5 Chemical Equilibrium

Question 1

Define ‘equilibrium.’

Answer 1

a state in which there are no observable changes over time

Question 2

What are the 4 main requirements of equilibrium?

Answer 2

a closed system
constant pressure and temperature
reversible process/reaction
rates of opposing changes are equal (i.e. Le Chatalier’s principle)

Question 3

Which two things describe an equilibrium system?

Answer 3

dynamic (in constant motion)

reversible (can approach from either direction but both reactants and products are present at all times)

Question 4

Looking at the following reaction, at what point has it reached equilibrium?

aA + bB → cC + dD

Answer 4

when the rate of the forward reaction equals the rate of the backward/reverse reaction (formation of reactants and products is the same)
quantities of reactants and products do not change any further

Question 5

State the ‘Law of Mass action.’

Answer 5

the rate of a reaction is proportional to the concentration of reactants

Question 6

What does the reaction quotient, Q, relate to?

Answer 6

the concentrations of products and reactants to each other at “any point” in a reaction

Question 7

For the general reaction: aA + bB → cC + dD state the formula for Q.

Answer 7

Q = [C]^c [D]^d/ [A]^a[B]^b

= [Products]/[Reactants]

Question 8

What is Kc?

Answer 8

the equilibrium constant
the value of the reaction quotient, Q, at equilibrium
does not depend on initial concentrations of reactants and products

Question 9

For the general reaction: aA + bB ⇌ cC + dD state the formula for Kc.

Answer 9

Kc = [C]^c [D]^d/ [A]^a[B]^b

= [Products]/[Reactants]

Question 10

State the three stages of writing out equilibrium expressions.

Answer 10

1. Write the balanced equation
2. Write the products as the numerator and the reactants as the denominator (do not include solids/solvents and square brackets [X] mean concentration, use them for Kc but not Kp)
3. Write the coefficient of each substance in the balanced equation as exponents

Question 11

What is ‘homogenous’ equilibrium? Give an example.

Answer 11

when reactants and products are in the same physical state
i.e. N₂(g) + H₂(g) → NH₃(g)
so all these components go into the Kc/Kp expression

Question 12

What is a ‘heterogeneous’ equilibrium? Give an example.

Answer 12

when reactants and products are in different physical states
i.e. CaCO₃(s) ⇌ CaO(s) + CO₂(g)
here only gases would be used:
Kc = [CO₂]
Kp = P(CO₂)

Question 13

For each of the following reactions write an expression for Kc:

a) CO₂(g) + H₂(g) → CO(g) + H₂O(l)
b) 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
c) SnO₂(s) + 2CO(g) → Sn(s) + 2CO₂(g)

Answer 13

a) Kc = [CO]/[CO₂][H₂]
b) Kc = [H₂]^4/[H₂O]^4
c) Kc =[CO₂]^2/[CO]^2

Question 14

What 2 things does Kc allow us to do?

Answer 14

predict how a reaction will progress to achieve equilibrium

calculate the concentration of reactants and products at equilibrium

Question 15

Calculate Kc and its units using the following information:
H₂(g) + I₂(g) → 2HI(g)
A closed system initially containing 1.00x10^-3M H₂ and 2.00x10-3M I₂ at 448℃ is allowed to reach equilibrium.
The equilibrium concentration of HI is 1.87x10^-3M.

Answer 15

Use the ICE method:
H₂ I₂ HI
I(mol) 1.00x10^-3 2.00x10^-3 0
C(mol) -1/2 x (1.87x10^-3) -1/2 x (1.87x10^-3) +1.87x10^-3
E(conc) 0.065x10^-3 1.065x10^-3 1.87x10^-3

Kc = [1.87x10^-3]/ [0.065x10^-3][0.065x10^-3]
= 51

Units = M^2/M^2 → NO UNITS

Question 16

How do we find the units for Kc or Kp?

Answer 16

By putting units into Kc/Kp brackets and cancelling them out like fractions.

Question 17

For each of the following relationships between Q and k values, state the initial state of the reaction and the change in equilibrium:

a) Q = 0
b) Q < k
c) Q = k
d) Q > k
e) Q ∞ k

Answer 17

a) initial state = all reactants  
    change = shift right 
b) initial state = mostly reactants  
    change = shift right 
c) initial state = equilibrium 
    change = none 
d) initial state = mostly products 
    change = shift left 
e) initial state = all products 
    change = shift left

Question 18

What can a Kc tell us?

Answer 18

the equilibrium position of a reaction and how far a reaction progresses

Question 19

What does a small Kc value tell us?

Answer 19

the reaction favours reactants
equilibrium lies to the left-hand side
little reaction occurs

Question 20

What does a large Kc value tell us?

Answer 20

the reaction favours products
equilibrium lies to the right-hand side
reaction nearly complete

Question 21

State the formula for finding ΔG from Kc.

Answer 21

ΔG = -RTln(Kc)
R - gas constant (8.314J/mol/K)
T - temperature (K)
ln(Kc) - Log to the base 10

Question 22

State Le Chatalier’s principle.

Answer 22

when a change is applied to a system in dynamic equilibrium, the system reacts in such a way as to oppose the effect of the change

Question 23

What does a change in equilibrium conditions do?

Answer 23

upsets the equilibrium of the system
a system under stress will shift to release the stress
there will be a change in the rate of the forward or reverse reaction to return the system to equilibrium

Question 24

Does reactant/product concentration affect a Kc value?

Answer 24

the Kc value is not affected by this at a constant temperature

Question 25

What does changing the amount/concentration of substance affect the value of?

Answer 25

the value of Q

Question 26

How is the value of Kc maintained?

Answer 26

the composition of the equilibrium mixture changes by shifting its position

Question 27

Explain what happens in equilibrium when reactant is added.

Answer 27

equilibrium shifts right
increases collisions involving reactants
rate of forward reaction increases

Question 28

Explain what happens in equilibrium when product is added.

Answer 28

equilibrium shifts left
decreases collisions involving reactants
rate of forward reaction decreases

Question 29

Give an example of where equilibrium has biological relevance.

Answer 29

Osmosis in erythrocytes - equilibrium position must be equal on both sides of the membrane
In a hypotonic solution, they will become lysed as water moves in
In an isotonic solution, they will maintain their shape as water moves in and out at an equal rate
In a hypertonic solution, they will become shrivelled as water moves out

Question 30

How does changing pressure affect reactions?

Answer 30

only affects reactions with different numbers of gas molecules on each side
the more particles you have in a given volume, the greater the pressure they exert
if pressure is increased, the system moves to decrease it, towards the side with fewer gas molecules
(pressure favours the direction of fewer gas molecules)

Question 31

Give an example of where the pressure in equilibrium has biological relevance.

Answer 31

i.e. Haemoglobin binding to and releasing oxygen molecules

Hb + O₂ ⇌ Hb-O₂

Question 32

How does adding a catalyst affect equilibrium?

Answer 32

does not affect the position of equilibrium
affects the rate of both forward and backward reaction equally
however, it does increase the rate of attainment of equilibrium

Question 33

Give an example of when a catalyst is used in attaining a biological equilibrium (using the formula equation shown on the document).

Answer 33

Fructose 1,6-phosphate ⇌ Dihydroxyacetone phosphate + Glyceraldehyde 3-phosphate

Question 34

Explain the effect of temperature on Kc.

Answer 34

temperature is the only thing that can change the value of Kc
altering temperature affects the rate of both backward and forward reactions but it alters the two rates to different extents
hereby, the equilibrium moves producing a new Kc value
the direction of movement depends on the sign of the enthalpy change

Question 35

How does the ΔH symbol affect the direction equilibrium will shift?

Answer 35

a negative ΔH means the forward reaction is exothermic and the backward reaction will decrease temperature so it will shift the equilibrium to the left-hand side
a positive ΔH means the forward reaction is endothermic and the backward reaction will increase temperature so it will shift the equilibrium to the right-hand side

Question 36

Complete the following table on changes and equilibrium:

Condition:                   Change:                Reaction:
Concentration
Pressure 
Temperature 
//
Catalyst

Answer 36

Complete the following table on changes and equilibrium:

Condition: Change: Reaction:
Concentration Add reactant Forward
Remove reactant Back
Add product Back
Remove product Forward

Pressure Increase Towards fewer
moles of gas
Decrease Towards more
moles of gas

Temperature Endothermic reaction → K changes
Increase ← K changes
Decrease
Exothermic reaction ← K changes
Increase → K changes
Decrease
//
Catalyst Increases rates equally No effect

Question 37

For a reaction at equilibrium, draw out the product and reactant lines on a:

a) concentration-time graph
b) rate-time graph

Answer 37

a) see document

b) see document