7.1 The characteristic properties of acids and bases

1. Reactions of acids and bases

Reactions involving acids

a) Acids react with reactive metals

Acid + reactive metal → salt + hydrogen gas

eg:

2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)

H₂SO₄(aq) + Fe(s) → FeSO₄

Reactive metals represent metals are above Hydrogen in activity series

b) Acids react with bases

Acid + base → salt + water

eg:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

c) Acids react with carbonates

Acid + carbonate → salt + carbon dioxide + water

eg:

CaCO₃ + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Na₂CO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)

Utilization: Do you still remember how to test the presence specified gases? (H₂, CO₂, etc?)

Reactions involving bases

a) Bases react with acids

same to reactions involving acids

b) Bases react with ammonium salts

Base + ammonium salt → salt + ammonia gas + water

eg:

NaOH(aq) + NH₄Cl(aq) → NaCl(aq) + NH₃(g) + H₂O(l)

Ba(OH)₂(s) + 2NH₄Cl(s) → BaCl₂(aq) + 2NH₃(g) + 2H₂O(l)

Effects on indicators

Indicator	Color in acidic solution	Color in neutral solution	Color in basic solution
Litmus	red	purple	blue
Thymolphthalein	colourless	colourless	blue
Methyl orange	red	orange	yellow

2. Definition of acid and base

Arrhenius definitions - in aqueous solution

Acids: can dissociate into H⁺ ions in aqueous solution.

eg:

HCl(aq) → H⁺(aq) + Cl^–(aq)

Bases: can dissociate into OH^– ions in aqueous solution.

eg:

NaOH(aq) → Na⁺(aq) + OH^–(aq)

Bronsted Lowry definitions - in terms of proton

Acids: proton (H⁺) donors.

Bases: proton (H⁺) acceptors.

Strong and weak - in terms of extent of dissociation

A strong acid: is an acid that is completely dissociated in aqueous solution.

HCl(aq) → H⁺(aq) + Cl^–(aq)

A weak acid: is an acid that is partially dissociated in aqueous solution.

Notice: dissociations of weak acids or bases should use equilibrium arrow “⇌“.

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO^–(aq)

Some common organic acids and mineral acids are listed in the following table.

Name	Formula	Strong or weak?	Where found or used
ethanoic acid	CH₃COOH	weak	in vinegar
methanoic acid	HCOOH	weak	in ant and nettle stings; used in kettle descaler
lactic acid	CH₃CH(OH)CO₂H	weak	in sour milk
citric acid	C₆H₈O₇	weak	in lemons, oranges and other citrus fruits
carbonic acid	H₂CO₃	weak	in fizzy soft drinks
hydrochloric acid	HCl	strong	used in cleaning metal surfaces; found as the dilute acid in the stomach
nitric acid	HNO₃	strong	used in making fertilisers and explosives
sulfuric acid	H₂SO₄	strong	in car batteries; used in making fertilisers, paints and detergents
phosphoric acid	H₃PO₄	intermediate to strong	in anti-rust paint; used in making fertilisers

3. Acid-base titration

Purpose

The purpose of titration is to determine the accurate concentration of one solution.

Apparatus

Procedure

Example: Use HCl to titrate NaOH

Step 1: 25 mL NaOH(aq) were placed in conical flask by pipette;

Step 2: Litmus was dropped into solution. (colour of solution: blue);

Step 3: 0.123 mol dm-3 HCl was placed in burette. Record the initial reading;

Step 4: Identify end-point (or called equivalence point) by colour change (turns purple)

Step 5: Record the final reading.

Step 6: Calculate concentration of NaOH.

Calculation