ACIDS AND BASES

…for it cannot be

But I am pigeon-liver’d and lack gall

To make oppression bitter… Hamlet

Learning objectives

Describe properties of acids and bases

Define acid and base using Arrhenius and Brønsted definitions

Identify Brønsted acids and bases in solution equilibria

Distinguish between strength and concentration of acids and bases

Estimate pH of acids and bases

Describe phenomenon of acid rain

Thinking about what is an acid

Acids are sour

Acids attack metals

Acids react with

bases and form salts

Acids turn litmus red

Common acids

Thinking about what makes a

base Bases taste bitter

Bases are slippery

Bases react with

acids to form salts

Bases turn litmus blue

Common bases

Acids and bases cancel:

Neutralization Neutralization involves reaction of acid with base:

ACID + BASE = SALT + WATER

Example in nature: Reaction of carbonic acid (rainwater -

CO2 in H2O) with the ocean to give limestone:

H2CO3 + Ca(OH)2 = CaCO3 + 2H2O

Arrhenius: it’s about water

The meaning of acid and base has

changed over the years

Arrhenius acid is one that generates

protons when dissolved in water

Arrhenius base is one that generates

hydroxide ions when dissolved in water

Hydronium ion is active ingredient

of acid in water

Protons (H+) do not exist in solution

CH3CO2H + H2O = H3O+ + CH3CO2

-

Vinegar in water produces hydronium ions

Hydroxide ion is active ingredient of

base in water

NH3 + H2O = NH4+ + OH-

Ammonia, a base, dissolves in water and

produces hydroxide ions

The essence of neutralization

Elimination of the components of acid and

base by combination to give H2O

H+ + OH- H2O

ACID BASE

Brønsted and Lowry:

All about protons

Broader definition of acids and bases

Reaction NH3 + HCl = NH4Cl has all

elements of acid-base neutralization but

no H2O

Brønsted acid donates a proton

Brønsted base accepts a proton

Brønsted acid

HCl + H2O = H3O+ + Cl-

Brønsted base

NH3 + H2O = NH4+ + OH-

water

NH3 + HCl = NH4+Cl-

No water

Substances can be both acids and

bases – depends on environment

Note that in one instance H2O behaves

like a base – accepting protons, and in

another, behaves like an acid – donating

protons

HCl + H2O = H3O+ + Cl-

In presence of an acid H2O is a base

NH3 + H2O = NH4+ + OH-

In presence of a base H2O is an acid

The products are themselves acids

and bases

Equilibrium: solution contains

mixture of all components

Identifying acids and bases:

Follow the protons

Salts

Products of acid-base neutralization

Contain metal cation and nonmetal anion

Acid + base = salt + water

HCl + NaOH = NaCl + H2O

HCl + KOH = KCl + H2O

HNO3 + KOH = KNO3 + H2O

2HCl + Ca(OH)2 = CaCl2 + 2H2O

HCN + NaOH = NaCN + H2O

Strong coffee (or concentrated?)

Equilibrium: not all acids

completely donate protons

to water molecules

HA + H2O A- + H3O+

Strength: Degree of

ionization

Concentration: Number of

moles per unit volume

- +

2 3HA + H O = A + H O

Strong and weak

Strong acid (HCl) – Fully ionized

equilibrium to right

All H+ and Cl-

– Corrosive

Weak acid (Acetic) – Weakly ionized

Equilibrium to left

Mostly CH3COOH

– Edible

- +

2 3HA + H O = A + H O

- +

2 3HA + H O = A + H O

Changing concentration does not

change strength Strength refers to degree of ionization: – Strong is completely ionized (100 %)

– Weak is partly ionized (1 % - 1:106)

Concentration refers to number of moles per unit volume

An acid (or base) can be strong and concentrated, weak and concentrated, strong and dilute, weak and dilute

- +

2 3HA + H O = A + H O

Ionization of water

Even in pure water some molecules are

ionized

Concentrations of OH- and H3O+ are equal

[H3O+] = [OH-]

Concentration

- +

2 2 3H O + H O = OH + H O

Equilibrium constant: in all aqueous solutions,

product of concentrations is constant

[H3O+][OH-] = constant

Add acid or base alters balance of [H3O

+] and [OH-]

Increasing [H3O

+] decreases [OH-]

(acidic conditions)

Increasing [OH-] decreases [H3O+]

(basic conditions)

The pH scale – reduces large

range of numbers to small

In water [H3O+][OH-] = 10-14

pH = - log10[H3O+]

Range of [H3O+]:

– 10 M (conc acid) – 10-15 M (conc base)

Range of pH:

– -1 (conc acid) to +15 (conc base)

Low pH = acid; high pH = basic

pH = 7 = neutral

pH scale and common substances

Relating pH to [H3O+]

For pH, take exponent of [H3O+], change sign

– Acid HCl(aq): [H3O+] = 1 x 10-1 M, pH = 1

– Pure H2O: [H3O+] = 1 x 10-7 M, pH = 7

– Base: NH3(aq): [H3O+] = 1 x 10-11 M, pH = 11

Note: change of 1 unit in pH is factor of ten in

[H3O+]

When [H3O+] is 1 x 10x, pH is whole number

Estimating pH

When [H3O+] is not 1 x 10x M pH is not

whole number

Estimating pH is often more useful than

doing exact calculations

Smaller pH value means larger H+

concentration

Estimating pH

Acidity and the environment

Rain is naturally weakly acidic because of CO2

Alkaline rocks – limestone – neutralize the acid

Granite substrate does not

neutralize the acid

Ocean water is naturally alkaline

Acid Rain

Acid rain is polluted by acid

in the atmosphere. Two

common pollutants acidify

rain: sulphur dioxide (SO2)

and nitrogen oxides (NOX)

Following information from

The Green LaneTM,

Environment Canada's

World Wide Web site -

www.ec.gc.ca/regeng.html

What’s the big deal?

Damage to aquatic

life

Damage to buildings

Damage to forests

Damage to air quality

Source of the problem

Sulphur dioxide (SO2)

byproduct of industrial

processes and

burning fossil fuels.

– Ore smelting

– coal-fired power

generators

– natural gas processing

Where do NOX emissions come

from?

Main source of NOX is

combustion of fuels in

motor vehicles,

residential and

commercial furnaces,

industrial and electrical-

utility boilers and engines.

NOX emissions were 2.5

million tonnes in 2000.

U.S. NOX emissions for

2000 were 21 million

tonnes.

Legislative success with acid rain

Eastern Canada Acid Rain program committed Canada to cap SO2 emissions at 2.3 million tonnes by 1994 - 40% reduction from 1980 levels

Targets achieved or exceeded

By 2001, emissions were 63% reduction from 1980 levels.

Would acid rain remain a problem

without further controls?

Yes. That is why The Canada-Wide Acid Rain

Strategy for Post-2000 calls for further emission

reductions in both Canada and the United

States.

In total, without further controls, almost 800,000

km2 in southeastern Canada-an area the size of

France and the United Kingdom combined-

would receive harmful levels of acid rain; that is,

levels well above critical load limits for aquatic

systems.