ACIDS AND BASES
A guide for A level students
KNOCKHARDY PUBLISHING
2008 SPECIFICATIONS
Acid & Bases
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm

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CONTENTS
Brønsted-Lowry theory of acids and bases
Lewis theory of acids and bases
Strong acids and bases
Weak acids
Weak bases
Hydrogen ion concentration and pH
Ionic product of water Kw
Relation between pH and pOH
Introduction to buffer solutions
Check list
Acid & Bases
Before you start it would be helpful to…

know the simple properties of acids, bases and alkalis
Acid & Bases
BRØNSTED-LOWRY THEORY

ACID proton donor HCl ——> H+(aq) + Cl¯(aq)

BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)

ACIDS AND BASES
BRØNSTED-LOWRY THEORY

ACID proton donor HCl ——> H+(aq) + Cl¯(aq)

BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)


Conjugate systems
Acids are related to bases ACID PROTON + CONJUGATE BASE

Bases are related to acids BASE + PROTON CONJUGATE ACID

ACIDS AND BASES
BRØNSTED-LOWRY THEORY

ACID proton donor HCl ——> H+(aq) + Cl¯(aq)

BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)


Conjugate systems
Acids are related to bases ACID PROTON + CONJUGATE BASE

Bases are related to acids BASE + PROTON CONJUGATE ACID


For an acid to behave as an acid, it must have a base present to accept a proton...

HA + B BH+ + A¯
acid base conjugate conjugate
acid base

example CH3COO¯ + H2O CH3COOH + OH¯
base acid acid base
ACIDS AND BASES
LEWIS THEORY

ACID lone pair acceptor BF3 H+ AlCl3

BASE lone pair donor NH3 H2O
ACIDS AND BASES
LONE PAIR DONOR
LONE PAIR
ACCEPTOR
LONE PAIR DONOR
LONE PAIR ACCEPTOR
STRONG
ACIDS completely dissociate (split up) into ions in aqueous solution

e.g. HCl ——> H+(aq) + Cl¯(aq) MONOPROTIC 1 replaceable H
HNO3 ——> H+(aq) + NO3¯(aq)

H2SO4 ——> 2H+(aq) + SO42-(aq) DIPROTIC 2 replaceable H’s
STRONG ACIDS AND BASES
STRONG
ACIDS completely dissociate (split up) into ions in aqueous solution

e.g. HCl ——> H+(aq) + Cl¯(aq) MONOPROTIC 1 replaceable H
HNO3 ——> H+(aq) + NO3¯(aq)

H2SO4 ——> 2H+(aq) + SO42-(aq) DIPROTIC 2 replaceable H’s




STRONG
BASES completely dissociate into ions in aqueous solution

e.g. NaOH ——> Na+(aq) + OH¯(aq)
STRONG ACIDS AND BASES
Weak acids partially dissociate into ions in aqueous solution

e.g. ethanoic acid CH3COOH(aq) CH3COO¯(aq) + H+(aq)

When a weak acid dissolves in
water an equilibrium is set up HA(aq) + H2O(l) A¯(aq) + H3O+(aq)

The water stabilises the ions

To make calculations easier
the dissociation can be written... HA(aq) A¯(aq) + H+(aq)


WEAK ACIDS
Weak acids partially dissociate into ions in aqueous solution

e.g. ethanoic acid CH3COOH(aq) CH3COO¯(aq) + H+(aq)

When a weak acid dissolves in
water an equilibrium is set up HA(aq) + H2O(l) A¯(aq) + H3O+(aq)

The water stabilises the ions

To make calculations easier
the dissociation can be written... HA(aq) A¯(aq) + H+(aq)



The weaker the acid the less it dissociates
the more the equilibrium lies to the left.

The relative strengths of acids can be expressed as Ka or pKa values

The dissociation constant for the weak acid HA is Ka = [H+(aq)] [A¯(aq)] mol dm-3
[HA(aq)]
WEAK ACIDS
Partially react with water to give ions in aqueous solution e.g. ammonia

When a weak base dissolves in water an equilibrium is set up

NH3 (aq) + H2O (l) NH4+ (aq) + OH¯ (aq)

as in the case of acids it is more simply written

NH3 (aq) + H+ (aq) NH4+ (aq)

WEAK BASES
Partially react with water to give ions in aqueous solution e.g. ammonia

When a weak base dissolves in water an equilibrium is set up

NH3 (aq) + H2O (l) NH4+ (aq) + OH¯ (aq)

as in the case of acids it is more simply written

NH3 (aq) + H+ (aq) NH4+ (aq)



The weaker the base the less it dissociates
the more the equilibrium lies to the left


The relative strengths of bases can be expressed as Kb or pKb values.
WEAK BASES
Hydrogen ion concentration [H+(aq)]
Introduction hydrogen ion concentration determines the acidity of a solution
hydroxide ion concentration determines the alkalinity

for strong acids and bases the concentration of ions is very much
larger than their weaker counterparts which only partially dissociate.
Hydrogen ion concentration [H+(aq)]
pH hydrogen ion concentration can be converted to pH pH = - log10 [H+(aq)]

to convert pH into hydrogen ion concentration [H+(aq)] = antilog (-pH)


pOH An equivalent calculation for bases converts
the hydroxide ion concentration to pOH pOH = - log10 [OH¯(aq)]

in both the above, [ ] represents the concentration in mol dm-3
Ionic product of water - Kw
Despite being covalent, water conducts electricity to a very small extent.

This is due to the slight ionisation ... H2O(l) + H2O(l) H3O+(aq) + OH¯(aq)
or, more simply H2O(l) H+(aq) + OH¯(aq)
Ionic product of water - Kw
Despite being covalent, water conducts electricity to a very small extent.

This is due to the slight ionisation ... H2O(l) + H2O(l) H3O+(aq) + OH¯(aq)
or, more simply H2O(l) H+(aq) + OH¯(aq)

Applying the equilibrium law
to the second equation gives Kc = [H+(aq)] [OH¯(aq)]
[ ] is the equilibrium concentration in mol dm-3 [H2O(l)]
Ionic product of water - Kw
Despite being covalent, water conducts electricity to a very small extent.

This is due to the slight ionisation ... H2O(l) + H2O(l) H3O+(aq) + OH¯(aq)
or, more simply H2O(l) H+(aq) + OH¯(aq)

Applying the equilibrium law
to the second equation gives Kc = [H+(aq)] [OH¯(aq)]
[ ] is the equilibrium concentration in mol dm-3 [H2O(l)]

As the dissociation is small, the water concentration is very large compared with the dissociated ions and any changes to its value are insignificant; its concentration can be regarded as constant.

This “constant” is combined with
(Kc) to get a new constant (Kw). Kw = [H+(aq)] [OH¯(aq)] mol2 dm-6
= 1 x 10-14 mol2 dm-6 (at 25°C)


Because the constant is based on an equilibrium, Kw VARIES WITH TEMPERATURE
Ionic product of water - Kw
The value of Kw varies with temperature because it is based on an equilibrium.

Temperature / °C 0 20 25 30 60
Kw / 1 x 10-14 mol2 dm-6 0.11 0.68 1.0 1.47 5.6
H+ / x 10-7 mol dm-3 0.33 0.82 1.0 1.27 2.37
pH 7.48 7.08 7 6.92 6.63
What does this tell you about the equation H2O(l) H+(aq) + OH¯(aq) ?
Ionic product of water - Kw
The value of Kw varies with temperature because it is based on an equilibrium.

Temperature / °C 0 20 25 30 60
Kw / 1 x 10-14 mol2 dm-6 0.11 0.68 1.0 1.47 5.6
H+ / x 10-7 mol dm-3 0.33 0.82 1.0 1.27 2.37
pH 7.48 7.08 7 6.92 6.63
What does this tell you about the equation H2O(l) H+(aq) + OH¯(aq) ?

• Kw gets larger as the temperature increases
• this means the concentration of H+ and OH¯ ions gets greater
• this means the equilibrium has moved to the right
• if the concentration of H+ increases then the pH decreases
• pH decreases as the temperature increases
Ionic product of water - Kw
The value of Kw varies with temperature because it is based on an equilibrium.

Temperature / °C 0 20 25 30 60
Kw / 1 x 10-14 mol2 dm-6 0.11 0.68 1.0 1.47 5.6
H+ / x 10-7 mol dm-3 0.33 0.82 1.0 1.27 2.37
pH 7.48 7.08 7 6.92 6.63
What does this tell you about the equation H2O(l) H+(aq) + OH¯(aq) ?

• Kw gets larger as the temperature increases
• this means the concentration of H+ and OH¯ ions gets greater
• this means the equilibrium has moved to the right
• if the concentration of H+ increases then the pH decreases
• pH decreases as the temperature increases

Because the equation moves to the right as the
temperature goes up, it must be an ENDOTHERMIC process
Relationship between pH and pOH
Because H+ and OH¯ ions are produced
in equal amounts when water dissociates [H+] = [OH¯] = 1 x 10-7 mol dm-3
their concentrations will be the same.

Kw = [H+] [OH¯] = 1 x 10-14 mol2 dm-6

take logs of both sides log[H+] + log[OH¯] = -14

multiply by minus - log[H+] - log[OH¯] = 14

change to pH and pOH pH + pOH = 14 (at 25°C)
Relationship between pH and pOH
Because H+ and OH¯ ions are produced
in equal amounts when water dissociates [H+] = [OH¯] = 1 x 10-7 mol dm-3
their concentrations will be the same.

Kw = [H+] [OH¯] = 1 x 10-14 mol2 dm-6

take logs of both sides log[H+] + log[OH¯] = -14

multiply by minus - log[H+] - log[OH¯] = 14

change to pH and pOH pH + pOH = 14 (at 25°C)


N.B. As they are based on the position of equilibrium and that varies with
temperature, the above values are only true if the temperature is 25°C (298K)

Neutral solutions may be regarded as those where [H+] = [OH¯].

Therefore a neutral solution is pH 7 only at a temperature of 25°C (298K)

Kw is constant for any aqueous solution at the stated temperature
Buffer solutions - Brief introduction

Definition “Solutions which resist changes in pH when
small quantities of acid or alkali are added.”


Acidic Buffer (pH < 7) made from a weak acid + its sodium or potassium salt ethanoic acid sodium ethanoate


Alkaline Buffer (pH > 7) made from a weak base + its chloride ammonia ammonium chloride


Uses Standardising pH meters
Buffering biological systems (eg in blood)
Maintaining the pH of shampoos
REVISION CHECK
What should you be able to do?
Recall the definition of acids and bases in the Brønsted-Lowry system
Recall the definition of acids and bases in the Lewis system
Recall and explain the difference between strong and weak acids
Recall and explain the difference between strong and weak bases
Recall the definition of pH
Recall the definition of the ionic product of water
Explain how and why pH varies with temperature
Recall the relationship between pH, [H+], [OH¯], pOH and Kw
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ACIDS AND BASES
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© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING