acids and bases a guide for a level students knockhardy publishing

ACIDS AND ACIDS AND BASESBASES

A guide for A level studentsA guide for A level students

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

Acid & BasesAcid & 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.argonet.co.uk/users/hoptonj/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


Before you start it would be helpful to…

• know the simple properties of acids, bases and alkalis


BRØNSTED-LOWRY THEORY

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

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

ACIDS AND BASESACIDS AND BASES




Conjugate systemsAcids are related to bases ACID PROTON + CONJUGATE BASE

Bases are related to acids BASE + PROTON CONJUGATE ACID





Conjugate systemsAcids 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


LEWIS THEORY

ACID lone pair acceptor BF3 H+ AlCl3

BASE lone pair donor NH3 H2O


LONE PAIR DONOR LONE PAIRACCEPTOR

LONE PAIR DONOR LONE PAIR ACCEPTOR

STRONGACIDS 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 BASESSTRONG ACIDS AND BASES

STRONGACIDS 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

STRONGBASES completely dissociate into ions in aqueous solution

e.g. NaOH ——> Na+(aq) + OH¯(aq)

STRONG ACIDS AND BASESSTRONG 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 inwater 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 ACIDSWEAK 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 inwater 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 ACIDSWEAK 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 BASESWEAK 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 dissociatesthe more the equilibrium lies to the left

The relative strengths of bases can be expressed as Kb or pKb values.

WEAK BASESWEAK BASES

Hydrogen ion concentration [HHydrogen ion concentration [H++(aq)(aq)]]

Introduction hydrogen ion concentration determines the acidity of a solutionhydroxide ion concentration determines the alkalinity

for strong acids and bases the concentration of ions is very muchlarger than their weaker counterparts which only partially dissociate.

Hydrogen ion concentration [HHydrogen ion concentration [H++(aq)(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 convertsthe hydroxide ion concentration to pOH pOH = - log10 [OH¯(aq)]

in both the above, [ ] represents the concentration in mol dm-3

STRONGLY ACIDIC

100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14

10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10-0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14pH

OH¯

[H+]

WEAKLY ACIDIC

NEUTRAL STRONGLY ALKALINE

WEAKLY ALKALINE

Ionic product of water - KIonic product of water - Kww

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 lawto the second equation gives Kc = [H+(aq)] [OH¯(aq)]

[ ] is the equilibrium concentration in mol dm-3 [H2O(l)]





Applying the equilibrium lawto 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


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 / 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


• 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




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


• 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 thetemperature goes up, it must be an ENDOTHERMIC process

Relationship between pH and pOHRelationship between pH and pOH

Because H+ and OH¯ ions are producedin 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 pOHRelationship between pH and pOH

Because H+ and OH¯ ions are producedin 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 withtemperature, 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 - Buffer solutions - BriefBrief introductionintroduction

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 metersBuffering biological systems (eg in blood)Maintaining the pH of shampoos

REVISION CHECKREVISION 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

CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO

You need to go over the You need to go over the relevant topic(s) againrelevant topic(s) again

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WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions

ACIDS AND ACIDS AND BASESBASES

THE ENDTHE END

© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

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