4.1 Energy changes and conservation of matter  (Page 3/3)

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This does not mean that hydrogen and oxygen always combine in a 2:1 ratio to form H 2 O. Multiple proportions are possible. For example, hydrogen and oxygen may combine in different proportions to form H 2 O 2 rather than H 2 O. In H 2 O 2 , the H:O ratio is 1:1 and the mass ratio of hydrogen to oxygen is 1:16. This will be the same for any molecule of hydrogen peroxide.

Volume relationships in gases

In a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure. This relationship is also known as Gay-Lussac's Law .

For example, in the reaction between hydrogen and oxygen to produce water, two volumes of H 2 react with 1 volume of O 2 to produce 2 volumes of H 2 O.

2H 2 + O 2 $\to$ 2H 2 O

In the reaction to produce ammonia, one volume of nitrogen gas reacts with three volumes of hydrogen gas to produce two volumes of ammonia gas.

N 2 $+$ 3H 2 $\to$ 2NH 3

This relationship will also be true for all other chemical reactions.

Summary

The following video provides a summary of the concepts covered in this chapter.

1. Matter does not stay the same. It may undergo physical or chemical changes.
2. A physical change means that the form of matter may change, but not its identity. For example, when water evaporates, the energy and the arrangement of water molecules will change, but not the structure of the water molecules themselves.
3. During a physical change, the arrangement of particles may change but the mass, number of atoms and number of molecules will stay the same.
4. Physical changes involve small changes in energy and are easily reversible.
5. A chemical change occurs when one or more substances change into other materials. A chemical reaction involves the formation of new substances with different properties . For example, magnesium and oxygen react to form magnesium oxide (MgO)
6. A chemical change may involve a decomposition or synthesis reaction. During chemical change, the mass and number of atoms is conserved, but the number of molecules is not always the same.
7. Chemical reactions involve larger changes in energy. During a reaction, energy is needed to break bonds in the reactants and energy is released when new products form. If the energy released is greater than the energy absorbed, then the reaction is exothermic. If the energy released is less than the energy absorbed, then the reaction is endothermic. Chemical reactions are not easily reversible.
8. Decomposition reactions are usually endothermic and synthesis reactions are usually exothermic .
9. The law of conservation of mass states that the total mass of all the substances taking part in a chemical reaction is conserved and the number of atoms of each element in the reaction does not change when a new product is formed.
10. The conservation of energy principle states that energy cannot be created or destroyed, it can only change from one form to another.
11. The law of constant composition states that in any particular compound, all samples of that compound will be made up of the same elements in the same proportion or ratio.
12. Gay-Lussac's Law states that in a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure.

Summary exercise

1. Complete the following table by saying whether each of the descriptions is an example of a physical or chemical change:
 Description Physical or chemical hot and cold water mix together milk turns sour a car starts to rust food digests in the stomach alcohol disappears when it is placed on your skin warming food in a microwave separating sand and gravel fireworks exploding
2. For each of the following reactions, say whether it is an example of a synthesis or decomposition reaction:
1. (NH 4 ) 2 CO 3 $\to$ 2NH 3 + CO 2 + H 2 O
2. 4Fe + 3O 2 $\to$ 2Fe 2 O 3
3. N 2 (g) + 3H 2 (g) $\to$ 2NH 3
4. CaCO 3 (s) $\to$ CaO + CO 2
3. For the following equation: CaCO 3 $\to$ CO 2 + CaO show that the 'law of conservation of mass' applies.

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