Topic 10 + 11: Equilibrium I, II

Question 1

What is dynamic equilibrium

Answer 1

A reversible reaction in a closed system where the rates of the forwards and backwards reaciton are equal and so the concentrations of reactants and products do not change.

Question 2

What is the affect of increasing the temperature on a reversible reaction?
The forward reaction is exothermic

Answer 2

The rate of reaction will increase because more reactants will have sufficient energy to reaction (more than or equal to Ea)
The position of equilibrium will shift to the left (the endothermic direction) in order to decrease the temperature of the system, thereby countering the change, according to Le Chatelier’s Principle

Question 3

What is the affect of increasing the concentration of products on a reversible reaction?

Answer 3

The position of equilibrium will shift to the left to decrease the concentration of the products, countering the change due to Le Chatelier’s Principle.

Question 4

What is the affect of increasing the pressure of a reaction:
2A(g) + B(g) < -> C(g) + D(s)

Answer 4

Rate of reaction increases because of increased pressure.
More moles of gas on the reactant side. Position of equilibrium will shift to the right to decrease the moles of gas and therefore decrease the pressure, countering the change due to Le Chatelier’s Principle.

Question 5

What is the affect of adding a catalyst to the position of equilibirum.

Answer 5

No change.
Increases the rate at which equilibrium is reached because catalysts increase the rate of reaction of the forwards and backwards reactions equally.

Question 6

What is Kc for the reaction:
aA + bB < = > cC + dD

Answer 6

Kc = ([C]^c x [D]^d) / ([A]^a x [B]^b)

Question 7

What is the only change in condition to affect the value for Kc

Answer 7

temperature

Question 8

what happens if you have a solid in the equation you need to find Kc for

Answer 8

IGNORE solids entirely

Question 9

What is the Kp expression written in terms of

Answer 9

partial pressures

Question 10

What is Kp for the reaction
aA + bB < => cC + dD

all of the state symbols are gases

Answer 10

Kp = [p(C)^c x p(D)^d] / [p(A)^a x p(B)^b]

where p(A) indicates the partial pressure of A at eqm

Question 11

What happens in heterogeneous reactions for Kp

Answer 11

Solids and liquids are ignored

Question 12

What changes the value for Kp

Answer 12

Change in temperature

Question 13

How do you calculate the partial pressures from the number of moles and total pressure

Answer 13

1. The number of moles of each gas should be used to first calculate the mole fractions (mole of each gas/ total number of moles)
2. The mole fractions are then multiplied by the total pressure to get the partial pressures

Question 14

For an endothermic reaction; what happnes to K(p/c) if the temperature increases?

Answer 14

Increases

Question 15

Why does the value of the equilibrium constant increase with increasing temperature for an endothermic reaction?

Answer 15

Increase temperature shifts position of equilibrium to the product side so the equilibrium concentrations (or partial pressures) of the products are larger and the reactants smaller so that the temperature is decreased (in accordance with Le Chatelier’s principle)

Question 16

Why does change in pressure not affect Kp

This is analagous to the changing concentrations and Kc

Answer 16

The quotient changes temporarily but the shift in eqm concentrations bring the value of Kp back to its original (constant) value.
Changes in pressure cause a shift in the position of equilibrium to a new position which restores the value of Kp

This is also the same for Kc and changing concentrations

Question 17

What is Le Chatelier’s Principle

Answer 17

Le Chatelier’s principle says that if a change is made to a system in dynamic equilibrium, the position of the equilibrium moves to minimise this change.