Buffers and Neutralisation A2 Flashcards
What are buffers? DEFINITION
Solutions that MINIMISE pH changes on the addition of small amounts of acid or base.
What are buffers composed of?
Weak acids and their conjugate bases.
HA <–> H+ + A-
When are buffers most and least effective?
Most effective when [HA] = [A-]
No longer effective when one of the components is completely used up.
HA <–> H+ + A-
How do buffers work generally?
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.
What happens when acid is added to a buffer?
1) [H+] increases
2) Conjugate base (A-) reacts with H+ to make HA.
3) Equilibrium shifts to the left.
4) [H+] decreases
What happens when alkali is added to a buffer?
1) [OH-] increases
2) H+ reacts with OH- to form H2O
3) Equilibrium shifts to the right
4) HA dissociates, restoring H+ concentration
What are 2 ways to form buffers?
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.
What is the name of the most important buffer in the blood? Give the equation.
Carbonic acid-hydrogencarbonate
H2CO3 <–> HCO3- + H+
What happens to a person if their blood becomes too acidic/alkali?
Too acidic = fatigue, shortness of breath, death
Too alkaline = light-headed, nausea, spasms
What does the pH of a buffer depend on?
The weak acid that is used and the concentration of each component in the buffer (e.g. [H+])
The operating pH of a buffer is +/- what of pKa?
pH = +/- 1 of pKa
e.g. pKa = 2.5, then pH = 1.50-3.50
Why are buffers most effective when [HA] = [A-]?
HA and A- cancel out in Ka expression so Ka = [H+]
So pH = pKa of HA
([H+] = ka)
When calculating the pH of a buffer, why can you not turn [H+][A-] into [H+^2] (like with weak acids)?
Because [H+] and [A-] are not equal because buffers are made by adding the conjugate salt or neutralisation causing excess acid.
What is the equivalence point on a pH curve?
The volume required to have completely reacted the acid and base together.
What does the equivalence point depend on?
The strength of the acid and base used.
Strong acid + strong base = 7
Strong acid + weak base = <7
Weak acid + strong base = >7
Give examples of strong and weak acids and bases
Strong acids: HCl, H2SO4, HNO3
Weak acids: Any organic acid
Strong base: All hydroxides
Weak base: NH3
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)?
12.5 cm^3 (only need half the volume to neutralize with double the concentration)
When a strong acid and base react together, why is the pH 7?
H+ ions completely neutralised by OH- ions to form only water and salt present.
Why does reacting a strong acid with a weak base (NH3) produce a pH < 7?
NH3 (weak base) has a strong conjugate acid, NH4+ which will react with water to produce H3O+.
Why does reacting a weak acid with a strong base produce a pH >7?
Weak acid (HA) will have a strong conjugate base (A-) which reacts with water to produce OH- ions.
A- + H2O –> HA + OH-
What are acid-base indicators? How do they work?
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.
What is the End Point? Why do indicators have different pH values for the end point?
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.
How can you determine the pH of the end point?
Find the pKa value. Because at end point [HA]=[A-] so Ka = [H+].
Most indicators are over a ______ range of pH.
small
