Characteristic Properties of Acids and Bases Notes

Cambridge IGCSE Chemistry: Revision notes

Key facts

  • Acids give H⁺ ions in water; alkalis give OH⁻ ions.
  • Acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide.
  • Neutralisation: HX++OHX−→HX2O\ce{H+ + OH- -> H2O}.
  • Strong acids dissociate completely; weak acids only partially.
  • Acids are proton donors; bases are proton acceptors.

Reactions of acids

Acids react with metals, bases and carbonates to form a salt plus hydrogen, water, or water and carbon dioxide.

Acids release H⁺ ions in aqueous solution. With a reactive metal: Mg+2 HCl→MgClX2+HX2\ce{Mg + 2HCl -> MgCl2 + H2}. With a base (neutralisation): HCl+NaOH→NaCl+HX2O\ce{HCl + NaOH -> NaCl + H2O}. With a carbonate: 2 HCl+CaCOX3→CaClX2+HX2O+COX2\ce{2HCl + CaCO3 -> CaCl2 + H2O + CO2}. These are diagnostic tests for an acid.

Acid +

  • Metal
  • Base
  • Carbonate

Products

  • Salt + hydrogen
  • Salt + water
  • Salt + water + carbon dioxide

Which gas forms when an acid reacts with a carbonate?

Indicators and pH

Indicators change colour in acid and alkali; the pH scale measures how acidic or alkaline.

Acids have a high concentration of H⁺ ions and alkalis a high concentration of OH⁻ ions. pH below 7 is acidic, 7 is neutral and above 7 is alkaline. The lower the pH, the more H⁺; the higher above 7, the more OH⁻. Match universal indicator paper to the colour scale to rank solutions.

Colour in acid

Litmus:
Red
Thymolphthalein:
Colourless
Methyl orange:
Red
Universal indicator:
Red (pH 1 to 3)

Original colour

Litmus:
Purple
Thymolphthalein:
Colourless
Methyl orange:
Orange
Universal indicator:
Green (pH 7)

Colour in alkali

Litmus:
Blue
Thymolphthalein:
Blue
Methyl orange:
Yellow
Universal indicator:
Blue/purple (pH 11 to 14)
0123456789101112131417 neutral13
The pH scale

Which solution is the most alkaline?

Bases and alkalis

A base is a metal oxide or hydroxide; an alkali is a base that dissolves in water.

All alkalis are bases, but not all bases are alkalis; the key difference is solubility. Bases such as CuO\ce{CuO} and AlX2OX3\ce{Al2O3} are insoluble; alkalis such as NaOH\ce{NaOH} and KOH\ce{KOH} produce OH⁻ ions in water. Alkali + ammonium salt → salt + ammonia + water, and ammonia is detected by its pungent smell.

Base

What it is:
Oxide or hydroxide of a metal
Solubility:
Not necessarily soluble
Examples:
CuO, Ca(OH)₂, Al₂O₃

Alkali

What it is:
A soluble base
Solubility:
Produces OH⁻ ions
Examples:
NaOH, KOH

2 NaOH+(NHX4)X2SOX4→NaX2SOX4+2 NHX3+2 HX2O\ce{2NaOH + (NH4)2SO4 -> Na2SO4 + 2NH3 + 2H2O}

Which statement is correct?

Strong and weak acids

Strong acids dissociate completely; weak acids only partially.

Neutralisation is HX+(aq)+OHX−(aq)→HX2O(l)\ce{H+(aq) + OH-(aq) -> H2O(l)}. A strong acid such as hydrochloric acid dissociates completely: HCl(aq)→HX+(aq)+ClX−(aq)\ce{HCl(aq) -> H+(aq) + Cl-(aq)}. A weak acid such as ethanoic acid is only partially dissociated: CHX3COOH(aq)⇌HX+(aq)+CHX3COOX−(aq)\ce{CH3COOH(aq) <=> H+(aq) + CH3COO-(aq)}. A strong acid gives more H⁺ than a weak acid of the same concentration.

Strong acid

Dissociation:
Complete
Equation arrow:
Single arrow (→)
H⁺ concentration:
High
pH:
Lower (for example 1 to 2)
Examples:
HCl, H₂SO₄, HNO₃

Weak acid

Dissociation:
Partial
Equation arrow:
Equilibrium arrow (⇌)
H⁺ concentration:
Low
pH:
Higher (for example 3 to 6)
Examples:
CH₃COOH, citric acid

Which acid is weak?

Proton transfer

An acid is a proton donor; a base is a proton acceptor.

The Brønsted–Lowry definition: acids donate H⁺ (a proton), bases accept it. In HX+(aq)+OHX−(aq)→HX2O(l)\ce{H+(aq) + OH-(aq) -> H2O(l)} the acid is the proton donor and the OH⁻ is the proton acceptor. Ammonia, NHX3\ce{NH3}, is also a base because it accepts a proton. This definition is broader because it covers non-aqueous solutions.

  1. 1

    Acid

    holds an H⁺ ion

  2. 2

    Proton transfers

    H⁺ passes to the base

  3. 3

    Base accepts it

    for example OH⁻ becomes H₂O

  4. 4

    Salt and water form

    acid and base are neutralised

Proton transfer in neutralisation

In neutralisation, the OH⁻ ion acts as a:

Try an exam question

Explain the difference between a strong acid and a weak acid of the same concentration.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.