Chemical reactions Study Guide

In chemistry, the 'mole' is a fundamental unit for measuring the amount of a substance. It provides a bridge between the macroscopic world (mass you can weigh) and the microscopic world (individual atoms and molecules). One mole of any substance contains approximately 6.022 x 10^23 particles (atoms, molecules, or ions), a value known as Avogadro's constant. The mass of one mole of a substance is called its molar mass, and it is numerically equal to the relative formula mass (Mr) of that substance, but expressed in grams per mole (g/mol).

To account for how the mass of a substance is related to the amount in moles, we use the following relationship:

Amount in moles (mol) = Mass (g) / Molar mass (g/mol)

This equation allows us to convert between mass and moles. For example, if you have 18 grams of water (H₂O), its relative formula mass is (2 x 1.0) + (1 x 16.0) = 18.0. Therefore, its molar mass is 18.0 g/mol. Using the formula, the amount of water in moles would be 18 g / 18.0 g/mol = 1 mole.

Conversely, if you know the amount of a substance in moles and its molar mass, you can calculate the mass:

Mass (g) = Amount in moles (mol) x Molar mass (g/mol)

For instance, if you have 0.5 moles of carbon dioxide (CO₂), its relative formula mass is 12.0 + (2 x 16.0) = 44.0, so its molar mass is 44.0 g/mol. The mass of 0.5 moles of CO₂ would be 0.5 mol x 44.0 g/mol = 22 grams. This relationship is crucial for stoichiometry, allowing chemists to predict reactant and product quantities in chemical reactions.

Oxidation and reduction are fundamental concepts in chemistry, particularly in understanding chemical reactions where electrons are transferred. At the Higher Tier level, it is crucial to define these processes in terms of the gain or loss of electrons. Oxidation is defined as the loss of electrons by an atom, ion, or molecule. This loss results in an increase in the oxidation state of the species. Conversely, reduction is defined as the gain of electrons by an atom, ion, or molecule. This gain results in a decrease in the oxidation state of the species. These two processes always occur simultaneously in a chemical reaction; one species loses electrons (is oxidised) while another species gains those electrons (is reduced). Such reactions are known as redox reactions.

To identify which species are oxidised and which are reduced, you need to analyse the change in the electron count for each reactant. For instance, if an element goes from being a neutral atom to a positive ion, it has lost electrons and is therefore oxidised. If a positive ion becomes a neutral atom, it has gained electrons and is therefore reduced. Similarly, if an ion becomes more positive, it has lost electrons (oxidised), and if it becomes less positive (or more negative), it has gained electrons (reduced). A helpful mnemonic to remember this is 'OIL RIG': Oxidation Is Loss of electrons, Reduction Is Gain of electrons. For example, in the reaction between zinc metal and copper(II) ions, Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), zinc loses two electrons to become a zinc ion (oxidised), while copper(II) ions gain two electrons to become copper metal (reduced). It is important to remember that the total number of electrons lost by one species must equal the total number of electrons gained by the other in any redox reaction.