S2.4 From models to materials
S2.4 From models to materials (4 hours)
The properties of materials are related to the type of bonding within the substance
This section describes the properties of materials such as melting point, electrical conductivity, thermal conductivity and malleability and how they are related to their bonding and structure. It looks at how their properties can be predicted depending upon where simple substances are located on a triangular bonding triangle, such as the van Arkel-Ketelaar triangle, and also covers alloys and polymers.
Guiding question
What role do bonding and structure have in the design of materials?
Pause for thought
It is the first time that triangular bonding diagrams have appeared on the core of the IB syllabus. Previously they were covered under the old Option A on materials and they have been on the Cambridge pre-U syllabus (another international examination for 16-19 year olds) for several years. I’m not convinced as to how useful they are. To me they do not really go much further than the old rule of thumb that if the electronegativity difference is greater than about 1.8 the compound is likely to be ionic. They remind me a bit of the octet rule. Both work for simple compounds but both are based on dubious assumptions and fail for more complex examples. Even if we can deduce the percentage of covalent to ionic bonding in a compound, does that really help to understand the properties? Metallic, covalent and ionic are nice labels and ways of classifying materials but their properties can only be generalised. For example metals “have high boiling points” but not all metals do – mercury is a liquid with an appreciable vapour pressure at room temperature. ‘‘Covalent substances have lower boiling points than ionic substances”– try telling that to diamond and sodium chloride!
One of the problems is electronegativity itself. It is not an absolute value and indeed there are different scales of electronegativity. The values are arrived at by considering how a bonding pair of electrons is shared between the two bonding atoms so in a sense the percentage of covalent and ionic bonding has already been taken into account. My three biggest criticisms though are:
1. In the triangular diagram given in the data booklet, ‘covalent’ only applies to simple covalent molecules and does not consider macromolecules like diamond, silica or graphite. Diamond and graphite both appear in the covalent section but what new knowledge is gained from that information? It does not enable us to deduce that one conducts electricity and one does not. The tetrahedral diagram (above right), which is not on the syllabus, does attempt to address this.
2. Relying on the bonding diagram to predict the properties of some simple compounds can give completely wrong results. Consider boron trifluoride, BF3. On the Pauling scale of electronegativities (given in Section 9 of the IB data booklet) the values for boron and fluorine are respectively 2.0 and 4.0. This gives a difference of 2.0 and an average value of 3.0. When these coordinates are plotted on the van Arkel-Ketelaar triangle, BF3 appears high up just on the right edge of the shaded area for polar covalent. This equates to approximately 60% ionic and 40% covalent bonding. It would therefore be expected that BF3 would have a reasonably high boiling point. In fact, boron trifluoride is a gas at room temperature as it boils at −100.3 °C. What the diagram fails to take into account is the geometry of the BF3 molecule. It is trigonal planar with bond angles of 120o and is therefore a non-polar molecule with only weak London dispersion attractive forces between the molecules, hence its low boiling point.
3. It does not distinguish between different oxidation states. Consider the chloride of a metal M, where M has an electronegativity value of 1.8. Chlorine’s value is 3.2 so the difference is 1.4 and the average is 2.5. This puts the chloride of M in the polar covalent region so we might conclude that it will not conduct electricity when molten and will have a reasonably high boiling point although lower than that of an ionic compound. The problem is that M is lead and lead forms two chlorides, lead(II) chloride, PbCl2, and lead(IV) chloride, PbCl4. Lead(II) chloride is ionic, conducts electricity when molten and boils at 950 oC, Lead(IV) chloride is a yellow non-conducting covalent oily liquid at room temperature and boils at 50 oC. The van Arkel – Ketelaar diagram is of absolutely no use whatsoever in deducing this information.
Nature of science
Can models of bonding be used to make specific predictions or are they only useful to make general predictions and even then have limitations?
Learning outcomesAfter studying this topic students should be able to: Understand:
Apply their knowledge to:
| Clarification notesA triangular bonding diagram is provided in the data booklet. To illustrate the relationship between bonding type and properties include examples of materials with varying percentage bonding character. Only binary compounds need to be considered. Calculations of percentage ionic character are not required. Electronegativity data are given in the data booklet. Illustrate alloys with common examples such as bronze, brass, and stainless steel. Include other examples of choice. Specific examples of alloys do not have to be learned. Examples of natural and synthetic polymers should be discussed. Examples of addition polymers should include polymerisation reactions of alkenes. Structures of monomers do not have to be learned but will be provided or will need to be deduced from the polymer. Linking questionsHow do the trends in properties of period 3 oxides reflect the trend in their bonding? What are the limitations of discrete bonding categories? Why do composites like reinforced concretes, which are made from ionic and covalently bonded compounds and steel bars, have unique properties? Why are alloys more correctly described as mixtures rather than as compounds? What are the structural features of some plastics that make them biodegradable? What functional groups in molecules can enable them to act as monomers for addition reactions? Why is the atom economy 100% for an addition polymerization reaction? |
Teaching tipsYou may already have covered some of the material in this topic as you were teaching ionic, covalent and metallic bonding but in some ways it makes sense to put all this together in a separate topic. The problem is that by including polymers, in particular the repeating units of addition polymers, it assumes that students know about alkenes and addition reactions. Many teachers may prefer to wait until they cover organic chemistry before teaching this part of the topic. Alloys are important but the syllabus clearly states that specific examples of alloys do not have to be learnt so focus more on how their properties are related to the general structure of alloys rather than cover too many specific examples. Bonding triangles are not difficult to explain but as I have shown in Pause for Thought they do have considerable limitations. Make sure that your students understand why caesium, caesium fluoride and fluorine are at the three apices of the triangle. There are many natural polymers that you can mention such as proteins, starch and other polysaccharides etc. but again none are listed as being required specifically. Because condensation polymers are covered at AHL there is no need to go into detail about requiring two reactive groups on a molecule to form condensation polymers like polyamides or polyesters. However do give students practice at drawing the repeating unit for addition polymers formed by polymerising ethene and substituted alkenes. | Study guide
Pages 37, 38 QuestionsFor ten 'quiz' multiple choice questions with the answers explained see MC Test: S4 From models to materials. For short-answer questions which can be set as an assignment for a test, homework or given for self study together with model answers see S2.4 From models to materials questions. Vocabulary listvan Arkel-Ketelaar triangle Putting the topic into contextSee Topic S2 Models of bonding and structure which covers all of Topic S2. |
Teaching slides
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Other resources
1. A well-illustrated video on using the van Arkel-Ketelaar triangle by Richard Thornley.
van Arkel-Ketelaar diagram2. An animated video on alloys and their uses also by Richard Thornley. It was originally made for the old core sub-topic 4.5 but is still relevant to this sub-topic.
3. A very simple explanation of how alkene monomers form addition polymers by BBLC.mov.