[19.3] controlling the position of equilibrium

Question 1

what is the effect of temperature on Kc?

Answer 1

• changing the temperature changes the value of the equilibrium constant Kc
• the effect on Kc depends on whether the reaction is exothermic or endothermic

Question 2

what happens if the forward reaction is exothermic and temperature is increased?

Answer 2

• equilibrium shifts to the left
• value of Kc decreases

Question 3

what happens if the forward reaction is endothermic and temperature is increased?

Answer 3

• equilibrium shifts to the right
• value of Kc increases

Question 4

explain why equilibrium shifts to the left when the temperature is increased in an exothermic reaction

Answer 4

• suppose @300K equilibrium constant = 100 and @100K equilibrium constant = 40
• ratio of partial pressures or equilibrium concentration needs to change to give the new lower equilibrium constant
• denominator term needs to increase to bring equilibrium constant down to new value
• equilibrium shifts to the left

Question 5

explain why equilibrium shifts to the left when the temperature is increased in an exothermic reaction

Answer 5

• suppose @300K equilibrium constant = 100 and @100K equilibrium constant = 40
• ratio of partial pressures or equilibrium concentrations need to change to give the new higher equilibrium constant
• numerator term needs to increase to bring equilibrium constant up to new value
• equilibrium shifts to the right

Question 6

when are equilibrium constants not changed?

Answer 6

• changes in concentration
• changes in pressure
• presence of a catalyst

Question 7

what happens in the reactions where equilibrium constants are not changed?

Answer 7

the equilibrium position needs to shift to maintain the value of the equilibrium constant

Question 8

what happens when the concentration of a product or reactant is decreased?

Answer 8

• product conc decreased: the equilibrium shifts to convert more of the reactants into products
• reactant conc decreased: the equilibrium shifts to favour the backward reaction

the ratio of products to reacts is restored and the value of Kc remains the same

Question 9

explain how equilibrium constants control equilibrium position on changing concentration (is increased)

Answer 9

• in the Kc equation, the denominator is higher than before
• the ration of [products] / [reactants] will give Kc a value less than the original
• to restore Kc, the concentration of the one not increased needs to increase and the one that increased needs to decrease
• equilibrium shifts (left / right)

Question 10

explain how equilibrium constants control equilibrium position on changing pressure (when increased)

Answer 10

• while at the same temperature, suppose the pressrure of the system was doubled
• doubling the pressure will double all of the partial pressures
• numerator affected more due to squared term in Kp expression
• ratio of partial pressures will increase value of Kp
• side with more moles p(A) needs to decrease and fewer moles p(B) needs to increase to restore equilibrium and Kp
• equilibrium shifts to left / right

Question 11

what happens when pressure is changed and there are equal moles on each side?

Answer 11

• a change in pressure affects numerator and denominator the same (raised by same power)
• ratio of partial pressure of products / reactants will still give same value for Kp
• equilibrium does not need to shift

Question 12

what is the effect of a catalyst on the rate of reaction?

Answer 12

• a catalyst increases the rate of reaction without being used up itself
• it increases the rate of the forward and backward reactions equally
• it does not affect the position of equilibrium, the yield or the value of the equilibrium constant
• it does increase the speed at which equilibrium is achieved