Sunday, 4 January 2015

3.5.1 Laboratory Activity : Determining the empirical formula of copper oxide

Laboratory Activity 3.5.1:
Determining the empirical formula of copper oxide
Aim: To determine the empirical formula for copper oxide
» Copper oxide
» Zinc pieces
» Dilute hydrochloric acid
» Anhydrous calcium chloride
» Flat-bottomed flask
» Asbestos paper
» Glass tube with stopper
» Electronic balance
» Thistle funnel
» U-tube
» Retort stand and clamp
» Combustion tube with a tiny hole at the end
Procedure and Results:

1. The animation below shows the arrangement and the results of the experiment.
2. A piece of asbestos paper is placed in a combustion tube. Weigh with a chemical balance.
3. A spatula of dry copper(II) oxide powder in placed on the asbestos paper and the combustion tube and is weighed again.
4. Thistle funnel dropper is opened to allow the acid to flow into the conical flask.
5. Dry hydrogen gas is flowed through the combustion tube for several minutes. Collect some gas from the exit of combustion tube with test tube. Test the gas with a burning splint. If there is no 'pop' sound, then all hydrogen has been removed.
Precautionary steps :Hydrogen gas must be allowed to pass through the combustion tube for several minutes before the copper oxide is heated. This action is to remove all the air in the combustion tube. A mixture of hydrogen and air can cause an explosion when lighted.
6. The copper(II) oxide in heated and the excess hydrogen which exits at the end of the tube is lit.
7. Copper(II) oxide will glow red-hot and react with hydrogen.
8. The heating is stopped when there are no more red-hot glow of copper(II) oxide seen.
Precautionary steps :The flow of hydrogen gas must be continued throughout the heating. This is to ensure that air does not enter the combustion tube. Thus, the hydrogen gas must be seen to be burning continuously at the end of the combustion tube.
9. The tube is cooled down. The product is weighed once again.
Precautionary steps :The hot copper metal is allowed to be cooled in a stream of hydrogen gas. This is to ensure the oxygen from the air does not oxidise the hot copper to copper oxide again.
10. The heating, cooling and weighting steps are repeated until a constant mass is obtained, to ensure that all the copper(II) oxide has been reacted.

Mass of combustion tube + asbestos crucible
= x g
Mass of combustion tube + asbestos crucible + copper oxide
= y g
Mass of combustion tube + asbestos crucible + copper = z g
Calculation :

Mass of copper obtained = (zx) g
Mass of oxygen that combines with copper = (yz) g
[Relative atomic mass: O, 16; Cu, 64]

Element Cu O
Mass (zx) (yz)
Number of moles z - x 64 y - z 16
Simplest ratio a b

The ratio of Cu : O = a : b
Thus the empirical formula is CuaOb
Discussion :

Copper oxide is black. When the reaction of copper oxide is completed, brown copper metal is formed.

The purpose of repeating the heating, cooling and weighing process is to ensure complete reaction of the copper oxide.

The empirical formula of copper oxide is CuO.

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  1. Why the excess hydrogen has to be lighted???

  2. after the reaction is completed, hydrogen gas is allowed to flow continuously until the copper is cooled to room temperature. could you explain why ?

    1. maybe this is too late but it is to prevent hot copper from reacting with oxygen in the air to form copper(II) oxide again

  3. why do we start off with copper oxide instead of allowing copper to react with oxygen in the air?

    1. copper has low reactivity that make reaction with oxygen very slow. It takes a long time to produce copper oxide. The other way round is faster.