Chemguide

Extra material needed for the 2010 exam - Chapter 12

This page lists the material from the old Edexcel syllabus which isn't covered by my new Edexcel IGCSE Chemistry book. There is quite a lot of additional material in this section relative to the new syllabus, although most of it is covered elsewhere in the book.

Group 1

The old syllabus says that candidates should be able to "discuss the simple physical and chemical properties of the hydroxides, halides, sulphates, nitrates and carbonates of these elements". Edexcel have issued guidance on this, and what they want is as follows:

• You are expected to know the solubility of the hydroxides, halides, sulfates, nitrates and carbonates of the elements of Group 1. This is easy.

  Apart from a few lithium compounds, Group 1 compounds are soluble.

  This is actually covered in the new book at the bottom of page 104. You wouldn't be expected to know the fine details of the less soluble lithium compounds.

• Group 1 oxides react with water to produce solutions of hydroxides, which are alkaline. For example, sodium oxide reacts with water to produce alkaline sodium hydroxide solution (turns red litmus paper blue).

  

Group 2

Knowledge of Group 2 chemistry is restricted to magnesium and calcium, although you would be expected to make predictions about elements further down the Group. Group 2 elements are sometimes referred to as the alkaline earth metals.

Reactions between the elements and water

The reactions between water and the elements magnesium and calcium are covered in the Reactivity Series chapter on pages 64-65. Notice that calcium is more reactive than magnesium. The reactivity continues to increase down the rest of the Group.

Simple chemical and physical properties of various Group 2 compounds

As with Group 1, the old syllabus wants you to "recall the simple physical and chemical properties of the oxides, hydroxides, chlorides, nitrates and carbonates of these elements". Again, Edexcel have given clarification of this as follows. (I've just noticed that there is a mismatch between their list in the syllabus and their clarification! What follows is based on the clarification.)

• You should know the solubility of the hydroxides, sulfates, nitrates and carbonates of the elements of Group 2. This is more complicated than for Group 1, but everything you need is in the table on page 82, which you should have learnt already. Remember that the syllabus is only talking about magnesium and calcium, although you should, of course, be aware that barium sulfate is insoluble (as is strontium sulfate).

• You should know that Group 2 oxides react with water to give hydroxides. The equation for magnesium hydroxide reacting with water is given on page 56.

The reaction between calcium oxide and water is described on page 121, together with the equation. There is a short quirky bit of video of this reaction on YouTube.

The hydroxides get increasingly soluble (although never very soluble) as you go down the Group, and their solutions are alkaline. The Group 2 hydroxides are much less soluble than the Group 1 hydroxides.

• You need to know about the thermal decomposition of the carbonates of the elements. Thermal decomposition is splitting something up by heating it.

  These carbonates decompose on heating to give the oxide and carbon dioxide. You will find this discussed for carbonates in general on page 122. You can ignore the stuff about energy changes for now. The magnesium carbonate equation is just like the others.

Transition metals

There is some copper and iron chemistry in the old syllabus which has been removed from the new one. I have postponed the iron chemistry until the metals chapter (because this is already going to be very long and tedious!), but have included the copper chemistry here, because most of it follows on from what we have just been looking at.

The existence of copper(I) compounds

You have to be aware (no more than that!) that copper forms a series of copper(I) compounds as well as the more familiar copper(II) compounds. You are already familiar with the fact that iron forms both iron(II) and iron(III) compounds - well, the copper is doing something similar.

A simple example is red copper(I) oxide - Cu₂O.

The ability to form more than one kind of compound is typical of the transition metals. It used to be described as variable valency. "Valency" is an old fashioned term, and you would now describe it as variable oxidation state. At this level, you can think of it as an ability to form ions with different charges.

Some simple physical and chemical properties of copper(II) compounds

This is the same sort of thing that we have just looked at for Groups 1 and 2. You have to be able to describe the simple physical and chemical properties of copper(II) oxide, hydroxide, nitrate, sulfate, carbonate, and chloride. The following is based on Edexcel's clarification:

The colour and solubility of some compounds

• Copper(II) oxide is black and insoluble in water.

• Copper(II) hydroxide is pale blue and insoluble in water.

• Copper(II) chloride is blue-green and dissolves in water to give a blue solution (tending to blue-green at higher concentrations).

• Copper(II) sulfate is blue and dissolves in water to give a blue solution.

• Copper(II) nitrate is blue and dissolves in water to give a blue solution.

• Copper(II) carbonate is green and insoluble in water.

This looks a bit daunting, but you have already met the solubility patterns on page 82. Most of the colours will be familiar from other parts of the course.

The reduction of copper(II) oxide to copper

This is just a simple Reactivity Series reaction. You can reduce copper(II) oxide to copper by heating it with a more reactive element such as magnesium (see page 60) or carbon (see page 61).

You can also reduce it by passing a stream of hydrogen over hot copper(II) oxide. The black powder glows red hot, and a heap of pink-brown copper powder is left behind.

Warning! Don't try this at home! Unless you do things in exactly the right way, there is a serious explosion risk.

The thermal decomposition of copper(II) carbonate and nitrate

The carbonate is easy. It is described, with a photo, on page 122.

Many nitrates (including copper(II) nitrate) decompose on heating to give the metal oxide, nitrogen dioxide gas, and oxygen gas.

In the case of copper(II) nitrate, the blue crystals turn black on heating, and give off a mixture of brown nitrogen dioxide gas and oxygen. There is a very short piece of video on YouTube which gives you all the important information.

The formation of a complex copper ion

We have already come across this in the additional material for Chapter 11, but it is worth repeating here, and adding to slightly.

If you add ammonia solution to a solution containing copper(II) ions, you get an initial pale blue precipitate of copper(II) hydroxide - similar to the one produced with sodium hydroxide solution.

However, when you add an excess of ammonia solution, this precipitate redissolves to give a very deep blue solution containing the ion [Cu(NH₃)₄(H₂O)₂]²⁺. This is an example of a complex ion.

A complex ion is any ion which contains a central ion to which are attached other atoms or groups of atoms known as ligands. These ions are very common, even though we tend to keep quiet about them at this level. The blue colour of copper(II) sulfate solution, for example, is actually due to the [Cu(H₂O)₆]²⁺ ion, although we normally just call it Cu²⁺(aq).

The transition metals form a huge range of complex ions.

Transition metal summary

The point of all this is to illustrate the syllabus statement: "understand that the study of these two metals and their compounds illustrates typical transition metal properties of variable valency, formation of coloured compounds and formation of complex ions". The two metals referred to include iron, of course.

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