Strong and weak acids Archives

Classification of Acids

Depending upon the elements present, acids may be classified as follows.

(i) Oxyacid : Acids that contain both hydrogen and oxygen are called oxyacids. For example, nitric acid (HNO₃), sulphuric acid (H₂SO₄) and phosphoric acid (H₃PO₄) are oxyacids.
(ii) Hydracid : Acids that contain hydrogen and other nonmetallic element(s), except oxygen, are called hydracids. For example, hydrochloric acid (HCl) and hydrocyanic acid (HCN) are hydracids.
Acids may also be classified as follows.

1. Organic and inorganic acids : All sour things that we use in our daily food contain acids. These acids are organic acids. Some of the common acids that are generally used in the laboratory are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃). These are inorganic acids, also called mineral acids. Hydrochloric acid is also present in the gastric juice in our stomach.

2. Concentrated and dilute acids : An acid solution may be concentrated or dilute depending upon the amount of the acid present in the solution. Concentrated and dilute solutions of acids are generally used in laboratories. Let us see what these acids are.
An acid is generally used as solution in water. When the solution contains a larger amount of the acid, it is said to be concentrated, whereas a dilute solution contains smaller amount of the acid.
Thus, concentrated and dilute solutions of an acid differ from each other only in the proportions of the acid and water in them.

3. Strong and weak acids : The strength of an acid is determined by the amount of hydrogen ions (H⁺) that the acid provides when dissolved in water.
Some of the acids, when dissolved in water, get almost completely dissociated to provide hydrogen ions. These acids are called strong acids. For example, hydrochloric acid (HCl), nitric acid (HNO₃) and sulphuric acid (H₂SO₄) are strong acids.
On the other hand, there are some acids which when dissolved in water, are only incompletely dissociated to give hydrogen ions. These are called weak acids. For example, carbonic acid (H₂CO₃) and acetic acid (CH₃COOH) are weak acids.

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How can we measure the strength of acids and alkalis?

1. An acid produces hydrogen ions in aqueous solution. The acidity of a solution is a measure of the concentration of the hydrogen ions in the solution.

A base produces hydroxide ions in aqueous solution. The alkalinity of a solution is a measure of the concentration of the hydroxide ions in the solution.
In 1909, Soren Sorensen proposed the pH scale for measuring acidity or alkalinity of an aqueous solution.

The pH scale

How can we measure the strength of acids and alkalis 1

The pH scale normally has a range of values from 0 to 14 to indicate how acidic or alkaline an aqueous solution is.
The pH value measures the concentration of hydrogen ions or hydroxide ions.
pH < 7 → acidi solution
pH = 7 → neutral solution
pH > 7 → alkaline solution
The lower the pH value, the higher the concentration of hydrogen ions.
The higher the pH value, the higher the concentration of hydroxide ions.
The pH of values some common solutions used in daily life are shown below.
The pH value of an aqueous solution can be measured by using
(a) Universal Indicator
(b) pH meter
(c) acid-base indicators

Table shows examples of acid-base indicators.

Indicator	pH range	Colour change
Indicator	pH range	Acid Neutral		Alkali
Methyl orange	3.0 – 5.0	Red	Orange	Yellow
Bromothymol blue	6.0 – 8.0	Yellow	Green	Blue
Phenolphthalein	8.0-10.0	Colourless	Colourless	Pink

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Strong and weak acids

The strength of an acid or alkali depends on the degree of dissociation of the acid or alkali in water.
The degree of dissociation measures the percentage of acid molecules that ionise when dissolved in water.
A strong acid is an acid which ionises or dissociates completely in water to produce a high concentration of hydrogen ions.
Hydrochloric acid is a strong acid. It is 100% ionised in water. All the hydrogen chloride molecules that dissolve in the water ionise completely into hydrogen ions and chloride ions.
A weak acid is an acid which ionises partially in water to produce a low concentration of hydrogen ions.
Ethanoic acid is a weak acid. Dilute ethanoic acid is about 0.4% ionised, only 4 out of every 1000 ethanoic acid molecules ionise. As fast as the acid molecules ionise to produce ions, the ions combine back again to give the original acid molecules in a reversible reaction.

Examples of strong and weak acids are shown in Table.

Acid	Name	Particles
Strong acid	Hydrochloric acid, HCl	H⁺, Cl^–
	Nitric acid, HNO₃	H⁺, NO₃^–
	Sulphuric acid, H₂S0₄	H⁺, HSO₄^–, SO₄^2-
Weak acid	Carbonic acid, H₂C0₃	H⁺, HCO₃^–, CO₃^2-, H₂CO₃
	Ethanoic acid, CH₃COOH	H⁺, CH₃COO^–, CH₃COOH
	Sulphurous acid, H₂SO₃	H⁺, HSO₃^–, SO₃^2-, H₂SO₃

Strong acid and weak acid are defined as follows.
A strong acid is completely ionised in water to produce a high concentration of hydrogen ions.
A weak acid is partially ionised in water to produce a low concentration of hydrogen ions.
The pH of an acid solution changes with concentration of the acid. The concentration of the acid affects the concentration of hydrogen ions produced. For example:
10 mol dm^-3 hydrochloric acid: pH = 1
01 mol dm^-3 hydrochloric acid: pH = 2
pH values can be used to compare the acid strength of different acids. All the acids to be compared must be of the same concentration.
For example:
10 mol dm^-3hydrochloric acid: pH = 1
10 mol dm^-3 ethanoic acid: pH = 3
For two different acids of the same concentration, the acid with the lower pH value is the stronger acid, i.e. higher degree of ionisation in water.

Strong and weak alkalis

Sodium hydroxide is a strong alkali. It ionises fully when dissolved in water.
A strong alkali is an alkali which is fully ionised in water to produce a high concentration of hydroxide ions.
A weak alkali is an alkali which ionises partially in water to produce a low concentration of hydroxide ions.
Ammonia is an example of a weak alkali. It is only partly ionised in water, which means the ionisation of ammonia in water is incomplete. Only a small amount of ammonia molecules are ionised in water to produce ammonium ions and hydroxide ions.

Examples of strong and weak alkalis are shown in Table.

Alkali	Name	Particles
Strong alkali	Sodium hydroxide, NaOH	Na⁺, OH^–
	Potassium hydroxide, KOH	K⁺, OH^–
	Barium hydroxide, Ba(OH)₂	Ba²⁺, OH^–
Weak alkali	Ammonia, NH₃	NH₃, NH₄⁺, OH^–
	Methylamine, CH₃NH₂	CH₃NH₂, CH₃NH₃⁺, OH^–
	Hydrazine, N₂H₄	N₂H₄, N₂H₅⁺, OH^–

Strong alkali and weak alkali are defined as follows.
A strong alkali is completely ionised in water to produce a high concentration of hydroxide ions.
A weak alkali is partially ionisesed in water to produce a low concentration of hydroxide ions.
The pH of an alkali solution changes with concentration of the alkali. The concentration of the alkali affects the concentration of hydroxide ions produced.
For example:
10 mol dm^-3m^-3 sodium hydroxide solution pH = 13
01 mol dm^-3 sodium hydroxide solution pH = 12
pH values can be used to compare the strength of different alkalis. All the alkalis to be compared must be of the same concentration. For example:
10 mol dm^-3 sodium hydroxide: pH = 13
10 mol dm^-3 ammonia solution: pH = 11
For two different alkalis of the same concentration, the alkali with the higher pH value is the stronger alkali, i.e. higher degree of ionisation in water.