Buffers and Neutralisation A2

Question 1

What are buffers? DEFINITION

Answer 1

Solutions that MINIMISE pH changes on the addition of small amounts of acid or base.

Question 2

What are buffers composed of?

Answer 2

Weak acids and their conjugate bases.

HA <–> H+ + A-

Question 3

When are buffers most and least effective?

Answer 3

Most effective when [HA] = [A-]

No longer effective when one of the components is completely used up.

HA <–> H+ + A-

Question 4

How do buffers work generally?

Answer 4

Buffers have INDEPENDENT RESERVOIRS (HA and A-) and so a reaction with one will cause a shift in the EQUILIBRIUM to change H+ concentration and therefore pH.

Question 5

What happens when acid is added to a buffer?

Answer 5

1) [H+] increases
2) Conjugate base (A-) reacts with H+ to make HA.
3) Equilibrium shifts to the left.
4) [H+] decreases

Question 6

What happens when alkali is added to a buffer?

Answer 6

1) [OH-] increases
2) H+ reacts with OH- to form H2O
3) Equilibrium shifts to the right
4) HA dissociates, restoring H+ concentration

Question 7

What are 2 ways to form buffers?

Answer 7

Adding solutions of weak acid and its conjugate salt (e.g. mixing ethanoic acid with Sodium ethanoate)

OR

Partial neutralisation of a weak acid
- Add an alkali (e.g. NaOH) to EXCESS weak acid.
- This will neutralise SOME of the acid to form the conjugate base (A-) and leaving some acid left over.
e.g. CH3COOH + NaOH -> CH3COONa (Na+ and CH3COO-) + H2O with excess acid left over.

Question 8

What is the name of the most important buffer in the blood? Give the equation.

Answer 8

Carbonic acid-hydrogencarbonate
H2CO3 <–> HCO3- + H+

Question 9

What happens to a person if their blood becomes too acidic/alkali?

Answer 9

Too acidic = fatigue, shortness of breath, death

Too alkaline = light-headed, nausea, spasms

Question 10

What does the pH of a buffer depend on?

Answer 10

The weak acid that is used and the concentration of each component in the buffer (e.g. [H+])

Question 11

The operating pH of a buffer is +/- what of pKa?

Answer 11

pH = +/- 1 of pKa

e.g. pKa = 2.5, then pH = 1.50-3.50

Question 12

Why are buffers most effective when [HA] = [A-]?

Answer 12

HA and A- cancel out in Ka expression so Ka = [H+]
So pH = pKa of HA

([H+] = ka)

Question 13

When calculating the pH of a buffer, why can you not turn [H+][A-] into [H+^2] (like with weak acids)?

Answer 13

Because [H+] and [A-] are not equal because buffers are made by adding the conjugate salt or neutralisation causing excess acid.

Question 14

What is the equivalence point on a pH curve?

Answer 14

The volume required to have completely reacted the acid and base together.

Question 15

What does the equivalence point depend on?

Answer 15

The strength of the acid and base used.

Strong acid + strong base = 7

Strong acid + weak base = <7

Weak acid + strong base = >7

Question 16

Give examples of strong and weak acids and bases

Answer 16

Strong acids: HCl, H2SO4, HNO3
Weak acids: Any organic acid

Strong base: All hydroxides
Weak base: NH3

Question 17

If you have 25cm^3 of 0.1 mol dm^-3 HNO3 and you add 50cm^3 of 0.2 mol dm^-3 NaOH with a burette, what volume of NaOH do you need to neutralize the acid (reach equivalence point)?

Answer 17

12.5 cm^3 (only need half the volume to neutralize with double the concentration)

Question 18

When a strong acid and base react together, why is the pH 7?

Answer 18

H+ ions completely neutralised by OH- ions to form only water and salt present.

Question 19

Why does reacting a strong acid with a weak base (NH3) produce a pH < 7?

Answer 19

NH3 (weak base) has a strong conjugate acid, NH4+ which will react with water to produce H3O+.

Question 20

Why does reacting a weak acid with a strong base produce a pH >7?

Answer 20

Weak acid (HA) will have a strong conjugate base (A-) which reacts with water to produce OH- ions.

A- + H2O –> HA + OH-

Question 21

What are acid-base indicators? How do they work?

Answer 21

Weak acids that have different coloured conjugate bases. The two are in equilibrium.

HIn <–> H+ + In-

In acidic conditions, the indicator equilibrium shifts to the left (H+ react with conjugate base) and as the system becomes more basic the equilibrium shifts to the right (H+ reacts with OH- forming water)- altering the colour.

Question 22

What is the End Point? Why do indicators have different pH values for the end point?

Answer 22

The point where the colour of an indicator changes. Equal [HIn] and [In-]. So colour lies between both extremes.

Each indicator will have a different pH value for the end point as they have different Ka values.

Question 23

How can you determine the pH of the end point?

Answer 23

Find the pKa value. Because at end point [HA]=[A-] so Ka = [H+].

Question 24

Most indicators are over a ______ range of pH.

Answer 24

small