C5.3 Equilibria

Question 1

What is the difference between reversible reactions and non-reversible reactions?

Answer 1

1) in reversible reactions, products can react together to form original reactants
2) in many reactions, products cannot easily be changed back into the reactants

Question 2

What is an example of a reversible reaction to know?

Answer 2

The reaction between copper (II) sulfate & water.
hydrated copper (II) sulfate ⇌ anhydrous copper (II) sulfate + water
CuSO4.5H2O (blue) ⇌ CuSO4 (white) + 5H2O

Question 3

Explain the reaction between copper(II) sulfate and water.

Answer 3

1) blue crystals of copper(II) sulfate are hydrated copper(II) sulfate, CuSO4.5H2O
2) each Cu^2+ ion surrounded by 5 water molecules
3) if you remove these water molecules by heating you get white anhydrous copper(II) sulfate, CuSO4
4) endothermic forward reaction because energy is transferred from the surroundings; backward is opp.
a) when water added, white anhydrous copper(II) sulfate becomes blue hydrated copper(II) sulfate
b) energy transferred to surroundings (is exo)
5) enough energy transferred by heating to prod. steam in reaction

Question 4

What does the ‘⇌’ symbol mean?

Answer 4

The ‘⇌’ symbol is used in equations for reversible reactions instead of an arrow.
Combines the forward reaction & backward reaction.
-> e.g.:
a) forward: CuSO4.5H2O -> CuSO4 + 5H2O
b) backward: CuSO4 + 5H2O -> CuSO4.5H2O

Question 5

What is an endothermic reaction?

Answer 5

A reaction where energy is transferred from the surroundings.

Question 6

What is an exothermic reaction?

Answer 6

A reaction where energy is transferred to the surroundings (exothermic reaction).

Question 7

What is hydrated copper(II) sulfate, CuSO4.5H2O more accurately called and why?

Answer 7

Copper(II) sulfate-5-water; the water molecules in the ionic lattice are the ‘water of crystallisation’.

Question 8

What is a dynamic equilibrium?

Answer 8

1) in closed system, no substances can enter or leave
2) when reversible reaction happens in closed system, rates of forward reaction & backward reaction become equal
-> situation is called equilibrium:
a) forward & backward reactions still happen so is dynamic equilibrium
b) conc.s of all reacting substances remain constant
-> conc.s of reacting substances constant at equilibrium, not changing but not necessarily equal

Question 9

What are examples of closed systems?

Answer 9

1) stoppered flask
2) beaker of solution in which all the reacting substances remain in the solvent

Question 10

True or False?: state changes are also reversible.

Answer 10

True.

Question 11

What is bromine in the liquid and gas states in a closed system an example of?

Answer 11

Equilibrium (state changes are also reversible).

Question 12

What is the equilibrium position?

Answer 12

1) Gives idea of ratio of equilibrium concentrations of products to reactants:
a) equilibrium position is on left when conc. of reactants greater than conc. products
b) equilibrium position on right when conc. of reactants less than conc. products
2) equilibrium position may change if there are changes in conditions
-> affects conc.s of reacting substances

Question 13

What happens if you change the pressure (equilibrium position)?

Answer 13

If pressure is increased, equilibrium position moves in direction of fewest moles of gas.
-> fewer moles of gas on left / right side so equilibrium position moves to left / right side if pressure increased

Question 14

What happens if you change the concentration (equlibrium position)?

Answer 14

If the conc. of substance is increased, equilibrium position moves in direction away from that substance.
-> equilibrium position moves to left / right side if conc. of (substance on) left / right side increased

Question 15

What happens if you change the temperature (equilibrium position)?

Answer 15

If temperature is increased, equilibrium position moves in direction of endothermic change.
-> equilibrium position moves in direction of endo if increased (+ve energy change) or exo is decreased (-ve energy change)

Question 16

If the forward reaction has an energy change of xkJ/mol, what will the energy change of the backward reaction be?

Answer 16

-xkJ/mol

Question 17

What does a positive energy change mean?

Answer 17

The reaction is endothermic.

Question 18

What does a negative energy change mean?

Answer 18

The reaction is exothermic.

Question 19

What happens to a yellow equilibrium mixture mostly containing K2CrO4(aq) when sulfuric acid is added?

Answer 19

It becomes an orange equilibrium mixture mostly containing K2Cr2O7(aq).

Question 20

What happens to tubes containing equilibrium mixtures of NO2 & N2O4 when the temperature increases?

Answer 20

The concentration of brown NO2 increases & the concentration of colourless N2O4 decreases.

Question 21

How is methanol made?

Answer 21

Methanol is made by reacting carbon monoxide with hydrogen:
CO(g) + 2H2(g) ⇌ CH3OH(g) (deltaH = -91kJ/mol)

Question 22

What is the equilibrium yield?

Answer 22

The amount of the desired product present in a reaction at equilibrium; depends on the:
a) pressure
b) temperature
c) conc. reactants

Question 23

How do you make plenty or product (in a reversible reaction)?

Answer 23

Supply plenty of reactants.
-> increasing conc.s of reactants in reaction mixture moves equilibrium position to right, forming higher equilibrium conc. product(s)

Question 24

What is Le Chatelier’s principle?

Answer 24

A principle which provides us with a rule to predict what happens to reactions at equilbrium when conditions change:
“When a change is made to a reaction at equilibrium, the position of the equilibrium moves to oppose the change,”
-> e.g. when conc. substance decreases, equilibrium position moves to increase its conc. again

Question 25

How do you choose a suitable temperature (use example of production of methanol)?

Answer 25

1) 1 mol of gas on RHS of balanced equation for making methanol but 3 mol on LHS -> means if pressure increased eq. position moves to right & equilibrium yield increases 2) but high pressures need £ equip to compress the gases, lot of energy to run & tough reaction vessels to w/stand pressure -> in practice, chem. engineers choose compromise pressure -> high enough to achieve reasonable equilibrium yield, but not so high as to be £ or hazardous

Question 26

How do you choose a suitable temperature (using example of production of methanol)?

Answer 26

1) forward reaction is exo so backward is endo -> means if T increased eq. position moves to left & equilibrium yield of methanol decreases 2) but at low Ts rate of reaction too low to prod. methanol quick enough to be profitable -> in practice: chem. engineers choose compromise T -> low enough to achieve reasonable eq. yield but high enough to achieve reasonable rate of reaction

Question 27

What is a use of methanol?

Answer 27

Fuel for monster trucks.

Question 28

What are features of industrial compressors?

Answer 28

Complex & require large amounts of energy.

Question 29

What are features of chemical factories?

Answer 29

Huge. Require lot of energy for heating, lighting, & moving machinery.