C5.3

Question 1

what are reversible reactions?

Answer 1

a reaction in which the products can react together to form the reactants

Question 2

what is the symbol used in reversible reactions?

and what does it mean

Answer 2

⇆

means forward reaction
ie. reactants to products

Question 3

how can a reversible reaction occur to produce either a product or a reactant? (give examples)

Answer 3

altering the reaction conditions

ie. adding water to a product, or heating it up

Question 4

what is dynamic equilibrium?

describe an analogy

Answer 4

when the forward and backward reaction happen at the same rate - so there is no observable change in amount of product/reactant

The concentration of all the reacting substances remain constant

(digging a hole, you dig it and don’t remove dirt, so the hole does not change)

Question 5

in what condition can dynamic equilibrium occur?

Answer 5

ONLY in a closed system (so no substances can enter or leave)

Question 6

is a substance stationary if it is in dynamic equilibrium

Answer 6

no, the forward and backward reactions are still happening, but they both flip sides at the same ‘rate’, so it looks like nothing is changing

Question 7

do reversible reactions happen by themselves?

Answer 7

sometimes they can be spontaneous but other times reaction conditions have to be changed (ie. heat it up, change conditions etc.)

Question 8

what is the equilibrium position?

Answer 8

a symbol which gives the idea of the ratio of the equilibrium concentration of products to reactants

ie. shows which one is more or less

Question 9

how do you calculate the effect of changing pressure on the equilibrium position?

Answer 9

1) calculate moles (of gas) on each side
2) therefore, the position would shift to side with least moles

IN ORDER TO TRY AND GO BACK TO EQUILIBRIUM

if correct attempt question on p188 - example one

Question 10

why does the equilibrium position shift in the first place?

Answer 10

since one of the conditions have changed, and in order to try and get the amount of products vs reactants at a dynamic equilibrium

Question 11

how do you calculate the effect of changing concentration on the equilibrium position?

Answer 11

(as it is usually in terms of increasing the concentration of one reactant)

• the equilibrium position shifts away from that substance,
  ie. if sulfuric acid is on left, equilibrium position shifts to the right

Question 12

describe the concept behind the effect of changing concentration on the equilibrium position

Answer 12

• ultimately, this reversible reaction wants to go back to equilibrium right?
• therefore, is one (ie. product’s) concentration increases, then more product is made to counter that in order to balance both substances out

Question 13

how do you calculate the effect of changing the temperature on the equilibrium position?

Answer 13

1) look at the enthalpy number ie. (∆H = -58kJ/mol)
2) so then write out where the heat is (ie. at reactant side or the product side)
3) the equilibrium position shifts to opposite side
where the heat is
OR
equilibrium shifts to the direction of the endothermic reaction

whole idea is which way would make the temperature go back to equilibrium

Question 14

how would the enthalpy number look for a

(a) exothermic reaction
(b) endothermic reaction

Answer 14

(a) (∆H = -58kJ/mol)
MINUS NUMBER as overall heat is lost

(b) (∆H = +58kJ/mol)
POSITIVE NUMBER as overall heat is gained

Question 15

Equilibrium yield

Answer 15

The amount of desired product present in a reaction at equilibrium.

Depends of pressure, temperate, conc. of reactants

Question 16

Le Chatelier’s principle

Answer 16

‘When a change is made to a reaction at equilibrium, the position of equilibrium moves to oppose the change’

Question 17

Choosing reaction conditions: high pressure

Answer 17

High pressure = equilibrium yield (in producing methanol)

BUT
High pressures need:
-expensive equipment to compress the gasses,
-a lot of energy to run,
-tough reaction vessels to withstand the pressure

SO compromise pressure chosen : high enough to achieve reasonable equilibrium yield, but not so Hugh to be expensive or hazardous

Question 18

Choosing reaction conditions: temperature

Answer 18

In Manufacture of methanol, forward reaction is endothermic - so high temps decreases equilibrium yield

BUT
Too low temperature means rate of reaction too low to produce methanol quickly enough to be profitable

SO comprise temperature chosen: low enough to achieve reasonable equiliburm yield, but high enough to achieve a reasonable rate of reaction

Question 19

Spec - recall something about dynamic equilibrium,

Answer 19

C5.3b recall that dynamic equilibrium occurs in a closed system when the rates of forward and reverse reactions are equal

Question 20

What is a reversible reaction - pb

Answer 20

One in which the products are able to react with each other to change back into the reactants WITHOUT ANY ADDITIONAL CHANGES

Question 21

Pb - define equilibrium

Answer 21

When the forwards rate of reaction is equal to the backwards rate of reaction, in a reversible reaction