GCSE Chemistry: Acids and Bases

Understanding Acids and Bases

In GCSE Chemistry, the study of acids and bases is crucial for understanding chemical reactions and the properties of various substances. This topic covers the pH scale, neutralisation reactions, and the preparation of salts.

The pH Scale

The pH scale ranges from 0 to 14 and is used to measure the acidity or alkalinity of a solution. A pH of 7 is considered neutral, while values below 7 indicate acidity and values above 7 indicate alkalinity. The scale is logarithmic, meaning each whole number change represents a tenfold change in acidity or alkalinity.

Acids and Bases

Acids are substances that release hydrogen ions (H+) in solution, while bases are substances that can accept hydrogen ions or donate hydroxide ions (OH-). Common examples include:

• Acids: Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric acid (HNO3)
• Bases: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide (Ca(OH)2)

Neutralisation Reactions

Neutralisation occurs when an acid reacts with a base to produce a salt and water. The general equation for a neutralisation reaction is:

Acid + Base → Salt + Water

For example, when hydrochloric acid reacts with sodium hydroxide, the reaction can be represented as:

HCl + NaOH → NaCl + H2O

Preparation of Salts

Salts can be prepared through various methods, including:

• Reacting an acid with a metal: Acid + Metal → Salt + Hydrogen
• Reacting an acid with a metal oxide: Acid + Metal Oxide → Salt + Water
• Reacting an acid with a metal hydroxide: Acid + Metal Hydroxide → Salt + Water
• Reacting an acid with a metal carbonate: Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

Titrations

Titration is a technique used to determine the concentration of an acid or base in a solution. It involves adding a solution of known concentration (the titrant) to a solution of unknown concentration until the reaction reaches the endpoint, indicated by a color change using an indicator.

Worked Example

Problem: How much 0.1 M hydrochloric acid is needed to neutralise 25 cm3 of 0.1 M sodium hydroxide?

Solution:

• Write the balanced equation: HCl + NaOH → NaCl + H2O
• From the equation, we see that 1 mole of HCl reacts with 1 mole of NaOH.
• Calculate moles of NaOH: Moles = Concentration × Volume = 0.1 mol/dm3 × 0.025 dm3 = 0.0025 moles
• Since the ratio is 1:1, moles of HCl needed = 0.0025 moles.
• Calculate volume of HCl needed: Volume = Moles / Concentration = 0.0025 moles / 0.1 mol/dm3 = 0.025 dm3 = 25 cm3.

Electrolysis

Electrolysis is a process that uses electricity to break down ionic compounds into their elements. This can occur in both molten and aqueous states. The reactions can be represented by half equations, which show the oxidation and reduction processes occurring at the electrodes.

Understanding acids, bases, and their reactions is fundamental in GCSE Chemistry, providing a basis for further studies in chemistry and related fields.