3.9 Acid-base equilibria

What is the Brønsted-Lowrey definition of an acid and a base?

Acids are proton donors, whereas bases are proton acceptors.

Whats the dissociation equations for the following:

1. HNO₃

2. H₂CO₃

3. CH₃COOH

1. HNO₃ ⇌ H⁺ + NO₃^-

2. H₂CO₃ ⇌ 2H⁺ + CO₃^2-

3. CH₃COOH ⇌ H⁺ + CH₃COO^-

3 steps to write ionic equation with the example of HCl and MgCO₃

1. Write full equation

2HCl_(aq) + MgCO_3(s) → MgCl_2(aq) + H₂O_(l) + CO_2(aq)

2. Split aqueous into ions:

2H⁺ + 2Cl^- + MgCO₃ → Mg²⁺ + 2Cl^- + H₂O + CO₂

3. Cancel out:

2H⁺ + MgCO₃ → Mg²⁺ + H₂O + CO₂

Label if acid, base or amphoteric:

1. NH₄⁺

2. NH₃

3. H₃O⁺

4. HClO₄

5. CO₃^2-

6. NO₃^-

1. Acid

2. Both

3. Acid

4. Acid

5. Base

6. Base

Which adds as an acid and a base in the following, and complete:

1. H₂O + NH₃ ⇌

2. CH₃COOH + HClO₄ ⇌

3. HCO₃^- + HSO₄^- ⇌

4. H₃O⁺ + OH^- ⇌

1. H₂O + NH₃ ⇌ NH₄⁺

2. CH₃COOH + HClO₄ ⇌ CH₃COOH₂⁺ + ClO₄^-

3. HCO₃^- + HSO₄^- ⇌ H₂O + CO2 + SO₄^2-

4. H₃O⁺ + OH^- ⇌ 2H₂O

What is a conjugate acid base pair?

Example with HNO₂ and water

An acid, once donated a proton, makes a conjugate base as you can add a proton to reverse and vice versa.

e.g. HNO₂ (A) + H₂O (B) ⇌ H₃O⁺ (CA) + NO₂^- (CB)

Define pH.

pH = -log₁₀[H⁺]

How to calculate [H⁺]

[H⁺] = 10^-pH

What is a monoprotic acid?

Strong acid where [H⁺] is the same as the acid, due to complete dissociation

What is a diprotic acid?

2 hydrogens, so pH = -log₁₀[2xacid] → double acid concentration

Equation for Ka for:

HA ⇌ H⁺ + A^-

Equal amounts of [H⁺] and [A^-], so can write as [H⁺]²

What are the units for Ka

Fixed units of moldm^-3

What will the Ka value of a strong acid be?

Large value.

Equation for pKa and when to use?

pKa = -log₁₀Ka, as it is an easier number to compare.

What does a smaller pKa indicate?

Stronger acid.

What is the equation for Kw?

Kw = [H⁺][OH^-]

As equilibrium is tiny and lies almost completely to the left.

Whats the kw value of water?

1×10^-14mol²dm^-6

Define a base and an alkali

A base is a species that accepts a proton.

An alkali is a soluble base that releases hydroxide ions.

What is the concentration of a base equal to?

The concentration of hydroxide ions.

What is a buffer?

Any system that minimises pH changes by addition of small amounts of acid or base.

They cannot prevent pH changes, but they’re minimised and rectified.

How to form an acidic vs basic buffer?

An acidic buffer is formed from a weak acid and one of its salts. A basic buffer is from a weak base and one of its salts.

Explain how buffers minimise pH change with the example of sodium ethanoate and ethanoic acid.

The salt fully dissociates: CH₃COONa → CH₃COO^- + Na⁺
The acid partially dissociates: CH₃COOH ⇌ H⁺ + CH₃COO^-
The high concentration of ethanoate ions from the salt means equilibrium lies heavily to the left. pH is maintained as adding H⁺ shifts equilibrium to further left, removing them and maintaining pH. If a base is added, it shifts equilibrium to the right to produce more H⁺, thus maintaining pH.

For basic buffers what is the difference?

Salt fully dissociates, and equilibrium has H⁺ on the left instead.
e.g. NH₃ + H⁺ ⇌ NH₄⁺

Equation to calculate the Ka of a buffer?

Salt completely dissociates.
[A^-] no longer equal to acid concentration due to the buffer containing a mixture of the acid and its salt
The acid barely dissociates

What is the Henderson-Hasselbach equation?

What is the equivalence point on an acid-base titration curve?

The vertical region telling the volumes needed to neutralise the acid. An indicator needs to have a colour change with a pH within this vertical region. This also shows the pH of the salt.

What is the buffer region and half equivalence point on an acid-base titration curve?

At the half equivalence point: pH = pKa

What is the graph for a strong-acid strong-base?

What is the graph for a weak acid-strong base?

What is the graph for strong acid-weak base

What is the graph for strong acid-weak base?

What is the graph for weak acid-weak base?

Define an indicator.

A substance that has different colours in low and high pH solutions. This can be used to differentiate between an acid and alkali but cannot distinguish between acids of different pH or alkalis of different pH.

How to find pH for weak acid-weak base?

No significant vertical region, so no indicator works. Use a pH probe instead.

How to calculate the pH of an alkali

14-pOH