Acids and pH

Question 1

What is an acid-base pair?

Answer 1

Pair of two species that transform into each other by gain or loss of a proton

Question 2

Draw titration curves for:

• weak acid strong base
• strong acid strong base
• strong acid weak base
• weak acid weak base

Answer 2

-

Question 3

What is neutralisation?

Answer 3

Chemical reaction in which an acid and a base react together to produce a salt and water

Question 4

Units of Ka

Answer 4

Always mol dm^-3

Question 5

What does a large Ka show?

Answer 5

A strong acid because lots of dissociation

Question 6

How does a buffer work?

Answer 6

An addition of acid the conjugate base reacts with H+ ions and equilibrium shifts to left removing H+ ions added
When alkali is added - conc. OH- ions increases - H+ ions react with the OH- ions, HA dissociates, equilibrium shifts to right to restore reacted H+ ions
HA H+ + A-

Question 7

What is a buffer?

Answer 7

A mixture that minimises pH change on addition of small amounts of acid or base. It is a mixture of a weak acid and its conjugate base. Equilibrium mixture formed contains a high conc. of undissociated weak acid and A-. Conc. H+ is very low.

Question 8

What is the equivalence point?

Answer 8

The point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution. It is the centre of the vertical section of a tritration curve.

Question 9

What is half neutralisation point?

Answer 9

Volume is half of volume at equivalence point, here pKa = pH

Question 10

What is an acid-base indicator and how is it represented?

Answer 10

A weak acid, represented as Hln
Hln H+ +ln-
Different colours in acid and conjugate base form

Question 11

What is equation when indicator is at its end point?

Answer 11

[Hln] = [ln-]

Question 12

How do you choose an indicator?

Answer 12

Chose one so that the pH at the end point is as close as possible to the titration’s equivalence point

Question 13

What assumption is made when doing pH calculations for a strong acid?

Answer 13

[H+] = [HA]

Question 14

Equation for [H+] of weak acid

Answer 14

square root of Ka x [HA]

Question 15

Equation for pH of a buffer system

Answer 15

-log (Ka x ([HA]/[A-]))

Question 16

Equation for ionic product of water

Answer 16

Kw = [OH-][H+] = 1.00 x 10^-14 mol2 dm-6 at 25 c

Question 17

[OH-] for a strong base

Answer 17

[OH-] = conc. base

Question 18

How does carbonic acid-hydrogen carbonate buffer system in blood work?

Answer 18

H2CO3 H+ + HCO3 -
(Then the carbonic acid is converted into aqueous carbon dioxide through the action of an enzyme, then converted to gas in the lungs and exhaled)

Question 19

What is standard enthalpy change of neutralisation?

Answer 19

The enthalpy change which accompanies the neutralisation of an aqueous acid by an aqueous base to form 1 mole of H2O under standard conditions

Question 20

What is a conjugate acid?

Answer 20

Species formed when a proton is added to a base

Question 21

What is a conjugate base

Answer 21

A species formed when a proton is added to an acid

Question 22

Ethanoic acid is a strong acid in liquid ammonia but is weak in water, why?

Answer 22

The solvents are acting as bases and ammonia is a better base than water

Question 23

Suggest why some acid are weak in water while others are strong

Answer 23

The weaker or more polar the bond between the hydrogen and the rest of the molecule, the stronger the acid is likely to be because it can dissociate better

Question 24

Under what circumstances, during the calculation of pH for strong acids and alkalis, is the assumption that [H+] and [OH-] due to ionisation of water is negligible valid and invalid?

Answer 24

Valid:
If the [H+] or [OH-] from the acid or alkali is very large compared with that of the water and INVALID if it is comparable to or smaller than