acids and bases

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3.1 Acids and bases
Acids
Chemical reactions involving acids and bases play an important role in our lives. They
occur in the kitchen, in the laundry, in the garden, in swimming pools and even inside the
body.
Most people think of acids as being corrosive and dangerous. This is true for some acids
we use in the laboratory, such as sulfuric acid. But some acids are not harmful and are
commonly found in our food.
For example, vinegar is a solution of ethanoic acid in water. Citrus fruits, such as oranges
and lemons, contain citric acid. The acids are responsible for the sour taste in these fruits.
In fact, the word 'acid' comes from the Latin word acidus meaning sour. All acids have a
sour taste.
However, some acids like the sulfuric acid used in car batteries, are dangerously
corrosive. The acids contain in ant stings and bee stings cause pain.
Definition of Acid
An acid is a substance which produces hydrogen ions as the only positive ions when it is
dissolved in water.
Example: HCl molecules dissolve in water to give hydrogen ion and chloride ion.
Acids are classified as weak or strong. Strong acids are man-made and very corrosive.
Large amounts of strong acids are used in industry for making useful products. Weak
acids are usually found in plants and animals. They are used in food and are not as
corrosive. The formulas of some common acids are given in the table below.
Strong acids
Common / Mineral Name
Hydrochloric acid
Nitric acid
Sulfuric acid
Chemical Name
Hydrogen chloride
Hydrogen nitrate
Hydrogen sulfate
Weak acids
Name of Acid
Source
Acetic acid (ethanoic acid) :vinegar
Citric acid
: lemon
Tartaric acid
: grape
Chemical Formula
HCl
HNO3
H2SO4
Strength of an acid
The strength of an acid depends on its degree of dissociation / ionisation in water to
form hydrogen ions.
Strong acids
A strong acid is one that ionises / dissociates completely in water to produce hydrogen
ions (H+). There are no molecules left. The solution contains a high concentration of
ions: good electrical conductor.
Pure HCl (g)
: covalent
HCl (aq) → H+ (aq) + Cl- (aq)
Pure HNO3 (l) : covalent
HNO3 (aq) → H+ (aq) + NO3 -(aq)
Pure H2SO4 (l)
: covalent
H2SO4 (aq) → 2H+ (aq) + SO42- (aq)
Weak acids
A weak acid is one that ionises / dissociates incompletely / partially in water to produce
few hydrogen ions (H+). Most of the acid molecules remain as molecules. The solution
contains a low concentration of ions: poor electrical conductor.
CH3COOH (aq)
CH3COO- (aq) + H+ (aq)
Importance of water for acidity
Pure acids exist as molecules instead of ions. Pure acids do not behave as acids as the
properties of acids are due to the presence of hydrogen ions. When acids are mixed with
water, ionization of acids occurs, and hydrogen ions are produced. Therefore, acids can
only behave as acids when they are dissolved in water.
Properties of Acids
An acid, when dissolved in water, forms a colourless solution. Solutions of acids have the
following properties:
1. sour taste.
2. change the colour of indicators (e.g. turn blue litmus paper red).
3. contain hydrogen ions and conduct electricity.
4. react with metals, carbonates and bases.
Uses of acids
The table below shows some common uses of acids:
Describing acids and bases
You can describe how acidic or basic a substance is by using the numbers on the pH
scale. The pH scale ranges from 0 to 14.
Low pH numbers (less than pH 7) mean that substances are acidic. High pH numbers
(more than pH 7) mean that substances are basic. If a substance has a pH of 7 it is said to
be neutral — neither acidic nor basic.
Acids and bases can be graded from strong to weak. pH values depend on the
concentration of acid/base and degree of dissociation. The use of pH in measuring the
strength of an acid is limited since its value changes with concentration. The higher the
concentration of the hydrogen ion, the lower the pH value.
For example, a strong acid has a very low pH (pH 0 or 1) and a strong base has a very
high pH
(pH 13 or 14).
The pH level of a substance can be measured using a pH meter or a special indicator
called universal indicator. Universal indicator is a mixture of indicators and it changes
colour as the strength of an acid or base changes. The colour range of universal indicator
is shown below.
The colour range of universal indicator. It is pink in strong acid (pH1), blue in strong
base (pH14) and green in neutral solutions (pH7).
The pH of most 'pure' water is not exactly 7 because it contains tiny amounts of
impurities. Even rain water has a pH less than 7. It has a pH of about 6 because carbon
dioxide from the air makes it slightly acidic.
The pH meter is an electrical meter for measuring the pH of a solution. It is more
accurate than indicators.
Bases and Alkalis
Bases have a bitter taste and feel slippery or soapy to touch. Some bases are very
corrosive, especially caustic soda. Caustic soda will break
down fat, hair and vegetable matter. Other bases are used in shampoos, toothpaste, and
cleaning agents like dishwashing liquid and cloudy ammonia. Bases that can be dissolved
in water are called alkalis.
Definition of bases and alkalis
Bases are the oxides or hydroxides of metal that reacts with an acid to form a salt and
water only. Most bases are insoluble in water. Bases that dissolve in water are called
alkalis.
An alkali is a metal hydroxide which is soluble in water and produces hydroxide ions
(OH-) in water.
All alkalis are bases but not all bases are alkalis.
Strength of an alkali
Alkalis can be weak or strong. When strong alkalis dissolve in water, they become OH(aq) ions in solution. Sodium hydroxide and potassium hydroxide are examples of strong
alkalis.
The strength of an alkali depends on its degree of dissociation / ionisation in water to
form hydroxide ions. The properties of alkalis are due to the hydroxides ions.
Strong alkali
A strong alkali is one that ionises / dissociates completely in water to produce hydroxide
ions (OH-). There are no molecules left. The common strong alkalis are sodium
hydroxide and potassium hydroxide.
NaOH (aq) → Na+ (aq) + OH- (aq)
KOH (aq) → K+ (aq) + OH- (aq)
Strong alkalis react with ammonium salts to produce ammonia gas.
Weak alkali
A weak alkali is one that ionises / dissociates incompletely / partially in water to
produce few hydroxide ions (OH-). Most of the alkali molecules remain as molecules.
Ammonia is the most common example of a weak alkali. When ammonia dissolves in
water, only a small fraction of the ammonia molecules react with the water to form OH(aq) ions. Most of the ammonia molecules remain unchanged. That is why you can smell
the ammonia molecules in a bottle of aqueous ammonia.
Properties of bases
1. Alkalis have a soapy feel and a bitter taste.
2. Alkalis can change the colour of indicators (e.g. turn red litmus paper blue).
3. Bases can react with acids to form salt and water only – Neutralization.
This means that the acid properties have been “destroyed”.
Base + acid → salt + water
Example: NaOH + HCl (aq) → NaCl (aq) + H2O (l)
Note: Neutralization – The formation of molecules of water from hydrogen ion of an acid
and hydroxide ion of an alkali.
H+ (aq) + OH- (aq) → H2O (l).
Uses of Bases and Alkalis
Alkalis are used in the home for two reasons. One is to neutralise acids. Toothpastes are
alkaline. The alkali neutralises acids on our teeth produced by bacteria when they feed on
sugars in our food. If the acid is not destroyed it corrodes the teeth causing them to decay.
Toothpaste usually contains magnesium hydroxide, which neutralises the acids in the
mouth.
Excess acid in the stomach can make you feel unwell. The medicine for treating this
contains alkalis and carbonates. This neutralises some of the acid.
The other reason for using alkalis is to dissolve dirt and grease. Soaps and detergents are
mild alkalis. Floor cleaners often contain sodium hydroxide, which is a powerful alkali.
Ammonia is used in liquids for cleaning glass windows.
The table below shows some other common uses of bases:
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