10.1 Conservation of matter  (Page 3/3)

Results:

Add the masses for the reactants for each reaction. Do the same for the products. For each reaction compare the mass of the reactants to the mass of the products. What do you notice? Is the mass conserved?

In the experiment above you should have found that the mass at the start of the reaction is the same as the mass at the end of the reaction. You may have found that these masses differed slightly, but this is due to errors in measurements and in performing experiments (all scientists make some errors in performing experiments).

Law of constant composition

In any given chemical compound, the elements always combine in the same proportion with each other. This is the law of constant proportion .

The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2:1 ratio. If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen. This mass proportion will be the same for any water molecule.

This does not mean that hydrogen and oxygen always combine in a 2:1 ratio to form $\mathrm{H}{}_2\mathrm{O}$ . Multiple proportions are possible. For example, hydrogen and oxygen may combine in different proportions to form $\mathrm{H}{}_2\mathrm{O}{}_2$ rather than $\mathrm{H}{}_2\mathrm{O}$ . In $\mathrm{H}{}_2\mathrm{O}{}_2$ , 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 $\mathrm{H}{}_2$ react with 1 volume of $\mathrm{O}{}_2$ to produce 2 volumes of $\mathrm{H}{}_2\mathrm{O}$ .

$2\mathrm{H}{}_2+\mathrm{O}{}_2\to 2\mathrm{H}{}_2\mathrm{O}$

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.

$\mathrm{N}{}_2+3\mathrm{H}{}_2\to 2\mathrm{NH}{}_3$

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 ( $\mathrm{MgO}$ )
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.

End of chapter exercises

1. For each of the following definitions give one word or term:
   1. A change that can be seen or felt, where the particles involved are not broken up in any way
   2. The formation of new substances in a chemical reaction
   3. A reaction where a new product is formed from elements or smaller compounds
2. State the conservation of energy principle.
   
3. Explain how a chemical change differs from a physical change.
   
4. 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

5. For each of the following reactions, say whether it is an example of a synthesis or decomposition reaction:
   1. $$({\mathrm{NH}}_4{)}_2{\mathrm{CO}}_3\to {\mathrm{NH}}_3+{\mathrm{CO}}_2+\mathrm{H}{}_2\mathrm{O}$$
   2. ${\mathrm{N}}_2\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\to 2{\mathrm{NH}}_3$
   3. ${\mathrm{CaCO}}_3\left(\mathrm{s}\right)\to \mathrm{CaO}+{\mathrm{CO}}_2$
6. For the following equation: ${\mathrm{CaCO}}_3\left(\mathrm{s}\right)\to \mathrm{CaO}+{\mathrm{CO}}_2$ show that the 'law of conservation of mass' applies.