Further Aspects Of Equilibria

Question 1

pH is defined as….

Answer 1

The negative logarithm to the base 10 of the hydrogen ion concentration. ( pH = -log₁₀[H] ).

Question 2

For a strong acid or base, the concentration of H or OH in a solution of the acid or base is….

Answer 2

(Approximately)The same as the concentration of the acid or the base.

Question 3

formula for ionic product of water. (Kw).

Answer 3

Kw = [H] [OH].

Question 4

What is Ka?

Answer 4

Acid dissociation constant. (Ka of HA–> Ka = [H][A] / [HA]. )

Question 5

A high value of Ka indicates that…..

Answer 5

The acid is almost completely ionised and exists mostly as the ions of the acid.

Question 6

A low value of Ka indicates…..

Answer 6

That the acid is only slightly ionised and exists mostly as the molecules of the acid.

Question 7

pKa values are used to compare strengths of acids. What is the equation to find it?

Answer 7

pKa = -log₁₀Ka.

Question 8

When calculating the value of Ka for a weak acid. (Assumptions).

Answer 8

• We ignore the Conc. of the H-ions produced by the ionisation of the water molecules present in the sol. As the Kw of water is negligible compared with most weak acids.
• We assume the ionisation of the weak acid is so small that the Conc. of undissociated HA molecules in eq. is approx. the same as that of the unaq. acid.

Question 9

What is an acid-base indicator?

Answer 9

This is a dye that changes colour over a specific PH range.

Question 10

On the graph of reaction of acid and base the end point is where?

Answer 10

At the midpoint of the sharp fall.

Question 11

Range of sharp fall in reaction between strong acid and strong base.

Answer 11

pH between 10.5 and 3.5.

Question 12

Range of sharp fall in reaction between a weak base and strong acid.

Answer 12

pH between 7.5 and 3.5.

Question 13

Range of sharp fall in reaction between a strong base and weak acid.

Answer 13

pH between 11 and 7.5.

Question 14

What is a buffer solution?

Answer 14

Is a solution in which the pH does not change significantly when small amounts of acids or alkalis are added. E.g a mixture of a weak acid and its salts (ethanoic acid & sodium ethanoate)

Question 15

How does a buffer solution work?

Answer 15

• If H(plus ions) are added to the solution the pos. of equ. shifts to the left until equilibrium is established.
• the large reserve supply of the anion (c.base)from the dissociation of the salt ensures the conc.of the anion doesn’t change significantly.
• The large reserve supply of the acid ensures conc.of the acid doesn’t change significantly.

Question 16

For an increase in OH ions what happens in a buffer solution?

Answer 16

• This reduces the conc. Of Hplus ions and the pos. of equ. shifts to the right.
• So the acid ionises to form the ions to restore equilibrium.
• The large reserve supply of the acid ensures there is no significant change in conc. of the acid.
• The large reserve supply of the anion doesn’t change significantly.

Question 17

In polluted areas rainwater that has CO₂ dissolved in it acts as a buffer solution as it forms carbonic acid (H₂CO₃).
But this is not so for areas polluted by acidic oxides of N and S , why?

Answer 17

The concentrations of the carbonic acid and its conjugate base are not high enough to cope with the large amounts of acidic pollution involved.

Question 18

Equations to calculate pH of buffer solution.

Answer 18

[H] = Ka × [Acid] / [Salt].
&
pH = pKa + log([salt] / [Acid])

Question 19

Uses of buffer solution.

Answer 19

• Many industrial processing like electroplating.
• Leather treatment & dye manu.
• Used to keep pH of some animals constant e.g in human blood.

Question 20

What is solubility?

Answer 20

This is the number of grams or moles needed to saturate 100g of water at a given temp.

Question 21

What is solubility product? (Ksp).

Answer 21

This is the product of the concentrations of each ion in a saturated solution of a sparingly soluble salt at 298K, raised to the power of their relative concentrations.

Question 22

Equation for solubility product.

Answer 22

E.g for AgCl
Ksp = [Ag(aq)][Cl(aq)]. ( it is raised to the power of their moles and their oxidation numbers are put on top of their formulas).

Question 23

How do you know if a precipitate is formed?

Answer 23

Let’s say compound is HA.

If [H][A] is greater than the value of Ksp given.

Question 24

What is the common ion effect?

Answer 24

The reduction in solubility of a dissolved salt achieved by adding a solution of a compound which has an ion in common with the dissolved salt. Often results in precipitation.

Question 25

How could the common ion effect cause precipitation?

Answer 25

It shifts the pos. of equ. to the left by adding more of the ion they have in common.

Question 26

What is partition coefficient?

Answer 26

It is the equ. constant which has to do with the separation technique in which a solute is partitioned between two immiscible solvents e.g water and organic solvent.

Question 27

Equation for partition coefficient.

Answer 27

Kpc = [x(solvent A)] / [(solvent B)].