Decomposition of copper carbonate - proving one of two equations.

0.1g CuCO3 → 19.433… = 19.43cm3 (2dp)

When an element like copper can form two oxides, which one forms is based on the stability of the compound formed. The more stable the compound, the more likely it is to form. The stability of a compound with respect to its elements can be predicted by the ΔHf (molar heat of formation). This is the energy change when 1 mole of a compound is formed from its elements. If it is exothermic (negative), then the compound is stable with respect to its elements. If it is endothermic (positive), then the compound is unstable with respect to its elements. In general, the lower the value of ΔHf, the greater the energetic stability of the compound with respect to its elements. The molar heat of formation of CuO is –155.2, and the molar heat of formation of Cu2O is –166.7.  So it seems as though Cu20 will be the more stable compound. However, this fails to take into account the kinetic stability of a compound. The kinetic stability of a compound is caused by the activation energy required to cause it to react. The greater the activation energy required, the greater the kinetic stability of a compound, since the likelihood of the activation energy being supplied and the compound reacting is slim. It is also not sufficient to simply consider the stability of a compound with respect to its elements. It is necessary to consider the stability of copper (I) oxide with respect to copper (II) oxide, as well as with respect to copper and oxygen. It would also be important to consider the stability of CuCO3 with respect to both oxides, to see which reaction is energetically most likely. So it is clear that the value of ΔHf suggests that Cu2O will be formed, but that this will not necessarily be the case because of the other factors involved.

        

Copper (II) compounds are by far the most common – they are called “cupric”. Copper (I) compounds – “cuprous” compounds are far less common. Copper atoms most readily go to oxidation state +2, by a variety of reactions. Cupric compounds are unstable in the presence of water, so must either be insoluble or form complexes with other molecules.

Pilot Experiment

Heat 0.1g of CuCO3 in the apparatus as set up above. Record the volume of gas collected in the gas burette. When no more gas is being evolved, remove the apparatus from the water (to prevent suck-back) and stop heating.

Pilot Results

Analysis of and Modifications to the Pilot

This result suggests that the reaction taking place has equation 2 – the volume of 21.15cm3 is approximately 9% different from the predicted value of 23.32cm3, whereas it is approximately 13% different from the expected value were the first reaction taking place. This is not in line with what was suggested by the molar heat of formation, but the other factors explained in the background theory could account for the discrepancy.

This result is fairly ambiguous, because the volume produced is only 13% different from equation 1, raising doubt over the reliability of the result. This ambiguity would be lessened by increasing the mass of copper carbonate used, which would reduce the overall effect of any inaccuracy on the result. I will calculate the maximum possible value for the mass which still enables all the gas to be collected in one burette. To improve the reliability of the experiment, it will be necessary to repeat the reaction several times and take an average, excluding anomalous results (those not within 10% of the other results).  One problem with the pilot method was ensuring that all the gas from the delivery tube was caught by the narrow opening of the gas burette. To overcome this, I will place an inverted funnel under the mouth of the gas burette to funnel the gas into it. Some excess gas was given out because of the expansion of the air in the test tube when heated. To measure the volume of gas caused by this is difficult – it would be possible using the gas laws, but this requires measuring the temperature inside the boiling tube, which is difficult using standard laboratory equipment. Instead a “control” experiment could be set up, in which no copper carbonate is used, but an empty boiling tube is heated for the same length of time, and the volume of the gas collected due to expansion of the air measured. This volume could then be subtracted from the volume obtained by the decomposition of the copper carbonate to give a more accurate result for the volume of gas given off.

Hypothesis

I predict that the CuO compound will be formed, because this is in line with the pilot results, and would be supported by the background theory. Copper most commonly forms compounds as a divalent  ion, so I think that the thermal decomposition will be no exception and the oxide will be CuO. The kinetic stability and the stability of CuO with respect to Cu2O could account for the fact that Cu2O is energetically more stable with respect to its elements.

Main Experiment Plan

Variables

Independent variable - Mass of copper carbonate

Dependent variable - Volume of gas produced

Controlled variables:

Mass of Copper Carbonate

The mass of copper carbonate was chosen to be the maximum convenient mass which would give out less than 40cm3 of gas, enabling it to be collected in a gas burette with room for expansion.

Equation 1: 2CuCO3 (s)  → Cu2O (s) + 2CO2 (g) + ½ O2 (g)

1 mole of any gas occupies 24dm3 at room temperature and pressure.

2 moles CuCO3 → 2.5 moles gas, ie 60dm3.

246.4g CuCO3 → 60 000cm3

Acceptable volume of gas is between 20 and 40cm3

ie: Between 0.0821g and 0.1642g

Equation 2: CuCO3 (s) → CuO (s) + CO2 (g)

1 mole CuCO3 → 1 mole gas, ie 24dm3

123.2g → 24 000cm3

Acceptable volume of gas is between 20 and 40cm3

ie: Between 0.1026g and 0.2053g

A good mass to use which would allow for both equations would be 0.16g

Apparatus

• Top pan balance – The top pan balance is accurate to 0.005g. For a mass of 0.16g, this is accuracy to within 3.1%.
• Boiling tube and bung – a tight-fitting bung will ensure that no gas is lost once heating has started.
• Gas burette – a burette is accurate to 0.05cm3. For a 25cm3 volume, this is accuracy to within 0.2%. The experiment needs to show which of the two equations is correct, and the volume of gas produced by each differs by just under 9cm3, so an inaccuracy of 0.05cm3 will still allow which reaction is taking place to be clearly discerned.
• Bunsen burner
• 2 Bosses, clamps and clamp stands for boiling tube and gas burette
• Trough and delivery tube and funnel – The two gases which could possibly be produced are oxygen and carbon dioxide. Both are only sparingly soluble at room temperature and pressure and so are suitable for being collected over water.

Total quantifiable inaccuracy is approximately 3.3%

Risk assessment

• Copper carbonate is harmful if swallowed. The powder will irritate lungs and eyes, so safety goggles will be worn.
• Copper oxides are also harmful if swallowed and irritating to lungs and eyes. They are dangerous with aluminium and magnesium, forming explosive mixtures.

So throughout the experiment, safety goggles will be worn, and care will be taken with potentially hazardous chemicals. In the case of ingestion, wash out mouth, give a glass or two of water, and seek medical attention. If the dust is inhaled, seek fresh air to rest, and seek medical attention if breathing is difficult. If dust gets in the eyes, flood the eye with tap water for 10 minutes and seek medical attention. If spilt on skin or clothes, remove contaminated clothing and wash the affected area thoroughly. In the case of spillage, alert the teacher and clear up carefuly without raising too much dust and wipe area with a damp cloth to remove all traces of the solid.

• When heating, stand apparatus on a heating mat, wear safety goggles, tie long hair back, and leave the bunsen burner on safety flame when not in use.

• When stopping heating, there is a risk of “suck-back” as the air in the test tube cools and contracts, causing a vacuum which can draw the water from the trough into the test tube. Cold water on the hot test tube can cause the test tube to shatter or explode, so it is important to remove the test tube and delivery tube from the trough immediately after stopping heating.

Method

1. Weigh out 0.16g of copper carbonate on the top pan balance into a boiling tube, placing it carefully using a spatula half way along the horizontal tube.
2. Set up the apparatus as shown in the diagram, and place the boiling tube into the clamp stand, ensuring that the bung is firmly in place and the clamp is holding the boiling tube as close to the bung end as possible to avoid setting the cork on the clamp alight with the bunsen burner
3. Take the initial reading from the gas burette, and ensure that the delivery tube is pointing straight up the funnel into the burette.
4. Heat the copper carbonate vigorously until all the green powder has turned black, and no more gas is being produced.
5. Stop heating, and immediately remove the delivery tube from the trough by lifting the clamp stand holding the boiling tube, and moving everything clear of the water to prevent suck back.
6. Take the final reading from the gas burette, and calculate the total volume of gas given off.

Theoretical volumes of gas given off by 0.16g of copper carbonate using each equation

Equation 1: 2CuCO3 (s)  → Cu2O (s) + 2CO2 (g) + ½ O2 (g)

Mr of CuCO3 is 123.5

1 mole of any gas occupies 24dm3 (rtp)

2 moles CuCO3 → 2.5 moles of gas, ie 60dm3

247g CuCO3 → 60 000cm3

0.16g CuCO3 → 38.8663… = 38.87cm3 (2dp)

Equation 2: CuCO3 (s) → CuO (s) + CO2 (g)

1 mole CuCO3 → 1 mole gas, ie 24dm3

123.5g CuCO3 → 24000cm3

0.16g CuCO3 → 31.0931… = 31.09cm3 (2dp)