Classwork Series and Exercises {Chemistry – SS1}: Acids

Chemistry SS 1 Week 1

Topic: Acids

Introduction

Acids form a class of chemical substances which contain hydrogen ions in aqueous solution, H⁺ (aq), as the only positive ion. Acids are usually classified into mineral (Inorganic) or organic acids.

Mineral Acids

• They are generally much stronger
• Most do not occur naturally
• They usually have simpler molecules

Examples are:

• Sulphuric Acid
• Nitric Acid
• Hydrochloric Acid
• Phosphoric Acid
• Carbonic Acid

• They naturally occur
• They are found in vegetables, fruit and other foodstuffs
• They are usually weaker and less corrosive

Examples are:

• Ethanoic Acid
• Citric Acid
• Lactic Acid
• Tartaric Acid
• Acetic Acid

Acids in Solution

Acids are substances that form hydrogen ions when dissolved in water. A hydrogen ion is actually a proton. Therefore, acids are called the proton donors.

Examples:

HCl → H⁺ + Cl⁻

HNO₃ → H⁺ + NO₃^–

H₂SO₄ → 2H⁺ + SO₄^2-

The hydrogen ions produced will combine with the water molecule to form hydroxonium ions (H₃O⁺)

H⁺ + H₂O → H₃O⁺

Therefore the reaction can also be written as:

HCl  +  H₂O → H₃O⁺ + Cl⁻

HNO₃  +  H₂O → H₃O⁺ + NO₃^–

H₂SO₄  +  H₂O → H₃O⁺ + SO₄^2-

Basicity of Acids

Basicity of an acid is the number of hydrogen ions which can be produced by one molecule of the acid.

There are 3 common types of Basicity of an acid, these are:

• Monobasic (monoprotic)
• Dibasic (diprotic)
• Tribasic (triprotic)

Monobasic Acids

The monobasic acids are the acids that produced one H⁺ ion from each acid molecule.

Examples:

HCl → H⁺ + Cl⁻

HNO₃ → H⁺ + NO₃^–

CH₃COOH → CH₃COO⁻ + H⁺

Dibasic Acids

The dibasic acids are the acids that produced two H⁺ ion from each acid molecule.

Examples:

H₂SO₄ → 2H⁺ + SO₄^2-

H₂SO₃  → 2H⁺ + SO₃^2-

H₂CO₃ → 2H⁺ + CO₃^2-

Tribasic Acids

The tribasic acids are the acids that produced three H⁺ ion from each acid molecule.

Example:

H₃PO₄  → 3H⁺ + PO₄^3-

Note: Acid containing more than three replaceable hydrogen ions in one molecule of the acid is called a polybasic acid

Strong Acids and Weak Acids

Acids are chemical substances that ionize/dissociate in the presence of water to produce hydrogen ions (or hydroxonium ions). Acids can be classified into 2 categories:

• Strong acids
• Weak acids

The strength of an acid depends on the degree of ionization/dissociation of the acid.

Strong Acids: Strong acids are acids that ionise completely to form hydrogen ions in water

Examples:

• Sulphuric acid
• Hydrochloric acid
• Nitric acid

Weak Acids: Weak acids are acids that partly ionise to form hydrogen ions in water

Examples:

• Ethanoic acid
• Phosphoric acid
• Citric acid

Concentrated and Diluted Acids

• Concentrated Acid: This is one in which a large amount of the acid is dissolved in little amount of water
• Dilute Acid: This is one in which a little amount of the acid is dissolved in large quantity of water

Physical Properties of Acids

Acids have the following physical properties:

• Tastes sour
• Turns moist blue litmus to red
• pH value is less than 7 (i.e. < 7)
• Can conduct electricity
• Corrosive

Note:

Colour of Litmus in Acids

Litmus can be used as acid/alkali indicator. Image below shows the colour of litmus paper when immerse in acid and alkali. The litmus turn red in acids and turn blue in alkali.

pH value of acids

pH value is quantity to measure the concentration of hydrogen ions in a solution. The higher the concentration of hydrogen, the lower the pH value of the solution. Pure water has pH value of 7. All acids has pH value lower than 7.

Chemical Properties of Acids

Acids have the following chemical properties:

1. Acid + Reactive Metal → Salt + Hydrogen gas

Acids react with a metal that is more electropositive than hydrogen in the electrochemical series to produce salt and hydrogen gas. Acids do not react with copper and silver. This is actually a displacement, where the metals that are placed above hydrogen in Electrochemical Series displace hydrogen from acid.

Acids + Reactive Metal → Salt + Hydrogen Gas

Examples:

Sulphuric acid + Zinc

H₂SO₄ + Zn → ZnSO₄ + H₂

Nitric acid + Lead

2HNO₃ + Pb → Pb(NO₃)₂ + H₂

• Acid + Metal Carbonate → Salt + Water + Carbon Dioxide gas

Acids react with metal carbonates produces salt, water and carbon dioxide

Acids + Metal Carbonate → Salt + Water + Carbon Dioxide Gas

Examples:

Sulphuric acid + Lime Stone

H₂SO₄ + CaCO₃ → CaSO₄ + CO₂ + H₂O

Nitric acid + Copper(II) Carbonate

2HNO₃ + CuCO₃ → Cu(NO₃)₂ + CO₂ + H₂O

• Acid + Base oxide → Salt + Water

Acids react with bases produces salt and water

Acids + Base Oxide → Salt + Water (Neutralisation)

Examples:

Sulphuric acid + Iron(II) Oxide

H₂SO₄ + FeO → FeSO₄ + H₂O

Hydrochloric acid + Magnesium Oxide

2HCl + MgO → MgCl₂ + H₂O

• Acid + Alkali → Salt + Water

Acids react with alkali produces salt and water only. This is called a neutralisation reaction.

Acids + Alkali → Salt + Water (Neutralisation)

Examples:

Ethanoic Acid + Sodium Hydroxide

CH₃COOH + NaOH → CH₃COONa + H₂O

Hydrochloric Acid + Ammonia Solution

HCl + NH₃ → NH₄Cl

Preparation of Acids

There are four methods which can be used to prepare acids. These are:

1. By dissolving acid anhydrides in water. Acid anhydrides are oxides of non-metals which will react with water to form the corresponding acids

Examples:

CO_2(g) + H₂O_(l) —–> H₂CO_3(aq)

SO_2(g) + H₂O_(l) —–> H₂SO_3(aq)

SO_3(g) + H₂O_(l) —–> H₂SO_4(aq)

1. By displacing a weaker more volatile acid form its sodium or potassium salt using a stronger but less volatile acid

Examples:

NaCl_(s) + H₂SO_4(aq) —–> NaHSO_4(aq) + HCl_(g)

KNO_3(aq) + H₂SO_4(aq) —-> NaHSO_4(aq) + HNO_3(g)

1. By precipitating insoluble sulphide of metal from a solution of the metallic salt using hydrogen sulphide gas

Examples:

(CH3COO)2Pb_(aq) + H₂S_(g) —–> PbS_(s) + 2CH₃COOH_(aq)

Cu(NO₃)_2(aq) + H₂S_(g) —–> CuS_(s) + 2HNO_3(aq)

1. By direct combination between the elements

H_2(g) + Cl_2(g) —–> 2HCl_(g)

H_2(g) + Br_2(g) —–> 2HBr_(g)

Uses of Acids

• Vinegar, used in the kitchen, is a liquid containing 3-6% acetic acid. It is used in pickles and in many food preparations.
• Lemon and orange juice contains citric acid. Citric acid is used in the preparation of effervescent salts and as a food preservative.
• Acids have been put to many uses in industry. Nitric acid and sulphuric acid are used in the manufacture of fertilizers, dyes, paints, drugs and explosives.
• Sulphuric acid is used in batteries, which are used in cars, etc. Tannic acid is used in the manufacture of ink and leather.
• Hydrochloric acid is used to make aqua regia, which is used to dissolve noble metals such as gold and platinum.
• Sulphuric acid is used in manufacturing fertilizers such as super phosphate, ammonium sulfate etc.