Chemguide: Support for CIE A level Chemistry

This statement is about how a knowledge of chemistry can help overcome environmental problems.

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

Very little had been asked about this in the first 13 exam sessions of the current syllabus up to June 2013. What has been asked so far is fairly trivial.

CFCs and the ozone layer

Of all the things that CIE might ask in this area, this is the one that contains the most chemistry. You have come across it before in learning outcome 10.3(e).

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 should also read the first half of the page about uses of halogenoalkanes because this gives some more useful information about CFCs and their replacements.

HFCs (hydrofluorocarbons) have no effect on the ozone layer. They tend to be broken down in the lower atmosphere by reaction with hydroxyl radicals, OH.

But even the molecules which reach the ozone layer obviously can't produce chlorine radicals - they don't contain chlorine. The fluorine-carbon bond is too strong to be broken by energies available in UV light, and so you don't get fluorine radicals either.

Oil spills

Spillage of oil at sea obviously causes major environmental damage when it occurs. There are several ways of dealing with such spills, some of which are more effective than others.

Simple physical methods

You can use booms to trap the oil and then skim it off the surface.

Detergents

Traditionally, powerful detergents have been used to break up the oil spill. These work by wrapping up tiny oil droplets in detergent molecules. The detergent molecules have an oil-soluble "tail" which dissolves in the oil, and a water-soluble "head" which allows the detergent-wrapped oil to dissolve in the water.

Actually, it isn't a true solution - it's an emulsion. A true solution will have individual molecules or ions throughout the water. An emulsion has droplets of a material suspended in the water.

The problem with this is that it doesn't really get rid of the oil - it disperses it over a wider area. Some detergents also cause environmental damage themselves.

Bioremediation

Bioremediation just means cleaning up an environmental problem using biological organisms like bacteria or plants.

The major oil spill in the Gulf of Mexico in 2010 disappeared much faster than anyone expected. There are natural bacteria in the area which use hydrocarbons as a source of fuel, unlike humans who have a biochemistry which uses carbohydrates.

A rapid population growth of these converted the hydrocarbons in the oil into carbon dioxide and water.

You will come across bioremediation again in a different context, using plants rather than bacteria, in a moment.

Sorbents

Sorbents are porous materials which soak up oil. They are much more effective if they don't soak up water at the same time. If they can also be designed to stay floating on the surface of the water when they are full of oil, that makes cleaning up afterwards relatively easy.

CIE asked a question about these in June 2009 paper 4 Q9(b). This was about a material called a hydrophobic aerogel.

An aerogel is a light, very porous material that can absorb as much as 200 times its own weight of liquid.

Hydrophobic literally means "water fearing". So these compounds won't absorb water, but they will absorb organic molecules like the hydrocarbons in oil.

You were told that they were based on a silicon dioxide network with some of the silicon atoms attached to groups containing fluorine atoms:

-O-Si-CH₂CF₃

You were asked what the word hydrophobic means - an easy mark.

You were then asked why the fluorine-containing groups allowed oil to be absorbed, but not water. This is actually quite tricky if you haven't thought about it before.

The answer wanted was that the fluorine atoms couldn't form hydrogen bonds with water molecules, but could experience van der Waals attractions to the hydrocarbon molecules in oil.

It isn't actually at all obvious why fluorine atoms attached to carbon won't form hydrogen bonds to water molecules - but they won't. Just remember that. I can't give you a simple reason why they don't.

Cleaning up contamination in ground water

Ground water is water that has soaked into the ground, and may eventually end up in drinking supplies. Some contamination may come from substances in the rocks, but it can also be caused by waste water from mining metals like copper, lead and zinc.

There are several different ways of solving this, and it is a waste of time trying to explore all of them. Judging from the only minor question (1 mark in 13 exam sessions!) that CIE had asked up to June 2013, there are two methods you could usefully be aware of - one chemical, one biological.

Precipitation of heavy metals like copper, lead, etc

It is better to stop the heavy metals from getting into the ground water to start with. Water flowing away from a mine is often acidic, and this helps to dissolve metal ions. By raising the pH of the water so that it is mildly alkaline, these metal ions will precipitate out again as metal hydroxides.

The waste water can be treated, for example, with slaked lime - calcium hydroxide. Copper ions will react with hydroxide ions to make copper(II) hydroxide, Cu(OH)₂, which would be left in a sludge. Other metal ions would behave similarly.

It may be possible to recover the useful metals from these sludges.

Bioremediation

Some plants have the ability to take up heavy metal ions, and store them. Provided the roots reach into the ground water, heavy metal ions will be removed. The plant growth can be harvested, and disposed of safely or possibly treated to recover the metals if they are in sufficient quantity to make it economic.

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