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8.4.1 Transition elements

What the transition elements are

Definition

Transition element

A metal in the central block of the Periodic Table, between Group II and Group III.

  1. The transition elements are the block of elements in the middle of the Periodic Table, between Group II and Group III.
  2. Familiar examples include chromium, manganese, iron, nickel and copper.
  3. Every transition element is a metal.
  4. They share four characteristic properties: high densities, high melting points, the formation of coloured compounds, and frequent action as catalysts.
  5. These properties set them apart from the Group I metals, which are soft, have low densities and low melting points, and form white or colourless compounds.

High density and high melting point

Definition

Density

The mass of a substance per unit volume, usually measured in g/cm3\text{g/cm}^3g/cm3.

Definition

Melting point

The temperature at which a solid changes into a liquid.

  1. High density: a piece of a transition element has a large mass for its volume.
    1. Iron has a density of 7.9 g/cm37.9\ \text{g/cm}^37.9 g/cm3 and copper has a density of 8.9 g/cm38.9\ \text{g/cm}^38.9 g/cm3.
    2. Sodium, by comparison, has a density of only 0.97 g/cm30.97\ \text{g/cm}^30.97 g/cm3.
  2. High melting point: a transition element has to be heated to a very high temperature before it melts.
    1. Iron melts at 1538 ∘C1538\ ^\circ\text{C}1538 ∘C, nickel at 1455 ∘C1455\ ^\circ\text{C}1455 ∘C and copper at 1085 ∘C1085\ ^\circ\text{C}1085 ∘C.
    2. Sodium melts at only 98 ∘C98\ ^\circ\text{C}98 ∘C and potassium at only 63 ∘C63\ ^\circ\text{C}63 ∘C.
  3. The atoms of a transition metal are packed tightly together, which gives a large mass in a small volume.
  4. The metallic bonding between the positive ions and the delocalised electrons is very strong, so a large amount of energy is needed to separate the ions and melt the metal.
  5. Transition metals are also harder and stronger than the Group I metals, which is why they cannot be cut with a knife.
Example

Comparing the mass of iron and sodium

  • Two blocks each have a volume of 10 cm310\ \text{cm}^310 cm3, one made of iron and one made of sodium.

mass=density×volume\text{mass} = \text{density} \times \text{volume}mass=density×volume

  • For iron, the mass is 7.9×10=79 g7.9 \times 10 = 79\ \text{g}7.9×10=79 g.
  • For sodium, the mass is 0.97×10=9.7 g0.97 \times 10 = 9.7\ \text{g}0.97×10=9.7 g.
  • The iron block has about eight times the mass of a sodium block of the same size, which shows how much denser the transition element is.

Coloured compounds

  1. Compounds of transition elements are usually coloured, both as solids and when dissolved in water.
  2. Copper(II) sulfate forms blue crystals and a blue solution.
  3. Nickel(II) sulfate forms a green solution.
  4. Potassium manganate(VII) forms a deep purple solution.
  5. Potassium dichromate(VI) forms an orange solution.
  6. Compounds of Group I and Group II metals, such as sodium chloride and calcium carbonate, are white solids, and any that dissolve give colourless solutions.
  7. A coloured compound is therefore a strong sign that a transition element is present.
Common Mistake
  • Do not write that transition elements are coloured, because the metals themselves are mostly grey or silvery and it is their compounds that are coloured.
  • Do not describe transition elements as having low densities or low melting points, because those are properties of the Group I metals.

Transition elements as catalysts

Definition

Catalyst

A substance that increases the rate of a reaction and is chemically unchanged at the end of the reaction.

  1. A catalyst increases the rate of a reaction and is chemically unchanged at the end of the reaction, so it can be used again and again.
  2. Transition elements often act as catalysts, both as the elements themselves and in their compounds.
  3. As an element: iron is the catalyst in the Haber process, which makes ammonia from nitrogen and hydrogen.
  4. In a compound: vanadium(V) oxide is the catalyst in the Contact process, which converts sulfur dioxide into sulfur trioxide.
  5. In a compound: manganese(IV) oxide speeds up the decomposition of hydrogen peroxide into water and oxygen.
  6. Catalysts allow industrial reactions to run quickly at lower temperatures, which saves energy and money.
  7. The word often matters, because not every transition element catalyses every reaction.
Practical

Manganese(IV) oxide as a catalyst

  • Aim: to show that manganese(IV) oxide, a compound of a transition element, speeds up the decomposition of hydrogen peroxide and is unchanged at the end of the reaction.
  • Apparatus: two conical flasks, a measuring cylinder, dilute hydrogen peroxide solution, manganese(IV) oxide powder, a spatula, a balance reading to 0.01 g0.01\ \text{g}0.01 g, a filter funnel and filter paper, a wooden splint, a Bunsen burner and eye protection.
  • Method:
    • Measure 25 cm325\ \text{cm}^325 cm3 of hydrogen peroxide solution into each of the two conical flasks.
    • Leave the first flask with no catalyst, so that it acts as a control.
    • Weigh 0.50 g0.50\ \text{g}0.50 g of manganese(IV) oxide onto a piece of filter paper and record the exact mass.
    • Tip the powder into the second flask and watch both flasks for one minute.
    • Light a splint, blow it out so that it glows, and hold it in the neck of each flask in turn.
    • When the fizzing in the second flask stops, filter the mixture to collect the black powder.
    • Rinse the powder with a little distilled water, leave it to dry completely, and then weigh it again.
  • Results:
    • The flask without manganese(IV) oxide shows only a few tiny bubbles.
    • The flask with manganese(IV) oxide fizzes rapidly and the solution may feel warm.
    • The glowing splint relights in the second flask, which shows that the gas is oxygen.
    • The dried black powder still has a mass of 0.50 g0.50\ \text{g}0.50 g.
  • Conclusion: manganese(IV) oxide increases the rate at which hydrogen peroxide decomposes and is not used up, so it is acting as a catalyst.
  • Equation: 2H2O2→2H2O+O22\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_22H2​O2​→2H2​O+O2​
  • Watch out: the powder must be completely dry before it is weighed again, because any water left on it makes the mass seem to increase.
  • Safety: hydrogen peroxide irritates the skin and eyes and manganese(IV) oxide is harmful if swallowed, so wear eye protection and wash off any splashes at once.
Exam technique

Describing transition elements

  • When asked to describe the transition elements, give the four properties in full: high density, high melting point, form coloured compounds and act as catalysts.
  • For the catalyst property, add that they act as catalysts as elements and in compounds, and name an example of each where you can.
  • Write "high density" rather than "heavy", because "heavy" describes a mass, not a property of the material.
Self review
  • Where are the transition elements found in the Periodic Table?
  • Give two physical properties of transition elements and compare them with the Group I metals.
  • What is the colour of a solution of copper(II) sulfate?
  • Name a transition element that acts as a catalyst as an element, and a transition element compound that acts as a catalyst.
  • How can you show that a catalyst is not used up in a reaction?
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8.4.1 Transition elements Revision Guide

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Revision notes for CIE IGCSE Chemistry 8.4.1 Transition elements: explanations and worked examples.

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