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ACIDS AND BASES A guide for A level students. 2008 SPECIFICATIONS. KNOCKHARDY PUBLISHING. Acid & Bases. INTRODUCTION

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slide1

ACIDS AND BASES

A guide for A level students

2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING

slide2

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

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard

slide3

Acid & Bases

  • 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
slide4

Acid & Bases

  • Before you start it would be helpful to…
  • know the simple properties of acids, bases and alkalis
slide5

ACIDS AND BASES

BRØNSTED-LOWRY THEORY

ACIDproton donor HCl ——> H+(aq) + Cl¯(aq)

BASEproton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)

slide6

ACIDS AND BASES

BRØNSTED-LOWRY THEORY

ACIDproton donor HCl ——> H+(aq) + Cl¯(aq)

BASEproton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)

Conjugatesystems

Acids are related to bases ACID PROTON + CONJUGATE BASE

Bases are related to acids BASE + PROTON CONJUGATE ACID

slide7

ACIDS AND BASES

BRØNSTED-LOWRY THEORY

ACIDproton donor HCl ——> H+(aq) + Cl¯(aq)

BASEproton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)

Conjugatesystems

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

slide8

ACIDS AND BASES

LEWIS THEORY

ACIDlone pair acceptor BF3 H+ AlCl3

BASElone pair donor NH3 H2O

LONE PAIR

ACCEPTOR

LONE PAIR DONOR

LONE PAIR DONOR

LONE PAIR ACCEPTOR

slide9

STRONG ACIDS AND BASES

STRONG

ACIDScompletely 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

slide10

STRONG ACIDS AND BASES

STRONG

ACIDScompletely 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)

slide11

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)

slide12

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)]

slide13

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)

slide14

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.

slide15

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.

slide16

[H+]

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

OH¯

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

pH

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

STRONGLY ACIDIC

WEAKLY ACIDIC

WEAKLY ALKALINE

STRONGLY ALKALINE

NEUTRAL

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

slide17

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)

slide18

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)]

slide19

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

slide20

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 equationH2O(l) H+(aq) + OH¯(aq) ?

slide21

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 equationH2O(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

slide22

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

slide23

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)

slide24

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

slide25

Buffer solutions - Briefintroduction

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

slide26

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

CAN YOU DO ALL OF THESE? YES NO

slide27

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

Click on the button to

return to the menu

slide28

WELL DONE!

Try some past paper questions

slide29

ACIDS AND BASES

THE END

© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING