Physical 6: Equilibria

Question 1

What are the conditions necessary for equilibrium?

Answer 1

1) Only in a closed system so that reactants and products cannot escape (i.e. solvent can’t evaporate)
2) Equilibrium can be approached from either direction
3) Rates of forwards and reverse reactions are the same
4) Concentrations of reactants and products remain constant (but can be in any proportion)

Question 2

What is dynamic equilibrium?

Answer 2

The state of a reversible reaction in which the forward and reverse reactions occur at the same rate and the reactant and product concentrations remain constant

Question 3

What is Le Chatelier’s Principle?

Answer 3

If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance

Question 4

What are the three conditions that have an effect on position of equilibrium?

Answer 4

Concentration of solutions
Pressure of gases
Temperature

Question 5

What effect does an increase in temperature have on equilibrium?

Answer 5

Increase in temperature causes a shift in the endothermic direction to oppose the increase
Which allows more heat to be absorbed, so the temperature of the equilibrium mixture decreases to reestablish equilibrium
This would increase the yield of the molecules on that side of the equation

Question 6

What effect does a decrease in temperature have on equilibrium?

Answer 6

Decrease in temperature causes a shift in the exothermic direction to oppose the decrease
Which allows more heat to be released, so the temperature of the equilibrium mixture increases to reestablish equilibrium
This would increase the yield of the molecules on that side of the equation

Question 7

What effect does an increase in pressure have on equilibrium?

Answer 7

Increase in pressure causes a shift to the side of the equation with fewer molecules to oppose the increase
Because fewer molecules exert less pressure, so the pressure of the system decreases to reestablish equilibrium
This would increase the yield of the side of the equation with fewer molecules

Question 8

What effect does a decrease in pressure have on equilibrium?

Answer 8

Decrease in pressure causes a shift to the side of the equation with more molecules to oppose the decrease
Because more molecules exert more pressure, so the pressure of the system increases to reestablish equilibrium
This would increase the yield of the side of the equation with fewer molecules

Question 9

What effect does an increase in concentration have on equilibrium?

Answer 9

An increase in concentration of molecules on one side of the equation would cause a shift to the other side of the equation to oppose the increase
To increase the concentration of the opposite side and therefore reestablish equilibrium
Increasing the yield of the molecules on that side of the equation

Question 10

What effect does a decrease in concentration have on equilibrium?

Answer 10

A decrease in concentration of molecules on one side of the equation would cause equilibrium position to shift to that side of the equation
To increase the concentration of that substance again and reestablish equilibrium
Increasing the yield of those molecules

Question 11

When would a change in pressure not have an effect on equilibrium?

Answer 11

If there were the same number of molecules on either side of the reversible reaction

Question 12

What is the general template to answer a question about changing conditions of equilibrium?

Answer 12

1) Change in position of equilibrium
2) Why does this happen?
3) To oppose the change and reestablish equilibrium
4) Increasing/decreasing yield

Question 13

What effect do catalysts have on equilibrium position?

Answer 13

Catalysts have no effect on the position of equilibrium because they affect the forward and reverse reactions equally
However they allow equilibrium to be reached more quickly by providing an alternative pathway with a lower activation energy
Catalysts also have no effect on Kc

Question 14

What are the raw materials for the Haber process?

Answer 14

1) Air: to provide nitrogen
2) Natural gas (methane) and water: to provide hydrogen

CH4 + H2O –> CO2 + 3H2
To generate hydrogen

Question 15

What is the catalyst for the Haber process?

Answer 15

Lumps of iron
Increases surface area so as many particles pass over the iron as possible

Question 16

What is the equation for the Haber process?

Answer 16

N2(g) + 3H2(g) –> 2NH3(g)
Reversible reaction
Forward reaction is exothermic

Question 17

What are the optimum conditions for the Haber process?

Answer 17

20,000kPa (lower pressure)
670K (high temperature)

Question 18

What are the uses of the Haber process?

Answer 18

Mostly in fertilisers like ammonium nitrate
Nylon
Explosives
Drugs
Dyes

Question 19

What is the raw material for the hydration of ethane?

Answer 19

Crude oil

Question 20

What is the catalyst for the hydration of ethene?

Answer 20

Phosphoric acid absorbed on silica

Question 21

What is the equation for hydration of ethene?

Answer 21

C2H4(g) + H2O(g) –> C2H6OH(g)

Forward reaction is exothermic

Question 22

What are the optimum conditions for the hydration of ethene?

Answer 22

570K (lower temperature)
6500kPa (high pressure)

Question 23

What are the problems caused by the optimum conditions for hydration of ethene?

Answer 23

1) High pressure causes ethene to polymerise
2) It is expensive to build machinery to supply high pressure, and is expensive to run them as well
3) Low temperature reduces reaction rate
4) Too much steam dilutes the phosphoric acid catalyst

Question 24

What are the uses of hydration of ethene?

Answer 24

Making cosmetics
Making detergents
Making inks

Question 25

What is the general Kc expression?

Answer 25

Where X,Y and Z are concentrations of products
And A,B and C are concentrations of reactants

Question 26

How do you calculate units of Kc?

Answer 26

1) Substitute moldm-3 into the Kc expression
2) Simplify

Question 27

What is the equilibrium constant?

Answer 27

An expression that can be applied to any reversible reaction and is a measure of the ratio of concentrations of products to reactants at equilibrium

Question 28

What happens to Kc when temperature increases?

Answer 28

1) The endothermic reaction is favoured
2) If the forward reaction is endothermic, the yield of products increases as equilibrium shifts to the right
3) So Kc increases as the value of the numerator in the Kc expression increases

Question 29

What happens to Kc if the concentration of a reactant increases?

Answer 29

A + B –> C + D

1) If the concentration of A increased, equilibrium would shift to the right to oppose the change
2) Increasing the yield of C and D
3) Which would increase the concentration of C and D
4) So value of Kc would increase