Chemistry 4.9-4.10 - Oxidation and recycling

Reactivity and resistance to oxidation

When a metal reacts with oxygen, the metal is oxidised: it gains oxygen and forms a metal oxide.

The reactivity series orders metals by how readily they react. A metal near the top is more reactive, so it tends to react with oxygen more readily. It therefore has lower resistance to oxidation. A metal near the bottom is less reactive, reacts with oxygen less readily and has higher resistance to oxidation.

This is a trend in chemical tendency, rather than a guarantee of visible speed. Aluminium is reactive, yet a thin, adherent oxide layer protects the underlying metal from further attack. Surface protection must be considered when interpreting a particular sample.

The two quantities run in opposite directions:

higher reactivity → more readily oxidised → lower resistance to oxidation

lower reactivity → less readily oxidised → higher resistance to oxidation

For example, magnesium is above copper in the reactivity series. Under comparable conditions, magnesium reacts with oxygen more readily than copper, so magnesium is less resistant to oxidation and copper is more resistant.

At the visible scale, a more vigorous reaction can produce a brighter glow, a faster surface change or oxide more quickly. At the particle scale, metal atoms and oxygen atoms become joined in a metal oxide. At the symbolic scale, balanced equations conserve every atom:

$\ce{2Mg + O2 -> 2MgO}$

$\ce{2Cu + O2 -> 2CuO}$

Both equations show oxidation because the metal gains oxygen. The equations show the substances and atom ratios, but they do not show which reaction is faster or more vigorous; that evidence comes from observations made under comparable conditions.

Gathering evidence safely

Pearson lists metal/oxygen reactions as suggested practical evidence, not as a core practical for this lesson. A teacher demonstration could compare small samples of different metals heated in air using identical heatproof containers.

1. Clean each metal surface in the same way and use samples with the same mass and, as far as possible, a similar physical form and surface area.
2. Heat each sample with the same flame position and heating time while keeping the air supply the same.
3. Record a defined dependent observation, such as the time until a visible change begins. A cooled mass gain can provide additional evidence because oxygen has been added to the metal.
4. Repeat each comparison and look for a consistent pattern rather than relying on one dramatic observation.

The independent variable is the metal. The dependent variable is the chosen oxidation evidence. Controlled variables include sample mass, surface area and form, surface cleaning, heating time, flame position and oxygen supply.

Wear eye protection, use small samples on a heatproof mat and handle hot apparatus with tongs. Do not look directly at burning magnesium, and allow products and containers to cool before weighing or touching them.

This method has limitations. Judging brightness or the first visible change is subjective; different forms of metal can have different surface areas; a pre-existing surface layer can slow what is observed; and some powdered oxide could escape before a final mass is measured. The observations can support the predicted order, but a fair conclusion must acknowledge these limits.