OxfordAQA International GCSE Chemistry: Transition metals (9202)

This study guide teaches section 3.7.2 Transition metals of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. It covers both statements in the section, 3.7.2 a and 3.7.2 b. The qualification is not tiered, so all of it applies to you. The specification says content from any part of it may be assessed on either paper.

Section 3.7.2 has no required practical. The five required practicals listed in section 6.2 of the specification belong to other sections.

The course hub lists every topic, and the printable checklist lets you tick off each statement. A free diagnostic can show you which topics to work on first.

What this unit covers

Specification	What you must be able to do
3.7.2 a	Locate the transition metals between Groups 2 and 3; state that many have ions with different charges, form coloured compounds and are useful as catalysts
3.7.2 b	Compare transition metals with Group 1: higher melting points (except mercury) and densities; stronger and harder; much less reactive, so they do not react as vigorously with water or oxygen

The section is short, but questions on it often bring in ideas from elsewhere in the course: ion tests (3.4.3), catalysts and rates (3.8.1) and the Haber and contact processes (3.8.3). The examples below show those links.

Where the transition metals are (3.7.2 a)

The transition metals make up the wide central block of the periodic table, lying between Groups 2 and 3. The first row runs from scandium to zinc, in the same period as potassium and calcium. Familiar examples include titanium, chromium, manganese, iron, cobalt, nickel, copper, silver, gold and mercury.

If a question asks you to locate them, “in the central block, between Groups 2 and 3” matches the specification’s wording.

Three typical properties (3.7.2 a)

The specification says that many transition metals have ions with different charges, form coloured compounds and are useful as catalysts. Each property is taken in turn below.

Notice the word “many”. Not every transition metal shows every property. Zinc sits in the central block but forms only one ion, Zn²⁺, and its compounds are white. Even copper has a white compound: anhydrous copper(II) sulfate is a white powder, and it only turns blue when water is added. So write “many” or “most”, never “all”.

Ions with different charges

Every Group 1 metal forms only 1+ ions. A transition metal can often lose different numbers of electrons, so the same element forms more than one positive ion. The charge is shown as a Roman numeral in the name of the compound.

Element	Common ions	Example compounds
iron	Fe²⁺, Fe³⁺	iron(II) hydroxide Fe(OH)₂; iron(III) nitrate Fe(NO₃)₃
copper	Cu⁺, Cu²⁺	copper(I) sulfide Cu₂S; copper(II) nitrate Cu(NO₃)₂
cobalt	Co²⁺, Co³⁺	cobalt(II) carbonate CoCO₃; cobalt(III) fluoride CoF₃
titanium	Ti⁴⁺ and others	titanium(IV) oxide TiO₂
vanadium	V⁵⁺ and others	vanadium(V) oxide V₂O₅

The Roman numeral is the charge on the metal ion. It does not tell you how many metal atoms are in the formula.

Worked example 1: charge from a formula

Find the charge on the metal ion in (a) Cu₂S and (b) CoF₃, and name each compound.

(a) Sulfide is S²⁻. Two copper ions must balance 2−, so each copper ion is 1+. The compound is copper(I) sulfide.

(b) Fluoride is F⁻. Three of them give 3−, so the cobalt ion is 3+. The compound is cobalt(III) fluoride.

Method: total negative charge ÷ number of metal ions = charge on each metal ion. Check that the positive and negative totals match.

Worked example 2: formula from a name

Write the formula of iron(III) nitrate.

1. Iron(III) means Fe³⁺. Nitrate is NO₃⁻.
2. Three nitrate ions are needed to balance one Fe³⁺.
3. Put brackets round the nitrate group: Fe(NO₃)₃.

Coloured compounds

Group 1 compounds are white, and their solutions are colourless (3.7.1 a). Many transition metal compounds are coloured, and the colour often changes with the charge on the ion. Some colours you will meet:

Compound	Colour
hydrated copper(II) sulfate, or its solution	blue
iron(III) oxide (as in rust)	red-brown
hydrated cobalt(II) chloride	pink
potassium manganate(VII)	purple

The ion tests in section 3.4.3 c use these colours directly. With sodium hydroxide solution:

• copper(II) ions give a blue precipitate, Cu(OH)₂;
• iron(II) ions give a green precipitate, Fe(OH)₂;
• iron(III) ions give a brown precipitate, Fe(OH)₃.

Iron(II) and iron(III) give different colours. Here, two of the typical properties work together: different charges on the ions, and different colours.

Worked example 3: which solutions contain transition metal ions?

Three solutions are tested with sodium hydroxide solution.

Solution	Colour of solution	With sodium hydroxide solution
P	colourless	white precipitate that dissolves in excess
R	yellow-brown	brown precipitate
T	pale green	green precipitate

(a) Identify the metal ion in each. (b) Write the ionic equation for solution R.

(a) P contains aluminium ions (3.4.3 b): white, dissolving in excess. Aluminium is in Group 3, so it is not a transition metal, and its solution is colourless. R contains iron(III) ions. T contains iron(II) ions.

(b) Fe³⁺ needs three hydroxide ions: Fe³⁺ + 3OH⁻ → Fe(OH)₃.

R and T contain the same element with different charges, and the two solutions are different colours.

Catalysts

A catalyst alters the rate of a reaction and the same amount of it is still there at the end (3.8.1 g). Many transition metals and their compounds are good catalysts. Two are named in this specification:

• iron in the Haber process, which makes ammonia from nitrogen and hydrogen at about 450 °C and about 200 atmospheres (3.8.3 c);
• vanadium(V) oxide, V₂O₅, in stage 2 of the contact process, where sulfur dioxide reacts with oxygen to make sulfur trioxide (3.8.3 d).

Industry uses catalysts because a faster reaction means more product in a given time, which reduces costs (3.8.1 h). The catalyst is not used up, so a small amount works for a long time.

Worked example 4: seeing a catalyst take part

You do not need to learn this reaction. It is here because it shows two typical properties at once.

A solution of potassium sodium tartrate and hydrogen peroxide is heated to about 75 °C. Very little happens. A few drops of pink cobalt(II) chloride solution are added. Within seconds the mixture turns green and fizzes strongly as gas is given off. About half a minute later the fizzing slows and the pink colour returns.

(a) Explain why cobalt(II) chloride is described as a catalyst. (b) Suggest what the green colour shows.

(a) The reaction becomes much faster once the cobalt compound is added. At the end the pink colour returns, so the cobalt(II) ions are still there: the catalyst has not been used up.

(b) The cobalt ions change while the reaction goes on. Pink Co²⁺ is changed to a green compound of Co³⁺, which is then changed back to Co²⁺. The cobalt takes part in the reaction but is regenerated. This is possible because cobalt forms ions with different charges.

Comparing transition metals with Group 1 (3.7.2 b)

Melting point and density

Compared with Group 1 metals, transition metals have higher melting points (except mercury) and higher densities. The values below are rounded standard data. You will not need to recall them, but you may be given figures like these.

Metal	Type	Melting point / °C	Density / g/cm³
sodium	Group 1	98	0.97
rubidium	Group 1	39	1.53
caesium	Group 1	28	1.93
titanium	transition	1668	4.51
silver	transition	962	10.5
tungsten	transition	3422	19.3
mercury	transition	−39	13.5

Mercury is the exception for melting point. It is a liquid at room temperature, and it melts at a lower temperature than any Group 1 metal. It is still much denser than any Group 1 metal, so the exception applies to melting point only.

Worked example 5: comparing two metals from data

Use the table to compare tungsten with caesium in melting point and density.

1. Melting point. Tungsten’s melting point is far higher: 3422 − 28 = 3394 °C higher.
2. Density. Tungsten is much denser: 19.3 ÷ 1.93 = 10.0, so tungsten is ten times as dense as caesium.

Each point names a property, states which metal is higher, and backs it with a figure.

Worked example 6: is mercury really an exception?

Using the table, show that mercury breaks the melting point pattern but not the density pattern.

1. Mercury melts at −39 °C and caesium at 28 °C. Mercury’s melting point is 28 − (−39) = 67 °C lower than caesium’s, so it does not fit “higher melting point”.
2. Mercury’s density is 13.5 g/cm³ and caesium’s is 1.93 g/cm³. 13.5 ÷ 1.93 = 7.0, so mercury is about seven times as dense. It fits “higher density”.

Strength and hardness

Transition metals are stronger and harder than Group 1 metals. Sodium and the other alkali metals are soft enough to cut with a knife. Titanium and iron (as steel) are used for aircraft parts, tools and bridges because they keep their shape under large forces. Strong means hard to break; hard means hard to scratch or dent. The specification lists both, so mention both.

Reactivity with water and oxygen

Compared with Group 1, transition metals are much less reactive, so they do not react as vigorously with water or oxygen.

With water

• Group 1 metals react with cold water, releasing hydrogen and forming an alkaline hydroxide solution. Rubidium reacts violently: 2Rb + 2H₂O → 2RbOH + H₂.
• Copper does not react with water, which is why it is used for water pipes (3.3.1 c). Iron reacts only very slowly at room temperature, and rusting needs oxygen as well as water. Hot iron reacts with steam: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

With oxygen

• A freshly cut surface of a Group 1 metal goes dull within seconds as an oxide forms, so these metals are stored under oil.
• Transition metals react slowly or only on heating. A bright iron nail stays shiny for days in dry air. Heated copper slowly forms a black layer of copper(II) oxide: 2Cu + O₂ → 2CuO.

“Much less reactive” does not mean “unreactive”. Iron does rust, and hot iron wool burns in oxygen. Use comparative words: less vigorous, slower, needs heating.

Linking properties to uses

Use	Key property	Problem with a Group 1 metal
titanium in aircraft frames	strong, high melting point	soft, weak, melts easily
tungsten lamp filaments	very high melting point	would melt far below working temperature
copper water pipes	does not react with water	reacts with cold water, releasing hydrogen
iron catalyst in the Haber process	catalytic activity; solid at 450 °C	sodium melts at 98 °C

Common errors

• Claiming transition metals are more reactive than Group 1. The reverse is true: they are much less reactive.
• Saying all transition metals form coloured compounds or are catalysts. The specification says many.
• Forgetting the mercury exception, or thinking it applies to density as well.
• Reading the Roman numeral as a number of atoms. Iron(III) oxide is Fe₂O₃, not Fe₃O.
• Calling transition metals “heavier”. Write denser.
• Swapping the precipitate colours: iron(II) green, iron(III) brown.
• Listing facts about one metal in a “compare” question without a comparative word.

Where to go next

Condense this page with the Transition metals revision notes, then try the Transition metals practice questions. The Metals study guide covers copper’s uses and the reactivity series, and the atomic structure study guide explains how the table is arranged.

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.