Equation 2: CuCOз ➔ CuO + CO
If 1 mole of CuCOз produces 24000cm³ of CO, to produce 75cm³ (for equation 2) we need:
Moles in gas = volume / 24000
= 75 / 24000
Moles in gas = 0.003125 moles
Ratio = CO : CuCOз
1 : 1
0.003125 : 0.003125
Moles of CuCOз = 0.003125 moles
R.A.M of CuCOз = 63.5 + 12 + (16 × 3)
= 123.5
Mass of CuCOз = 0.003125 × 123.5
= 0.3859375
Mass of CuCOз needed to be decomposed = 0.39g (2 s.f)
Equation 1: 2CuCOз ➔ CuO + 2CO + ½O
For equation 1, volume of gas that will be produced using 0.39g of CuCOз will be:
Moles of 2CuCOз = mass / R.A.M
= 0.39 / 123.5
Moles of 2CuCOз = 0.003157894 moles
Ratio = 2CuCOз : 2CO
= 2 : 2
= 0.00315784 : 0.00315784
Moles of 2CO = 0.00315784 moles
Volume of 2CO = moles × 24000
= 0.00315784 × 24000
= 75.78816cm³
Volume of 2CO = 76cm³
APPARATUS
- 0.39g of CuCOз
- Bunsen burner
- Heat proof mat
- Tripod
- Gauze
- Bung
- Conical flask
- Delivery tube
- Gas syringe
- Clamp
- Scales (capable of weighing out 0.01g)
METHOD
- Collect and set up apparatus as shown in the drawing
- Ensure the gas syringe is at 0 before the experiment begins.
- Place the weighed mass of CuCOз into the conical flask and insert the bung tightly so that no gas escapes.
- Light the Bunsen burner and place it underneath the test tube (as shown in the drawing).
- Heat the conical flask until the CuCOз has fully decomposed. (This will be indicated by the fact that the bubbling will stop, and there will be a significant colour change of the CuCOз form bluish green to black or red).
- Continue to heat even after the bubbling has stopped for about 1 minute. This will ensure that the reaction has completely ended
- Leave the apparatus unattended to for a few seconds to ensure all gas has been released.
- Then record the reading from the gas syringe which shows the volume of gas produced.
- Repeat theses stages thrice, to get a more accurate result. Then take the average for the results.
WHICH EQUATION IS CORRECT?
In previous calculation, 0.39g of CuCO³ should produce 75cm³ of gas, if equation 2 is correct. If this volume of gas or a slightly less volume (to allow for slight gas loss) is obtained during the experiment, then equation 2 is correct. But if more than 75cm³ of gas is produced, then it means that another gas has been produced which will be oxygen. This would then mean that equation 1 is correct.
Also, after the decomposition has fully occurred, you can try to guess which equation is happening through looking at the colour of the substance after decomposition. If it’s a red compound, it may be equation 1 occurring due to the CuO which is a red crystalline. Whereas if it’s a black compound then it may be equation 2 occurring as CuO is a black powder.
SAFETY
CuCOз is harmful; ingestion may cause nausea or vomiting, and inhalation may cause irritation (4).
Care must be taken when dealing with Bunsen burner to avoid been burnt.
REFRENCES
(1)
(2) OCR Chemistry 1 - p20
(3)
(4)