Chemical reactions Revision Notes

The pH Scale

The pH scale is a numerical scale used to specify the acidity or alkalinity of an aqueous solution. It ranges from 0 to 14. A pH of 7 indicates a neutral solution. Values below 7 indicate acidity, with lower numbers representing stronger acids. Values above 7 indicate alkalinity (basicity), with higher numbers representing stronger alkalis. For example, a pH of 1 is very acidic, while a pH of 14 is very alkaline. Pure water has a pH of 7.

Acids

Acids are substances that have a pH less than 7. The strength of an acid increases as its pH value decreases. For example, lemon juice typically has a pH of about 2-3, making it acidic. Strong acids, such as hydrochloric acid, can have a pH as low as 0 or 1. Acids taste sour, react with metals to produce hydrogen gas, and turn litmus paper red. They also neutralise alkalis.

Alkalis (Bases)

Alkalis, also known as bases, are substances that have a pH greater than 7. The strength of an alkali increases as its pH value increases. For instance, soap solutions usually have a pH of about 9-10, making them alkaline. Strong alkalis, like sodium hydroxide, can have a pH of 13 or 14. Alkalis feel soapy, taste bitter, and turn litmus paper blue. They also neutralise acids.

Measuring pH

The pH of a solution can be measured using indicators or a pH meter. Indicators are chemical substances that change colour depending on the pH of the solution. Common indicators include litmus paper (red for acid, blue for alkali) and universal indicator (shows a range of colours for different pH values). A pH meter provides a more accurate and precise digital reading of the pH, often used in scientific experiments and industry.

The Importance of Balanced Equations

A balanced chemical equation shows the exact ratio in which reactants combine and products are formed. This ratio is expressed in moles. For example, in the reaction 2H₂ + O₂ → 2H₂O, two moles of hydrogen react with one mole of oxygen to produce two moles of water. This precise stoichiometric relationship is what allows us to perform mass calculations, as it ensures the conservation of atoms and thus mass throughout the reaction. Without a balanced equation, any mass calculations would be inaccurate and scientifically invalid.

Steps for Mass Calculations (Higher Tier)

To calculate the mass of a reactant or product from a balanced equation, follow these steps:

1. Write down the balanced chemical equation.
2. Calculate the relative formula mass (Mr) for all substances involved in the calculation.
3. Convert the known mass of a substance into moles using the formula: moles = mass / Mr.
4. Use the molar ratio from the balanced equation to find the number of moles of the unknown substance. For instance, if 1 mol of A produces 2 mol of B, and you have 0.5 mol of A, you will form 1 mol of B.
5. Convert the moles of the unknown substance back into mass using the formula: mass = moles  Mr. Always double-check your calculations and units.

Relative Formula Mass (Mr) (Higher Tier)

The relative formula mass (Mr) of a substance is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. For example, to calculate the Mr of H₂SO₄: Ar(H) = 1, Ar(S) = 32, Ar(O) = 16. So, Mr(H₂SO₄) = (2  1) + (1  32) + (4  16) = 2 + 32 + 64 = 98. It is crucial to correctly calculate Mr values, as any error will propagate through the subsequent mass calculations, leading to an incorrect final answer. Ensure you use the correct number of atoms for each element in the formula.

Example Calculation (Higher Tier)

Consider the reaction: CaCO₃ → CaO + CO₂. If 100 g of calcium carbonate (CaCO₃) decomposes, what mass of carbon dioxide (CO₂) is produced?

1. Balanced equation: CaCO₃ → CaO + CO₂ (already balanced).
2. Mr values: Mr(CaCO₃) = 40+12+(316) = 100. Mr(CO₂) = 12+(216) = 44.
3. Moles of CaCO₃: moles = 100 g / 100 g/mol = 1 mol.
4. Molar Ratio: From the equation, 1 mole of CaCO₃ produces 1 mole of CO₂. So, 1 mol of CO₂ is produced.
5. Mass of CO₂: mass = 1 mol  44 g/mol = 44 g. This structured approach helps prevent errors.

Defining Concentration (Higher Tier)

Concentration is a measure of the amount of solute dissolved in a given volume of solution. It quantifies how much 'stuff' (solute) is packed into a certain amount of 'liquid' (solution). The most common unit for concentration at GCSE is grams per cubic decimeter (g/dm³) or grams per litre (g/L). Understanding concentration is vital for many chemical processes, such as preparing solutions for experiments, controlling reaction rates, and interpreting results from chemical analyses. For instance, a more concentrated acid can dissolve a metal faster than a dilute one.

The Concentration Formula (Higher Tier)

The relationship between mass of solute, volume of solution, and concentration is given by a simple formula: Concentration = Mass of solute / Volume of solution. It's crucial to use consistent units. If the mass of solute is in grams (g) and the volume of solution is in cubic decimeters (dm³), then the concentration will be in g/dm³. If the volume is given in cubic centimeters (cm³), it must first be converted to dm³ by dividing by 1000, since 1 dm³ = 1000 cm³. For example, 500 cm³ is equal to 0.5 dm³.

Impact of Mass and Volume on Concentration (Higher Tier)

Concentration is directly proportional to the mass of the solute: increasing the mass of solute (keeping volume constant) increases the concentration. This is like adding more sugar to the same amount of tea – it becomes sweeter. Concentration is inversely proportional to the volume of the solution: increasing the volume of solution (keeping solute mass constant) decreases the concentration. This is like adding more water to a sugary tea – it becomes less sweet. These relationships are key to understanding dilution and solution preparation.

Rearranging the Formula for Calculations (Higher Tier)

The concentration formula can be rearranged to find any of the three variables if the other two are known. For example, to find the mass of solute needed to make a solution of a specific concentration and volume: Mass of solute = Concentration × Volume of solution. To find the volume of solution that can be made with a certain mass of solute at a particular concentration: Volume of solution = Mass of solute / Concentration. Always ensure units are consistent before performing any calculation to avoid errors.

Defining Oxidation and Reduction

In chemistry, oxidation and reduction are processes that involve the transfer of oxygen atoms. It's important to remember these definitions clearly. Oxidation is specifically defined as the gain of oxygen by a substance. When a substance gains oxygen, it is said to be oxidised. Conversely, reduction is defined as the loss of oxygen from a substance. When a substance loses oxygen, it is said to be reduced. These two processes always happen together in what are called redox reactions.

Identifying Oxidised Species

To identify which species has been oxidised in a reaction, you need to look for the reactant that has gained oxygen in forming its product. For example, if carbon reacts with oxygen to form carbon dioxide, the carbon has gained oxygen and is therefore oxidised. The carbon started as C and ended as CO2. Another example is the rusting of iron, where iron metal reacts with oxygen to form iron oxide; the iron has been oxidised.

Identifying Reduced Species

To identify which species has been reduced, you need to look for the reactant that has lost oxygen in forming its product. A common example is the extraction of metals from their oxides using carbon. In the reaction of copper(II) oxide with carbon to form copper and carbon dioxide, the copper(II) oxide loses oxygen to become copper. Therefore, copper(II) oxide is the reduced species. It started as CuO and ended as Cu.

Simultaneous Nature of Redox Reactions

It's a key concept that oxidation and reduction always occur at the same time in a chemical reaction. You cannot have one without the other. If one substance gains oxygen (is oxidised), then another substance must have provided that oxygen, meaning the provider lost oxygen (is reduced). Therefore, in any overall reaction involving oxygen transfer, you will always be able to identify both an oxidised species and a reduced species among the reactants.