Equilibria ( How Far)

Question 1

What is the symbol for the equilibrium constant?

Answer 1

Kc

Question 2

What is the equation for Kc? (Equilibrium constant)

Answer 2

Kc = [C]c [D]d / [A]a [B]b

Question 3

What are the units of Kc?

Answer 3

Mol dm3 (but you have to work it out and sometimes there are no units)

Question 4

If the reaction is endothermic, is the change in enthalpy +ve or -ve?

Answer 4

Enthalpy change is positive

Question 5

If the reaction is exothermic, is the enthalpy change +ve or -ve

Answer 5

Enthalpy change is negative

Question 6

If the temperature was increased in an endothermic reaction, what would happen to the equilibrium constant?

Answer 6

It would increase

Question 7

If the temperature was increased in an exothermic reaction, what would happen to the equilibrium constant?

Answer 7

Equilibrium constant would decrease.

Question 8

What is not included in Kc?

Answer 8

Solids and pure liquids arent included.

It is for the concentration of gas and aqueous solutions only.

Question 9

What is this called? And why?
Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g)

Answer 9

A heterogeneous equilibrium because the physical states are all different.

Question 10

What is Kc?

Answer 10

It is the equilibrium constant for a reversible reaction at a specific temperature

Question 11

What is Kp?

Answer 11

The equilibrium constant of a gaseous system calculated using the partial pressures.

Question 12

How to find out the mole fraction of a gas?

Answer 12

Number of moles of A / total number of moles in gas mixture.

Question 13

What is the equation of partial pressure?

Answer 13

Partial pressure = mole fraction x total pressure

Question 14

What is the ideal gas equation?
What is the T measured in?
Wha is the V measured in?
What is R?

Answer 14

PV=nRT
T measured in kelvin
V measured in m3
R is 8.314, measured in Jmol-1 K-1

Question 15

What is the definition of a partial pressure of a gas?

Answer 15

The partial pressure of a gas is the pressure it would exert if it alone occupied the container at the same temperature and volume.

Question 16

What affects Kp?

Answer 16

Kp is constant and is only changed when the temperature of the reaction changes

Question 17

What is ignored in a Kp equilibrium expression?

Answer 17

Solids and liquids are ignored. It is only for gases.

Question 18

In a Kp equation, what should not be used?

Answer 18

Square brackets as these represent concentration and therefore imply a Kc expression.

Question 19

What is the Kp equation?

Answer 19

Kp = (PC)c x (PD)d / (PA)a x (PB)b

Question 20

What is the effect of an increase in temperature on the value of Kc?

Answer 20

If temperature increases for a reaction that is exothermic in the forward reaction, equilibrium will shift from the right to the left.

The value of Kc decreases because the ratio of [products] to [reactants] decreases

If temperature increases for a reaction that is endothermic in the forward direction, equilibrium shifts from left to right.

The value of Kc increases because the ratio of [products] to [reactants] increases.

Question 21

What is the effect of an increase in temperature on the value of Kp?

Answer 21

An increase in temperature for a reaction that is exothermic in the forward direction shifts equilibrium from right to left. Value of Kp decreases as the ratio of [products] to [reactants] decreases.

A decrease in temperature for a reaction that is endothermic in the forward reaction shifts equilibrium from left to right. Value of Kp increases as the ratio of [products] to [reactants] increases.

Question 22

What is the effect of increased pressure on the value of Kc?

Answer 22

No effect because Kc is not affected by any changes in pressure as this Kc only depends on temperature. A change in temperature would change the concentrations temporarily, but the system re-establishes equilibrium so the ratio in the Kc expression returns to the same value.

Question 23

What is the effect of an increased pressure on the value of Kp?

Answer 23

Kp is not affected by pressure, it depends only on temperature. Changes in pressure cause a change in partial pressure temporarily, but a new equilibrium is re-established therefore the value of Kp remains constant.

Question 24

What is the effect of a catalyst on the value of Kp?

Answer 24

Kp isnt affected by a catalyst because they only speed up the forward and reverse reaction at the same rate, so the ratio of [products] to [reactants] unchanged.

Catalysts only cause a reaction to reach equilibrium faster therefore has no effect on the position of equilibrium.