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HS Chemistry - Working With Chemical Reactions

Chemical Reaction Equations

Overview of The Page

This page will cover:

  • How are chemical reaction equations written?

When dealing with chemical reactions, we usually describe what is happening in terms of an equation. When the products are added together shown with a \+ sign, they form shown with a → sign the products. Thus, a chemical reaction equation takes the form:

A + B → C + D

Where A & B are the reactants, and C & D are the products. There can, of course, be more than two reactants or two products in a reaction or even less than two, but the equation is written out in the same way, with the reactants on one side, the products on the other side, and the arrow pointing from the reactants to the products, showing the direction of the reaction.

In a chemical reaction equation, no mass is lost. The left-hand side must equal the right-hand side in terms of both the types of atoms present, how many there are of each atom, and the total charges. Additionally, it is recommended to note the state of matter solid, liquid, gas, or aqueous \[dissolved\] for each of the reactants and products.

But how do we write the reactants and products?

Molecular Formula

The molecular formula for a molecule, metal atom, or ionic compound shows what the atoms present inside 1 molecule of that substance are. For example, the formula H2O shows how in an H2O molecule, there are 2 Hydrogen molecules connected to 1 Oxygen molecule.

Molecular formulae are very important in chemical reactions, as they show what atoms are present on each side, as well as how many there are of each atom.

Empirical Formula

Closely related to the molecular formula is the empirical formula, the ratio of atoms in a molecule.

There are two important things to note about empirical formulae:

  1. Molecules with different molecular formulae can have the same empirical formula. For example:

    • Molecule 1 molecular formula: C6H12
    • Molecule 2 molecular formula: C2H4


    • Molecule 1 empirical formula: CH2
    • Molecule 2 empirical formula: CH2

    Thus, the molecular formula and empirical formula are not interchangeable.

  2. In an ionic compound, the ionic formula is the same as the empirical formula. This is covered more in depth in Unit 3