5: Chemical equilibrium

Question 1

What is are Activities (generally)?

Answer 1

Dimensionless numbers defined relative to standard states so that a substance in its standard state has activity equal to 1

Question 2

What is the activity for a solute in a dilute solution?

Answer 2

For a solute in a dilute solution the standard concentration is 1 M, and the activity is the concentration of the solute divided by the standard concentration. So the activity is numerically the same as the concentration in M but with no units.

Question 3

What is the activity for a solvent in a dilute solution?

Answer 3

For the solvent in a dilute solution the standard state is pure solvent and the activity of the solvent is effectively 1.

Question 4

What is the activity for a solid?

Answer 4

For a solid, the standard state is the pure solid, and so if a solid is involved in the equilibrium the activity is 1.

Question 5

What is the activity for a gas?

Answer 5

For a gas, the standard pressure is 1 bar, and the activity is the pressure of the gas divided by the standard pressure.

Question 6

What is the Law of Mass Action?

Answer 6

The equilibrium constant for a solution-phase reaction is the ratio of the activities of products to reactants, each raised to the power of the stoichiometric numbers

Question 7

When would you exclude water from an equilibrium constant reaction?

Answer 7

If water is the solvent then it would normally be excluded from this ratio, as its activity is taken to be 1

Question 8

How can you describe a saturated solution?

Answer 8

As a system in equilibrium (between the solid and the aqueous ions or molecules)

Question 9

What is the equilibrium constant of a saturated solution called ( and why )?

Answer 9

The solubility product ( because the activity of the solid is 1 and so the denominator of the Kc expression is 1 leaving just the product of the product concentrations )

Question 10

What is Dalton’s law?

Answer 10

The total pressure exerted by a mixture of gases is the sum of the partial pressures of the gases.

Question 11

What does Dalton’s law mean for the partial pressure of a gas?

Answer 11

The partial pressure of a gas is therefore equal to the total pressure multiplied by the mole fraction of that component.

Question 12

What is the equilibrium constant for the self-ionisation of water more commonly known as?

Answer 12

The ionic product of water (Kw)

Question 13

What is the ionic product of water?

Answer 13

Kw = [OH-][H3O+]

Question 14

What is the value of the ionic product of water at 25 celcius?

Answer 14

1.0×10−14 mol2 dm−6

Question 15

How does pKa vary with the strength of acids?

Answer 15

The lower the value of pKa, the larger the value of Ka and the greater the dissociation of the acid in water. A weak acid has a high value of pKa, while a strong acid has a low value of pKa
.

Question 16

What are the two sources of disorder during a reaction?

Answer 16

Entropy and the mixing of products and reactants

Question 17

What is the relationship between the position of equilibrium and Gibbs free energy?

Answer 17

Equilibirum is reached when the total Gibbs energy reaches a minimum.

Question 18

The standard Gibbs energy change and equilibrium constant are linked by the expression…

Answer 18

ΔGƟ = −R T lnK
where lnK is the natural log of the equilibrium constant