Unit 4 – Environmental
Chemistry
Acids and Bases
Definitions

There are many different definitions of what makes acids and bases

We will be using the Arrhenius Definition
Arrhenius Acid

Produces a hydrogen ion, H+ ion, or hydronium ion (H3O+) when dissolved in
water. The general formula is:
HA(aq) + H2O(l)  A-(aq) + H3O+(aq)

HA is a polar covalent molecule that contains a proton (H+) that easily
dissociates and bonds with a water molecule.

H = Hydrogen

A = Anything that is negatively charged
Examples of Arrhenius Acids
Name
Formula
Reaction in Water
(Dissociation Reaction)
Hydrofluoric Acid
HF(aq)
HF(aq) + H2O(l)  F-(aq) + H3O+(aq)
Hydrochloric Acid
HCl(aq)
Hydrobromic Acid
Hydroiodic Acid
Examples of Arrhenius Acids – Polyatomic
Ions
Name
Formula
Reaction in Water
(Dissociation Reaction)
Nitric Acid
HNO3(aq)
HNO3(aq) + H2O(l)  NO3-(aq) + H3O+(aq)
Carbonic Acid
Sulfuric Acid
Phosphoric Acid
H2SO4(aq)
Arrhenius Base

Produces a hydroxide ion (OH-) when dissolve in water. There are two ways
this can happen
1. A hydroxide ion dissociates from an ionic compound:
BOH(aq)  B+(aq) + OH-(aq)
2. A covalent molecule WITH A LONE PAIR (i.e. a Nitrogen atom) steals a proton
from a water molecule
B(aq) + H2O(l)  BH+(aq) + OH-(aq)
Examples of Bases – Type 1 (Ionic
Hydroxide)
Name
Formula
Reaction in Water
Lithium Hydroxide
LiOH
LiOH(s)  Li+(aq) + OH-(aq)
Sodium Hydroxide
NaOH
Magnesium Hydroxide
Mg(OH)2
Beryllium Hydroxide
Calcium Hydroxide
Examples of Bases – Type 2 (Nitrogenous)
Name
Formula
Reaction in Water
Ammonia
NH3
NH3 + H2O  NH4+(aq) + OH-(aq)
Hydrazine
(dinitrogen tetrahydride)
N2H4
The pH Scale
The pH Scale

The pH of a solution is a measure of how acidic or how basic something

The pH scale is a scale ranging from 0-14, 0 being the most acidic and 14
being the most basic.

If a solution is neither acidic nor basic, it is said to be neutral. Neutral
solutions have a pH of 7. Pure water has a pH of 7
Water

Water is not strictly H2O molecules, it also contains H3O+ and OH- ions at
low concentrations due to ionization
H2O + H2O  H3O+ + OH
Pure water has these ions in equal concentrations of:
[H3O+] = [OH-] = 1×10-7 mol/L

pH (power of hydronium ions) is a measure of H3O+ ions within a solution.

pH is a (negative) logarithmic scale… each number differs by a power of a 10

That is: pH = 1 is 1000 times more acid than pH 4
Strong and Weak Acids

The strength of an acid referes to its ability to dissociate in a solution.\

Consider: HCl


Write the dissociation reaction
In this case, 100% of the HCl will dissociate in water. The concentration of
H3O+ ions will be equal to the initial amount of HCl present

There is no HCl left at the end of the reaction

Some strong acids:

HCl

HNO3

H2SO4

HBr

HI

HClO4

BUT, some acids (like acetic acid, vinegar) do not dissociate very well (i.e.
most CARBOXYLIC ACIDS).
RXN:


In acetic acid the dissociation of acetic acid is only about 0.5%
Other weak acids include HNO, HCN, HF

In a similar fashion, there are strong and weak bases

Some strong bases:


LiOH, NaOH, KOH, Ca(OH)2, Ba(OH)2
Some weak bases:

Include all amines (organic bases)

NH3, CH3NH2, (CH3)2NH
Neutralization Reactions
Acid-Base Indicators

An indicator is a substance that changes colour depending on whether it is in
an acid or a base. You only need a drop for dramatic colour changes
Acid-Base Neutralization

Neutralization reactions are a type of double displacement reaction
Acid + Base  Salt + Water
Example: Hydrochloric Acid and Potassium Hydroxide
Neutralization reactions are important for many chemical reactions, including in
environmental chemistry and throughout human systems
Acid Rain

Normal rain is slightly acidic (pH = 5.6) due to natural levels of CO2 in the air

CO2 reacts with water to yield dilute carbonic acid

Normal occurrences of CO2 include: respiration, forest fires, volcanic eruptions

Nitrogen oxides from lightning strikes and plant decay also decrease pH by
creating nitric acid (strong) and nitrous acid (weak) when they react with
water

Sulfur oxides from volcanic eruptions also make sulfuric acid (strong) and
sulfurous acid (weak) when they react with water

Acid precipitation: any form of natural precipitation (rain, snow, etc.) that
has an unusually high acidity (pH < 5.6)

Acid Deposition: acid forming pollutants, including acid precipitation as well
as dust, and other dry particulate matter

Acid rain is harmful to the environment: makes waters acidic and cause them
to absorb aluminum that makes its way from soil into lakes and streams;
damages trees and wildlife; damages building materials (particularly
concrete) and sculptures
Hard vs. Soft Water
Drinking Water

Undesirable materials need to be removed from water before it is fit for
consumption


Colloidal clay, microscopic organisms, chemicals which cause tastes or odours, and
acidic substances
Large filtration process: sendimentation, sand filter beds, chlorine to kill
bacteria, small amounts of sulfur dioxide, aluminum sulphate, carbon slurries,
and lime slurries
Hard and Soft Water

Compounds dissolve in water with a variety of consequences

Water which readily gives a lather with soap (not detergents) is described as soft
water

Dissolved substances make water hard

Wastes soap a causes soap scum – insoluble calcium and magnesium salts formed by a
reaction of soap molecules and calcium and magnesium ions


Usually from chalk and limestone (naturally); or Epsom salts (magnesium sulphates)
Hard water can be made soft by removing the dissolved calcium and magnesium
ions (precipitation or other types of reactions); also sometimes through boiling

Hard water is not always bad, contains dissolved calcium, etc., which is useful in bone
development
Plastics and the Environment
Atmospheric Pollutants