higher the yield of ethanol. However, the rate will be too low so 300ºC is a compromise between yield and rate of reaction to obtain maximum profit.

Question 9

Manufacture of ethanol: C2H4(g) + H2O(g) <> C2H5OH(g)

Delta(H) = -46kJmol-1

Why is a pressure of 60 atm used?

Answer 9

The higher the pressure, the more the equilibrium will shift to the right, producing more ethanol.

High pressure will increase the rate too.

However, very high pressures are expensive.

Question 10

Manufacture of ethanol: C2H4(g) + H2O(g) <> C2H5OH(g)

Delta(H) = -46kJmol-1

Which catalyst is used?

Answer 10

Phosphoric acid (H₃PO₄)

Question 11

Manufacture of ethanol: C2H4(g) + H2O(g) <> C2H5OH(g)

Delta(H) = -46kJmol-1

What temperature is used?

Answer 11

300ºC

Question 12

Manufacture of ethanol: C2H4(g) + H2O(g) <> C2H5OH(g)

Delta(H) = -46kJmol-1

What pressure is used?

Answer 12

60 atm

Question 13

For homogeneous equilibria, what would be the expression for Kc if:

nA + nB <> nC + nD

Answer 13

[C]ⁿ x [D]ⁿ / [A]ⁿ x [B]ⁿ

Where [A] is the molar concentration of A.

Question 14

For heterogeneous equilibria, what would the expression for Kc look like if:

nA_(g) + nB_(s) <> nC_(g) + nD_(g)

Answer 14

[C]ⁿ x [D]ⁿ / [A]ⁿ

Question 15

For heterogeneous equilibria, why isn’t teh concentration of solids or pure liquids not taken into account?

Answer 15

They’re practically constant.