Transition elements and catalysts

The metals that make other reactions go

Every car exhaust, every bag of fertiliser and every tub of margarine depends on a metal from the middle of the periodic table that is not used up. How can a metal help a reaction and come out unchanged?

The elements of groups 3 to 12, in the centre of the table, are the d-block or transition elements. All of them are metals. They are often hard, with high densities and high melting and boiling points. They show variable oxidation states, often form coloured compounds, and are malleable and ductile.

Transition metals and their compounds are catalysts in many important reactions: the metal holds other substances on its surface and activates them. Iron speeds up the reaction of nitrogen and hydrogen to make ammonia. In the Contact process, V₂O₅ now turns sulphur dioxide into sulphur trioxide; platinum was used first, but arsenic impurity made it inactive.

A catalytic converter in a car's exhaust uses platinum, palladium and rhodium to turn harmful engine gases into gases that do not pollute the air. Nickel catalyses the hydrogenation of oils to make solid margarine, which spoils less easily than butter.

Transition elements: hard, dense, high-melting metals with coloured compounds and variable oxidation states; many are catalysts.

Key terms

Transition element

Naming the central block.

Catalyst

Industrial processes.

Variable oxidation state

A mark of transition metals.

Catalytic converter

Cutting car pollution.

Short questions with model answers

1. Q1. Which compound do you expect to be coloured: KCl, BaCl₂, AlCl₃ or NiCl₂? (Exercise ii, p. 126)

   • Find the metal
   • Apply the property

   NiCl₂, because nickel is a transition metal.

2. Q2. Write the two steps of the Contact process given in the book and name the catalyst.

   • 2SO₂(g) + O₂(g) → 2SO₃(g)
   • SO₃ + H₂O → H₂SO₄

   The catalyst is vanadium pentoxide, V₂O₅.

Common mistakes

• ✗ Some transition elements are non-metals.

  ✓ All transition elements are metals (p. 122).

• ✗ The iron catalyst is used up as ammonia forms.

  ✓ A catalyst speeds up the reaction without being consumed.

• ✗ Platinum is still the preferred catalyst in the Contact process.

  ✓ Arsenic poisoned platinum; V₂O₅ is now preferred (p. 123).

MCQs

1. 1. Which compound do you expect to be coloured? (Exercise ii, p. 126)

   1. (a) KCl
   2. (b) BaCl₂
   3. (c) AlCl₃
   4. (d) NiCl₂
   Show answer

   (d) Ni is a transition metal (p. 122).

2. 2. Which catalyst is used to make margarine from oil?

   1. (a) Iron
   2. (b) Nickel
   3. (c) Platinum
   4. (d) Vanadium pentoxide
   Show answer

   (b) Hydrogenation of oils (p. 123).

3. 3. Transition elements are found in groups:

   1. (a) 1 and 2
   2. (b) 3 to 12
   3. (c) 13 to 17
   4. (d) 18
   Show answer

   (b) The d-block in the centre (p. 122).

4. 4. Which metals are the catalysts in a catalytic converter?

   1. (a) Fe, Ni, Cu
   2. (b) Pt, Pd, Rh
   3. (c) Na, K, Li
   4. (d) V, Cr, Mn
   Show answer

   (b) Platinum, palladium and rhodium (p. 123).

5. 5. Why was platinum replaced by V₂O₅ in the Contact process?

   1. (a) Platinum is too light
   2. (b) Arsenic impurity made platinum inactive
   3. (c) Platinum reacts with water
   4. (d) V₂O₅ is a noble gas
   Show answer

   (b) p. 123.

Quick revision

• Transition elements (groups 3 to 12) are all metals: hard, dense and high-melting.
• They show variable oxidation states and often form coloured compounds.
• Catalysts: Fe for ammonia, V₂O₅ in the Contact process, Pt/Pd/Rh in converters, Ni for margarine.