Chemical Equations

Definition of Chemical Equations

A chemical equation uses symbols and formulas to represent a chemical reaction. It shows the reactants (substances you start with) on the left and the products (new substances formed) on the right.

The arrow (→) means 7react to form8 or 7yield8. For example, in the reaction between hydrogen and oxygen:

$\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O}$

This means hydrogen gas reacts with oxygen gas to form water.

Writing Chemical Equations

When writing chemical equations, it is important to use the correct chemical formulas for each element or compound. Each element is represented by its chemical symbol (e.g. H for hydrogen, O for oxygen), and compounds are shown by combining these symbols with numbers (subscripts) to show the number of atoms.

State symbols are added in brackets after each formula to show the physical state of the substances:

• (s) 6 solid
• (l) 6 liquid
• (g) 6 gas
• (aq) 6 aqueous (dissolved in water)

For example, the reaction of zinc with hydrochloric acid can be written as:

$\text{Zn (s)} + 2\text{HCl (aq)} \rightarrow \text{ZnCl}_2\text{ (aq)} + \text{H}_2\text{ (g)}$

Here, zinc is a solid, hydrochloric acid is aqueous, zinc chloride is aqueous, and hydrogen is a gas.

Balancing Chemical Equations

Chemical equations must obey the conservation of mass, meaning no atoms are lost or gained during a reaction. The number of atoms of each element must be the same on both sides of the equation.

To balance an equation, we adjust the numbers in front of formulas called coefficients. These tell us how many molecules or moles of each substance are involved.

For example, the unbalanced combustion of methane is:

$\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

Counting atoms:

• Left: C = 1, H = 4, O = 2
• Right: C = 1, H = 2, O = 3 (2 from CO9 + 1 from H2)

Hydrogen and oxygen atoms are not balanced. To fix this, add coefficients:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Now atoms balance:

• C: 1 each side
• H: 4 each side (2 7 2)
• O: 4 each side (2 7 2 on left, 2 + 2 on right)

Interpreting Chemical Equations

Chemical equations give useful information beyond just what reacts and what forms:

• Mole ratios: The coefficients show the ratio of moles of reactants and products. For example, in $\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O}$, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
• Predicting product amounts: Knowing the amount of one reactant allows calculation of how much product forms, using mole ratios.
• Identifying reaction types: Equations can show if reactions are combustion, displacement, acid-base, etc., by recognising the reactants and products.

For instance, the reaction:

$\text{Mg (s)} + 2\text{HCl (aq)} \rightarrow \text{MgCl}_2\text{ (aq)} + \text{H}_2\text{ (g)}$

is an example of a metal reacting with an acid to produce a salt and hydrogen gas.

Understanding mole ratios helps predict how much product forms. For example, if 3 moles of magnesium react, the equation shows 2 moles of HCl react per mole of Mg, so 6 moles of HCl are needed, producing 3 moles of hydrogen gas.

Example calculation: If 4 moles of hydrogen gas react with oxygen, how many moles of water are produced? Using the balanced equation $\text{2H}_2 + \text{O}_2 \rightarrow \text{2H}_2\text{O}$, 2 moles of hydrogen produce 2 moles of water, so 4 moles of hydrogen produce 4 moles of water.