Chemical equilibrium

L.O.:

Use Kc (the equilibrium constant ) to work out the composition of an equilibrium mixture.

for an equilibrium reaction of the form...

aA + bB cC + dD

then (at constant temperature) [C]c . [D]d = a constant, (Kc)

[A]a . [B]b

where [ ] denotes the equilibrium concentration in mol dm-3

Kc is known as the Equilibrium Constant

THE EQUILIBRIUM CONSTANT KTHE EQUILIBRIUM CONSTANT Kcc

VALUE OF Kc

AFFECTED by a change of temperature

NOT AFFECTED by a change in concentration of reactants or productsa change of pressureadding a catalyst

for an equilibrium reaction of the form...

aA + bB cC + dD

then (at constant temperature) [C]c . [D]d = a constant, (Kc)

[A]a . [B]b

where [ ] denotes the equilibrium concentration in mol dm-3

Kc is known as the Equilibrium Constant

THE EQUILIBRIUM CONSTANT KTHE EQUILIBRIUM CONSTANT Kcc

2A(g) + B(g) 2C (g)At equilibrium there were found to be 3 × 10-3 mol of A, 2 × 10-3 mol of B and 6.2 × 10-3 mol of C with a total volume of 2 dm3.Calculate Kc

Example 2Consider the equilibrium P + 2Q R + S (all species are aqueous)One mole of P and one mole of Q are mixed. Once equilibrium has been achieved 0.6 moles of P are present. How many moles of Q, R and S are present at equilibrium ?

P + 2Q R + SInitial moles 1 1 0 0At equilibrium 0·6 0·2 0·4 0·4

(0·4 reacted) (2 x 0·4 reacted) (get 1 R and 1 S for every P that reacts) 1- 0·6 remain 1- 0·8 remain

Explanation • if 0.6 mol of P remain of the original 1 mol, 0.4 mol have reacted

• the equation states that 2 moles of Q react with every 1 mol of P

• this means that 0.8 (2 x 0.4) mol of Q have reacted, leaving 0.2 mol

• one mol of R and S are produced from every mol of P that reacts

• this means 0.4 mol of R and 0.4 mol of S are present at equilibrium

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

Calculating amounts at equilibrium

A2.CHEM4.2.001.doc

A2.CHEM4.2.002.doc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

Example 1One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc.

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

Example 1One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc.

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

Example 1One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc.

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

moles (initially) 1 1 0 0moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3

Initial moles of CH3COOH = 1 moles reacted = 2/3 equilibrium moles of CH3COOH = 1/3

For every CH3COOH that reacts; a similar number of C2H5OH’s react (equil moles = 1 - 2/3)

a similar number of CH3COOC2H5’s are produced

a similar number of H2O’s are produced

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

Example 1One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc.

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

moles (initially) 1 1 0 0moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 equilibrium concs. 1/3 / V 1/3 / V 2/3 / V 2/3 / V

V = volume (dm3) of the equilibrium mixture

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

Example 1One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc.

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

moles (initially) 1 1 0 0moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 equilibrium concs. 1/3 / V 1/3 / V 2/3 / V 2/3 / V

V = volume (dm3) of the equilibrium mixture

Kc = [CH3COOC2H5] [H2O]

[CH3COOH] [C2H5OH]

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

• construct the balanced equation, including state symbols (aq), (g) etc.• determine the number of moles of each species at equilibrium• divide moles by volume (in dm3) to get the equilibrium concentrations in mol dm-3

(If no volume is quoted, use a V; it will probably cancel out)• from the equation constructed in the first step, write out an expression for Kc.

• substitute values from third step and calculate the value of Kc with any units

Example 1One mole of ethanoic acid reacts with one mole of ethanol at 298K. When equilibrium is reached it is found that two thirds of the acid has reacted. Calculate the value of Kc.

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

moles (initially) 1 1 0 0moles (at equilibrium) 1 - 2/3 1 - 2/3 2/3 2/3 equilibrium concs. 1/3 / V 1/3 / V 2/3 / V 2/3 / V

V = volume (dm3) of the equilibrium mixture

Kc = [CH3COOC2H5] [H2O] = 2/3 / V . 2/3 / V = 4

[CH3COOH] [C2H5OH] 1/3 / V . 1/3 / V

CALCULATIONS INVOLVING KCALCULATIONS INVOLVING Kcc

A(g) + B(g) C(g)0.50 mol of A and 0.40 mol of B were mixed in a 10 dm3 container. At equilibrium there were 0.2 mol of A. What is the value of Kc?

Finding equilibrium concentrations from Kc values

For the following equilibrium system, Kc = 0.1X(g) Y(g)

If 10 moles of X were allowed to reach equilibrium, how many moles ofY would there be in the equilibrium mixture?

4.2 Exercise 1 - Kc

Q 4-11

Note: you can find similar examples in Q4 of chem factsheet no 21

Chemical equilibrium

L.O.:

Use Kc (the equilibrium constant ) to work out the composition of an equilibrium mixture.