Equilibrium

Question 1

What happens at equilibrium?

Answer 1

At equilibrium, [reactants] and [products] dont change over time, however this doesnt mean no further transformations are occurring, it simply means that the rate of backwards and forwards reactions are equal.

Depending on starting concentrations the reaction might proceed forwards or backwards to reach the equilibrium

Question 2

What is the formula of Keq for a given reversible reaction?

Answer 2

Keq = ( [C]^c x [D]^d ) / ([A]^a x [B]^b)

for reaction aA + bB >< cC + dD

Note that Keq is unitless

Question 3

What does a small Keq indicate about the equilibrium?

Answer 3

Equilibrium favours reactants

Question 4

What does a large Keq indicate about the equilibrium?

Answer 4

Equilibrium favours products

Question 5

What is the reaction quotient, Q?

Answer 5

The reaction quotient ( Q ) measures the relative amounts of products and reactants present during a reaction at a particular point in time. It is able to indicate where the reaction will shift to achieve equilibrium.

Question 6

What does Q < Keq mean

Answer 6

Reaction will shift right (forward) to achieve equilibrium (increasing products and decreasing reactants)

Question 7

What does Q > Keq mean

Answer 7

Reaction will shift left (reverse) to achieve equilibrium (decreased products and increased reactants)

Question 8

Why are there no units in Keq?

Answer 8

This is because the complete expression for Keq is in terms of ratios of concentrations of standard state concentration c° , where c° = 1 mol / L, so that we can simply ignore units. In the end everything cancels out.

Question 9

What happens to the Keq if we half the original equation?

Answer 9

Root Keq

Question 10

What happens to the Keq if we reverse the original equation?

Answer 10

1/Keq

Question 11

What happens to the Keq if we multiply each component in an equation by ‘n’?

Answer 11

Keq^n

Question 12

What happens if we add chemical reactions together in terms of the Keqs?

Answer 12

Keqs of the two reactions are multiplied together

Question 13

Do we include liquids and solids in the equilibrium constant?

Answer 13

We dont include them. The concentration of a solid is its density, which is independent of how much substance is present. Thus, this constant can be incorporated by the equilibrium constants.

The concentration of water in liquid water is also a constant and is not changed by exchange between liquid and vapour, thus it doesn’t have to be included.

Question 14

How do we do equilibrium calculations

Answer 14

Use rice table. however the general approach:

1) work out original concentration

2) Let x = change in concentration

3) write equilibrium equation in terms of x

4) solve for x

5) substitute x to work out final concentrations or amounts

Question 15

How can we disturb a system?

Answer 15

Changing concentrations
Changing volume
Changing pressure
Changing temperature

Question 16

How can we determine how systems respond in the different disturbances of the system?

Answer 16

Q –> Keq and Δs total > 0 helps to determine how systems responds in each case

Question 17

What is ΔG formula?

Answer 17

ΔG = ΔH - TΔS = -TΔS (total)

Question 18

What happens when ΔG = 0

Answer 18

It corresponds to the equilibrium state

Question 19

How can you tell if a reaction is spontaneous?

Answer 19

ΔS total > 0 or ΔG < 0

Question 20

What happens if ΔS > 0, ΔH < 0

Answer 20

Always spontaneous (based on ΔG = ΔH - TΔS)

Question 21

What happens if ΔS > 0, ΔH > 0

Answer 21

At high temperatures (based on ΔG = ΔH - TΔS)

Question 22

What happens if ΔS < 0, ΔH < 0

Answer 22

At low temperatures (based on ΔG = ΔH - TΔS)

Question 23

What happens if ΔS < 0, ΔH > 0

Answer 23

Never (based on ΔG = ΔH - TΔS)

Question 24

Which reaction is favoured when we lower temperature? WHy

Answer 24

The exothermic direction of the reaction.

Could be explained through either LCP or using the gibbs free energy and enthalpy table

Question 25

Which reaction is favoured when we increase temperature? Why?

Answer 25

The endothermic direction of a reaction

Could be explained through either LCP or using the gibbs free energy and enthalpy table

Question 26

What is the response to an increase in concentration in equilibrium? Why?

Answer 26

System acts to consume some of it.

This could be explained through change in concentrations, atltering the Q and the reaction shifts so that Q –> K. Could also be explained using Le Chatelier’s Principle

Question 27

What is the response to a decrease in concentration in equilibrium? Why?

Answer 27

System acts to produce some of it

This could be explained through change in concentrations, altering the Q and the reaction shifts so that Q –> K. Could also be explained using Le Chatelier’s Principle

Question 28

How could pressure be changed?

Answer 28

Adding (or removing) a gaseous reactant or product

Adding a gas not involved in reaction. This doesnt affect concentration of reactants or products (i.e. inert gas). Q isnt changed, reaction doesnt shift

Changing volume of container

Question 29

What is the response to a decrease in volume on equilibrium? Why?

Answer 29

If volume decreases, system acts to decrease number of moles present (favours reaction which produces least moles)

Changing volume alters Q, and reaction shifts so that Q –> K. Can also be explained via LCP.

Question 30

What is the response to an increase in volume on equilibrium? Why?

Answer 30

If volume increases, system acts to increase the number of moles present (favours reaction which produces most moles)

Changing volume alters Q, and reaction shifts so that Q –> K. Can also be explained via LCP.

Question 31

Does increasing the temperature favour the endothermic or exothermic reaction

Answer 31

Favours the endothermic reaction

Question 32

Does decreasing the temperature favour the endothermic or exothermic reaction?

Answer 32

Favours the exothermic reaction