How do reactivity and properties change down Group II and Group VII?
Trends in reactivity, melting point, solubility and thermal stability down Group II, the reactions of Group II metals and compounds, the trends in Group VII including colour, volatility and oxidising power, displacement reactions and tests for halide ions.
A CCEA A-Level Chemistry answer on Group II and Group VII, covering the trends in reactivity and solubility down Group II and the reactions of its compounds, and the trends in colour, volatility and oxidising power down Group VII, halogen displacement reactions and the tests for halide ions.
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What this dot point is asking
CCEA wants you to describe and explain the trends down Group II (reactivity, solubility of the hydroxides and sulfates, thermal stability) and the reactions of Group II metals and compounds, and the trends down Group VII (colour, volatility, oxidising power), halogen displacement reactions, and the tests for halide ions.
Group II trends
Group II metals react with water to form hydroxides and hydrogen, reacting more vigorously down the group, for example . Magnesium reacts only very slowly with cold water but readily with steam to give the oxide and hydrogen. Their oxides and hydroxides are basic and react with acids to form salts and water.
Solubility and thermal stability explained
The two solubility trends run in opposite directions and are worth learning carefully. The hydroxides become more soluble down the group (magnesium hydroxide is almost insoluble, but barium hydroxide is fairly soluble), which is why calcium hydroxide solution, limewater, is used as a test for carbon dioxide. The sulfates become less soluble down the group (magnesium sulfate is very soluble, but barium sulfate is insoluble), which is why acidified barium chloride is the test for sulfate ions. The thermal stability of the carbonates and nitrates increases down the group, because the larger cations lower down have a smaller charge density and polarise the anion less, so the anion holds together at higher temperatures.
Group VII trends and oxidising power
A displacement reaction confirms the trend: chlorine displaces bromine and iodine from their solutions, and bromine displaces iodine, because the higher halogen is the stronger oxidising agent. For example (or as an ionic equation ). The colour of the displaced halogen confirms the reaction: bromine is orange, iodine is brown in solution (or a grey-black solid). Chlorine also disproportionates in water and in cold dilute alkali (its oxidation number going to both and ), which is the basis of bleach and of water chlorination.
Tests for halide ions
Examples in context
Example 1. Treating indigestion with Group II compounds. Antacid tablets often contain magnesium hydroxide or calcium carbonate, both basic Group II compounds that neutralise excess hydrochloric acid in the stomach: and . The carbonate also releases carbon dioxide, which is why some antacids cause burping. This uses the basic character of Group II oxides, hydroxides and carbonates that the dot point covers.
Example 2. Barium meals in medical imaging. A patient swallows a suspension of barium sulfate before an X-ray of the gut. Barium ions are toxic, but barium sulfate is safe to use because it is essentially insoluble (the Group II sulfate at the bottom of the group, the least soluble), so almost no barium dissolves into the body. This is a direct medical application of the trend that Group II sulfate solubility decreases down the group, and it is the same insolubility exploited in the qualitative test for sulfate ions.
Try this
Q1. Explain why reactivity increases down Group II. [2 marks]
- Cue. Atomic radius and shielding increase, so the outer electrons are lost more easily.
Q2. State the colour of the silver halide precipitate formed with bromide ions. [1 mark]
- Cue. Cream.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20205 marksChlorine is bubbled through separate solutions of potassium bromide and potassium iodide. Describe what you would observe in each case, write an ionic equation for each reaction, and explain the trend the results illustrate.Show worked answer →
Markers want the observations, both ionic equations, and the oxidising-power trend.
With potassium bromide, the colourless solution turns orange or yellow as bromine is displaced:
.
With potassium iodide, the colourless solution turns brown (or a black solid of iodine may form) as iodine is displaced:
.
In both cases chlorine displaces the lower halogen because chlorine is a stronger oxidising agent: it removes electrons from the halide ions more readily. This illustrates the trend that oxidising power decreases down Group VII, because the larger atoms lower down gain an electron less easily (more shielding and a larger radius).
Markers reward the two colour changes, the two ionic equations, and the explanation in terms of decreasing oxidising power down the group.
CCEA 20184 marksDescribe how the thermal stability of the Group II carbonates changes down the group, and explain the trend in terms of the polarising power of the cations.Show worked answer →
A trend-plus-explanation question.
The thermal stability of the Group II carbonates increases down the group: magnesium carbonate decomposes at a lower temperature than calcium carbonate, which decomposes at a lower temperature than barium carbonate. They all decompose to the oxide and carbon dioxide, for example .
The explanation is polarising power. Smaller cations higher up the group () have a high charge density, so they polarise (distort) the carbonate ion strongly, weakening a bond and making it decompose at a lower temperature. Going down the group the cations get larger, their charge density falls, they polarise the carbonate ion less, and so the carbonate is more stable and needs a higher temperature to decompose.
Markers reward the correct direction of the trend, the decomposition products, and the explanation in terms of decreasing polarising power (charge density) of the larger cations down the group.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)