TOPIC: LAWS OF CHEMICAL COMBINATION

PERIOD 1: CHEMICAL LAWS OF COMBINATIONS

There are four laws of chemical combination which describe the general features of a chemical change.

Law of conservation of mass: This law was established by Lavoisier, a French chemist. The law of conservation of mass states that matter is neither created nor destroyed during chemical reaction, but changes from one form to another.

Experiment to verify the law of conservation of matter (mass)

Theory:

The equation of the chemical reaction chosen for study is as follows;

Silver nitrate + sodium chloride Silver chloride + Sodium trioxonitrate(v)

(White precipitate)

Method:

1. Put some sodium chloride solution in a conical flask

2. Fill a small test tube with silver trioxonitrate (iv) solution of string, suspend it in a conical flask as shown below:

Insert the stopper and weight the whole apparatus on a balance, note the mass of the whole system.

Mix the two liquids by pulling the string attached to the bottom end of the small test tube.

Weigh the whole apparatus again.

Result: When the two reactants are mixed together, a white precipitate is formed indicating that a chemical reaction has taken place. The new substances formed are known as the products of the chemical reaction. The masses of the system taken before and after the reaction are found to be the same, indicating that the mass of the reactants equals that of the products.

CONCLUSION: Since there is no overall change in mass when the products are formed,we can infer that matter is neither created nor destroyed during the chemical reaction. The law is, hence valid.

EVALUATION:

1. Mention another compound that could be used instead of silvertrioxo-nitrate(v) with sodium chloride

2. State the law of conservation of mass/matter.

PERIOD 2: LAW OF DEFINITE PROPORTION OR LAW OF CONSTANT COMPOSITION

The second law of chemical combination which is supported by the Atomic theory was proposed by provost (1755-1826) known as the Law of definite proportions or constant composition.

The law of definite proportions states that all pure samples of a particular chemical compound contain similar elements combined in the same proportion by mass. It is based on the fact that when elements combine to form a given

compound, they do so in fixed proportions by mass, so that all pure samples of that compound are identical in composition by mass. Water for example: chemical analyses showed that as long as it is pure, its composition is always in the ratio of one mole of oxygen to two moles of hydrogen. i.e. 32g of O to 4g of H. Irrespective of whether the water comes from river, sea, rain or anywhere.

Experiment to verify the law of definite proportion

Method: Prepare two samples of black copper (ii) oxide, each by a different method as given below:

Sample A: Place some coppers turning in a crucible and add some concentrated trioxonitrate (v) acid, a little at a time, until the copper dissolves completely. Evaporate the resulting green solution of copper II oxide trioxonitrate (v) to dryness; continue to heat the residue until it decomposes to give a black solid which is copper II oxide. Keep the black residue dry in desiccator.

Sample B:Place some copper (i) trioxocarbonate (iv) in a crucible and decompose it into copper (ii) oxide and carbon (iv) oxide store the residue in a desiccator.

ANALYSES:

Determine the amount of copper present in the two samples of copper oxide by reducing the oxide in a stream of hydrogen or carbon II oxide as follows.

1.Weigh two clean metal boats.

2.Add a reasonable amount of sample A to one and sample B to the other

3.Reweigh and determine the mass of each sample. Place the boats inside a hard glass tube as shown. Heat the samples stronglywhile passing a stream of dry hydrogen gas through the tube. After some time, a reddish- brown copper residue is left in each boat. Remove the flame, but continue passing the hydrogen as the copper residues cool down. This presents the re-oxidation of the hot copper residue by atmospheric oxygen. Any water formed during the reaction is absorbed by the fused calcium chloride in the adjacent U-tube.

Result:

Sample	A	B
Mass of copper II oxide	3.55g	3.02g
Mass of copper residue	2.81g	2.42g
Percentage of copper present in copper (ii) oxide	×100	× 100
	79.2%	80.1%

Assignment