3.2 Inorganic chemistry

3.2.1 Periodicity

The Periodic Table provides chemists with a structured organisation of the known chemical elements from which they can make sense of their physical and chemical properties. The historical development of the Periodic Table and models of atomic structure provide good examples of how scientific ideas and explanations develop over time.

3.2.1.1 Classification

Content

Opportunities for skills development

An element is classified as s, p, d or f block according to its position in the Periodic Table, which is determined by its proton number.

 

3.2.1.2 Physical properties of Period 3 elements

Content

Opportunities for skills development

The trends in atomic radius, first ionisation energy and melting point of the elements Na–Ar

The reasons for these trends in terms of the structure of and bonding in the elements.

Students should be able to:

  • explain the trends in atomic radius and first ionisation energy
  • explain the melting point of the elements in terms of their structure and bonding.
 

3.2.2 Group 2, the alkaline earth metals

The elements in Group 2 are called the alkaline earth metals. The trends in the solubilities of the hydroxides and the sulfates of these elements are linked to their use. Barium sulfate, magnesium hydroxide and magnesium sulfate have applications in medicines whilst calcium hydroxide is used in agriculture to change soil pH, which is essential for good crop production and maintaining the food supply.

Content

Opportunities for skills development

The trends in atomic radius, first ionisation energy and melting point of the elements Mg–Ba

Students should be able to:

  • explain the trends in atomic radius and first ionisation energy
  • explain the melting point of the elements in terms of their structure and bonding.

The reactions of the elements Mg–Ba with water.

The use of magnesium in the extraction of titanium from TiCl4

The relative solubilities of the hydroxides of the elements Mg–Ba in water.

Mg(OH)2 is sparingly soluble.

The use of Mg(OH)2 in medicine and of Ca(OH)2 in agriculture.

The use of CaO or CaCO3 to remove SO2 from flue gases.

The relative solubilities of the sulfates of the elements Mg–Ba in water.

BaSO4 is insoluble.

The use of acidified BaCl2 solution to test for sulfate ions.

The use of BaSO4 in medicine.

Students should be able to:

  • explain why BaCl2 solution is used to test for sulfate ions and why it is acidified.

AT c and k

PS 2.2

Students could test the reactions of Mg–Ba with water and Mg with steam and record their results.

AT d and k

PS 2.2

Students could test the solubility of Group 2 hydroxides by mixing solutions of soluble Group 2 salts with sodium hydroxide and record their results.

Students could test the solubility of Group 2 sulfates by mixing solutions of soluble Group 2 salts with sulfuric acid and record their results.

Students could test for sulfate ions using acidified barium chloride and record their results.

Research opportunity

Students could investigate the use of BaSO4 in medicine.

3.2.3 Group 7(17), the halogens

The halogens in Group 7 are very reactive non-metals. Trends in their physical properties are examined and explained. Fluorine is too dangerous to be used in a school laboratory but the reactions of chlorine are studied. Challenges in studying the properties of elements in this group include explaining the trends in ability of the halogens to behave as oxidising agents and the halide ions to behave as reducing agents.

Content

Opportunities for skills development

The trends in electronegativity and boiling point of the halogens.

Students should be able to:

  • explain the trend in electronegativity
  • explain the trend in the boiling point of the elements in terms of their structure and bonding.

The trend in oxidising ability of the halogens down the group, including displacement reactions of halide ions in aqueous solution.

The trend in reducing ability of the halide ions, including the reactions of solid sodium halides with concentrated sulfuric acid.

The use of acidified silver nitrate solution to identify and distinguish between halide ions.

The trend in solubility of the silver halides in ammonia.

Students should be able to explain why:

  • silver nitrate solution is used to identify halide ions
  • the silver nitrate solution is acidified
  • ammonia solution is added.

AT d and k

PS 2.2

Students could carry out test-tube reactions of solutions of the halogens (Cl2, Br2, I2) with solutions containing their halide ions (eg KCl, KBr, KI).

Students could record observations from reactions of NaCl, NaBr and NaI with concentrated sulfuric acid.

Students could carry out tests for halide ions using acidified silver nitrate, including the use of ammonia to distinguish the silver halides formed.

3.2.3.2 Uses of chlorine and chlorate(I)

Content

Opportunities for skills development

The reaction of chlorine with water to form chloride ions and chlorate(I) ions.

The reaction of chlorine with water to form chloride ions and oxygen.

Appreciate that society assesses the advantages and disadvantages when deciding if chemicals should be added to water supplies.

The use of chlorine in water treatment.

Appreciate that the benefits to health of water treatment by chlorine outweigh its toxic effects.

The reaction of chlorine with cold, dilute, aqueous NaOH and uses of the solution formed.

Research opportunity

Students could investigate the treatment of drinking water with chlorine.

Students could investigate the addition of sodium fluoride to water supplies.

Required practical 4Carry out simple test-tube reactions to identify:
  • cations – Group 2, NH4+
  • anions – Group 7 (halide ions), OH, CO32–, SO42–
 

3.2.4 Properties of Period 3 elements and their oxides (A-level only)

The reactions of the Period 3 elements with oxygen are considered. The pH of the solutions formed when the oxides react with water illustrates further trends in properties across this period. Explanations of these reactions offer opportunities to develop an in-depth understanding of how and why these reactions occur.

Content Opportunities for skills development

The reactions of Na and Mg with water.

The trends in the reactions of the elements Na, Mg, Al, Si, P and S with oxygen, limited to the formation of Na2O, MgO, Al2O3, SiO2, P4O10, SO2 and SO3

The trend in the melting point of the highest oxides of the elements Na–S

The reactions of the oxides of the elements Na–S with water, limited to Na2O, MgO, Al2O3, SiO2, P4O10, SO2 and SO3, and the pH of the solutions formed.

The structures of the acids and the anions formed when P4O10, SO2 and SO3 react with water.

Students should be able to:

  • explain the trend in the melting point of the oxides of the elements Na–S in terms of their structure and bonding
  • explain the trends in the reactions of the oxides with water in terms of the type of bonding present in each oxide
  • write equations for the reactions that occur between the oxides of the elements Na–S and given acids and bases.

AT a, c and k

PS 2.2

Students could carry out reactions of elements with oxygen and test the pH of the resulting oxides.

3.2.5 Transition metals (A-level only)

The 3d block contains 10 elements, all of which are metals. Unlike the metals in Groups 1 and 2, the transition metals Ti to Cu form coloured compounds and compounds where the transition metal exists in different oxidation states. Some of these metals are familiar as catalysts. The properties of these elements are studied in this section with opportunities for a wide range of practical investigations.

3.2.5.1 General properties of transition metals (A-level only)

Content

Opportunities for skills development

Transition metal characteristics of elements Ti–Cu arise from an incomplete d sub-level in atoms or ions.

The characteristic properties include:

  • complex formation
  • formation of coloured ions
  • variable oxidation state
  • catalytic activity.

A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.

A complex is a central metal atom or ion surrounded by ligands.

Co-ordination number is number of co-ordinate bonds to the central metal atom or ion.

 

3.2.5.2 Substitution reactions (A-level only)

Content

Opportunities for skills development

H2O, NH3 and Cl can act as monodentate ligands.

The ligands NH3 and H2O are similar in size and are uncharged.

Exchange of the ligands NH3 and H2O occurs without change of co-ordination number (eg Co2+ and Cu2+).

Substitution may be incomplete (eg the formation of [Cu(NH3)4(H2O)2]2+).

The Cl ligand is larger than the uncharged ligands NH3 and H2O

Exchange of the ligand H2O by Cl can involve a change of co-ordination number (eg Co2+, Cu2+ and Fe3+).

Ligands can be bidentate (eg H2NCH2CH2NH2 and C2O42–).

Ligands can be multidentate (eg EDTA4–).

Haem is an iron(II) complex with a multidentate ligand.

Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.

Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.

Bidentate and multidentate ligands replace monodentate ligands from complexes. This is called the chelate effect.

Students should be able to:

  • explain the chelate effect, in terms of the balance between the entropy and enthalpy change in these reactions.

AT d and k

PS 1.2

Students could carry out test-tube reactions of complexes with monodentate, bidentate and multidentate ligands to compare ease of substitution.

AT d and k

PS 2.2

Students could carry out test-tube reactions of solutions of metal aqua ions with ammonia or concentrated hydrochloric acid.

3.2.5.3 Shapes of complex ions (A-level only)

Content

Opportunities for skills development

Transition metal ions commonly form octahedral complexes with small ligands (eg H2O and NH3).

Octahedral complexes can display cis–trans isomerism (a special case of E–Z isomerism) with monodentate ligands and optical isomerism with bidentate ligands.

Transition metal ions commonly form tetrahedral complexes with larger ligands (eg Cl).

Square planar complexes are also formed and can display cis–trans isomerism.

Cisplatin is the cis isomer.

Ag+ forms the linear complex [Ag(NH3)2]+ as used in Tollens’ reagent.

MS 4.1 and 4.2

Students understand and draw the shape of complex ions.

MS 4.3

Students understand the origin of cis–trans and optical isomerism.

Students draw cis–trans and optical isomers.

Students describe the types of stereoisomerism shown by molecules/complexes.

3.2.5.4 Formation of coloured ions (A-level only)

Content

Opportunities for skills development

Transition metal ions can be identified by their colour.

Colour arises when some of the wavelengths of visible light are absorbed and the remaining wavelengths of light are transmitted or reflected.

d electrons move from the ground state to an excited state when light is absorbed.

The energy difference between the ground state and the excited state of the d electrons is given by:

E = hν = hc

Changes in oxidation state, co-ordination number and ligand alter ∆E and this leads to a change in colour.

The absorption of visible light is used in spectroscopy.

A simple colorimeter can be used to determine the concentration of coloured ions in solution.

PS 3.1 and 3.2

Students could determine the concentration of a solution of copper(II) ions by colorimetry.

MS 3.1 and 3.2

Students determine the concentration of a solution from a graph of absorption versus concentration.

AT a, e and k

Students could determine the concentration of a coloured complex ion by colorimetry.

3.2.5.5 Variable oxidation states (A-level only)

Content

Opportunities for skills development

Transition elements show variable oxidation states.

Vanadium species in oxidation states IV, III and II are formed by the reduction of vanadate(V) ions by zinc in acidic solution.

The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH and by the ligand.

The reduction of [Ag(NH3)2]+ (Tollens’ reagent) to metallic silver is used to distinguish between aldehydes and ketones.

The redox titrations of Fe2+ and C2O42– with MnO4

Students should be able to:

  • perform calculations for these titrations and similar redox reactions.

AT d and k

PS 1.2

Students could reduce vanadate(V) with zinc in acidic solution.

AT b, d and k

PS 4.1

Students could carry out test-tube reactions of Tollens' reagent to distinguish aldehydes and ketones.

AT a, d, e and k

PS 2.3, 3.2 and 3.3

Students could carry out redox titrations.

Examples include, finding:
  • the mass of iron in an iron tablet
  • the percentage of iron in steel
  • the Mr of hydrated ammonium iron(II) sulfate
  • the Mr of ethanedioic acid
  • the concentration of H2O2 in hair bleach.

3.2.5.6 Catalysts (A-level only)

Content

Opportunities for skills development

Transition metals and their compounds can act as heterogeneous and homogeneous catalysts.

A heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface.

The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost.

V2O5 acts as a heterogeneous catalyst in the Contact process.

Fe is used as a heterogeneous catalyst in the Haber process.

Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency; this has a cost implication.

A homogeneous catalyst is in the same phase as the reactants.

When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species.

Students should be able to:

  • explain the importance of variable oxidation states in catalysis
  • explain, with the aid of equations, how V2O5 acts as a catalyst in the Contact process
  • explain, with the aid of equations, how Fe2+ ions catalyse the reaction between I and S2O82–
  • explain, with the aid of equations, how Mn2+ ions autocatalyse the reaction between C2O42– and MnO4

AT d and k

PS 4.1

Students could investigate Mn2+ as the autocatalyst in the reaction between ethanedioic acid and acidified potassium manganate(VII).

3.2.6 Reactions of ions in aqueous solution (A-level only)

The reactions of transition metal ions in aqueous solution provide a practical opportunity for students to show and to understand how transition metal ions can be identified by test-tube reactions in the laboratory.

Content

Opportunities for skills development

In aqueous solution, the following metal-aqua ions are formed:

[M(H2O)6]2+, limited to M = Fe and Cu

[M(H2O)6]3+, limited to M = Al and Fe

The acidity of [M(H2O)6]3+ is greater than that of [M(H2O)6]2+

Some metal hydroxides show amphoteric character by dissolving in both acids and bases (eg hydroxides of Al3+).

Students should be able to:

  • explain, in terms of the charge/size ratio of the metal ion, why the acidity of [M(H2O)6]3+ is greater than that of [M(H2O)6]2+
  • describe and explain the simple test-tube reactions of: M2+(aq) ions, limited to M = Fe and Cu, and of M3+(aq) ions, limited to M = Al and Fe, with the bases OH, NH3 and CO32–

AT d and K

PS 1.2

Students could carry out test-tube reactions of metal-aqua ions with NaOH, NH3 and Na2CO3

AT d and k

PS 2.2

Students could carry out test-tube reactions to identify the positive and negative ions in this specification.

PS 1.1

Students could identify unknown substances using reagents.

Required practical 11

Carry out simple test-tube reactions to identify transition metal ions in aqueous solution.