Ch 2 · Oxides · O Level Chemistry 5070
Topic 2 of 3 in Acids, Bases and Salts — O Level Chemistry 5070 — 47 lessons.
Introduction
Almost every element combines with oxygen, and the oxide it forms tells you a great deal about the element. Metals form basic oxides, non-metals form acidic oxides, and a few elements near the boundary form oxides that behave as both. Being able to classify an oxide and write the equation for its reaction with water, an acid or an alkali is a routine Paper 1 and Paper 2 skill.
Key ideas — Acidic oxides
Acidic oxides are the oxides of non-metals. Examples are carbon dioxide (CO_2), sulfur dioxide (SO_2), sulfur trioxide (SO_3), nitrogen dioxide (NO_2) and phosphorus(V) oxide (mathrmP_4O_10).
🧑🏫 Teacher's explanation
Think of oxides as a 'marriage' between an element and oxygen. The behavior of this oxide depends heavily on the 'personality' of the element involved. Non-metals are 'electron-hungry' (electronegative); when they bond with oxygen, they form oxides that are looking to give away protons or accept electrons, which is the hallmark of acidity.
📖 Definitions
🔍 Worked example — Easy
Identify the nature of the following oxides based on the element type: CO_2, CaO, and Al_2O_3.
🔍 Worked example — Standard
Write a balanced chemical equation for the reaction between sulfur dioxide (SO_2) and sodium hydroxide (NaOH) in aqueous solution.
🔍 Worked example — Exam-style
A sample of SO_3 gas is bubbled through water. Write the balanced equation for this reaction and state the type of solution formed.
⚠️ Common errors
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🎯 Exam question
Key ideas — Basic oxides
Basic oxides are the oxides of metals. Examples are copper(II) oxide (CuO), calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na_2O) and iron(III) oxide (Fe_2O_3).
🧑🏫 Teacher's explanation
Think of oxides as a chemical 'marriage' between an element and oxygen. The behavior of this marriage depends entirely on the personality of the metal involved. Metals are 'generous' with their electrons, and when they bond with oxygen, they create compounds that are eager to donate hydroxide ions (OH^-) or accept protons (H^+) from acids. This 'generous' nature is what we call 'basic'.
📖 Definitions
🔍 Worked example — Standard
Write the balanced chemical equation for the reaction between Magnesium oxide (MgO) and Sulfuric acid (H_2SO_4).
🔍 Worked example — Exam-style
A student reacts 0.50 mol of Sodium oxide (Na_2O) with excess water. Calculate the volume of the resulting Sodium hydroxide (NaOH) solution if the concentration is 2.0 mol/dm^3.
⚠️ Common errors
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🎯 Exam question
Key ideas — Oxides and the Periodic Table
The type of oxide an element forms reflects its metallic or non-metallic character. Across a period, elements change from metals on the left to non-metals on the right, so their oxides change from basic through amphoteric to acidic. In Period 3, sodium oxide and magnesium oxide are basic, aluminium oxide is amphoteric, and the oxides of silicon, phosphorus, sulfur and chlorine are acidic.
🧑🏫 Teacher's explanation
Think of the Periodic Table as a sliding scale of 'personality'. On the far left, we have metals, which are generous and like to give away electrons; their oxides are 'basic' and act like sponges for acids. On the far right, we have non-metals, which are greedy for electrons; their oxides are 'acidic' and act like sponges for bases.
📖 Definitions
🔍 Worked example — Easy
Predict the type of oxide formed by Magnesium (a metal) and state its reaction with HCl.
🔍 Worked example — Standard
A sample of SO_2 is dissolved in water. Write the balanced equation for this reaction and state the pH change of the solution.
🔍 Worked example — Exam-style
Aluminum oxide (Al_2O_3) is amphoteric. Write the balanced equation for its reaction with Hydrochloric acid (HCl).
⚠️ Common errors
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🎯 Exam question
(a) Classify the oxide as acidic, basic, or amphoteric. [1] (b) Write a balanced chemical equation for the reaction of this oxide with sulfuric acid. [2] [3 marks]
Key ideas — Amphoteric oxides
amphoteric oxides. An amphoteric oxide reacts with both acids and bases to produce a salt and water. The two you must know are aluminium oxide, Al_2O_3, and zinc oxide, ZnO. Aluminium and zinc are metals, but they lie close to the metal to non-metal boundary, so their oxides show both kinds of behaviour.
🧑🏫 Teacher's explanation
In chemistry, most oxides are 'one-sided': they are either acidic (non-metal oxides) or basic (metal oxides). However, some oxides are 'double agents'. These are called amphoteric oxides. Think of them like a person who can act as both a defender and an attacker depending on who they are interacting with.
📖 Definitions
🔍 Worked example — Easy
Write a balanced equation for the reaction between Zinc oxide and Hydrochloric acid.
🔍 Worked example — Standard
Calculate the mass of Al_2O_3 required to react completely with 12.0 g of HCl.
🔍 Worked example — Exam-style
A student adds excess NaOH(aq) to a solution containing Zn(OH)_2(s). Write the equation for the reaction between the hydroxide and the alkali.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
\n(a) Write a balanced chemical equation for this reaction. [2] (b) Explain why Al_2O_3 is classified as amphoteric. [1] [3 marks]
Worked example
Question. Oxide A is a white solid that dissolves in dilute hydrochloric acid and also in hot sodium hydroxide solution. Oxide B is a colourless gas that turns damp blue litmus red. Oxide C is a black solid that reacts with warm dilute sulfuric acid to give a blue solution but does not react with sodium hydroxide.
🔍 Worked example — Easy
Classify the following oxides: CO_2, MgO, and Al_2O_3.
🔍 Worked example — Standard
Write a balanced equation for the reaction between copper(II) oxide (CuO) and dilute sulfuric acid (H_2SO_4).
🔍 Worked example — Exam-style
Oxide X is a white solid that reacts with both HCl and NaOH. Identify the type of oxide and write the equation for its reaction with HCl.
⚠️ Common errors
Common mistakes
Practice
Answers Magnesium oxide is basic because magnesium is a metal [B1]; sulfur trioxide is acidic because sulfur is a non-metal [B1]. (a) SO_2 + H_2O arrow H_2SO_3 [B1]; (b) CaO + H_2O arrow Ca(OH)_2 [B1]. CO_2 + 2KOH arrow K_2CO_3 + H_2O [B1]; potassium carbonate [B1]. An oxide that reacts with both acids and bases [B1] to form a salt and water [B1]; Al_2O_3 + 2NaOH arrow 2NaAlO_2 + H_2O [B1].