Chapter 15
Acids and Bases
Learning Outcomes
After completing this chapter, you should be able to
 define acids and alkalis in terms of the ions they produce in
aqueous solution and their effects on indicators
 describe the characteristic properties of acids in reactions
with metals, bases and carbonates
 state some uses of acids including sulfuric acid
 describe the characteristic properties of bases in reactions
with acids, metal ions and ammonium compounds
Learning Outcomes
 describe the difference between strong and weak
acids and alkalis in terms of the extent of ionisation
 describe the pH scale as a measure of relative
acidity and alkalinity
 construct ionic equations
What are Some Common Acids?
Laboratory Acids
 Three common acids used
in the school laboratory
are:
 Hydrochloric acid
 Sulfuric acid
 Nitric acid
 Concentrated acids contain
a large amount of pure acid
dissolved in water.
 Dilute acids contain small
amounts of acid in water.
Properties of Dilute Acids
 Acids have a sour taste.
 Acids are hazardous.
 Acids change the colour of
indicators.
Properties of Dilute Acids
 Acids react with metals to produce
hydrogen gas.
Mg(s) + 2HCl(aq)
H2(g)
MgCl2(aq) +
Properties of Dilute Acids
 Acids react with carbonates and hydrogencarbonates
to produce carbon dioxide gas.
CaCO3(s) + 2HCl(aq)
CaCl2(aq) + CO2(g) + H2O(l)
NaHCO3(s) + HCl(aq)
NaCl(aq) + CO2(g) + H2O(l)
Properties of Dilute Acids
 Acids react with metal oxides and hydroxides
forming a salt and water.
CuO(s) + H2SO4(aq)
CuSO4(aq) + H2O(l)
Storage of Acids
 Acids do not react with
materials such as sand,
glass, brick and plastic.
 As metals can react with
acids, acidic foods are not
usually stored in metal
containers.
 Rather, acidic foods are
stored in glass, plastic jars
or clay pots.
Uses of Acids
Skills Practice
Answers
 1. The 5 mol/dm3 acid is more concentrated; it has more solute (acid)
in each
unit volume (dm3) of solution.
 2. Copper does not react with dilute hydrochloric acid or dilute sulfuric
acid as
it is below hydrogen in the reactivity series.
 3. (a) Bubbles of a colourless gas (hydrogen) are produced very slowly.
(b) Bubbles of a colourless gas (carbon dioxide) are produced.
 4. (a) Mg(s) + H2SO4(aq)
MgSO4(aq) + H2(g)
(b) MgO(s) + 2HCl(aq)
MgCl2(aq) + H2O(l)
(c) Cu(OH)2(s) + H2SO4(aq)
CuSO4(aq) + 2H2O(l)
(d) ZnCO3(s) + 2HNO3(aq)
Zn(NO3)2(aq) + CO2(g) + H2O(l)
(e) 2NaHCO3(s) + H2SO4(aq)
Na2SO4(aq) + 2CO2(g) + 2H2O(l)
 5. (a) Acids do not react with glass. Also, as glass is transparent, it
enables us
to see how much acid is in the bottle.
(b) Copper, unlike aluminium, does not react with dilute acids.
Why is Water Important for
Acids?
Hydrogen Ions
 Pure acids consist of small
covalent molecules.
 In the presence of water, the acid
molecules form ions.
 So, when hydrogen chloride gas
dissolves in water, hydrogen ions
are produced.
 But when hydrogen chloride gas
dissolves in organic solvent, the
gas remains as molecules.
 The properties and reactions of
acids are due to the hydrogen
ions.
 Acids only behave as acids when
they are dissolved in water.
The Definition of an Acid
 An acid is a substance that produces hydrogen ions
when dissolved in water.
Basicity of an Acid
 The maximum number of hydrogen ions produced by a
molecule of an acid is called the basicity of the acid.
Strong and Weak Acids
 A strong acid is one that completely ionises in water.
 Hydrochloric acid, sulfuric acid and nitric acid are
examples of strong acids.
 A weak acid is one that partially ionises in water.
 An example is ethanoic acid.
Reactivity of strong and weak
acids
 Strong acids react more
vigorously than weak acids.
Comparing acids
 Strong and weak refer to the extent of ionisation.
 Concentrated and dilute refer to the amount of solute
in the solution.
Skills Practice
Answers
 1. The double arrow shows that the reaction is reversible/proceeds in
both
directions. (Students will learn more about reversible reactions when
studying the Haber Process in Chapter 17.)
 2.
Note: Water molecules are not drawn but are represented by the
background shading.
 3. A strong acid is one that is completely ionised in water. A weak acid
is one
that is only partially ionised in water.
Answers
 4. (a) Hydrogen ions, H+(aq) and sulfate ions, SO42-(aq).
acid
(b) Hydrogen ions, H+(aq), ethanoate ions, CH3COO-(aq) and ethanoic
molecules, CH3COOH(aq).
(c) Hydrogen ions, H+(aq) and nitrate ions, NO3-(aq)
 5. (a) Carbon dioxide gas
acid.
(b) The reaction with nitric acid will be faster as nitric acid is a strong
What are Bases?
 Bases are the oxides or
hydroxides of metals.
 The soluble bases are
called alkalis.
Laboratory Alkalis
 Three alkalis commonly
found in the school
laboratory are:
 Sodium hydroxide
 Aqueous ammonia
 Calcium hydroxide
 An alkali is a substance
that produces hydroxide
ions in water.
 The properties of alkalis are
due to the presence of these
hydroxide ions.
Properties of Alkalis
 Alkalis feel slippery.
 Alkalis are hazardous.
 Alkalis change the colour of
indicators.
 Alkalis react with acids.
 Alkalis react with
ammonium compounds to
produce ammonia gas.
NH4Cl(s) + NaOH(aq)
NaCl(aq) + NH3(g) + H2O(l)
Properties of Alkalis
 Alkalis react with solutions of
metal ions.
2NaOH(aq) + CuSO4(aq)
Cu(OH)2(s) + Na2SO4(aq)
Strong and Weak Alkalis
 When strong alkalis are added to water, they form
OH- ions in solution.
 Sodium hydroxide and potassium hydroxide are
examples of strong alkalis.
 Ammonia is the most common example of a weak
alkali.
 When ammonia dissolves in water, only a small
fraction of the ammonia molecules react with the
water to form OH- ions.
Uses of Bases and Alkalis
Skills Practice
Answers
 1. Two common alkalis are sodium hydroxide and ammonia solution.
is
Sodium
hydroxide is found in oven and floor cleaners while ammonia solution
found in window cleaners.
 2. (a) ZnCl2(aq) + 2NaOH(aq)
(b) CuSO4(aq) + 2NaOH(aq)
(c) FeCl2(aq) + 2KOH(aq)
(d) Fe(NO3)3(aq) + 3KOH(aq)
 3. (NH4)2SO4(aq) + 2NaOH(aq)
Zn(OH)2(s) + 2NaCl(aq)
Cu(OH)2(s) + Na2SO4(aq)
Fe(OH)2(s) + 2KCl(aq)
Fe(OH)3(s) + 3KNO3(aq)
Na2SO4(aq) + 2NH3(g) + 2H2O(l)
 4. (a) is a dilute solution of a strong alkali.
(b) is a concentrated solution of a strong alkali.
(c) is a dilute solution of a weak alkali.
(d) is a concentrated solution of a weak alkali.
Indicators
 An indicator is a substance that has different colours
in acidic and alkaline solutions.
Indicators
 Many brightly coloured flowers, vegetables and
berries make good indicators.
The pH Scale
 pH is a measure of the acidity or alkalinity of a
solution.
How Do We Measure the pH of a
Solution?
Universal Indicator
 Universal Indicator is a mixture
of indicators.
 It comes either in the form of a
solution or paper.
 The indicator gives different
colours at different pH values.
 The pH of a solution can be
measured by dipping a piece of
Universal Indicator paper in the
solution.
 The pH is then found by
comparing the colour obtained
with a colour chart.
How Do We Measure the pH of a
Solution?
pH meter
 We can use a pH meter to obtain accurate values.
 The probe is dipped into the solution and the meter
shows the pH either on a scale or digitally.
How Do We Measure the pH of a
Solution?
pH sensor and computer
 A pH sensor connected to a computer through an
interface can be used to measure the pH of a solution.
 The pH reading is displayed on the computer screen.
[
pH of Some Common
Substances
Why is pH Important?
pH and the body
 Substances in the body have different pH values.
 Acidic conditions in the stomach and alkaline
conditions in the small intestine are needed for good
digestion.
 Blood going to the heart and lungs contains carbon
dioxide which makes the blood slightly acidic.
Why is pH Important?
pH and food preservation
 Many fresh foods quickly go bad.
 This is mainly due to micro-
organisms which are present in
the food.
 Micro-organisms do not grow well
in solutions of low pH.
 Therefore, acids can be used to
preserve foods.
Why is pH Important?
pH and the soil
 The pH of soil is important for good plant growth.
 Most plants grow best in soil that is weakly acidic.
 However, some plants grow well in more acidic soil or
soil that is more alkaline.
Why is pH Important?
pH and hair
 Normal hair is weakly acidic with a pH of about 5.
 At this pH, the hair is smooth, strong and healthy.
 Alkaline solutions make hair curly.
 The hair becomes duller, weaker and easier to break
after it has been treated with alkaline solutions.
 To change the shape of hair, for example, to make it
wavy, a process called perming is used.
 Perm solutions are always alkaline.
Skills Practice
Answers
 1. Acids are corrosive or irritants and may harm tissue in the mouth.
 2. (a) Sour milk consists of lactic acid, which gives it a sour taste.
(b) Sour milk has a pH 3~4, as lactic acid is a weak acid.
 3. Yiwu is correct. The red litmus paper will remain red with water and
must
with
other neutral substances. To be sure that a substance is acidic, we
observe a change in colour. Therefore we should also test the solution
blue litmus paper.
 4. The solution with pH 1 is strongly acidic.
The
The
The
The
solution with
solution with
solution with
solution with
pH
pH
pH
pH
4 is weakly acidic.
7 is neutral.
9 is weakly alkaline.
13 is strongly alkaline.
Answers
 5. Cola – pH 4
Distilled water – pH 7
Seawater – pH 8
Liquid soap – pH 10
Lemon juice – pH 3
 6. pH paper is red in dilute hydrochloric acid, yellow in lemonade and
green in
distilled water.
What are Ionic Equations?
 An ionic equation is an equation involving ions in an
aqueous solution; only the ions formed or changed
during the reaction are included.
 In ionic equations:
 Formulae of ions that change are included; ions that do
not change are not included.
 Formulae of solids, liquids and gases are written in full.
Reactions between metals and
acids
 An example is the reaction between magnesium and
dilute hydrochloric acid.
Mg(s) + 2HCl(aq)
MgCl2(aq) + H2(g)
 HCl consists of H+ ions and Cl- ions and MgCl2
consists of Mg2+ ions and Cl- ions.
Mg(s) + 2H+(aq) + 2Cl-(aq)
Mg2+(aq) + 2Cl-(aq) +
H2(g)
from
from
2HCl(aq)
MgCl2(aq)
 Cl- ions do not change and hence they are not
involved in the reaction.
Reactions between metals and
acids
 Because the purpose of an ionic equation is to show
changes, we can cancel these ions.
Mg(s) + 2H+(aq)
Mg2+(aq) + H2(g)
 This is the ionic equation.
 It shows that the reaction is essentially between
magnesium and hydrogen ions.
Reactions between metals and
acids
Reactions between acids and
alkalis
 An example is the reaction between hydrochloric acid
and a solution of sodium hydroxide.
HCl(aq) + NaOH(aq)
NaCl(aq) + H2O(l)
 Hydrochloric acid, sodium hydroxide and sodium
chloride are ionic compounds.
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)
(aq)
from HCl(aq)
from NaOH(aq)
Na+(aq) + Clfrom NaCl(aq)
+ H2O(l)
Reactions between acids and
alkalis
 In this reaction, the Na+ ions and Cl- ions do not
change and are not included.
H+(aq) + OH-(aq)
H2O(l)
Reactions between insoluble
ionic compounds and acids
 An example is the reaction between sulfuric acid and
copper(II) oxide.
CuO(s) + H2SO4(aq)
CuSO4(aq) + H2O(l)
 Sulfuric acid and copper(II) sulfate are aqueous
solutions containing H+ ions, Cu2+ ions and SO42ions.
 However, the SO42- ions do not change and so are not
included.
CuO(s) + 2H+(aq)
Cu2+(aq) + H2O(l)
Reactions producing
precipitates
 An example is the reaction
between copper(II) sulfate
solution and sodium
hydroxide solution.
CuSO4(aq) + 2NaOH(aq)
Cu(OH)2(s) + Na2SO4(aq)
 In the reaction, the Cu2+
ions and OH- ions react to
produce the precipitate.
Reactions producing
precipitates
 The Na+ ions and SO42- ions do not react.
 Therefore, the ionic equation is:
Cu2+(aq) + 2OH-(aq)
Cu(OH)2(s)
Displacement reactions
 An example is the reaction between zinc and copper(II)
sulfate solution.
Zn(s) + CuSO4(aq)
ZnSO4(aq) + Cu(s)
 The CuSO4 and ZnSO4 consist of separate ions in
solution.
Zn(s) + Cu2+(aq) + SO42-(aq)
Cu(s) from
CuSO4(aq)
Zn2+(aq) + SO42-(aq) +
from
ZnSO4(aq)
 As the SO42- ions do not change, they are cancelled.
Zn(s) + Cu2+(aq)
Zn2+(aq) + Cu(s)
Skills Practice
Answers
 1. Mg(s) + 2H+(aq)
acid is
Mg2+(aq) + H2(g)
The ionic equation for the reaction between magnesium and sulfuric
the same as that for the reaction between magnesium and
hydrochloric
acid. The Cl-(aq) and SO42-(aq) ions are only spectator ions. As they
are not
involved in the reaction, they are omitted from the ionic equation.
 2. (a) Zn(s) + 2H+(aq)
(b) MgO(s) + 2H+(aq)
(c) Fe3+(aq) + 3OH-(aq)
(d) Cu(s) + 2Ag+(aq)
(e) Cl2(g/aq) + 2I-(aq)
Zn2+(aq) + H2(g)
Mg2+(aq) + H2O(l)
Fe(OH)3(s)
Cu2+(aq) + 2Ag(s)
2Cl-(aq) + I2(aq)