# 0.6 Physical and chemical change  (Page 3/7)

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## Results:

You should observe a gas bubbling up into the second test tube. This reaction happens quite rapidly.

## Conclusions:

When hydrogen peroxide is added to manganese dioxide it decomposes to form oxygen and water. The chemical decomposition reaction that takes place can be written as follows:
$2{\mathrm{H}}_{2}{\mathrm{O}}_{2}\to 2\mathrm{H}{}_{2}\mathrm{O}+{\mathrm{O}}_{2}$
Note that the manganese dioxide is a catalyst and is not shown in the reaction. (A catalyst helps speed up a chemical reaction.)

The previous experiment used the downward displacement of water to collect a gas. This is a very common way to collect a gas in chemistry. The oxygen that is evolved in this reaction moves along the delivery tube and then collects in the top of the test tube. It does this because it is lighter than water and so displaces the water downwards. If you use a test tube with an outlet attached, you could collect the oxygen into jars and store it for use in other experiments.

The above experiment can be very vigourous and produce a lot of oxygen very rapidly. For this reason you use dilute hydrogen peroxide and only a small amount of manganese dioxide.

## Synthesis reactions

During a synthesis reaction , a new product is formed from elements or smaller compounds. The generalised equation for a synthesis reaction is as follows:

$\mathrm{A}+\mathrm{B}\to \mathrm{AB}$

One example of a synthesis reaction is the burning of magnesium in oxygen to form magnesium oxide( [link] ). The equation for the reaction is:

$2\mathrm{Mg}+{\mathrm{O}}_{2}\to 2\mathrm{MgO}$

## Aim:

To demonstrate the synthesis of iron sulphide from iron and sulphur.

## Apparatus:

5,6 g iron filings and 3,2 g powdered sulphur; porcelain dish; test tube; Bunsen burner

## Method:

1. Measure the quantity of iron and sulphur that you need and mix them in a porcelain dish.
2. Take some of this mixture and place it in the test tube. The test tube should be about 1/3 full.
3. This reaction should ideally take place in a fume cupboard. Heat the test tube containing the mixture over the Bunsen burner. Increase the heat if no reaction takes place. Once the reaction begins, you will need to remove the test tube from the flame. Record your observations.
4. Wait for the product to cool before breaking the test tube with a hammer. Make sure that the test tube is rolled in paper before you do this, otherwise the glass will shatter everywhere and you may be hurt.
5. What does the product look like? Does it look anything like the original reactants? Does it have any of the properties of the reactants (e.g. the magnetism of iron)?

## Results:

1. After you removed the test tube from the flame, the mixture glowed a bright red colour. The reaction is exothermic and produces energy .
2. The product, iron sulphide, is a dark colour and does not share any of the properties of the original reactants. It is an entirely new product.

## Conclusions:

A synthesis reaction has taken place. The equation for the reaction is:
$\mathrm{Fe}+\mathrm{S}\to \mathrm{FeS}$

## Apparatus:

Bunsen burner, 4 test tubes, a test tube rack and a test tube holder, small spatula, pipette, magnet, a birthday candle, $\mathrm{NaCl}$ (table salt), $0,1\phantom{\rule{2pt}{0ex}}\mathrm{M}\phantom{\rule{4pt}{0ex}}{\mathrm{AgNO}}_{3}$ , $6\phantom{\rule{2pt}{0ex}}\mathrm{M}\phantom{\rule{4pt}{0ex}}\mathrm{HCl}$ , magnesium ribbon, iron filings, sulphur.

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