Equilibrium

Question 1

Why is a dynamic equilibrium so called?

Answer 1

The forward and back reactions are still happening but their rates are balanced, so the amounts of each substance in the mixture do not change.

Question 2

Approach to equilibrium:-

Answer 2

Assume that left is A and right is B. Initially no A- forward = zero. Lots of B so relatively fast back reaction.
A starts to form so forward reaction now starts, rate increasing relative to amount of A. Simultaneously, B amount will decrease and thus so will back reaction rate. Eventually reaction rates equalise and amounts of A and B stop changing. Equilibrium has been established.

Question 3

Equilibrium constant:-

Answer 3

Function of conc of each component of reaction mixture.

Question 4

Composition of an equilibrium mixture generally depends on:-

Answer 4

How favourable the forward reaction is compared to the reverse reaction and how much of each substance was initially present.

Question 5

Key features of Kc:- (3)

Answer 5

• Kc has a fixed value for any particular reaction at a given temp, independent of starting comp or other variables.
• Kc expression value computed from concs in mixture will not equal Kc if mixture isn’t at equilibrium.
• Kc changes when temp changes.

Question 6

Kc units:-

Answer 6

Add up powers in top line or Kc expression and subtract powers on bottom line. Call this X. Units are mol^(x)dm^-(3x). If mols are equal on either side of equation, conc units will cancel out and so Kc will have no units.

Question 7

Calculating Kc from experimental data:-

Answer 7

Generally difficult to measure all concs in subsfance so usual approach is to start with mixture where we know initial concs and measure the change in conc of one reactant, then use equation for the redt.

Question 8

Examples of ways to follow conc change:- (3)

Answer 8

• change in absorption of light, if one substance is coloured.
• pH change, if one is acid/alkali.
• titration, provided time taken for titration is short compared to the time taken for equilibrium to shift.

Question 9

Knowing when equilibrium is reached:-

Answer 9

Parameter being measured will stop changing.

Question 10

Value of Kc interpretation:-

Answer 10

Higher Kc = further to the right equilibrium = greater yield.

Question 11

Changing temp effect on equilibria:-

Answer 11

Kc decreases for exothermic as temp rises, increases for endo.

Question 12

Temp equilibrium shift:-

Answer 12

Increase = shift in endothermic direction (since this will take away heat and oppose the change).

Question 13

Catalyst on equilibria:-

Answer 13

No effect as both rates increase.

Question 14

Pressure on Kc:-

Answer 14

No change.

Question 15

Heterogeneous equilibria:-

Answer 15

Solutions + gases included in Kc, solids and liquids not.

Question 16

Mole fraction:-

Answer 16

Moles of X/total moles of all substances

Question 17

Partial pressure:-

Answer 17

The pressure a gas would exert were it the only one present.

Question 18

Kp equation:-

Answer 18

P products divided by p reactants. P is to the power of how many moles the substance is. No brackets. Only gases usef.

Question 19

Partial pressure equation:-

Answer 19

Px = mole fraction of A x total pressure

Question 20

Kp units:-

Answer 20

Top - bottom powers for power. Units = atm^ , Pa^ or kPa^

Question 21

Kp value:-

Answer 21

• depends only on temp.

* bigger value = further to right.