KEY2CHEM

Oxidation Reduction (Redox) Reactions

An oxidation reduction (redox) reaction results from the net movement of electrons. One species loses electrons (is oxidized) and another species gains the electrons (is reduced). Tracking oxidation numbers allows for the evaluation of which species is oxidized (oxidation number gets more positive) and which species is reduced (oxidation number gets more negative).

Example 1.

What is oxidation number of $$\require{mhchem}\ce{S}$$ in $$\require{mhchem}\ce{SO4^{2-}}$$?

A. $$+8$$

B. $$+6$$

C. $$-2$$

Solution

A. $$+6$$

The sum of the oxidation numbers must add to the overall charge ($$-2$$, in this case). Since each $$\require{mhchem}\ce{O}$$ atom has an oxidation number of $$-2$$, the $$\require{mhchem}\ce{S}$$ must contribute $$+6$$.

Example 2.

Which is a redox reaction?

A. $$\require{mhchem}\ce{C(s) + O2(g) -> CO2(g)}$$

B. $$\require{mhchem}\ce{H+(aq) + OH^{-}(aq) -> H2O(l)}$$

C. $$\require{mhchem}\ce{Ag+(aq) + Cl^{-}(aq) -> AgCl(s)}$$

Solution

A. $$\require{mhchem}\ce{C(s) + O2(g) -> CO2(g)}$$

A redox reaction is one where a change in oxidation number occurs, as this indicates a net movement of electrons. Chemical reactions can certainly occur without a change in oxidation number; they are just not redox reactions. Examples include precipitation and acid-base reactions.

Example 3.

In the combustion of methane ($$\require{mhchem}\ce{CH4}$$), which species is oxidized?

$$\require{mhchem}\ce{ CH4(g) + 2O2(g) -> CO2(g) + 2 H2O(g) }$$

A. $$\require{mhchem}\ce{CH4}$$

B. $$\require{mhchem}\ce{O2}$$

C. $$\require{mhchem}\ce{CO2}$$

Solution

A. $$\require{mhchem}\ce{CH4}$$

The oxidation number of carbon in $$\require{mhchem}\ce{CH4}$$ is $$-4$$. The oxidation number of carbon in $$\require{mhchem}\ce{CO2}$$ is $$+4$$. Since the oxidation number of carbon gets more positive (starts at $$-4$$ and ends at $$+4$$), $$\require{mhchem}\ce{CH4}$$ (which contains the carbon that is being transformed) is oxidized.