Acid-Base Equilibria Flashcards
Define a Brønsted-Lowry acid:
A substance which donates a proton in a reaction; it is a proton donor
Define a Brønsted-Lowry base:
A substance which accepts a proton in a reaction; it is a proton acceptor
What are conjugate bases?
The ion/molecule remaining after the acid has lost a proton
What are conjugate acids?
The species created when the base accepts the proton
What is the formulae to find the pH?
-log[H+(aq)] mol dm-3
What classifies as a strong acid?
In water, strong acids are nearly completely dissociated into their ions
Give examples of strong acids in water:
- Hydrohalic acids - HCl, HBr, HI (not HF it is a weak acid)
- Nitric acid - HNO3
- Sulphuric acid - H2SO4
- Perchloric acid - HClO4
- Phosphoric acid - H3PO4
Define monoprotic:
Dissociates to give one mole H+ per mole of acid
What are the majority of acids?
Weak acids
So at normal concentration in water they are partially ionised
Give an example of a strong base?
OH
Where does equilibrium lie for strong acids?
Far to the right
Give examples of strong bases in water:
NaOH
LiOH
KOH
Ca(OH)2
Define monobasic/monoacidic base
Dissociates to give one mole OH- per mole of alkali
Define dibasic/diacidic base
Dissociates to give two moles OH- per mole of alkali
What is the neutralisation equation:
H3O+ (aq) + OH- ⇌ 2H2O (l)
Simplified to: H+ (aq) + OH- (aq) ⇌ H2O (l)
What is the formula to find the concentration of a strong acid from pH?
[H+] = 10^-pH
What is the definition of Kw?
Kw = [H+][OH-] mol2 dm-6
What is the definition of Ka?
Ka = [H+(aq)][A-(aq)]/[HA(aq)]
What Ka is considered a weak acid?
Ka < 1 classed as weak acid
What are the two assumptions for the approximation of Ka?
Assumption 1: The [HA(aq)] can be replaced by [HA]tot which is the total orignial concentration of HA
Assumption 2: The [H+] can be assumed to arise solely from the dissociation of the acid and not from the ionisation of water, therefore [H+] is equal to [A-]
What is the weak-acid approximation:
Ka = [H+]^2/[HA]tot
Can only be used in situations where the acid alone is added to water
What is the definition of pKa?
pKa = -log Ka
In relative terms - the stronger the acid…..
- Equilibrium shifts to RHS
- More [H+][A-]
- Less [HA]
- Bigger Ka
- Smaller pKa
What is a titration used for?
Used to find the concentration of a solution by gradually adding it to a second solution which reacts with it. One of the solutions needs to be of a known concentration and the equation of the reaction must also be known
What can be used to measure the pH in acid-base titrations?
pH meter - indicator not needed
What can the change in pH during titration be used for?
To determine the equivalence point
Define equivalence-point:
The point on a titration curve at which stoichiometrically equivalent amounts of acid and base have been added
How do you find the equivalence point from a pH curve?
It lies at the mid-point of the extrapolated vertical position of the titration curve
What does the pH curve for a strong acid-strong base titration look like and explain the shape of the curve?
At the equivalence point it reaches pH 7 because the only major ions present are Cl-(aq) Na+ (aq) which have no effect on the pH, plus [H+] = [OH-]. Resulting solution pH = 7
What does the pH curve for a weak acid-strong base titration look like and explain the shape of the curve?
Equivalence point lies above pH 7
- For a weak acid at the strart of the titration the ph rises more steeply before flattening
- Equivalence point is not ph 7
- Before equivalence the pH rises less steeply
- After equivalence the pH follows same trace of a acid-strong base titration
What does the pH curve for a strong acid-weak base titration look like and explain the shape of the curve?
At the equivalence point the pH lies below pH 7
- For a week base at the start of the titration the pH falls more steeply before flattening
- Equivalence point is not pH 7
- Before equivalence the pH follows same trace of a strong acid-strong base titration after equivalence
What does the pH curve for a weak acid-weak base titration look like and explain the shape of the curve?
End point less clear therefore most indicators are not suitable
What are acid-base indicators?
Water-soluble, weak organic acids that have different coloured acid and base forms.
What are the properties of indicators?
- Colour change sharp at end point (only 1 drop acid or base needed to give change)
- End point must be same as equivalence point, otherwise titration would give wrong answer
- Distinct colour change
Colour change and pH range for methyl orange:
Low pH: red
Transition pH range: 3.1-4.4
High pH base: yellow
Colour change and pH range for phenolphthalein:
Low pH: colourless
Transition pH range: 8.2-10.0
High pH base: pink
Colour change and pH range for bromothymol blue:
Low pH: yellow
Transition pH range: 6.0-7.6
High pH base: blue
What does the indicator equilibrium look like?
Hln ⇌ H+ + ln-
What happens to the indicator equilibrium at low pH?
Low pH (excess H+) - more Hln due to H+ from acid pushing equilibrium to left
- Colour of Hln
What happens to the indicator equilibrium at high pH?
High pH (excess OH-) - more ln- due to OH- reacting with H+ and equilibrium being pushed to right
- Colour of ln-
When does the colour change of most indicators take place?
Over a pH range of 2 units, centered around the pKa for the indicator
What indicators are suitable for strong acid-strong base titrations?
Both indicators (methyl orange and phenolpthalein)
What indicators are suitable for strong acid-weak base titrations?
Methyl orange
What indicators are suitable for weak acid-strong base titrations?
Phenolpthalein
What indicators are suitable for weak acid-weak base titrations?
No suitable indicator
Define half-equivalence
Exactly one-half of the equivalence volume of strong base has been added to the weak acid
- the pH of a solution of a weak acid has the same value as pKa
Define buffer solution:
One that is able to resist change in pH when small amounts of acid or base are added. It is also able to maintain its pH in the face of dilution with water
Define acidic buffer:
One which maintains a solution at a pH below 7 and, typically. consists of a weak acid and one of its salts (to provide an anion, which acts as a base)
Define basic buffer:
One which maintains a solution at a pH above 7 and, typically, consists of a weak base and one of its salts (to provide a cation, which acts as an acid)
What happens when you add alkali to an acidic buffer?
The OH- ions will react with H+ to produce water. The weak acid equilibrium moves to the right to oppose the decrease in H+. The [H+] is restored and the OH- have been removed so the pH remains almost the same
What happens when you add acid to an acidic buffer?
The added H+ react with the A- to produce HA. The weak acid equilibrium moves to the left. The [H+] is restored and the H+ have been removed so the pH remains almost the same
Define buffer region:
The region (volume) over which a weak acid/base can show buffer action
Define buffer range:
The pH range over which a weak acid/base can show buffer action
What happens when a small amount of OH- is added?
- There is a large reservoir of weak acid and the salt
- OH- reacts with the weak acid (CH3CH2COOH + OH- –> CH2CH3COO- + H2O)
- [HA] decreases slightly, [A-] increases slightly
- But ratio [HA]/[A-] only changes slightly
- So [H+] only changes slightly as [H+] is proportional to [HA]/[A]
