An Experiment Into the Thermal Decomposition of Metal Carbonates

3. Mass of carbonate

Instead of working out the moles and then the equivalent mass needed, I have decided that it would be more advantageous to work it out by determining how much gas that I want to produce. I have decided to make the volume of CO2 produced 100 cm3. 100/2400 = number of moles which is 0.004. This is the number of moles that will be needed for each carbonate. Below are the calculations to work out the mass of each carbonate needed to produce 100 cm3 of CO2.

Potassium Carbonate (K2CO3)

Mr = (39)2 + 60 = 138

0.004 * 138 = 0.552

Mass to produce 100 cm3 gas = 0.55g

Sodium Carbonate (Na2CO3)

Mr = (23)2 + 60 = 106

0.004 * 106 = 0.424

Mass to produce 100 cm3 gas = 0.42g

Zinc Carbonate (ZnCO3)

Mr = 65 + 60 = 125

0.004 * 125 = 0.5

Mass to produce 100 cm3 gas = 0.5g

Lead Carbonate (PbCO3)

Mr = 207 + 60 = 267

0.004 * 267 = 1.068

Mass to produce 100 cm3 gas = 1.07g

Copper Carbonate (CuCO3)

Mr = 64 + 60 = 124

0.004 * 124 =.0496

Mass to produce 100 cm3 gas = 0.5g

Apparatus

• Boiling tubes * 5
• Delivery tubing
• Water bath
• 100cm3 measuring cylinder
• Bunsen burner
• Heat tile
• Stopwatch

• Retort stands *2

Method

Experiment 1

        Fill the water bath, allowing room for at least an extra 100ml of water. Fill the measuring cylinder up, invert it, placing fingers over the top of the cylinder and place in water bath, making sure that no air bubbles enter the cylinder. Measure out exactly 0.55g of Potassium carbonate, and then put it into a boiling tube. Put a holed bung with a delivery tube in it onto the top of the boiling tube, secure that with a retort stand, as well as the measuring cylinder that is still inverted in the water bath. Put the Bunsen burner on a blue, non-roaring flame, and heat the bottom of the boiling tube. As soon as you start to heat the tube, press the start button on the stopwatch, and every 10 seconds, get down to and measure the amount of water displaced. Put the information in the 10 second intervals in the table dawn out. Repeat the experiment once more.

Experiment 2

        Fill the water bath, allowing room for at least an extra 100ml of water. Fill the measuring cylinder up, invert it, placing fingers over the top of the cylinder and place in water bath, making sure that no air bubbles enter the cylinder. Measure out exactly 0.42g of Sodium carbonate, and then put it into a boiling tube. Put a holed bung with a delivery tube in it onto the top of the boiling tube, secure that with a retort stand, as well as the measuring cylinder that is still inverted in the water bath. Put the Bunsen burner on a blue, non-roaring flame, and heat the bottom of the boiling tube. As soon as you start to heat the tube, press the start button on the stopwatch, and every 10 seconds, get down to and measure the amount of water displaced. Put the information in the 10-second intervals in the table dawn out. Repeat the experiment once more.

Experiment 3

        Fill the water bath, allowing room for at least an extra 100ml of water. Fill the measuring cylinder up, invert it, placing fingers over the top of the cylinder and place in water bath, making sure that no air bubbles enter the cylinder. Measure out exactly 0.5g of Zinc carbonate, and then put it into a boiling tube. Put a holed bung with a delivery tube in it onto the top of the boiling tube, secure that with a retort stand, as well as the measuring cylinder that is still inverted in the water bath. Put the Bunsen burner on a blue, non-roaring flame, and heat the bottom of the boiling tube. As soon as you start to heat the tube, press the start button on the stopwatch, and every 10 seconds, get down to and measure the amount of water displaced. Put the information in the 10 second intervals in the table dawn out. Repeat the experiment once more.

Experiment 4

        Fill the water bath, allowing room for at least an extra 100ml of water. Fill the measuring cylinder up, invert it, placing fingers over the top of the cylinder and place in water bath, making sure that no air bubbles enter the cylinder. Measure out exactly 1.07g of lead carbonate, and then put it into a boiling tube. Put a holed bung with a delivery tube in it onto the top of the boiling tube, secure that with a retort stand, as well as the measuring cylinder that is still inverted in the water bath. Put the Bunsen burner on a blue, non-roaring flame, and heat the bottom of the boiling tube. As soon as you start to heat the tube, press the start button on the stopwatch, and every 10 seconds, get down to and measure the amount of water displaced. Put the information in the 10 second intervals in the table dawn out. Repeat the experiment once more.

Experiment 5

        Fill the water bath, allowing room for at least an extra 100ml of water. Fill the measuring cylinder up, invert it, placing fingers over the top of the cylinder and place in water bath, making sure that no air bubbles enter the cylinder. Measure out exactly 0.5g of Copper carbonate, and then put it into a boiling tube. Put a holed bung with a delivery tube in it onto the top of the boiling tube, secure that with a retort stand, as well as the measuring cylinder that is still inverted in the water bath. Put the Bunsen burner on a blue, non-roaring flame, and heat the bottom of the boiling tube. As soon as you start to heat the tube, press the start button on the stopwatch, and every 10 seconds, get down to and measure the amount of water displaced. Put the information in the 10 second intervals in the table dawn out. Repeat the experiment once more.

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Preliminary Experiment Notes

        In a preliminary experiment I was able to find helpful information regarding the way in which the experiment will be carried out. In this experiment I used 0.005 moles of copper carbonate, with a gas syringe to collect the carbon dioxide into. Thanks to this preliminary investigation I was able to find out a number of things that helped me writ up the final method:

1. Do not use a gas syringe, because when the carbon dioxide gets hot and delivered directly into the syringe, it does not give an accurate reading, as the gas expands when hot. Also the gas syringe tends to stick, and not give an accurate enough reading. It is best to use an inverted test tube, so that the gas cools as it gores through the water into the cylinder, which has no moving parts that can stick and therefore gives a far more accurate reading.

2. Take care not to take the flame away during or after the experiment, because this causes an effect called a ‘suck back’. This happens when gas and air inside the boiling tube that has been heated, has the flame taken away. The heat of the gas will get lower, as will the volume, the drop in atmospheric pressure inside the boiling tube will drop, displacing some water by crating a partial vacuum. When the water is ‘sucked’ into the boiling tube, this can cause it to explode. This is another hazard.

3. It is best to work out how much gas you would like to produce, then work back to see how many moles of the carbonate that you need, rather than the other way around.

Safety

• Wear safety goggles at all times.
• Do not take the flame away at any point during the experiment, before you have removed the end of the delivery tube form the water.
• Do not run in the laboratory.

Prediction

        I predict that the more reactive the original metal is in the group, the less the readiness to thermally decompose. This is, I believe, as stated in the science at the beginning of the investigation, because of the need of the elements to gain a stable outer shell of electrons. Something that reacts readily with carbon and oxygen will have a stronger covalent bond, whereas, in the case of something that does not will have a weak bond to the carbon and oxygen and should therefore thermally decompose far more easily.

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