Chapter 1: Chemical Reactions and Equations

Introduction

Chemical reactions are processes in which one or more substances change into new substances with different properties. The substances that take part in a reaction are called reactants, while the new substances formed are called products. Chemical reactions occur around us every day, such as digestion of food, rusting of iron, burning of fuels, and the preparation of medicines.

Recognizing a Chemical Reaction

A chemical reaction may be identified by one or more observable changes. These include a change in colour, formation of a gas, change in temperature, formation of a precipitate, or a change in state. For example, when magnesium burns in air, it produces a bright white flame and forms a white powder called magnesium oxide.

Word equation: Magnesium + Oxygen → Magnesium oxide

Chemical equation: 2Mg + O₂ → 2MgO

Writing Chemical Equations

A chemical equation represents a reaction using symbols and formulae. The reactants are written on the left and the products on the right. An arrow separates them and shows the direction of the reaction. Conditions such as heat, light, or a catalyst may be written above the arrow.

For example, hydrogen reacts with oxygen to form water:

H₂ + O₂ → H₂O

This equation is not balanced because the number of oxygen and hydrogen atoms is different on both sides. A balanced equation is:

2H₂ + O₂ → 2H₂O

Balancing Chemical Equations

Equations must be balanced according to the law of conservation of mass. This law states that matter cannot be created or destroyed during a chemical reaction. The number of atoms of each element must therefore be equal on both sides of the equation.

To balance an equation, change only the coefficients placed before chemical formulae. Never change the small numbers, or subscripts, inside a formula because doing so changes the substance itself.

Types of Chemical Reactions

• Combination reaction: Two or more substances combine to form one product.
  CaO + H₂O → Ca(OH)₂
• Decomposition reaction: One compound breaks down into simpler substances.
  CaCO₃ → CaO + CO₂
• Displacement reaction: A more reactive element displaces a less reactive element.
  Zn + CuSO₄ → ZnSO₄ + Cu
• Double displacement reaction: Two compounds exchange ions to form new compounds.
  AgNO₃ + NaCl → AgCl + NaNO₃
• Combustion reaction: A substance reacts with oxygen and usually releases heat and light.
  CH₄ + 2O₂ → CO₂ + 2H₂O

Oxidation and Reduction

Oxidation may involve the addition of oxygen or removal of hydrogen, while reduction may involve the removal of oxygen or addition of hydrogen. These processes occur together and are called redox reactions. Rusting is an example of oxidation, in which iron reacts with oxygen and moisture to form rust.

Chapter Summary

Chemical reactions produce new substances and can be represented using equations. Equations must be balanced to follow the law of conservation of mass. Important reaction types include combination, decomposition, displacement, double displacement, and combustion. Observing changes and writing balanced equations helps us understand chemical processes in the laboratory and in everyday life.