Equilibrium

Question 1

How do reactions reach an equilibrium state?

Answer 1

when the rate of product formation equals the rate of the reactant reformation (i.e. the concentrations of reactants and products remain constant)
e.g. rate of forward reaction equals the rate of the reverse reaction

Question 2

Why is equilibrium dynamic?

Answer 2

because it is in constant equal and opposite motion at the molecular level

Question 3

Macroscopic observations at equilbrium

Answer 3

macroscopic properties include colour and density
these do not change as they depend on the concentrations of the components of the mixture

Question 4

Equilibrium Constant

Answer 4

is important because it tells us whether there are mostly products or reactants present when the reaction has reached equilibrium
K»»1 reaction has gone to completion
K>1 equilibrium position favours products
K=1 reactant and product concentrations are similar
K<1 equilibrium position favours reactants

Question 5

Reaction Quotient

Answer 5

calculated by subbing in the concentrations of the reactants and products at a specific time into the equilibrium constant expression
it is used to see if a reaction is at equilibrium
if Q=K the reaction is at equilibrium
if Q>K reaction proceeds to the left in favour of reactants
if Q

Question 6

What happens to Kc if you reverse the reaction?

Answer 6

inverts the expression
1/Kc

Question 7

What happens to Kc if you double the reaction coefficients?

Answer 7

squares the expression
Kc^2

Question 8

What happens to Kc if you triple the reaction coefficients?

Answer 8

cubes the expression
Kc^3

Question 9

What happens to Kc if you halve the reaction coefficients?

Answer 9

square roots the expression
(Kc)^1/2

Question 10

What happens to Kc if you add two reactions together?

Answer 10

multiplies the two expressions
Kc1 X Kc2

Question 11

Le Chatelier’s Principle

Answer 11

if a system at equilibrium is subjected to a change, it will respond in such a way as to minimise the effect of the change.

used to predict the qualitative effects of the changes of temperature, pressure and concentration on the position of equilibrium and the value of the equilibrium constant

Question 12

Changes in concentration on the position of equilibrium

Answer 12

shift away from the species being increased in order to lower the concentration
Kc does NOT change

Question 13

Removal of a species effect on position of equilibrium

Answer 13

shift towards that species in order to maintain its concentration, this is usually accomplished by adding something else that reacts with only that species
Kc does NOT change

Question 14

Increasing the pressure effect of the position of equilibrium

Answer 14

Shift towards the side with fewer moles of gas
Kc does NOT change

Question 15

Adding a Catalyst effect on position of equilibrium

Answer 15

No effect as it lowers teh activation energy for both the forward and reverse reaction meaning the rate of both reactions is increased by the same amount. You will only reach equilibrium faster
Kc does NOT change

Question 16

Adding an inert gas effect on the position of equilibrium

Answer 16

No effect because it does not effect the partial pressure of a reactant or product
does NOT effect Kc

Question 17

Changing the temperature effect on the position of equilibrium

Answer 17

a temperature increase favours the endothermic reaction
a temperature decrease favours the exothermic reaction
does effect Kc

Question 18

The Haber Process

Answer 18

nitrogen + hydrogen (in eq. with) ammonia
usually occurs at 450 degrees and at 200 atm
however a low temperature and high pressure would give the greatest yield
but this means that the reaction is too slow and the high pressure is costly so a compromise is made for both these conditions

Question 19

Equilibrium law

Answer 19

links the equilibrium concentration of the reactants and products to the equilibrium constant
only consider homogeneous equilibria where all species are in same phase
use and ICE chart:
Initial conc.: for react; normally given and prod. normally 0
Change in conc: for react. its negative and prod. positive
Equilibrium conc.: initial - change for react. for prod. initial + change

Question 20

Free energy and equilibrium

Answer 20

G = negative, the reaction proceeds in the forward direction
G = positive, the reaction proceeds in the backward direction
G=0, the reaction is at equilibrium

Question 21

G, spontaneity, entropy and equilibrium

Answer 21

when G is minimum (zero) the system is at equilibrium
the entropy at equilibrium is maximum

a reaction with G that is large and negative appears to occur spontaneously and has equilibrium mix of mainly products
a reaction with G that is large and positive, appears to occur non-spontaneously and has equilibrium mix of mainly reactants