Classification of Chemical Reactions

The Three Fundamental Mechanisms

• Every time you mix vinegar and baking soda, or watch an iron nail rust, you are witnessing one of just three fundamental ways atoms interact.
• According to the AQA specification, all chemical reactions can be classified into proton transfer, electron transfer, or electron sharing.
• Regardless of which mechanism occurs, energy is always conserved during the reaction.
• In terms of chemical bonding, "transfer" generally leads to the formation of ionic bonds, while "sharing" creates covalent bonds.

Proton Transfer (Acid-Base Reactions)

• A hydrogen atom consists of exactly one proton and one electron. When it loses its single electron to become a positive hydrogen ion ($H^+$), only the proton remains, meaning an $H^+$ ion is simply a proton.
• Proton transfer is the movement of an $H^+$ ion from an acid to a base. An acid is defined as a proton donor, while a base is a proton acceptor.
• When an acid dissolves in water, it donates a proton to a water molecule to form a hydroxonium ion ($H_3O^+$). Water is amphoteric, meaning it can act as either an acid or a base depending on the reaction.
• Neutralisation is a specific type of proton transfer where $H^+$ ions react with hydroxide ions ($OH^-$) from an alkali to produce water:

$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$

Electron Transfer (Redox Reactions)

• Electron transfer involves the movement of electrons from one chemical species to another. This defines redox (reduction-oxidation) reactions.
• Oxidation is the loss of electrons, while reduction is the gain of electrons. These processes always occur simultaneously.
• An oxidising agent is a substance that gains electrons (and is therefore reduced), while a reducing agent loses electrons (and is oxidised).
• To highlight the transfer of electrons, we use ionic equations that omit spectator ions (ions that do not change charge or state). For example, in a displacement reaction between zinc and copper sulfate:

$$Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$$

• Higher Tier (HT) students must represent this using a half-equation to show the exact number of electrons ($e^-$) transferred. For example, the oxidation of zinc is written as:

$$Zn \rightarrow Zn^{2+} + 2e^-$$

Electron Sharing (Covalent Bonding)

• You can snap a piece of chalk, but try snapping a diamond — the secret lies in how their atoms connect. Electron sharing occurs when two non-metal atoms provide one or more electrons each to be held in a shared space, forming a molecule.
• This creates a covalent bond, which is a very strong electrostatic attraction between the positively charged nuclei and the negatively charged shared pair of electrons.
• Simple covalent molecules do NOT conduct electricity. This is because all their outer electrons are localised within the bonds, meaning there are no free ions or delocalised electrons to carry an electrical charge.
• When atoms share electrons, they can form single, double, or triple bonds. For example, hydrogen and chlorine share one pair of electrons to form hydrogen chloride:

$$H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$$

Comparing the Mechanisms in Action

• Examiners frequently test your ability to classify equations, and the exact same elements can participate in different mechanisms depending on the context.
• When hydrogen and chlorine gases react to form $HCl(g)$, the mechanism is electron sharing. However, when that $HCl$ gas dissolves in water to act as an acid, it donates an $H^+$ ion, meaning the mechanism becomes proton transfer.
• Similarly, a metal reacting with an acid (e.g., $Mg + 2HCl \rightarrow MgCl_2 + H_2$) involves $H^+$ ions, but it is actually electron transfer (redox) because the magnesium loses electrons and the hydrogen ions gain them.