3.2.3 chemical equilibrium

Question 1

What does an arrow represent in an equation

Answer 1

The reaction going to completion (all reactants used up)

Question 2

Why do some reactions not go to completion

Answer 2

As they’re reversible - products can’t reacts to re-form original reactants

Question 3

What’s dynamic equilibrium

Answer 3

When rate of forward + reverse reaction are equal and concentrations of reactants and products are constant

Question 4

Where will dynamic equilibrium only occur

Answer 4

In closed systems (nothing allowed to escape from reaction vessel)

Question 5

E.g of a homogenous equilibrium reaction (all products and reactants in same physical state)

Answer 5

Esterification reactions

Gas phase reactions -> the haber process

Question 6

E.g of a heterogenous equilibrium reaction (products and reactants in different physical states)

Answer 6

Production of quicklime CaO, used on building industry

Question 7

What is Le Chatelier’s Principle

Answer 7

If any factor is changed which affects an equilibrium, the position of the equilibrium will shift so as to oppose the change

Question 8

3 factors that can affect the position of equilibrium

Answer 8

Pressure (gases)
Temperature
Concentration

Question 9

2 factors that surprisingly don’t affect the position of equilibrium

Answer 9

Catalysts

Surface area

Question 10

What tells you the pressure of a molecule

Answer 10

Big number in front

Question 11

What does the equilibrium do if pressure is increased (when more moles on left)

Answer 11

Forward reaction is favoured as this will reduce number of moles and pressure
This restored the pressure to its original lower value

so the equilibrium shifts to the right

More moles on left -> pressure lowered by equilibrium shift to the right

Question 12

What does the equilibrium do if pressure is decreased (when more moles on left)

Answer 12

Reverse reaction is favoured as this will increase moles + pressure

Equilibrium shift left

Question 13

If each side has equal number of moles and pressure is increased what is the effect

Answer 13

No effect as each side has equal number of moles

Question 14

What does the equilibrium do if temperature is increased (when reaction is exothermic)

Answer 14

Backward (endothermic reaction) is favoured as will absorb added heat + oppose temp increase

Equilibrium shifts left

Question 15

What does the equilibrium do if temperature is decreased (when reaction is exothermic)

Answer 15

Forward (exothermic) reaction is favoured as it releases heat energy, opposing the temp decrease

Equilibrium shifts right

Question 16

What does the equilibrium do if concentration is increased (of a reactant)

Answer 16

Equilibrium shift right

Causes removal of added reactant , restoring it to original lower concentration

Question 17

What does the equilibrium do if concentration is decreased(a reactant removed)

Answer 17

Equilibrium shift left

System attempts to replace removed reactant

Question 18

What does a catalyst do to both forward and backward reactions

Answer 18

Increases rate of them both equally

Question 19

Why does a catalyst have no effect on position of equilibrium
However what does it affect

Answer 19

Has no effect as both the reaction rates are equally increased
But it will increase rate at which equilibrium is reached

Question 20

What is the aim for industries

Answer 20

To produce the best yield of product with minimum costs and time

Question 21

What is the problem with high pressure in reaction vessels in the haber process

Answer 21

Reaction vessel have to be very strong (costly)
Expensive to maintain such high pressure
Risk of explosion

Question 22

What’s the pressure used for the haber process as a compromise

Answer 22

200atm

Question 23

What’s the temperature used for the haber process as a compromise
Why

Answer 23

450*c

As a low temperature would cause a very slow rate

This is high enough to give acceptable rate but not too high that equilibrium would be driven too far left, reducing yield

Question 24

What pressure and temperature is most efficient for most reactions

Answer 24

High pressure

Low temperature

Question 25

What’s ethanol a greener alternative for

Answer 25

Petrol

Question 26

What’s pure methanol used as

Answer 26

A fuel in Indy Car Racing

Question 27

What’s the general reaction for the equilibrium constant, Kc

Answer 27

aA + bB xX + yY

Question 28

What’s the equation for Kc

Answer 28

Kc = [products/reactants]

[X]x [Y]y / [A]a [B]b

Question 29

What do the [ ] tell us in Kc

Answer 29

That the concentration of that substance is in moldm-3

Question 30

How do we work out the overall moldm-3 concentration in Kc

Answer 30

By cancelling out the moles

Question 31

What always goes on top of the Kc fraction

Answer 31

The products

Question 32

For an exothermic reaction, what does increasing the temp do to Kc

Answer 32

Decreases it -> equilibrium move left

Question 33

For an endothermic reaction, what does increasing the temp do to Kc

Answer 33

Increases the Kc -> equilibrium move right

Question 34

2 things Kc isn’t affected by

Answer 34

Concentration

Catalysts

Question 35

What is the Kc of the reverse reaction

Answer 35

The inverse

Question 36

What is said to be happening in reactions with Kc greater than 10*10

Answer 36

Going to completion e.g combustion’s

Question 37

What is said to be happening in reactions with Kc less than 10*-10

Answer 37

They arent taking place at all (non-existent)