MYP 9 Unit 3

Structure and bonding: 9.3.5: Ionic solution reactions

Objectives

  • To understand that the ions in a solution are 100% dissociated
  • To know that solubilities are difficult to predict, but that there are empirical rules
  • To carry out small scale precipitation reactions
  • To construct equations showing precipitation reactions

Activity

The students are provided with bottles of ionic solutions and asked to mix pairs of ionic solutions in test tubes (small scale). Note that better results are obtained when different ions are chosen. In other words it is better to choose to mix iron(II) sulfate with sodium hydroxide than iron(II) sulfate with copper(II) sulfate.

They are to identify the formula of each ionic compound used and the ions that are present in each solution.

Example:

Copper(II) sulfate, CuSO4 solution contains Cu2+(aq) and SO42-(aq) 100% dissociated in solution.

Observations are recorded before and after mixing

Where a precipitate appears the students are to construct word and balanced equations for the reaction.

Example

Copper(II) sulfate solution + sodium hydroxide solution

A light blue precipitate is observed

Copper(II) hydroxide is insoluble as most metal hydroxides are insoluble except group 1 and barium sulfate from group 2

CuSO4(aq) + NaOH(aq) --> Cu(OH)2(s) + Na2SO4(aq)

Student follow up

Write up of the experiment with suitably recorded observations and equations.

Teacher's notes

Solutions suggested:

  • Copper(II) sulfate
  • Zinc sulfate
  • Iron(II) sulfate
  • Iron(III) chloride
  • Lead(II) nitrate
  • Sodium hydroxide
  • Potassium nitrate
  • Potassium iodide
  • Cobalt(II) chloride

The lesson must include a discussion on solubility and some empirical solubility "rules".

Soluble

  • 1. All group 1 and ammonium compounds are soluble (oxides react with water making hydroxides)
  • 2. All nitrates and ethanoates are soluble

Insoluble

  • 1. All carbonates except group 1 and ammonium
  • 2. All hydroxides except group 1
  • 3. Sulfates of group 2 except magnesium
  • 4. All silver and lead compounds except nitrates and ethanoates

Note that a mixture of copper(II) sulfate and potassium iodide causes a redox reaction in which copper(I) sulfate and iodine are formed. This is not precipitation, but serves to point out that other reactions are possible apart from simple ionic combinations.

Safety

Lead solution residues are toxic and must not be disposed of in the sink. There must be a residues container provided.

E-assessment

C2.1 Structure and bonding

Simple particle theory is developed in this unit to include atomic structure and bonding. The arrangement of electrons in atoms can be used to explain what happens when elements react and how atoms join together to form different types of substances.

Candidates should use their skills, knowledge and understanding to:

  • write formulae for ionic compounds from given symbols and ionic charges
  • represent the electronic structure of the ions in sodium chloride, magnesium oxide and calcium chloride in the following form: for sodium ion (Na+)

C2.1.1 Structure and bonding

a) Compounds are substances in which atoms of two or more elements are chemically combined.

b) Chemical bonding involves either transferring or sharing electrons in the highest occupied energy levels (shells) of atoms in order to achieve the electronic structure of a noble gas.

c) When atoms form chemical bonds by transferring electrons, they form ions. Atoms that lose electrons become positively charged ions. Atoms that gain electrons become negatively charged ions. Ions have the electronic structure of a noble gas (Group 0).

Candidates should be able to relate the charge on simple ions to the group number of the element in the periodic table.

d) The elements in Group 1 of the periodic table, the alkali metals, all react with non-metal elements to form ionic compounds in which the metal ion has a single positive charge.

Knowledge of the chemical properties of alkali metals is limited to their reactions with non-metal elements.

e) The elements in Group 7 of the periodic table, the halogens, all react with the alkali metals to form ionic compounds in which the halide ions have a single negative charge.

Knowledge of the chemical properties of the halogens is limited to reactions with alkali metals

f) An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions.

These forces act in all directions in the lattice and this is called ionic bonding.

Candidates should be familiar with the structure of sodium chloride but do not need to know the structures of other ionic compounds

Candidates should know the bonding in the examples in the specification for this unit, and should be able to recognise simple molecules and giant structures from diagrams that show their bonding

Suggested ideas for practical work to develop skills and understanding include the following:

  • modelling electron transfer and electron sharing using computer simulations
  • Group 1 and Group 7 reactions, eg sodium with chlorine
  • the reactions of bromine, chlorine and iodine with iron wool
  • growing metal crystals by displacement reactions using metals and salts

C2.2 How structure influences the properties and uses of substances

Substances that have simple molecular, giant ionic and giant covalent structures have very different properties. Ionic, covalent and metallic bonds are strong. However, the forces between molecules are weaker, eg in carbon dioxide and iodine.

C2.2.2 Ionic compounds

a) Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces in all directions between oppositely charged ions. These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.

Knowledge of the structures of specific ionic compounds other than sodium chloride is not required

b) When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current.

Suggested ideas for practical work to develop skills and understanding include the following:

  • investigating the properties of ionic compounds, eg NaCl:
  • melting point, conductivity, solubility, use of hand lens to study crystal structure
  • making metal crystals by displacement reactions, eg copper wire in silver nitrate solution

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