Ch 1 · Similarities and trends in the properties of the Group 2 metals and their compounds · AS & A Level Chemistry 9701

Topic 1 of 1 in Group 2 — AS & A Level Chemistry 9701 — 58 lessons.

Introduction

The Group 2 metals (Mg, Ca, Sr, Ba) show a graded family of reactions with oxygen, water and acids, and their compounds show clear, examinable trends in solubility and thermal stability down the group. Every trend traces back to the same cause: increasing ionic radius weakens electrostatic attraction to a neighbouring ion, whether that neighbour is a hydroxide ion, a sulfate ion, or the lattice holding a solid together.

Key ideas — Reactions of the elements with oxygen, water and dilute acid

All Group 2 metals burn in oxygen to form white ionic oxides, 2M(s) + O_2(g) arrow 2MO(s), and react with dilute acids to give a salt and hydrogen, e.g. Mg(s) + 2HCl(aq) arrow MgCl_2(aq) + H_2(g). Reactivity with water increases down the group. Mg reacts only very slowly with cold water (readily with steam, Mg(s) + H_2O(g) arrow MgO(s) + H_2(g)); Ca, Sr and Ba react increasingly vigorously with cold water: Ca(s) + 2H_2O(l) arrow Ca(OH)_2(aq) + H_2(g). This trend arises because first and second ionisation energies decrease down the group (the outer electrons are further from the nucleus and more shielded), so the metals lose electrons and form ions more readily.

🧑‍🏫 Teacher's explanation

Think of Group 2 metals like a group of energetic athletes. As you move down the group, the athletes get 'larger' and their outer electrons are held more loosely by the nucleus. Because the outer electrons are further away and shielded by more inner shells, it becomes much easier for the metal to 'give up' those electrons to react.

📖 Definitions

🔍 Worked example — Easy

Calculate the mass of Magnesium required to react completely with 50.0 cm^3 of 1.00 mol/dm^3 ceHCl according to the equation: ceMg(s) + 2HCl(aq) arrow MgCl_2(aq) + H_2(g).

🔍 Worked example — Standard

Write the balanced equation for the reaction of Calcium with steam and identify the products.

🔍 Worked example — Exam-style

Predict the trend in the temperature required to thermally decompose Group 2 carbonates (ceMCO_3) and explain why.

⚠️ Common errors

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🎯 Exam question

Key ideas — Reactions of the oxides, hydroxides and carbonates

The oxides react with water to give the hydroxide: MO(s) + H_2O(l) arrow M(OH)_2(aq/s), and with dilute acid to give a salt and water: MgO(s) + 2HCl(aq) arrow MgCl_2(aq) + H_2O(l). The hydroxides are bases and neutralise dilute acids: Ca(OH)_2(aq) + 2HCl(aq) arrow CaCl_2(aq) + 2H_2O(l). The carbonates are insoluble but react with dilute acid, releasing carbon dioxide: MgCO_3(s) + 2HCl(aq) arrow MgCl_2(aq) + H_2O(l) + CO_2(g).

🧑‍🏫 Teacher's explanation

Think of Group 2 oxides and hydroxides as a 'chemical staircase' of reactivity. As you move down the group, the metal ions get larger. This change in size affects how tightly the ions are held together and how much they 'like' to stay dissolved in water or react with acids.

📖 Definitions

🔍 Worked example — Easy

Write a balanced equation for the reaction between magnesium oxide and sulfuric acid.

🔍 Worked example — Standard

A student reacts calcium carbonate with dilute hydrochloric acid. Write the balanced equation for this reaction.

🔍 Worked example — Exam-style

Predict the trend in thermal stability for Group 2 carbonates from MgCO_3 to BaCO_3. Explain why.

⚠️ Common errors

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🎯 Exam question

Key ideas — Thermal decomposition of the nitrates and carbonates

All Group 2 carbonates decompose on heating to the oxide and carbon dioxide: MCO_3(s) arrow MO(s) + CO_2(g). The nitrates decompose to the oxide, nitrogen dioxide and oxygen: 2M(NO_3)_2(s) arrow 2MO(s) + 4NO_2(g) + O_2(g). Thermal stability of both the carbonates and the nitrates increases down the group: Mg salts decompose at the lowest temperature, Ba salts at the highest. The smaller, more highly charged mathrmMg^2+ ion polarises the large carbonate or nitrate anion strongly, distorting its electron cloud and weakening a bond within the anion enough for it to break down on heating; down the group, the cation becomes larger with a lower charge density, polarises the anion less, and a higher temperature is needed to decompose it.

🧑‍🏫 Teacher's explanation

Think of thermal stability as how much 'heat energy' you need to break a molecule apart. Imagine you are trying to pull two magnets apart. If one magnet is very small and powerful (high charge density), it pulls on the other object so hard that it distorts its shape, making it much easier to snap.

📖 Definitions

🔍 Worked example — Easy

Write a balanced chemical equation for the thermal decomposition of magnesium carbonate, MgCO_3(s).

🔍 Worked example — Standard

Predict whether CaCO_3 or BaCO_3 is more thermally stable and explain why using the concept of charge density.

🔍 Worked example — Exam-style

A sample of Mg(NO_3)_2(s) is heated. Write the balanced equation for the decomposition and state the observation regarding the gas produced.

⚠️ Common errors

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🎯 Exam question

Key ideas — Solubility trends: hydroxides and sulfates

Solubility trends run in opposite directions for these two anions. Mg(OH)_2 is only sparingly soluble, while Ba(OH)_2 is considerably more soluble: hydroxide solubility increases down the group. MgSO_4 is freely soluble, while BaSO_4 is essentially insoluble: sulfate solubility decreases down the group. This reflects the balance between lattice energy and hydration energy: both fall in magnitude as the cation gets larger, but because OH^- is a small anion, lattice energy falls faster than hydration energy down the group, favouring dissolving; because mathrmSO_4^2- is a large anion, lattice energy changes little down the group while hydration energy of the cation falls, favouring the solid.

🧑‍🏫 Teacher's explanation

Imagine you are trying to pull a group of people apart from a tight crowd. To dissolve a solid, you need to overcome the 'Lattice Energy' (the glue holding the crystal together) using 'Hydration Energy' (the energy released when water molecules surround the ions). If the glue is stronger than the water's pull, the substance stays solid.

📖 Definitions

🔍 Worked example — Easy

Predict the relative solubility of Mg(OH)_2 and Ba(OH)_2 based on the trend in hydroxide solubility.

🔍 Worked example — Standard

Explain why BaSO_4 is insoluble while MgSO_4 is soluble.

🔍 Worked example — Exam-style

A student is given two white powders, Ca(OH)_2 and CaSO_4. Which one is expected to be more soluble in water?

⚠️ Common errors

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🎯 Exam question

Key ideas — Predicting Group 2 trends and their applications

Because reactivity with water and thermal stability of carbonates/nitrates both increase down the group in a predictable way, an unfamiliar Group 2 reaction or decomposition temperature can be placed correctly relative to known members. These trends also underlie everyday and industrial uses: Mg(OH)_2 and CaCO_3 neutralise excess stomach acid; Ca(OH)_2 (slaked lime) is used to raise the pH of acidic soil; BaSO_4's extreme insolubility (and hence non-toxicity, despite mathrmBa^2+ being poisonous) allows it to be used as a "barium meal" in medical X-ray imaging.

🧑‍🏫 Teacher's explanation

Think of Group 2 elements like a group of siblings growing older. As you go down the group (from Magnesium to Barium), the atoms get larger because they add more electron shells. Because the outer electrons are further from the nucleus, they are held less tightly, making the metals more reactive. It is like a person trying to hold onto a ball; the further away they reach, the easier it is for someone to snatch it away.

All chapters in Group 2

Similarities and trends in the properties of the Group 2 metals and their compounds