Chemical Reactions, Equations, Acids, Bases, Salts

Chemical Reactions and Equations

1. Chemical Reaction Definition

The process in which a substance undergoes change to produce new substances with new properties is known as chemical reaction, e.g., magnesium carbonate when heated produces magnesium oxide and carbon dioxide (i.e., new substances with new properties).

A chemical change is generally accompanied by a change of state, change of colour, evolution of a gas, or change of temperature, etc.

2. Chemical Equation Definition

The qualitative representation of a chemical reaction in a shorthand or concise form, in terms of symbols and formulae, is called a chemical equation.

3. Skeletal Chemical Equation

A chemical equation written in the form of symbols and formulae is called a skeletal chemical equation or symbol equation.

4. Balanced Chemical Equation

A chemical equation in which the number of atoms of each element on the L.H.S. (i.e., reactants) and R.H.S. (i.e., products) is equal is called a balanced chemical equation.

5. Balancing Chemical Equations

The process of making the number of different elements equal on both sides of the equation is known as balancing chemical equations.

6. Types of Chemical Reactions

Combination Reactions

Combination reactions are those in which one element reacts with another to form a compound. These types of reactions are also known as synthesis reactions, e.g., hydrogen combines with oxygen to give water.

2H₂(g) + O₂(g) → 2H₂O(l)

Decomposition Reactions

Decomposition reactions are those reactions in which a compound breaks down into simpler compounds (or substances). This type of reaction is simply the reverse of combination reactions. These reactions require energy in the form of heat, light, electricity, etc. E.g., Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.

CaCO₃(s) → CaO(s) + CO₂(g)

Single Displacement Reactions

Single displacement reactions (or single substitution reactions) are reactions in which an atom, or group of atoms, present in a molecule is displaced by another atom. This type of reaction can be represented as follows:

AB + C → AC + B

For example, iron when added to a solution of copper sulfate displaces copper metal.

CuSO₄ + Fe → FeSO₄ + Cu

Double Displacement Reactions

Double displacement reactions (or double decomposition reactions) are reactions in which mutual exchange of radicals takes place. As a result of double decomposition reactions, two new substances are formed.

E.g., sodium chloride reacts with sulfuric acid.

2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl

Double-displacement reactions have two major features in common:

• First, two compounds exchange ions or elements to form new compounds.
• Second, one of the products is either a compound that will separate from the reaction mixture in some way (commonly as a solid or gas) or a stable covalent compound, often water.

Double-displacement reactions can be further classified as precipitation, gas formation, and acid-base neutralization reactions.

Oxidation-Reduction Definitions

Oxidation reaction: Oxidation is defined as a process which involves the addition of oxygen or removal of hydrogen.

Reduction reaction: The term reduction is defined as a process which involves the removal of oxygen or addition of hydrogen.

Redox Reactions

Redox reactions are reactions in which oxidation and reduction take place simultaneously, e.g.,

Exothermic and Endothermic Reactions

Chemical reactions usually proceed with either the liberation of heat or the absorption of heat.

When a chemical reaction liberates heat to the surroundings, it is said to be an exothermic reaction, and when it absorbs heat from the surroundings, it is said to be an endothermic reaction.

N₂(g) + 3H₂(g) → 2NH₃(g) + Energy (Exothermic)

CaCO₃(s) + Energy → CaO(s) + CO₂(g) (Endothermic)

7. Corrosion

Corrosion (erosion by chemical action) is the degradation of metals and is generally called rust. Corrosion causes damage to car bodies, iron railings, ships, and to all objects made of metals, especially those of iron. Corrosion of iron is a serious problem.

8. Rancidity

The most important cause of deterioration in fats and fatty foods is oxidation of fats when exposed to air, light, or moisture, or by bacterial action, resulting in an unpleasant change in the flavor and odor of a food. The whole process is called rancidity.

Acids, Bases, and Salts

Introduction to Acids, Bases, and Salts

The term acid comes from the Latin term acere, which means "sour". In everyday life, we come across many compounds that chemists classify as acids. Bases are compounds which taste bitter, e.g., milk of magnesia. Salts also have wide applications, e.g., ammonium chloride is used as an electrolyte in dry cells, sodium bicarbonate (baking powder) in the manufacture of glass, etc.

2. Chemical Properties of Acids

Action with Metals

Metals generally react with dilute acids to form their respective salts and liberate hydrogen gas.

Metal + Acid → Salt + Hydrogen

Action with Metal Oxides (Basic Oxides)

Metal oxides are generally basic oxides. These oxides get neutralized when they react with acids. These reactions are mostly carried out upon heating.

Basic oxide + Acid → Salt + Water (Neutralization reaction)

Action with Carbonates and Bicarbonates

Acids react with carbonates and hydrogen carbonates to form their respective salts, water, and carbon dioxide gas.

Carbonate/Bicarbonate + Acid → Salt + Water + Carbon Dioxide

3. Chemical Properties of Bases

Reaction with Metals

Metals (e.g., Zn, Al, Sn) dissolve in NaOH (an alkali) to liberate hydrogen gas.

Zn + 2NaOH → Na₂ZnO₂ + H₂↑

(Sodium Zincate)

Action with Acids

Bases combine with acids to form salt and water only. It is a neutralization reaction.

Base + Acid → Salt + Water

4. Strength of Acids and Bases

The strength of an acid or a base can be easily estimated by making use of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colors at different concentrations of hydrogen ions in solution.

5. pH Scale

It is a scale that is used for measuring H⁺ ion (Hydrogen ion) concentration of a solution.

The term pH stands for "potential of hydrogen". It is the amount of hydrogen ions in a particular solution.

• For acids: pH < 7
• For bases: pH > 7
• For neutral substances: pH = 7

6. Importance of pH in Daily Life

Blood pH

For proper functioning, our body needs to maintain blood pH between 7.35 to 7.45. Values of blood pH greater than 7.8 or less than 6.8 often result in death.

Acid Rain

When the pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into rivers, it lowers the pH of river water.

pH in the Digestive System

We know that hydrochloric acid (HCl) produced in our stomach helps in the digestion of food without harming the stomach. However, excess acid causes indigestion and leads to pain as well as irritation. To get rid of this, people use bases called antacids.

pH of the Soil

For healthy growth, plants require a specific pH. Soils with high peat content or iron minerals or with rotting vegetation tend to become acidic, and the soil pH can reach as low as 4.

pH and Tooth Decay

Tooth decay starts when the pH of the mouth is lower than 5.5.

Self-Defense by Animals and Plants

Bee stings leave an acid (formic acid or methanoic acid, HCOOH) which causes pain and irritation. Use of a mild base like baking soda on the stung area gives relief.

7. Salts

A salt is an ionic compound which dissociates to yield a positive ion other than a hydrogen ion (H⁺) and a negative ion other than a hydroxyl ion (OH⁻), e.g., NaCl. Salts are formed by the reaction of an acid and a base, which is also known as a neutralization reaction.

Sodium Hydroxide (Caustic Soda)

It is prepared on a commercial scale by the electrolysis of a strong solution of sodium chloride (NaCl), also called brine. The process is called the chlor-alkali process. The overall reaction taking place is:

2NaCl(aq) + 2H₂O(l) → H₂(g) + Cl₂(g) + 2NaOH(aq)

Uses:

• Sodium hydroxide is mostly used as a base in the laboratory.
• It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soap, and detergents, etc.

Baking Soda (Sodium Bicarbonate)

Sodium bicarbonate, commonly known as baking soda, is a chemical compound with the formula NaHCO₃. It is a salt composed of a sodium cation and a bicarbonate anion.

The overall reaction for its preparation is:

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

When NaHCO₃ is heated, the following reaction occurs:

2NaHCO₃(s) + Heat → Na₂CO₃(s) + H₂O(g) + CO₂(g)

This reaction occurs when baking soda is heated during cooking.

Uses:

• In baking powder: The most practical use of baking soda is as a leavening agent in baking.
• As an antacid: Baking soda reacts with acid due to its alkaline nature and neutralizes acidity (i.e., acts as an antacid).
• In fire extinguishers: It is used in soda-acid fire extinguishers.

Washing Soda (Sodium Carbonate)

Sodium carbonate can be obtained by heating baking soda. Recrystallization of sodium carbonate gives washing soda.

It is also a basic salt.

Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O

Uses:

• Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
• It is used for removing permanent hardness of water.

Bleaching Powder (Calcium Hypochlorite)

Bleaching powder, or calcium hypochlorite, is a chemical compound with the formula CaOCl₂. It is a yellowish powder with a smell of chlorine. It is widely used for water treatment and as a bleaching agent.

2Ca(OH)₂ + 2Cl₂ → CaOCl₂ + CaCl₂ + 2H₂O

Uses:

• Calcium hypochlorite is used for the disinfection of drinking water or swimming pool water.

Plaster of Paris

Plaster of Paris (CaSO₄·½H₂O) can be obtained by heating gypsum (CaSO₄·2H₂O).

CaSO₄·2H₂O + Heat → CaSO₄·½H₂O + 1½H₂O

Plaster of Paris is a white powder, and on mixing with water, it changes back to gypsum, giving a hard solid mass.

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

Uses:

• It is used for making molds or casts for toys, pottery, ceramics, etc.
• It is used in surgical bandages for setting fractured bones.