Which reactions are redox reactions? (a) \(\mathrm{Al}(s)+3 \mathrm{Ag}^{+}(a q) \longrightarrow \mathrm{Al}^{3+}(a q)+3 \mathrm{Ag}(s)\) (b) \(4 \mathrm{~K}(s)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{~K}_{2} \mathrm{O}(s)\) (c) \(\mathrm{SO}_{3}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{SO}_{4}(a q)\) (d) \(\mathrm{Mg}(s)+\mathrm{Br}_{2}(l) \longrightarrow \mathrm{MgBr}_{2}(s)\)

Understanding oxidation states is fundamental in determining whether a chemical reaction is a redox reaction. The oxidation state, often referred to as oxidation number, is a theoretical charge assigned to an atom based on a set of rules. These rules include assigning a charge of zero to a pure element, as well as specific charges to atoms in common states (e.g., oxygen is usually -2, hydrogen is usually +1).

The oxidation state can help us track how electrons are redistributed in a reaction. In the given examples, aluminum goes from 0 to +3 (losing electrons), and silver goes from +1 to 0 (gaining electrons), indicating electron transfer and confirming a redox reaction. Simplifying complex reactions to the movement of electrons allows for a clearer understanding and easier identification of redox processes.

Electron transfer is the core of redox reactions and involves the movement of electrons from one atom to another. This exchange can be a direct transfer, as in ionic compounds, or through a shared electron pair in covalent bonds.

In redox chemistry, the substance that gives away electrons is oxidized, while the substance that gains electrons is reduced. These reactions are vital to many biological processes and industrial applications. For example, in reaction (b), potassium atoms each lose one electron and are oxidized, while the oxygen molecule gains electrons, hence reduced, to form potassium oxide.

To systematically identify if a reaction is redox, we check for changes in the oxidation states of the elements involved. Not all reactions are redox; some involve acid-base chemistry where proton transfer occurs without any exchange of electrons.

In reaction (c), sulfur trioxide reacts with water to form sulfuric acid without any change in the oxidation numbers of sulfur, hydrogen, or oxygen. Instead, this process is identifiable as an acid-base reaction where proton transfer is the main event, thus excluding it from being classified as a redox reaction.

Analyzing the changes in oxidation numbers is crucial in identifying redox reactions. An increase in the oxidation number indicates oxidation (loss of electrons), while a decrease suggests reduction (gain of electrons).

For instance, in reaction (d), magnesium's oxidation number increases from 0 to +2, indicating that it has lost two electrons and has been oxidized. Concurrently, the bromine molecules each gain an electron, which decreases their oxidation number from 0 to -1, signifying reduction. This signifies that electrons were transferred between the reactants, thus confirming the redox nature of the reaction.