OxfordAQA International GCSE Chemistry: Transition metals (9202) – Revision Notes

These notes condense section 3.7.2 Transition metals (statements a and b) of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. The qualification is not tiered, and the specification says either paper may assess any part of it. Section 3.7.2 has no required practical.

The Transition metals study guide has the full explanations and worked examples. When you are ready, test yourself with the Transition metals practice questions. Tick off statements on the printable checklist and find other topics on the course hub. A free diagnostic shows which topics need the most work.

The two statements in one line each

• 3.7.2 a: transition metals sit in the centre of the periodic table, between Groups 2 and 3; many have ions with different charges, form coloured compounds and are useful as catalysts.
• 3.7.2 b: compared with Group 1 they have higher melting points (except mercury) and higher densities, are stronger and harder, and are much less reactive, so they do not react as vigorously with water or oxygen.

Key words

• Transition metal: an element in the middle block of the periodic table, between Groups 2 and 3.
• Ion: a charged particle. Metals form positive ions by losing electrons.
• Roman numeral in a name: the charge on the metal ion. Copper(I) = Cu⁺; vanadium(V) = V⁵⁺.
• Catalyst: alters the rate of a reaction; the same amount is left at the end.
• Density: mass per unit volume, in g/cm³.
• Strong: hard to break. Hard: hard to scratch or dent.

Group 1 against transition metals

Property	Group 1 metals	Transition metals
Melting point	low	high (mercury is the exception)
Density	low; the first three float on water	high
Hardness	soft enough to slice with a knife	hard
Strength	weak	strong
With water	react with cold water, release hydrogen, form an alkali	no reaction (copper) or very slow (iron); hot iron reacts with steam
With oxygen	tarnish within seconds; stored under oil	slow, or only on heating
Ions	always 1+	many form ions with different charges
Compounds	white; colourless solutions	many coloured

The mercury exception

Mercury is liquid at room temperature. It melts at −39 °C, lower than any Group 1 metal. Its density, 13.5 g/cm³, is still far higher than any Group 1 metal’s. The exception is for melting point only.

Method: a “compare” answer

1. Take one property at a time.
2. Say which metal is higher, harder or more reactive, using a comparative word.
3. If data are given, quote both values or work out a difference or a ratio.
4. Move on. Never make the same point twice in different words.

Small reminder. Silver (10.5 g/cm³) against rubidium (1.53 g/cm³): 10.5 ÷ 1.53 = 6.9, so silver is about seven times as dense.

Ions with different charges

Element	Ions	Example
iron	Fe²⁺, Fe³⁺	iron(III) nitrate Fe(NO₃)₃
copper	Cu⁺, Cu²⁺	copper(I) sulfide Cu₂S
cobalt	Co²⁺, Co³⁺	cobalt(III) fluoride CoF₃
manganese	Mn²⁺ and others	manganese(II) sulfate MnSO₄
titanium	Ti⁴⁺ and others	titanium(IV) oxide TiO₂
vanadium	V⁵⁺ and others	vanadium(V) oxide V₂O₅

Method: charge from a formula

1. Add up the negative charge (O²⁻, S²⁻, SO₄²⁻, F⁻, Br⁻, NO₃⁻).
2. Divide by the number of metal ions.
3. The answer is the charge on each metal ion, and the Roman numeral in the name.

Worked reminder: V₂O₅ has 5 × 2− = 10− in total. Two vanadium ions share 10+, so each is V⁵⁺: vanadium(V) oxide.

Method: formula from a name

1. Read the metal ion’s charge from the numeral.
2. Choose enough negative ions to balance it.
3. Use brackets for a group ion that appears more than once: Fe(NO₃)₃.

Coloured compounds

Compound or ion	Colour
copper(II) sulfate, hydrated or in solution	blue
anhydrous copper(II) sulfate	white
iron(III) oxide	red-brown
hydrated cobalt(II) chloride	pink
potassium manganate(VII)	purple
zinc compounds	white

Zinc and anhydrous copper(II) sulfate are the reason the specification says many, not all.

Sodium hydroxide test (3.4.3 c)

Ion	Precipitate	Ionic equation
Cu²⁺	blue	Cu²⁺ + 2OH⁻ → Cu(OH)₂
Fe²⁺	green	Fe²⁺ + 2OH⁻ → Fe(OH)₂
Fe³⁺	brown	Fe³⁺ + 3OH⁻ → Fe(OH)₃

The number of hydroxide ions equals the charge on the metal ion. Iron(II) and iron(III) are the same element but give different colours.

Catalysts

Process	Catalyst	Reaction
Haber process (3.8.3 c)	iron	nitrogen + hydrogen ⇌ ammonia, about 450 °C, about 200 atmospheres
Contact process, stage 2 (3.8.3 d)	vanadium(V) oxide, V₂O₅	2SO₂ + O₂ ⇌ 2SO₃, about 450 °C, atmospheric pressure

Why industry uses catalysts: faster reactions give more product in a given time and reduce costs (3.8.1 h). The catalyst is not used up, so it can be used again and again.

How to spot a catalyst from an experiment: with it, the reaction goes faster, and at the end you recover the same mass of it (or see the same colour of solution).

Must-know distinctions

• Less reactive (transition) vs more reactive (Group 1). Not “unreactive”: iron rusts.
• Denser vs heavier. Density compares equal volumes.
• Strong vs hard. Name both when asked about strength and hardness.
• Iron(II) Fe²⁺, green precipitate, vs iron(III) Fe³⁺, brown precipitate.
• Catalyst (not used up) vs reactant (used up).
• Aluminium (Group 3, white precipitate that dissolves in excess) vs a transition metal ion (coloured precipitate).

Property to use

Exam questions may name a use and ask for the property behind it. Name the property, then say why a Group 1 metal would fail.

Use	Property	Why not a Group 1 metal
titanium in aircraft frames	strong; high melting point	soft and weak; melts easily
tungsten lamp filaments	very high melting point	would melt at a low temperature
copper hot-water pipes	does not react with water	reacts with cold water, releasing hydrogen
iron catalyst in ammonia production	catalyst; solid at about 450 °C	would be liquid, and is far too reactive
coloured pottery glazes	coloured compounds	compounds are white

Where 3.7.2 links to other sections

Linked section	What the question might ask
3.3.1 c	why copper is used for wiring and plumbing
3.4.3 c	precipitate colours for Cu²⁺, Fe²⁺ and Fe³⁺
3.7.1 a	Group 1 compounds are white and dissolve to give colourless solutions
3.8.1 g and h	what a catalyst is, and why industry uses one
3.8.3 c and d	iron in the Haber process; V₂O₅ in the contact process

Small reminder (catalyst data). A transition metal powder doubles the volume of gas collected in a fixed time, and the same mass is recovered at the end. It is a catalyst: faster and not used up. If the mass fell, it was being used up as a reactant.

Quick self-test

1. Between which two groups are the transition metals found?
2. State the three typical properties that many transition metals have.
3. Give the charge on the metal ion in TiO₂.
4. Name the compound CuBr.
5. Write the formula of manganese(II) sulfate.
6. Which transition metal has a lower melting point than any Group 1 metal?
7. Tungsten melts at 3422 °C and rubidium at 39 °C. Calculate the difference.
8. Why does an iron nail stay shiny for days in dry air when a freshly cut piece of sodium does not?
9. A solution gives a green precipitate with sodium hydroxide solution. Name the ion.
10. Name the catalyst in stage 2 of the contact process.
11. Give one reason why industrial processes use catalysts.
12. True or false: every compound of a transition metal is coloured.

Answers

1. Group 2 and Group 3.
2. Ions with different charges; coloured compounds; useful as catalysts.
3. Two O²⁻ give 4−, so the ion is Ti⁴⁺.
4. Br⁻ is 1−, so copper is Cu⁺: copper(I) bromide.
5. Mn²⁺ and SO₄²⁻: MnSO₄.
6. Mercury (melting point −39 °C).
7. 3422 − 39 = 3383 °C.
8. Iron is much less reactive than sodium, so it reacts with oxygen much more slowly.
9. Iron(II), Fe²⁺.
10. Vanadium(V) oxide, V₂O₅.
11. It speeds up the reaction, so more product is made in a given time and costs are reduced.
12. False. Many are coloured, but zinc compounds and anhydrous copper(II) sulfate are white.

Where marks are usually lost

• Writing that transition metals are more reactive than Group 1, or that they are unreactive.
• Using “all” instead of “many” for the three typical properties.
• Forgetting mercury when asked about melting points, or applying the exception to density.
• Treating the Roman numeral as a number of atoms.
• Writing the wrong number of hydroxide ions in an ionic equation, for example Fe³⁺ + 2OH⁻.
• Swapping the iron(II) and iron(III) precipitate colours.
• Calling aluminium a transition metal because it forms a precipitate.
• Giving a fact about one metal only when the question says “compare”.
• Saying a catalyst is used up, or that it changes the amount of product rather than the rate.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. Section 3.7.2 Transition metals, statements a and b.