Ch 1 · The Characteristic Properties of Acids and Bases · O Level Chemistry 5070
Topic 1 of 3 in Acids, Bases and Salts — O Level Chemistry 5070 — 58 lessons.
Introduction
Acids and bases are everywhere: citric acid in fruit, hydrochloric acid in your stomach and calcium hydroxide spread on acidic soil. This lesson gives you the definitions, the indicator colours and the characteristic reactions that examiners expect word for word. candidates also need the proton-transfer definitions and the difference between strong and weak acids.
Key ideas — Acids, bases, alkalis and indicators
An acid is a substance that produces hydrogen ions, H^+, when dissolved in water. Common laboratory acids are hydrochloric acid (HCl), sulfuric acid (H_2SO_4), nitric acid (HNO_3) and ethanoic acid (CH_3COOH).
🧑🏫 Teacher's explanation
Think of acids and bases as a game of 'hot potato' played with protons (H^+ ions). An acid is like a person holding a hot potato (a proton) who is eager to throw it away. A base is like a person with a glove, ready to catch that proton. When an acid gives away its proton, it becomes a negative ion; when a base catches it, it becomes a negative ion.
📖 Definitions
🔍 Worked example — Easy
Identify the products of the reaction between hydrochloric acid and magnesium metal: 2HCl(aq) + Mg(s) arrow ? + ?
🔍 Worked example — Standard
Calculate the concentration of H^+ ions in a solution where the concentration of H_2SO_4 is 0.05 mol/dm^3. Assume complete dissociation of the acid.
🔍 Worked example — Exam-style
A student adds excess calcium carbonate to dilute nitric acid. Write the balanced equation for the reaction and state the observations.
⚠️ Common errors
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🎯 Exam question
Key ideas — The three characteristic reactions of acids
Every acid reacts with metals, bases and carbonates in the same way, and the salt formed takes its name from the acid: chlorides from hydrochloric acid, sulfates from sulfuric acid, nitrates from nitric acid.
🧑🏫 Teacher's explanation
Think of an acid as a chemical 'giver' that is looking to get rid of its hydrogen ions (H^+). When an acid meets a metal, it's like a high-energy collision where the metal pushes the hydrogen out, resulting in a salt and hydrogen gas. When it meets a base, they perform a 'handshake' called neutralisation, where the H^+ from the acid and the OH^- from the base combine to form stable water (H_2O).
📖 Definitions
🔍 Worked example — Easy
Write a balanced equation for the reaction between magnesium metal and dilute sulfuric acid. State the states of all reactants and products.
🔍 Worked example — Standard
Calculate the mass of calcium carbonate (CaCO_3) required to react completely with 20.0 g of nitric acid (HNO_3). Assume M_r values: Ca=40, C=12, O=16, H=1, N=14.
🔍 Worked example — Exam-style
A student reacts 25.0 cm^3 of 0.10 mol/dm^3 HCl with 25.0 cm^3 of NaOH. Calculate the concentration of the NaOH solution if the resulting solution is neutral.
⚠️ Common errors
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🎯 Exam question
Key ideas — The characteristic reactions of bases
Base + acid → salt + water. This is the same neutralisation viewed from the other side:
🧑🏫 Teacher's explanation
Think of an acid-base reaction like a tug-of-war over a hydrogen ion (H^+). An acid is like a person holding a ball (the proton) and wanting to give it away, while a base is like a person waiting to catch it. When they meet, the base 'accepts' the proton, resulting in a neutral salt and water.
📖 Definitions
🔍 Worked example — Easy
Write a balanced equation for the reaction between aqueous sodium hydroxide and hydrochloric acid.
🔍 Worked example — Standard
A student reacts solid ammonium sulfate with aqueous sodium hydroxide. Write the balanced equation for the products formed, including the gas produced.
🔍 Worked example — Exam-style
Calculate the mass of sodium hydroxide required to completely react with 14.0 g of hydrochloric acid in a neutralisation reaction.
⚠️ Common errors
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🎯 Exam question
(a) Write a balanced chemical equation for this reaction, including state symbols. [2] (b) Describe a chemical test to confirm the identity of the gas produced. [2] [4 marks]
Key ideas — Neutralisation, pH and universal indicator
In any reaction between an acid and an alkali the hydrogen ions from the acid combine with the hydroxide ions from the alkali to make water. The ionic equation is the same whatever the acid and alkali:
🧑🏫 Teacher's explanation
Think of an acid-base reaction like a dance where two people are looking for a partner to become stable. An acid is like someone looking to give away a gift (a proton, H^+), while a base is looking to receive one. When they meet, they 'neutralize' each other's reactive tendencies by forming a stable molecule, water.
📖 Definitions
🔍 Worked example — Easy
A solution has a pH of 3. Another solution has a pH of 6. Which solution has a higher concentration of H^+ ions?
🔍 Worked example — Standard
Write the ionic equation for the neutralisation reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH).
🔍 Worked example — Exam-style
A student adds universal indicator to a solution. The solution turns dark purple. State the relative alkalinity of this solution compared to a solution that turns light blue.
⚠️ Common errors
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🎯 Exam question
(a) The student finds that Solution A turns the indicator red. (b) Solution B turns the indicator purple. (c) Write the ionic equation for the reaction between Solution A and Solution B. [1] (d) Explain why Solution A is more acidic than Solution C, which turns the indicator yellow. [2] [3 marks]
Key ideas — Proton transfer and acid strength
proton donors and acceptors. An acid is a proton donor and a base is a proton acceptor. A hydrogen ion is a proton: hydrogen chloride donates one to water when it dissolves, and ammonia accepts one.
🧑🏫 Teacher's explanation
Imagine you are at a crowded party. An 'acid' is like a person carrying a handful of tennis balls (protons) who is eager to throw them to anyone nearby. A 'base' is like a person with a glove, ready to catch those balls. The 'strength' of the acid isn't about how many people are at the party (that is concentration), but how likely each person is to actually throw the ball.
📖 Definitions
🔍 Worked example — Easy
Calculate the concentration of H^+ ions in 0.1 mol/dm^3 of HCl(aq), assuming it is a strong acid.
🔍 Worked example — Standard
A solution of ethanoic acid CH_3 extCOOH has a concentration of 0.5 mol/dm^3. If only 1\% of the molecules dissociate into CH_3 extCOO^- and H^+, calculate the concentration of H^+ ions.
🔍 Worked example — Exam-style
Compare the pH of 0.1 mol/dm^3 HCl and 0.1 mol/dm^3 CH_3 extCOOH. Which has a higher pH and why?
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student adds a spatula of copper(II) carbonate to dilute sulfuric acid until no more reacts.
🔍 Worked example — Easy
Predict the products when magnesium reacts with dilute hydrochloric acid. Write the balanced symbol equation.
🔍 Worked example — Standard
A student uses universal indicator to test a solution. The solution turns red. State the pH range and describe the concentration of H^+ ions relative to a neutral solution.
🔍 Worked example — Exam-style
A student adds excess copper(II) carbonate to dilute sulfuric acid. (a) State two observations. (b) Write a balanced symbol equation. (c) The student repeats the experiment with ethanoic acid of the same concentration. State and explain how the rate of reaction differs.
⚠️ Common errors
Common mistakes
Practice
Answers Methyl orange is red [B1]; thymolphthalein is colourless [B1]. Zn + H_2SO_4 arrow ZnSO_4 + H_2 [M1 formulae, A1 balanced]; hydrogen gives a squeaky pop with a lighted splint [B1]. Ammonia [B1]; (NH_4)_2SO_4 + 2NaOH arrow Na_2SO_4 + 2H_2O + 2NH_3 [M1 formulae, A1 balanced]. An acid is a proton donor [B1]; hydrochloric acid is a strong acid and is completely dissociated, whereas ethanoic acid is only partially dissociated [B1], so the hydrochloric acid has a higher concentration of hydrogen ions and a lower pH [B1].