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# Assessed Practical Essay Sample

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## Assessed Practical Essay Sample

The aim of this experiment is to find out which reaction is correct for the thermal decomposition of copper carbonate out of the following two equations.

2CuCO3(s) ==> Cu2O(s) + 2CO2(g) + 1/2O2(g)

CuCO3(s) ==> CuO(s) + CO2(g)

Avagadro’s constant states that one mole of gas under standard conditions will fill 24dm3 under standard conditions so it is possible to find the amount of gas evolved by measuring the volume. From this it is also possible to find which version of the reaction has taken place.

How much copper carbonate should be used?

The first equation will produce more gas so that is the maximum amount to be taken into account when deciding how much copper carbonate needs to be used. If one mole of copper carbonate is used then one mole of CO2 and 0.25 moles of O2 will be evolved. This will occupy 30dm3 in total. One mole of copper carbonate has a mass of 123.664g.

Because most gas syringes are 100cm3 the amount of gas evolved needs to be less than this. To make the experiment such that 60cm3 will be evolved the mass of copper carbonate needs to be divided by 500. Having 60cm3 gas evolved will leave room in the gas syringe in case the conditions are not perfect for the experiment. If the volume was calculated so that 90cm3 gas would be evolved there is a chance that the gas syringe would not be able to take it.

30000/500 = 60

123.664/500 = 0.247

Therefore the amount of copper carbonate that should be used has a mass of 0.247g. As it is very difficult to measure to this degree of accuracy, this number can be rounded to 0.250g

Equipment, Apparatus and Chemicals

Bunsen burner

Tripod

Stand and clamp

Heatproof gauze

Heatproof mat

Gas syringe (100cm3)

Rubber tubing and bung

Copper carbonate

Safety

Because a Bunsen burner is being used certain precautions need to be taken whilst carrying out the experiment. When a Bunsen burner is turned on it should be with the safety flame, in order to make everyone close by aware that the Bunsen burner is turned on. The apparatus should not be handled straight after heating or it will cause burns. Copper carbonate is an irritant to the eyes so eye protection should be worn throughout the experiment, to protect the eyes from the Bunsen burner flame and the copper carbonate. Copper carbonate should not be swallowed, and is irritant.1 Copper (II) oxide (CuO) is also irritant and should not be ingested or allowed near the eyes.2 Likewise, copper (I) oxide (Cu2O) should not be ingested.

Conditions

The experiment must be carried out under standard conditions – 1 atmosphere of pressure and a temperature of 298K4. This is because when the expected volumes of gas are calculated the calculations will assume standard conditions. If the experiment is carried out under different conditions than those assumed in the calculations it will be impossible to draw an accurate conclusion, as temperature and pressure will both affect the volume of gas. It is less important to ensure that pH is neutral because this does not affect gas volume.

Procedure

* Place the gas syringe in the clamp, held exactly horizontally.

* Place 0.25g copper carbonate in the conical flask.

* Place the conical flask on the heatproof gauze, all on the tripod and over the Bunsen burner.

* Connect the gas syringe to the conical flask with the rubber bung and tubing.

* Heat the conical flask with the copper carbonate inside it until the reaction is complete. This is when no more gas is being given off.

* Turn off the Bunsen burner.

* Leave the apparatus to cool.

* Observe the volume of gas using the gas syringe.

* Repeat the experiment 3 times. For each experiment use a new conical flask and reset the gas syringe.

Calculations

If the equation “2CuCO3(s) ==> Cu2O(s) + 2CO2(g) + 1/2O2(g)” is correct, the amount of gas evolved should be around 61 cm3 using the calculation,

(M / Mr) x 30000

This is because for each mole of copper carbonate that reacts 30000 cm3 gas is evolved.

(0.25 / 123.664) x 30000 = 60.648

For the equation “CuCO3(s) ==> CuO(s) + CO2(g)” there is only 24000 cm3 gas evolved per mole of copper carbonate that reacts, so the formula for finding the volume is,

(M / Mr) x 24000

(0.25 / 123.664) x 24000 = 48.519

Therefore if this equation is correct approximately 49 cm3 gas is evolved.

References:

1 http://en.wikipedia.org/wiki/copper_carbonate

3 http://ptcl.chem.ox.ac.uk/MSDS/CU/cuprous_oxide.html

4 Chemistry 1 – Cambridge.

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