Chemical Reactions and the Conservation of Matter: Mechanism

Mechanism

Follow the mechanism step by step and distinguish what causes the change from what is merely observed.

This extension applies that lens specifically to Chemical Reactions and the Conservation of Matter.

In a chemical reaction the starting substances — the reactants — are rearranged into new substances, the products. We write this with an arrow that means becomes:

A word equation names them: magnesium $+$ oxygen $\rightarrow$ magnesium oxide.

Signs that a chemical (not merely physical) change has happened:

• a new substance with new properties appears;
• there is a colour change, a gas is given off, or a solid forms in a clear liquid;
• energy is released or absorbed (the container gets hot or cold);
• it is usually difficult to reverse.

Melting ice, dissolving sugar and boiling water are all physical changes: no new substance, and easy to reverse. Rusting iron, burning wood and baking bread are chemical changes.

The law of conservation of matter. In a chemical reaction atoms are only rearranged — never created and never destroyed. So the total mass of the products always equals the total mass of the reactants.

This is why a chemical equation must be balanced: the same number of each kind of atom on both sides.

Worked example — balance $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$.

Count first: on the left, 1 Mg and 2 O. On the right, 1 Mg and 1 O. The oxygen does not match.

1. Put a 2 in front of $\text{MgO}$ to fix the oxygen: $\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. Now O is 2 and 2. But Mg is 1 on the left and 2 on the right.
2. Put a 2 in front of $\text{Mg}$: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$.

Check: Mg is 2 and 2; O is 2 and 2. Balanced.

Worked example — the masses. $12\ \text{g}$ of magnesium burns completely in $8\ \text{g}$ of oxygen. No atoms are lost, so