3.7 - Equilibria

Question 1

Features of a dynamic equilibrium

Answer 1

• forwards and backwards reaction happen at same rate
• concentrations of reactants and products do not change
• closed system

Question 2

Enthalpy change of forward and backward reaction

Answer 2

same but one is exo and one ends (so one is + and one -)

Question 3

Effect of increasing temperature

Answer 3

shifts equilibrium position in endothermic direction
(always say whether forward reaction is exo/endo)

Question 4

Effect of increasing pressure

Answer 4

shifts equilibrium position to side with fewest molecules of gas

Question 5

Effect of catalyst

Answer 5

increases rate of both forward and reverse reactions by same amount so equilibrium position unchanged

Question 6

Importance of compromise between chemical equilibrium and reaction rate

Answer 6

• high pressures expensive to generate
• high temps increase energy costs
• high pressures have safety risk
• rate may be too slow with too low temps

Question 7

Define homogeneous equilibria

Answer 7

contains equilibrium species all in the same state/phase

Question 8

Define heterogeneous equilibria

Answer 8

contains equilibrium species that have different states/phases

Question 9

Which physical states of chemicals appear in Kc and which don’t?

Answer 9

yes: gas or aq
no: liquids and solids

why? conc of solids/liquids essentially constant

Question 10

Define mole fraction

Answer 10

the proportion by volume of a gas to the total volume of the gases in a mixture
- sum of mol fractions in mixture is always 1

Question 11

Define partial pressure

Answer 11

contribution a gas makes towards total pressure
- sum of PPs = total pressure
- units: kPa, Pa, atm

Question 12

Only factor that affects Kc

Answer 12

temperature

Question 13

What is Kp for?

Answer 13

gases only

Question 14

Equation for partial pressure

Answer 14

mol fraction x total pressure

Question 15

Effect of Kc on equilibrium position

Answer 15

Kc>1
- shifts right
- higher product yield

Kc<1
- shifts left

Question 16

How Kc is affected by temperature

Answer 16

forward reaction endo: Kc increases as temperature increases

forward reaction exo: Kc decreases as temperature increases

Question 17

How does e.g increasing pressure affect Kp and equilibrium position
(assume more molecules of gas on left)

Answer 17

• denominator increases more than numerator
• to restore Kp, denominator must decrease and numerator must increase
• increasing pressure causes equilibrium to shift right towards side with fewest gas molecules

Question 18

Only k_ with curved brackets

Answer 18

Kp

Question 19

State Le Chatelier’s principle

Answer 19

position of equilibrium shifts to minimise effect of any change

Question 20

How to find delta H from graph

Answer 20

(-) gradient x R

Question 21

How to find delta S from graph of ln Kp against 1/T

Answer 21

• extrapolate line to y-intercept
• delta S = intercept x R

Question 22

When can mol (not concs) be used to calculate Kc?

Answer 22

same no.mol on both sides of eq/volume cancels