MYP 9 Unit 3

Structure and bonding: 9.3.7: The properties of covalent substances

Objectives

  • To understand that covalent substances are simple molecular
  • To know that the majority of covalent substances with low relative mass are gases at room temperature
  • To know that simple molecules have only weak forces between them
  • To know that covalent substances have low melting and boiling points.
  • To know that covalent substances have soft waxy structures
  • To know that there are no free electrons or charges to carry electrical current.
  • To understand that covalent substances may be polar
  • To understand the rule of thumb that "like dissolves like".

Activity

Circus of experiments involving covalent elements and compounds

Covalent substances circus

Student follow up

The students should write up observations and inferences from the experiment circus.

Teacher's notes

Remind the students that atoms cannot exist on their own under normal conditions.

The non-metallic elements must be covalent.

Properties of covalent substances

  • Soft crystals
  • Low m.p. (often liquids or gases at RT)
  • Not usually soluble in water, soluble in non-polar (covalent) solvents
  • non-conductors when molten or in solution

Resources

Covalent bonding

Covalent properties

E-assessment

Unit 2: Chemistry 2

Throughout this unit candidates will be expected to write word equations for reactions specified. Higher tier candidates will also be expected to write and balance symbol equations for reactions specified throughout the unit.

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+)

represent the covalent bonds in molecules such as water, ammonia, hydrogen, hydrogen chloride, methane and oxygen, and in giant structures such as diamond and silicon dioxide, in the following forms:

represent the bonding in metals in the following form:

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.

g) When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong. Some covalently bonded substances consist of simple molecules such as H2, Cl2, O2, HCl, H2O, NH3 and CH4. Others have giant covalent structures (macromolecules), such as diamond and silicon dioxide.

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

  • molecular modelling
  • modelling electron transfer and electron sharing using computer simulations

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. Metals have many uses. When different metals are combined, alloys are formed. Shape memory alloys have a range of uses. There are different types of polymers with different uses. Nanomaterials have new properties because of their very small size.

Candidates should use their skills, knowledge and understanding to:

  • relate the properties of substances to their uses
  • suggest the type of structure of a substance given its properties
  • evaluate developments and applications of new materials, eg nanomaterials, fullerenes and shape memory materials.

C2.2.1 Molecules

a) Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.

b) Substances that consist of simple molecules have only weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.

Candidates need to be able to explain that intermolecular forces are weak in comparison with covalent bonds.

c) Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge

C2.2.3 Covalent structures

a) Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points.

Candidates should be able to recognise other giant structures or macromolecules from diagrams showing their bonding

b) In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.

c) In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other because there are no covalent bonds between the layers and so graphite is soft and slippery. Explain the properties of graphite in terms of weak intermolecular forces between the layers.

d) In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.

Candidates should realise that graphite is similar to metals in that it has delocalised electrons.

e) Carbon can also form fullerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, eg in tennis rackets.

Candidates' knowledge is limited to the fact that the structure of fullerenes is based on hexagonal rings of carbon atoms

C2.2.5 Polymers

a) The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced using different catalysts and reaction conditions.

b) Thermosoftening polymers consist of individual, tangled polymer chains. Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when they are heated.

Higher Tier candidates should be able to explain the properties of thermosoftening polymers in terms of intermolecular forces.

C2.2.6 Nanoscience

a) Nanoscience refers to structures that are 1-100 nm in size, of the order of a few hundred atoms. Nanoparticles show different properties to the same materials in bulk and have a high surface area to volume ratio, which may lead to the development of new computers, new catalysts, new coatings, highly selective sensors, stronger and lighter construction materials, and new cosmetics such as sun tan creams and deodorants.

Candidates should know what is meant by nanoscience and nanoparticles and should consider some of the applications of these materials, but do not need to know specific examples or properties.

Questions may be set on information that is provided about these materials and their uses.

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

  • demonstration of heating sulfur and pouring it into cold water to produce plastic sulfur
  • investigating the properties of covalent compounds: - simple molecules, eg wax, methane, hexane - macromolecules, eg SiO2(sand)
  • investigating the properties of graphite

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