Acids and bases (A-level only) revision notes

Acids and Bases in A Level Chemistry

Introduction

Acids and bases are fundamental concepts in chemistry that play a crucial role in various chemical reactions and processes. Understanding their properties, behaviors, and calculations is essential for advanced studies in chemistry.

Brønsted-Lowry Acids and Bases

• Definition: The Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors.
• Conjugate Acid-Base Pairs: When an acid donates a proton, it forms its conjugate base, while a base that accepts a proton forms its conjugate acid. For example, in the reaction of acetic acid (CH₃COOH) with water (H₂O):

CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

• Here, CH₃COOH is the acid, H₂O is the base, CH₃COO⁻ is the conjugate base, and H₃O⁺ is the conjugate acid.

Proton Transfer Equations

• Writing equations that show proton transfer is essential for understanding acid-base reactions. For instance, the dissociation of hydrochloric acid (HCl) in water can be represented as:

HCl + H₂O → Cl⁻ + H₃O⁺

• This equation illustrates the transfer of a proton from HCl to water, forming hydronium ions (H₃O⁺) and chloride ions (Cl⁻).

Distinguishing Acid Strength from Concentration

• Acid Strength: Refers to the ability of an acid to donate protons, which is a property of the acid itself.
• Concentration: Refers to the amount of acid present in a solution. A strong acid like HCl completely dissociates in water, while a weak acid like acetic acid only partially dissociates.

pH and Kw Calculations

• Calculating pH: The pH of a solution can be calculated from the hydrogen ion concentration using the formula:

pH = -log[H⁺]

• Conversely, the hydrogen ion concentration can be calculated from pH:

[H⁺] = 10^(-pH)

• Using Kw: The ion product of water (Kw) at 25°C is 1.0 x 10⁻¹⁴. It can be used to calculate the concentrations of hydrogen ions or hydroxide ions in a solution:

Kw = [H⁺][OH⁻]

Strong and Weak Acids

• Strong Acids: Completely dissociate in solution (e.g., HCl, H₂SO₄). Their pH can be calculated directly from their concentration.
• Weak Acids: Partially dissociate, and their pH is calculated using the acid dissociation constant (Ka). The expression for Ka is:

Ka = [H⁺][A⁻] / [HA]

• Where [HA] is the concentration of the undissociated acid, [H⁺] is the concentration of hydrogen ions, and [A⁻] is the concentration of the conjugate base.

pH Curves and Titrations

• Titration Calculations: Acid-base titrations involve the gradual addition of an acid to a base (or vice versa) to determine the concentration of one solution. The equivalence point is where the number of moles of acid equals the number of moles of base.
• pH Curves: The pH curve of a titration shows how the pH changes as the titrant is added. For strong acid-strong base titrations, the curve has a steep vertical section at the equivalence point. For weak acid-strong base titrations, the curve is more gradual.

Indicators and Their Selection

• Choosing Indicators: Indicators are substances that change color at a specific pH range. The choice of indicator depends on the pH at the equivalence point of the titration. For example, phenolphthalein is suitable for strong acid-strong base titrations, while methyl orange is better for strong acid-weak base titrations.

Buffer Solutions

• Acidic Buffers: Resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base.
• Basic Buffers: Similar to acidic buffers but consist of a weak base and its conjugate acid. They also resist changes in pH.
• Calculating pH of Buffers: The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([A⁻]/[HA])

• Where pKa is the negative logarithm of Ka.

Applications of Buffer Solutions

• Buffers are essential in biological systems, such as maintaining the pH of blood and cellular environments. They are also used in various industrial processes to control pH levels.

Conclusion

Understanding acids and bases is crucial for advanced chemistry. Mastery of these concepts allows for the application of acid-base chemistry in various scientific fields, including biochemistry, environmental science, and industrial chemistry.

A-Level Chemistry focus

Use Acids and Bases in A Level Chemistry to connect the exact AQA A-Level Chemistry 7405 subtopic to calculation, mechanism, evidence, practical reasoning, or explanation depth. Avoid generic GCSE-level statements.

How to use this revision note

Start by naming the chemical idea, then identify the relevant equation, observation, mechanism, trend, or practical method. Where calculations are involved, show the formula, substitution, working, final answer, and unit.

Exam focus

Strong A-Level answers justify each step. They separate evidence from conclusion, mechanism from product, observation from interpretation, and mathematical working from the final statement.

Common mistake

Do not rely on a memorised phrase if the question asks for reasoning. Check the subtopic wording, use precise terminology, and make sure each conclusion follows from the data or chemical principle given.