Chemical Equilibria and Kc

Question 1

Why is equilibrium only achieved in a closed system

Answer 1

Reactants or products may be lost in an open system

Question 1

What are the characteristics of a dynamic equilibrium

Answer 1

The rate of the forwards reaction is equal to the rate of the reverse reaction.

At equilibrium the concentrations of the reactants and products remains constant

Question 2

Homogenous equilibrium

Answer 2

When all the reactants and products are in the same phase (e.g all gases)

Question 3

Heterogenous equilibrium

Answer 3

When all the reactants and products are in different phases

Question 4

What factors affect the position of equilibrium

Answer 4

Concentration
Pressure
Temperature

Question 5

State and explain the effect on the position of equilibrium if more reactants are added to the mixture

Answer 5

The position of equilibrium will move to the right to use up the added reactants. The equilibrium moves to oppose the increase of reactants.

Question 6

State and explain the effect on the position of equilibrium if the products are removed

Answer 6

The position of equilibrium will shift to the right to produce more of the products. The equilibrium moves to oppose the loss of the products

Question 7

What effect does adding dilute HCl have on the position of equilibrium

Answer 7

Addition of hydrochloric acid increases the concentration of H+ ions, so the position of equilibrium shifts to the right to use up the added H+ ions.

Question 8

What effect does the addition of alkali have on the position of equilibrium if there are H+ ions already on the right hand side.

Answer 8

The OH- ions react with the H+ ions, so the position of equilibrium shifts to the right to produce more H+ ions.

Question 9

How can the pressure of an equilibrium system be increased

Answer 9

Adding more gas into the same volume
Decreasing the volume

Question 10

State with a reason, the effect of an increase in the pressure on the position of equilibrium.

Answer 10

The position of equilibrium will shift to the side with fewer gas molecules. The equilibrium moves in the direction that decreases the pressure.

Question 11

State with a reason, the effect of a decrease in the pressure on the position of equilibrium.

Answer 11

The position of equilibrium will shift to the side with more gas molecules. The equilibrium moves in the direction that increases the pressure.

Question 12

If the number of gaseous molecules if the same on both sides what effect does increasing/decreasing the pressure have on the position of equilibrium

Answer 12

The position of equilibrium does not change as there is no change in the number of gaseous molecules.

Question 13

What is the advantage of increasing the pressure in an equilibrium if there are the same number of gaseous molecules on both sides

Answer 13

The rate of reaching equilibrium is faster

Question 14

If the forwards reaction was exothermic, state with a reason the effect of an increase in temperature on the position of equilibrium

Answer 14

The position of equilibrium shifts to the left in the endothermic direction to absorb the added heat

Question 15

If the forwards reaction was exothermic, state with a reason the effect of a decrease in temperature on the position of equilibrium

Answer 15

The position of equilibrium shifts to the right in the exothermic direction to give out heat

Question 16

If the forwards reaction was endothermic, state with a reason the effect of an increase in temperature on the position of equlibrium

Answer 16

The position of equilibrium shifts to the right in the endothermic direction to absorb the added heat

Question 17

Why is the rate of reaching equilibrium faster when temp is increased

Answer 17

At a higher temperature more particles have energy greater than the activation energy, therefore there are more successful collisions in a given time.

Question 18

What effect does a catalyst have on equilibria

Answer 18

A catalyst increases the rate at which equilibrium is reached

Question 19

State and explain the effect on the yield/position of equilibrium that a catalyst has

Answer 19

A catalyst has no effect on the yield as it increases the rate of the forwards and reverse reaction by the same amount

Question 20

Kc =

Answer 20

[Concentration of products in mol dm^-3] / [Concentration of reactants in mol dm^-3]

Question 21

What affects the value of the equilibrium constant

Answer 21

Temperature ONLY

NOT CONCENTRATION OR CATALYST

Question 22

Why are solids not included in the expression for Kc

Answer 22

The concentration of a solid remains constant

Question 23

Why might a Kc in a reaction have no units

Answer 23

There are the same number of moles on both sides of the equation, so the volumes cancel

Question 24

How is 2HCl represented in a Kc expression

Answer 24

[HCl]^2

Question 25

What does a Kc value of greater than 1 mean

Answer 25

The equilibrium lies to the right

Question 26

What does a Kc value of smaller than 1 mean

Answer 26

The equilibrium lies to the left

Question 27

What does a Kc value of greater than 1x10^10 mean

Answer 27

The equilibrium lies to the far right

Question 28

How do you use ICE tables to calculate Kc

Answer 28

Draw a table with initial moles, change in moles and equilibrium moles.
Write down all the information given for each compound in the write section of the table
Unless told, the initial moles for a product is 0.
Work out the change in moles.
The sign for change of moles is opposite for products and reactants.
If products increases by +0.5 and the ratio is 1:1 , the reactants decrease by -0.5 moles.
Using the equilibrium moles and volume, find out the equilibrium conc and sub into Kc equation

Question 29

If the forwards reaction was endothermic, state and explain the effect on the value of Kc and hence on the position of equilibrium if a higher temperature was used

Answer 29

Kc increases as temperature increases. Kc is now greater than the quotient so the quotient must increase. This happens if more products are being formed and less reactants, so the position of equilibrium moves to the right to equal the new Kc.

Question 30

What affects the value of Kc

Answer 30

ONLY a change in temperature

Question 31

4NH3 + 5O2 = 4NO + 6H2O
The forwards reaction is exothermic.
State and explain the effect on the value of Kc and hence on the position of equilibrium if NO is added.

Answer 31

The value of Kc does not change.
When NO is added the value of [NO]^4[H2O]^6 / [NH3]^4[O2]^5 increases.
The quotient is now greater than unchanged Kc so the quotient must decrease. This happens if more reactants are being formed and less products so the position of equilibrium moves to the left to decrease the quotient until it equals Kc again.

Question 32

4NH3 + 5O2 = 4NO + 6H2O
The forwards reaction is exothermic.
State and explain the effect on the value of Kc and hence on the position of equilibrium if the temperature is decreased.

Answer 32

As the forwards reaction is exothermic, Kc increases as the temperature decreases.
Kc is now greater than the quotient, so the quotient must increase. This happens if more products are being formed and less reactants, so the position of equilibrium moves to the right to equal the new Kc .

Question 33

4NH3 + 5O2 = 4NO + 6H2O
The forwards reaction is exothermic.
State and explain the effect on the value of Kc and hence on the position of equilibrium if the temperature is increased

Answer 33

As the forwards reaction is exothermic, Kc decreases as the temperature increases.
Kc is now smaller than the quotient, so the quotient must decrease. This happens if more reactants are being formed and less products, so the position of equilibrium moves to the left to equal the new Kc .

Question 34

State and explain two benefits to the environment resulting from the use of catalysts in industrial processes.

Answer 34

Less fuel/ raw materials/ natural resources are used because the reaction has a lower activation energy.

Enables the use of an alternative process with a higher atom economy therefore less raw materials are needed.

Question 35

Give two features of a reaction in dynamic equilibrium

Answer 35

Rate of the forwards reaction is equal to the rate of the reverse reaction.
Concentrations of products and reactants remain constant.