Physical - 1.6 Equilibria & Kc

Question 1

Define a catalyst

Answer 1

A substance that increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy.

Question 2

Define closed system

Answer 2

A system where there is only heat exchange occurring between the system and its surroundings. No matter can enter or the exit system.

Question 3

Define dynamic equilibrium

Answer 3

Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the backward reaction.The concentrations of the reactants and products remain constant.

Question 4

What is the effect of changing concentration on equilibrium?

Answer 4

If the concentration of a reactant increases, more products will be formed to re-establish the equilibrium.

If the concentration of a product increases, more reactants will be formed to re-establish the equilibrium.

Question 5

What is the effect of changing pressure an equilibrium?

Answer 5

If pressure is increased, the position of equilibrium shifts toward the side with the fewest number of moles.

If the pressure is decreased, the position of equilibrium shifts towards the side with the greatest number of moles.

Question 6

What is the effect of changing temperature on equilibrium?

Answer 6

If the temperature of a system in equilibrium is increased, there will be an increase in the relative amount of products for an endothermic reaction and a decrease for an exothermic reaction.

Question 7

Define equilibrium constant

Answer 7

A value that expresses the relationship between the concentration of products and reactants present at equilibrium in a reversible reaction.

Question 8

State Le Chatelier’s principle

Answer 8

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

Question 9

Define reversible reaction

Answer 9

Reactions in which the products from the reaction can react together to form the original reactants. The direction of reversible reactions can be changed by changing the conditions.

Question 10

What three factor factors affect the rate of equilibrium?

Answer 10

Temperature
Pressure
Concentration

Question 11

What is the effect of a catalyst on the position of equilibrium?

Answer 11

A catalyst has no effect on the position of equilibrium that allows the reaction to get to the equilibrium faster.

A catalyst increases the rate of the forward and reverse reactions equally so equilibrium is attained much more quickly in the presence of a catalyst but it does not affect the position of equilibrium.

Question 12

What is the equilibrium constant?

Answer 12

Kc

Question 13

How is Kc expressed?

Answer 13

aA + bB ⇌ cC + dD

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

Kc = [products]/[reactants]

Question 14

How are the units of Kc calculated?

Answer 14

Units of Kc = (Moldm-3)(c+d)/(Moldm-3)(a+b)

Units are in terms of concentration, but the overall power depends on the balancing numbers in the equation for the reaction

Question 15

In what situation would Kc have no units?

Answer 15

When there are an equal number of moles on both side sides of the equation and they cancel each other out in the Kc expression

Question 16

How would you calculate equilibrium moles?

Answer 16

Using:

I
C
E

Question 17

How would you calculate the equilibrium concentrations?

Answer 17

If the volume is given in the example, the equilibrium moles are divided by the volume in dm3 to give equilibrium concentration in moldm-3

Question 18

What is the only factor that affects the value of Kc?

Answer 18

Temperature is the only factor that affects the value of Kc for an equilibrium reaction

Question 19

Do changes in volume or total pressure have an effect on Kc?

Answer 19

No