Chemical reactions Revision Notes

Defining Oxidation and Reduction (Higher Tier)

In chemistry, specifically at the Higher Tier, oxidation and reduction are defined based on electron transfer. Oxidation is the process where a chemical species (an atom, ion, or molecule) loses electrons. This loss results in an increase in the oxidation state of the species. For example, a neutral magnesium atom (Mg) losing two electrons to form a magnesium ion (Mg²⁺) is an oxidation process. Reduction, conversely, is the process where a chemical species gains electrons. This gain results in a decrease in the oxidation state of the species. For example, a copper(II) ion (Cu²⁺) gaining two electrons to form a neutral copper atom (Cu) is a reduction process. These two processes always occur together in what is known as a redox reaction.

Identifying Oxidised and Reduced Species (Higher Tier)

To identify which species is oxidised and which is reduced in a redox reaction, you must track the changes in electron ownership. If a reactant loses electrons, it is the species that has been oxidised. If a reactant gains electrons, it is the species that has been reduced. A simple mnemonic for remembering this is 'OIL RIG': Oxidation Is Loss of electrons, Reduction Is Gain of electrons. Consider the reaction: Fe²⁺ + MnO₄⁻ + H⁺ → Fe³⁺ + Mn²⁺ + H₂O. Iron(II) ions (Fe²⁺) lose an electron to become iron(III) ions (Fe³⁺), so Fe²⁺ is oxidised. Manganese in the permanganate ion (MnO₄⁻) gains electrons, changing from an oxidation state of +7 to +2 in Mn²⁺, so MnO₄⁻ is reduced. Always look for the electron transfer to determine which species undergoes which process.

Redox Reactions: Simultaneous Processes (Higher Tier)

It is essential to understand that oxidation and reduction are complementary and happen hand-in-hand. One cannot occur without the other. This pairing of processes is why such reactions are termed 'redox' reactions (reduction-oxidation). The electrons lost by the oxidised species are precisely the electrons gained by the reduced species. The overall electron count in the reaction remains balanced. For instance, when solid sodium reacts with chlorine gas, 2Na(s) + Cl₂(g) → 2NaCl(s), each sodium atom loses one electron (oxidised) and each chlorine atom gains one electron (reduced). The electrons are transferred directly from sodium to chlorine.

Introduction to Electrolysis of Aqueous Solutions

When an ionic compound dissolves in water, the solution contains its positive metal ions and negative non-metal ions. Crucially, water itself dissociates slightly to produce hydrogen ions (H+) and hydroxide ions (OH-). This means that during the electrolysis of an aqueous solution, there are always at least two types of positive ions and two types of negative ions present, leading to competing reactions at each electrode. These competing reactions determine which products are formed.

Reactions at the Cathode (Negative Electrode)

At the cathode, positive ions gain electrons (reduction). The species most easily reduced will react. There is a competition between metal cations (from the ionic compound) and hydrogen ions (H+) from water. If the metal is less reactive than hydrogen (e.g., copper, silver, lead), the metal ions will be reduced to form the pure metal. For example, Cu^2+ + 2e^- -> Cu. If the metal is more reactive than hydrogen (e.g., sodium, potassium, calcium, magnesium), hydrogen ions will be reduced to form hydrogen gas. For example, 2H+ + 2e^- -> H2. This is because it is energetically more favourable to reduce hydrogen ions than very reactive metal ions.

Reactions at the Anode (Positive Electrode)

At the anode, negative ions lose electrons (oxidation). The species most easily oxidised will react. There is a competition between non-metal anions (from the ionic compound) and hydroxide ions (OH-) from water. If halide ions (Cl-, Br-, I-) are present in a sufficiently high concentration, they will be oxidised to form halogen gases (e.g., 2Cl- - 2e^- -> Cl2). If other anions like sulfate (SO4^2-) or nitrate (NO3^-) are present, or if the halide concentration is very low, then hydroxide ions will be oxidised to form oxygen gas and water. For example, 4OH- - 4e^- -> O2 + 2H2O. This occurs because sulfate and nitrate ions are very stable and difficult to oxidise compared to hydroxide ions.

The Mole: A Unit of Amount (Higher)

The mole (symbol: mol) is the SI unit for the amount of a substance. It represents a specific number of particles (atoms, molecules, ions, electrons, etc.), which is Avogadro's constant, approximately 6.022 x 10^23 particles per mole. This huge number allows us to link the number of individual, invisibly small particles to a measurable mass in grams. One mole of any substance will always contain the same number of particles, regardless of the substance. This concept is fundamental for understanding chemical reactions and calculating quantities.

Molar Mass (Higher)

Molar mass (symbol: M) is the mass of one mole of a substance. It has units of grams per mole (g/mol). For any element, its molar mass in g/mol is numerically equal to its relative atomic mass (Ar) found on the periodic table. For compounds, the molar mass in g/mol is numerically equal to its relative formula mass (Mr). For example, the relative atomic mass of carbon is 12.0, so its molar mass is 12.0 g/mol. The relative formula mass of H₂O is 18.0, so its molar mass is 18.0 g/mol. This direct relationship is key to converting between mass and moles.

Calculating Moles from Mass (Higher)

The amount of a substance in moles can be calculated if its mass and molar mass are known. The formula for this conversion is: Amount of substance (mol) = Mass (g) / Molar mass (g/mol). It is essential to ensure that the mass is in grams and the molar mass is in g/mol for the units to cancel correctly, leaving moles. For instance, to find the moles in 50 g of calcium carbonate (CaCO₃), first calculate its molar mass: (40.1 + 12.0 + 3 x 16.0) = 100.1 g/mol. Then, 50 g / 100.1 g/mol ≈ 0.50 mol.

Calculating Mass from Moles (Higher)

Conversely, if the amount of a substance in moles and its molar mass are known, the mass can be calculated. The rearranged formula is: Mass (g) = Amount of substance (mol) x Molar mass (g/mol). This calculation is vital when you need to weigh out a specific number of moles for an experiment or determine the mass of products formed in a reaction. For example, to find the mass of 0.25 moles of sodium chloride (NaCl), calculate its molar mass: (23.0 + 35.5) = 58.5 g/mol. Then, 0.25 mol x 58.5 g/mol = 14.625 g.

What is a Reaction Profile Diagram?

A reaction profile diagram is a graphical representation of the energy changes that occur during a chemical reaction. It plots energy (y-axis) against the progress of the reaction (x-axis). These diagrams are essential for understanding whether a reaction releases or absorbs energy and the energy required to start the reaction. Key points on the diagram include the initial energy of reactants, the final energy of products, and the transition state where activation energy is supplied.

Exothermic Reactions and their Profiles

In an exothermic reaction, energy is released to the surroundings, typically as heat. This means the products have less energy than the reactants. On a reaction profile, the energy level of the products will be lower than the energy level of the reactants. The overall energy change ($Δ$H) for an exothermic reaction is negative. The activation energy is shown as the 'hump' from the reactant energy level to the peak, representing the minimum energy needed to initiate the reaction.

Endothermic Reactions and their Profiles

In an endothermic reaction, energy is absorbed from the surroundings. This results in the products having more energy than the reactants. On a reaction profile, the energy level of the products will be higher than the energy level of the reactants. The overall energy change ($Δ$H) for an endothermic reaction is positive. Like exothermic reactions, an activation energy is required, shown as the 'hump' from the reactant energy level to the peak before reaching the product energy level.

Labelling Reaction Profile Diagrams

When drawing or labelling a reaction profile diagram, ensure the following are clearly indicated:

1. Y-axis: Labelled 'Energy' or 'Enthalpy'.
2. X-axis: Labelled 'Reaction pathway' or 'Progress of reaction'.
3. Reactants: Horizontal line indicating their initial energy level.
4. Products: Horizontal line indicating their final energy level.
5. Activation Energy: The energy difference between the reactants' energy level and the peak of the curve.
6. Overall Energy Change ($Δ$H): The energy difference between the reactants' energy level and the products' energy level.