acids bases and buffers

Question 1

What is a Bronsted-Lowry acid

Answer 1

proton donor

Question 2

What is a Bronsted-Lowry base

Answer 2

proton acceptor

Question 3

How do you calculate pH and pKa

Answer 3

pH= -log(H+)
pKa= -log(Ka)

Question 4

How does the pH and pKa of strong acids compare with weak

Answer 4

The pH and pKa values for for strong acids are lower as they fully dissociate (ie, [H+]final = [HA]initial)

Question 5

How can the weak acids be quantified? What assumptions are made in doing so

Answer 5

Weak acids form an equilibrium mixture: HA ⇌ H+ + A-.
Ka= [H+][A-]/[HA]

1. We can assume [H+] = [A-] so all the H+ ions come from the dissociation of the acid and not the water.
2. The dissociation is so small that [HA]initial = [HA]eqm (this doesn’t work for ‘stronger’ weak acids).
   The first assumption doesn’t work for buffers whilst the second doesn’t work for stronger weak acids

Question 6

How should you tackle a
buffers question? (not a
calculation)

Answer 6

1.
Write down the equation:
a. Eg, HCOOH (high) ⇌ HCOO- (low) + H+ (low)
State what happens when acid is added:
a. H+ ions reacts with HCOO- so equlibrium (a) shifts left
to make more HCOOH (by Le Chatlier’s). State what happens when base is added:
a. HO- ions reacts with H+ forms H2O so equlibrium (a) shifts right to to replace H+

Question 7

How can the ionic product of water be used?

Answer 7

1. To work out the pH of water since, for pure water, [H+] = [HO- ].
2. To convert between the [H+] or [HO-] for a dissolved substance.
   Good for working out the [H+] ions in strong alkalis.

Question 8

What MUST BE true for any neutral solution?

Answer 8

H+] = [HO-]
Since temp. changes the value of Kw ⇒ [H+] and [HO-] must change ⇒ [H+] is no longer 10-7 ⇒ pH no longer 7 ⇒ pH of a neutral solution @ any temp. =/= 7. Ultimately, the pH scale changes with temp.

Question 9

What does a buffer solution do

Answer 9

Minimise pH changes (1) when SMALL amounts of acid or base is added (1)

Question 10

Describe both ways of making acidic buffers using examples and comment on how this is similar for basic buffers

Answer 10

1. React a weak acid with a salt of its conjugate base:
   ○ Since CH3COOH ⇌ CH3COO- + H+ and CH3COO-Na+ → CH3COO- + Na+, combing the two forms CH3COOH ⇌ CH3COO- + H+ of which there is lots of weak acid and
   ions.
2. React an excess of weak acid with a strong base:
   ○ So you get CH3COOH + NaOH → CH3COO-Na+ + H2O where the salt will dissociate.
   ● Similarly, for basic buffers, you can react a weak base with a salt of its conjugate weak acid or react a weak base with a strong acid.

Question 11

How are buffers used in the body

Answer 11

● To keep the pH of the blood between 7.35 and 7.45.
● An equilibrium exists between H2CO3 (carbonic acid) and HCO3- (hydrogen carbonate).
● The amount of H2CO3 is controlled by respiration: H2CO3 ⇌ H2O + CO2

Question 12

What is the equivalence point of a titration?

Answer 12

When there are equal moles of acid and base.

Question 13

How should you choose an indicator for a titration appropriately?

Answer 13

Make sure the equivalence point lies within the colour change range.

Question 14

Which indicator is required for a weak-acid weak-base titration?

Answer 14

None as they won’t work as there’s no sharp pH change.

Question 15

How do indicators work?

Answer 15

● Considered as weak acids. ● HIn (aq) ⇌ In- (aq) + H+ (aq) ● Colour A ⇌ Colour B
● Using Le Chatelier’s, if H+ acid is added, it will shift to the left and vice versa for HO- base

Question 16

Which is more acidic and why, CH3COCl or CH3COOH?

Answer 16

CH3COCl as it will form HCl in water.