An Introduction to ACIDS AND BASES
Acids and bases have been defined by a variety of methods in the history of science. Initially, they were
defined by their properties. For example, acids produced hydrogen gas when reacted with metals, taste
sour and turn blue litmus red, while bases taste bitter, turn red litmus blue, and feel slippery.
Realizing that a better definition of acids and bases would refer to their chemical behaviour, Svante
Arrhenius defined an acid as a chemical compound which
________________________________________________________________________________
_____________________________________________________ while a base is a compound which
_______________________________________________________________________________
__________________________________________________________. Common examples of Arrhenius
acids and bases are hydrochloric acid, HCl(aq) and sodium hydroxide, NaOH.
While acids form hydrogen ions (_____) in solution, the hydrogen ion is so reactive it does not exist in
solution on its own. It quickly reacts with water to form the hydronium ion (_________). Since hydrogen
ions act chemically the same as hydronium ions, we often use them interchangeably. Write the equation
for the formation of the hydronium ion below.
We can identify if substances are acids or bases using Acid-base indicators. These are actually weak acids
and/or bases that change color when they react
A common indicator, Litmus, is a naturally occurring indicator obtained from seaweed. Most of the
indicators we commonly use in chemistry are synthetic (man-made) compounds.
INDICATOR
INDICATOR COLOURS
ACID COLOUR
BASE COLOUR
LITMUS
BROMOTHYMOL BLUE
PHENOLPHTHALEIN
Universal
(Note: Indicators are actually weak acids and/or bases that change color when they react. This is why it is
important to only add a few drops of an indicator).
Arrhenius acids and bases produce the hydronium and hydroxide ions in solution through dissociation.
Dissociation refers to breaking down in solution. For example, write the dissociation equation for
hydrochloric acid, nitric acid, and sodium hydroxide.
Strong acids and bases are _____________ dissociated. For example, a 1.0 mol/L HCL solution produces
1.0 mol/L of hydronium ions in solution. Weak acids and bases are ______________________________
dissociated. Vinegar, (acetic acid) for example, is approximately 5% dissociated.
What concentration of hydronium ions would therefore be formed from a 1.0 mol/L solution of acetic acid
solution?
Most acids and bases are weak. Some of the important strong acids and bases are hydrochloric acid,
sulphuric acid (H2SO4), nitric acid (HNO3), sodium hydroxide (NaOH), potassium hydroxide (KOH).
A common source of confusion for acids and bases are the terms strong and weak vs. concentrated and
dilute. Strong and weak refer to percent dissociation, while concentrated and dilute refer to concentration.
Problems with the Arrhenius Definitions
Using the Arrhenius definition, we could correctly predict that HCl(aq) and HC2H3O2(aq) are acids.
(They are _____________________ acid and _________________________ acid). We would also
predict that NaOH(aq) and Ca(OH)2(aq) are bases. (They are ________________ hydroxide and
______________ hydroxide). However, what about compounds like CaO(aq), NH3(aq), Na2CO3(aq) and
Al(NO3)3(aq)? We would probably (incorrectly) predict that they are neutral.
Paul Giguere of Laval University, Quebec, revised the definition of acids and bases to say an acid is a
substance that directly reacts with water to produce the hydronium ion (H3O+(aq)) in solution, and bases are
substances that react with water to produce the hydroxide ion (OH-) in solution. All of the above chemicals
somehow react with water to produce these ions. Sometimes these are easy to predict, other times they
are not.
Write the reaction for ammonia with water.
Bronsted-Lowry Theory of Acids and Bases
Research has shown that the revised Arrhenius definitions of acids and bases still did not cover all
substances that show acid and base properties. For example, reactions of acids and bases do not always
include water.
The Bronsted-Lowry theory focuses on the reactions that acids and bases undergo. Acids are
defined as a hydrogen _______________________, and a base is a hydrogen ________________________.
Some examples:
Hydrochloric Acid in Water
Ammonia in water
Nitric Acid in water
Carbonate in water
By the Bronsted-Lowry theory some substances can be amphiprotic. This means a substance can both
________________ or ____________________ hydrogen, depending on what other molecules or ions they
are in solution with.
For example, the hydrogen carbonate ion (HCO3-) acts like a base if it is in a solution that contains
hydronium ions:
It acts like an acid in the presence of hydroxide ions:
The most common amphiprotic molecule is water.
Conjugate Acid-Base Pairs
Molecules or ions related by the transfer of a proton are known as conjugate acid-base pairs. An acid
becomes a conjugate base when a proton is removed.
List the two acid/base pairs from the reactions below.
HBr
+
H2O

H3O+
PH3
+
H2O

PH4+
Br-
+
+
OH-
Reaction of Acids with Metals
In the last unit you reacted various metals with dilute hydrochloric acid. Some metals react; other do not.
How do you tell whether or not a metal will react with dilute hydrochloric acid?
If a metal does react with a dilute acid, the general equation is:
METAL + (DILUTE) ACID  SALT + HYDROGEN
In the case of hydrochloric acid the salts formed will be chlorides, in the case of sulphuric acid, the salts
formed will be sulphates, and in the case of nitric acid, the salts formed will be nitrates.
Write balanced equations for each of the following reactions of metals with (dilute) acids:
1. Magnesium + hydrochloric acid
2. Calcium + sulphuric acid
3. Aluminum + hydrochloric acid
4. Zinc + nitric acid
Reaction of Acids with Carbonates
Another characteristic property of acids is that they liberate carbon dioxide gas when they react with
carbonates. In general:
Carbonate + (dilute) acid  Salt + Carbon dioxide + Water
Write both the word equations and the balanced equations for the reactions of each of the following
carbonates with (dilute) acids:
1. Copper carbonate + sulphuric acid
2. Sodium carbonate + hydrochloric acid
3. Aluminum carbonate + nitric acid
Questions:
1.
Write dissociation equations for the following acids:
a) HNO3
b) HBr
c) H2SO4
d) CH3COOH
e) HClO4
2.
Write dissociation equations for these Arrhenius bases:
a) LiOH
b) NH4OH
c) Al(OH)3
3.
Identify both conjugate acid-base pairs in these reactions:
a) H2CO3(aq) + H2O (l)
--
HCO3 (aq) + H3O+ (aq)
b) HCOOH (aq) + CN- (aq) -- HCOO- (aq) + HCN (aq)
c) H2PO4- (aq) + OH- (aq) --- HPO42- (aq) + H2O (l)
4.
What is the name and formula for the conjugate acid and conjugate base of water?
5.
Hydrofluoric acid dissolves in water to produce hydronium ions (H3O+) and fluoride (F-) ions.
a) Write a Bronsted-Lowry acid-base equation to represent this.
b) Label both conjugate acid-base pairs.
6.
Two acids are dissolved in water at a concentration of 1 mol/L. When the conductivity of each
solution is tested using a conductivity meter, one acid is found to be an excellent conductor while
the other is a very weak conductor. Explain these observations.
7.
Create a summary table to compare the properties of acids and bases. Include: taste; pH range;
conductivity; reaction with indicators (litmus, phenolphthalein, bromothymol blue); formation
equation; Arrhenius definition; Bronsted-Lowry definition; difference between strong and weak
acids and bases; examples of strong and weak acids and bases.