Decomposition of Copper Carbonate

Decomposition of Copper Carbonate
Introduction Copper has two oxides, Cu2O and CuO. Copper carbonate, CuCO3, decomposes on heating to form one of these oxides and an equation can be written for each possible reaction. Equation 1: 2CuCO3 (s) à Cu2O (s) + 2CO2 (g) + ½O2 (g) Equation 2: CuCO3 (s) à CuO (s) + CO2 (g) Aim === The aim of this experiment is to prove which of these two equations is correct by measuring the volume of gas given off by the decomposition of CuCO3. Background Theory -??????? Cu2O is known as cuprous oxide. It is a red crystalline material, which can be produced by electrolytic or furnace methods. CuO is cupric oxide. This, a black powder, can be prepared by the ignition of suitable salts such as the carbonate, the hydroxide, or the nitrate of copper, or by heating of cuprous oxide*. The planning of this experiment requires previous knowledge about the mole.
The mole is the chemist?s unit of amount; it is the mass of substance that has the same number of particles as there are atoms in exactly 12g of carbon-12. One mole of any substance, be it element or molecular compound, contains Avogadro?s number of atoms. Avogadro?s number is a constant, L, and is approximately 6.02 × 1023 mol-1**. Avogadro discovered that, at room temperature (25ºC) and pressure (1 atmosphere), all gases occupy that same volume. One mole of any gas will occupy a volume of 24dm3. This information is useful for working what amount of copper carbonate should be decomposed. Another essential equation will be required in order to do this: Moles = Mass Mr Therefore: Mass = Moles x Mr Calculating the Correct Mass ======== The best method would be to take one of the two equations and try to prove or disprove it. Since Equation 2 has only 1 mole of every substance, this would be the easier one to work with in terms of ratio. It can be said that: ?1 mole of CuCO3 will decompose to 1 mole of CuO and 1 mole of CO2? This can be assumed because the equation is "stoichiometric"***, meaning that exactly one mole of copper carbonate decomposes to exactly one mole of copper oxide and exactly one mole of carbon dioxide. So: CuCO3 (s) à CuO (s) + CO2 (g) 1mol 1mol 1mol And because 1 mole of any gas occupies 24dm³ or 24000cm³: CuCO3 (s) à CuO (s) + CO2 (g) 1mol 1mol 24000cm³ However, 24000cm³ would be far too much gas to be produced in a school laboratory, as that size of apparatus is not available. The largest gas syringe available can hold 100cm³. It would not, however, be sensible to attempt to produce 100cm³ because if Equation 1 turns out to be the correct one, which would mean oxygen is also produced and there would consequently be more gas. Aiming to produce 75cm³ of gas would mean that there is room for error, and for the possibility of Equation 1 occurring. It would also be a lot easier to read the measurement. So, if 1 mole of CuCO3 produces 24000cm³ of CO2, to produce 75cm³ we need: 1 × 75 moles 24000 = 3.125 × 10-3 moles Mr of CuCO3 = 63.5 + 12 + (16 × 3) = 123.5 Therefore: Mass = (3.125 × 10-3) x 123.5 = 0.386 (3s.f.) This will be the mass of copper carbonate that will need to be decomposed. Apparatus -??? Scales capable of weighing out 0.01g Gas syringe Test tube Bung Glass tubing Bunsen burner Heatproof mat 0.39g Copper Carbonate Safety goggles Lab. coat Clamp and clamp stand Method -?-
Collect and set up apparatus as below:
Place the weighed mass of CuCO3 into the test tube and tightly
insert the bung so that no gas can escape. Attach the test tube to
the clamp.
? Light the Bunsen burner and place it on the heatproof mat, underneath the test tube. ? Heat until the copper carbonate has entirely decomposed. This will be indicated by the fact that the bubbling will stop, and there will be a colour change from the green of the copper carbonate to the black or red colour of the possible oxides. ? Take the reading from the gas syringe and note down the volume of gas produced. ? Repeat these stages a further 3 times, then take an average of the 4 results. Fair Test -??? Minimizing gas loss: ==== Using a gas syringe as opposed to an upturned tube of water in a trough is, in itself, a more careful way of ensuring no gas is lost as the gas syringe has airtight seals. The bung must also be inserted tightly into the test tube before the decomposition begins. Keeping measurements accurate: The gas syringe must be at 0 before the experiment begins. It must be ensured that the copper carbonate has fully decomposed by the colour change, the lack of bubbling, and the fact the gas syringe has stopped being filled. The apparatus should be left assembled for a minute before the result is taken so all the gas can be released. The gas syringe should be left to cool so the gas inside can return to room temperature and pressure. The experiment should be repeated to obtain an accurate average and highlight anomalous results. If there are a few anomalies, they should be repeated. Safety -?- Copper carbonate is harmful. Ingestion may cause nausea and vomiting and inhalation may cause irritation on prolonged contact. Skin contact could cause skin irritation and possible skin discolouration and eye contact may cause mild eye irritation****. Care should be taken when dealing with the substance ? goggles and a lab. coat must be worn and if contact occurs the area should be rinsed. Care should also be taken when dealing with the Bunsen burner and the heated test tube. How to tell which equation is correct It was worked out in a previous calculation that 0.39g of CuCO3 should produce 75cm³ of gas, if Equation 2 is correct. If this volume, or a little less (to allow for slight gas loss) is obtained, then Equation 2 is correct. If more than this volume has been produced, it means that another gas, oxygen, has also been produced. This would suggest that the correct equation is Equation 1. Prediction I consider Equation 2 to be the most likely. Cu2O isproduced by electrolytic or furnace methods whereas CuO is produced by the ignition of suitable salts such as the carbonate, the hydroxide, or the nitrate of copper, or by heating of cuprous oxide*. Although the substance was not ignited, copper carbonate was decomposed, which may have a similar effect as ignition would.

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