acids and bases a guide for a level students knockhardy publishing
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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
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
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 & BasesAcid & Bases
Before you start it would be helpful to…
• know the simple properties of acids, bases and alkalis
Acid & BasesAcid & Bases
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
BRØNSTED-LOWRY THEORY
ACID proton donor HCl ——> H+(aq) + Cl¯(aq)
BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)
Conjugate systemsAcids are related to bases ACID PROTON + CONJUGATE BASE
Bases are related to acids BASE + PROTON CONJUGATE ACID
ACIDS AND BASESACIDS AND BASES
BRØNSTED-LOWRY THEORY
ACID proton donor HCl ——> H+(aq) + Cl¯(aq)
BASE proton acceptor NH3 (aq) + H+(aq) ——> NH4+(aq)
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
ACIDS AND BASESACIDS AND BASES
LEWIS THEORY
ACID lone pair acceptor BF3 H+ AlCl3
BASE lone pair donor NH3 H2O
ACIDS AND BASESACIDS AND BASES
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)
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)]
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)]
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 - KIonic product of water - Kww
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 - KIonic product of water - Kww
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 - KIonic product of water - Kww
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 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
Click on the button toClick on the button toreturn to the menureturn to the menu
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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