ACIDS & BASES

Acids and Bases reactions occur in everyday life and are essential for understanding our world.

How does pH value affect our environment?

Why is it important to monitor and maintain the pH of the water in aquariums, soil and our blood?

What exactly is pH? How is it measured?

Milk of magnesia is a medicine that usually relieves uncomfortable gastrointestinal symptoms within 30 minutes and constipation within six hours.Why is the milk of magnesia an antacid?

Keywords Acidity Basicity (Monoprotic, diprotic, triprotic) Bronsted-Lowry Theory - Proton donor/acceptor- Acid-base Conjugate pair- Amphiprotic Lewis Theory - Lone pair electrons - Dative/Coordinate bond- Dissociation constant (Ka)- Enthalpy of neutralisation

What is an acid?

A solution that contains __________ ions (protons).

OLD THEORY

Weak acid like ____________does not have the power to neutralise strong acid like sodium hydroxide.

hydrogen

ethanoic acid

What is a base/alkali?• A base is a substance like ___________ and

______________that react with acid to form salt and water only.

• An alkali is a soluble base which in solution produces ________ ions.

• Most bases are insoluble in water. 3 soluble bases are NaO/NaOH, KO/KOH, CaO/Ca(OH)2

Both acids and alkalis are _______.

metal oxidemetal hydroxide

hydroxide

soluble

What causes acidity?• It is the _____________that give an acid its acidic properties when they dissolve in water and _________ into ions.

E.g. HCl gas is a _________ compound.

When dissolves in water, it forms HCl acid which dissociate to form ions.

hydrogen ionsdissociate

covalent

What is basicity (proticity)?Basicity• refers to the no.of _________ atoms in one molecule of acid that can be replaced by a ______.

E.g. HCl (monobasic), H2SO4(dibasic), H3PO4(tribasic)

hydrogenmetal

Bronsted-Lowry theory

An acid is defined as a molecule or ion that acts as a proton ______.

A base is defned as a molecule or ion that acts as a proton ________.

donor

acceptor

Types of acids Acids that have single proton to donate – ___________. E.g. HCl(aq), HNO3(aq), HNO2(aq) Acids that have 2 protons to donate –

__________ . E.g. H2SO4(aq), H2SO3(aq), H2CO3(aq) H3PO4(aq) is _________.

monobasic

dibasic

tribasic

Hydrogen chloride gas dissolved in water (solvent)

HCl(g) + H2O(l) H3O+(aq) + Cl-(aq)

The equation can be split into(i) HCl(aq) Cl-(aq) + H+(aq)

(ii) H2O(l) + H+(aq) H3O+(aq)

Acidic behaviour is a transfer reaction in different solvents.

acid conjugate base

base conjugate acid

CH3COOH(l) + H2O(l) H3O+ (aq) + CH3OOO-(aq)

donates H+ acid base

Acid-base conjugate pair

conjugate base

conjugate acid

NH3(g) + H2O(l) NH4+(aq) + OH-(aq)

Water is sometimes described as _______________ because it can accept or donate a proton.

donates H+

amphiprotic

Acid Strength & pKa

Acid strength is the tendency of an acid to donate a proton.

The more readily a compound donates a proton, the stronger is an acid.

AcidityAcidity is measured by an equilibrium constant, Keq.When a Bronsted-Lowry acid H-A is dissolved in

water, an acid-base reaction occurs, and an equilibrium constant can be written for the reaction.

H H-A + H-O-H A- + H-O-H

:Keq

=[products] [reactants]

[H3O+][A -] [HA][H2 O]

=

Acidity and pKa

The concentration of the solvent H2O is essentially constant,

More convenient when describing acid strength to use “pKa” values than Ka.

=[H2 O]Keq

[H3O+][A -] [HA

]=Dissociation constant, Ka

Competition between acid/base and its conjugate(i) HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) acid base conjugate acid conjugate

base

(ii) CH3COOH(l) + H2O(l) H3O+ (aq) + CH3OOO-(aq)

acid base conjugate acid conjugate base

(i) Water is a much stronger base than chloride ion and has a stronger tendency to accept _______.The equilibrium shifts more to the _______.

(ii) Ethanote ion (CH3OOO-) is a much stronger base than water molecule. The equilbrium shifts to the _______.

protonsright

left

Strong acids have weak conjugate bases. Weak acids have strong conjugate bases.

(i) HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) acid base conjugate acid conjugate base

(ii) CH3COOH(l) + H2O(l) H3O+ (aq) + CH3OOO-

(aq) acid base conjugate acid

conjugate base

If HA is a strong acid in water, HA is a successful donor of H+ in water the reverse reaction hardly happens A- is a poor acceptor of H+ Ka (dissociation constant) is big

HA + H2O H3O + + A-

Equilibrium lies to the right. Strong acid , weak conjugate base

Weak acid , strong conjugate base. Equilibrium lies to the left

Common acids & conjugate bases in order of strengths

Lewis theory A Lewis acid is defined as a substance

that can accept a pair of _________ from another atom to form a _______(coordinate) covalent bond.

A Lewis base is defined as a substance that can __________ a pair of electrons to another atom to form a dative covalent bond.

B: H+ +BHLewis _____ Lewis ______

electronsdative

donate

acidbase

Lewis theory Boroader definition that include

compounds that do not have protons but exhibit acid/base behaviour.

E.g. AlCl3(aq) + Cl-(aq) AlCl4-(aq) Lewis acid Lewis base

Electron rich species react with electron poor species.

All Bronsted Lowry acids are also Lewis acids

Examples

Reaction between ammonia, NH3 and proton

H3N: H+ +NH4 Reaction between NH3 and BF3. H F H F H N B H N B H F F H F FBF3 is a good Lewis ______ as there are _______electrons around the central

boron atom which leaves room for 2 more electrons. Other common Lewis acid includes AlCl 3 and transition metal ions in aqueous solution.

acid 6

Reaction between a water molecule and proton

H2O: H+ H3O+

Lewis bondingIn complex ions formed by transition metals

The 6 water molecules, each donate a lone pair electrons from oxygen of their water molecules to (the empty 3d orbitals of) iron.

What does each water molecule and iron(III) ion act as in the reaction above?

Water acts as Lewis base. Fe(III) acts as Lewis acid

Dative (Coordinate) bond A dative covalent bond is always formed

in a Lewis acid-base reaction. For a substance to act as a base, it must

have space to accept the _________of electrons.

lone pair

Strong and weak acids and bases

Strong acid When strong acid (HA)

dissolves, virtually all acid molecules react with the water to produce hydronium ions (H3O+).

HA + H2O(l) H3O+(aq) + A-(aq) or HA H+(aq) + A-(aq) 0% 100% 0% 100%

Examples : HCl, H2SO4,HNO3, HClO4

Strong and weak acids and bases

Weak acid When a weak acid dissolves in

water, only a small % of its molecules (typically 1%) react with water molecules to release hydrogen or hydronium ions. The equilibrium lies on the ________ side of the equation.

HA + H2O(l) H3O+(aq) + A-(aq) or HA H+(aq) + A-(aq) 99% 1% 99% 1%

Examples : CH3COOH, aqueous carbon dioxide

left

Distinguish between strong and weak acids

Base on the information above, how do we distinguish between strong and weak acids of the same concentration (e.g. HCl and CH3COOH)?

0.1 mol dm-3 HCl(aq)

0.1 mol dm-3 CH3COOH (aq)

[H+(aq)] 0.1 mol dm-3 0.0013 mol dm-3 pH 1.00 2.89Electrical conductivity high lowRelative rate of reaction with magnesium

fast slow

Relative rate of reaction with calcium carbonate

fast slow

How to distinguish between strong and weak acids? A weak acid has a lower concentration of ___ and hence a

higher _____ than a stronger acid of the same concentration. Due to the lower concentration of hydrogen ions, a weak acid

has poorer ___________________ than a stronger acid of the same concentration (equimolar).

Weak acids react more _______ with reactive metals, metal oxides, metal carbonates and metal hydrogencarbonates than strong acids of the same concentration.

Strong and weak acids can also be distnguished by measuring and comparing their enthalpies of neutralisation.

What is the difference between the strength (strong and weak) and the concentrated (concentrated or dilute)?

H+ pH

electrical conductivity

slowly

Strong and weak acids and bases

Strong base A strong base undergoes almost 100%

____________________ when in dilute aqueous solution.

BOH B+(aq) + OH-(aq) 0% 100%

Examples : NaOH, KOH, Ba(OH)2

dissociation / ionisation

Strong and weak acids and bases

Weak base All bases are weak except the hydroxides of

groups _______ in the Periodic Table. In general for a weak molecular base, BOH

The equiibrium lies on the _____ side of the equation.

BOH + (aq) B+(aq) + OH-

(aq)

Examples : aqueous ammonia, ethylamine, caffeine, bases of nuclei acids

1 and 2

left

The pH (power of hydrogen) indicator

scale that measures the ________ of an acid and alkali.

pH of a substance is measured when it is dissolved in water.

[H+] = 1 x 10-n moldm-3 ( n = pH number)

strength

The pH Scale

pH probe and meterAn accurate method of measuring pH value.A pH probe is dipped into the solution being

testedand the pH value is then read directly from

the meter.

pH Calculation pH is a measure of the concentration of H+ ions

in a solution. pH = -log10[H+(aq)]

Example:If the concentration of H+ is (a) 1.0 x 10-3 moldm-3 (b) 1.0 x 10-2 moldm-3 (c) 2.50 x 10-3 moldm-3 , what is the pH?Compare (a) & (b)

2.60

Example:

Calculate the concentration of H+ of a solution that has a pH = 3.2.

6.31 x 10-4

Example:

Calculate the concentration of H+ and hence the pH of a 1.00 x 10-3 moldm-3 NaOH

Example:

(a) What is the pH of 10cm3 of 0.1 moldm-3 HCl?

(b) If 90cm3 of water is added to the acid, what happens to the pH? (c) If the solution from (b) is diluted by a factor of 105 , what is the approximate pH?

[(a)1,(b)2,(c)7]

Buffer A buffer resists changes in _____ when

small amounts of acid and alkali are added to it.

pH

Acidic Buffer An acidic buffer solution can be made by

mixing a weak ______ together with the _______ of the weak acid and a strong _______.

(1) CH3COOH(aq) H +(aq) + CH3COO-

(aq)

(2) CH3COONa(aq) Na+(aq) + CH3COO-

(aq)

acidsaltbase

Acidic Buffer If an acid is added, the extra H+ from the

acid react with the excess ethanoate ions in (2) and are _________ from the solution as ethanoic acid molecules (these have no effect on the pH). Hence the pH stays the same.

CH3COO-(aq) + H +(aq) CH3COOH(aq)

new

removed

Acidic Buffer If an alkali is added, the OH- from the alkali

react with the ________ions from (1) removing them from the right hand side. There is, however, a large reservoir of ethanoic acid on the left hand side of this equilibrium able to dissociate and make more hydrogen ions, restoring the pH.

CH3COOH(aq)+OH -(aq) CH3COO-(aq)+H2O(l)

hydrogen

Alkali Buffer An alkali buffer with a fixed pH greater

than 7 can be made from a weak base together with the salt of the base with a strong acid.

E.g. Ammonia and ammonium chlorideNH4Cl(aq) NH4

+(aq) + Cl-(aq)NH3(aq) + H2O(l) NH4

+(aq) + OH-(aq)

Alkali Buffer(1) NH4Cl(aq) NH4

+(aq) + Cl-(aq)(2) NH3(aq) + H2O(l) NH4

+(aq) + OH-(aq)If H+ ions are added they will combine with _____ (from 2) to form

water and more of the ammonia will dissociate to replace themAdding more OH- ions that can react with the free ____(from 1)

producing more ammonia (as in 2) and effectively being removed from the system. The ammonia molecules have no effect on ____ an therefore the pH remains the same.

In both cases, the hydroxide ion concentration and the hydrogen ion concentration remain constant.

OH-

NH+

pH