Chemguide: Support for CIE A level Chemistry

Learning outcomes 16.2(a), 16.2(b) and 16.2(c)

These statements deal with the lack of reactivity of the halogenoalkanes and some of the uses of fluorine-containing ones.

Before you go on, you should find and read the statements in your copy of the syllabus.

I am unsure exactly what CIE want for this despite what the syllabus says, and the limited amount of support material available. Very little has been asked about it in exam papers so far. I will modify this page if or when it becomes more obvious what they are looking for.

What follows explores around the topic enough, I hope, for you to be able to answer any questions which arise.

Statement 16.2(a)

You should have already read the necessary bits about this. Have another quick look at:

  • "the importance of bond strengths", about 3/4 of the way down the page introducing halogenoalkanes;

  • "comparing halogenoalkane reactivities", about half-way down the page about reactions involving halogenoalkanes and silver nitrate solution. Just read the "Background" and "Comparing the reaction rates as you change the halogen" sections.

    The syllabus talks about hydrolysis, and the silver nitrate reaction involves hydrolysis by water in the solution, followed by reaction of the silver ions with liberated halide ions.

All you need to realise from this is that the reactivity increases as the halogen gets bigger. That means that halogenoalkanes containing chlorine and, more particularly, fluorine are relatively unreactive. In fact, the energy needed to break the carbon-fluorine bond is so high that fluoroalkanes are virtually unreactive. A fluoroalkane is a halogenoalkane containing fluorine.

Statements 16.2(b) and 16.2(c)

For some uses and comments about some commonly used halogenoalkanes containing fluorine and chlorine, read the page uses of halogenoalkanes.

Concentrate on the CFCs (what the syllabus describes as fluorohalogenoalkanes) and fluoroalkanes (HFCs).

If you think about the uses (for example as refrigerants, or in aerosols, or in making foamed plastics), it is important that the compounds should be as chemically unreactive as possible.

You will have read that CFCs have (or, increasingly, once had) a lot of different uses partly because of their low reactivity. Carbon-chlorine bonds in the molecules are, however, broken by UV light in the high atmosphere and have caused problems with the ozone layer.

They are being replaced by HFCs which are simple fluoroalkanes. The fluorine-carbon bond isn't easily broken in the atmosphere.

You will find a detailed account of how CFCs destroy ozone on a page in the Physical Chemistry section of Chemguide about types of catalysis. You will find a section called "The destruction of atmospheric ozone" about 3/4 of the way down that page. You don't need to read the rest of the page for now.

There is one term that you may not have come across before - homogeneous catalysis. This is explained further up the page, but to save you the trouble of finding it, all you need to know is that in homogeneous catalysis, the catalyst is in the same physical state as the reactants. In this case, the catalyst and the reactants are all gases.

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© Jim Clark 2010 (last modified June 2014)