Lecture 1: Identifying Chemical reactions and Notations

1. What is a Chemical Reaction?

A chemical reaction occurs whenever a chemical change takes place. In a chemical change, the initial substance changes its fundamental nature and identity.

2. Evidence of a Chemical Reaction

You can determine if a chemical reaction has occurred by observing any of these four changes:

• Change in state (solid, liquid, gas).
• Change in color.
• Evolution of a gas.
• Change in temperature (either heating up or cooling down).

For example, when zinc granules react with dilute sulfuric acid, you observe the evolution of hydrogen gas and a change in temperature (the test tube becomes warm).

3. Writing Chemical Equations

A description of a reaction can be long, so we use equations for conciseness.

A. Word Equations: This is the simplest form. Reactants are the substances that undergo the chemical change, and products are the new substances formed.

• The reactants are written on the Left-Hand Side (LHS) with a plus sign (+) between them.
• The products are written on the Right-Hand Side (RHS) with a plus sign (+) between them.
• The arrow points toward the products, indicating the direction of the reaction.

Example: \text{Magnesium} + \text{Oxygen} \longrightarrow \text{Magnesium oxide}

B. Chemical Equations: To make the equation even more concise and useful, we use chemical formulae instead of words.

Example (Skeletal Equation): \text{Mg} + \text{O}_2 \longrightarrow \text{MgO}

This initial representation is called a skeletal chemical equation. We will discuss why skeletal equations are often considered “unbalanced” in our next module.