Thermal
Decomposition
Thermal decomposition, or thermolysis, is a chemical
decomposition caused by heat and chemical
decomposition is the breaking down of a chemical compound into elements
or simpler compounds or both. The chemical reaction, thermal decomposition, is
usually endothermic as heat is required to break chemical bonds, such as ionic,
covalent and metallic bonds, in the compounds undergoing decomposition. All
compounds will thermally decompose, it is just whether sufficient heat has been
applied to the substance for it to thermally decompose. Therefore, when compounds, such as group I compounds, are
called more thermally stable, it just means more heat needs to be applied, as
more energy is required, to break the chemical bonds between the compounds,
which is why they do not decompose easily and only thermally decompose at
higher temperatures that the Bunsen Burners in Science laboratories are unable
to reach.Therefore, this links back to the reactivity table. Group I metals being the most reactive metals in the reactivity series, whose compounds are the most thermally stable, would mean that the metals that are very unreactive such as sliver and gold’s compounds would decompose very easily and do not require strong heating for them to decompose, as they require very little energy to overcome the chemical bonds existing in their compounds, therefore it would imply that they are not thermally stable when compared to Group I compounds.
Therefore, the more reactive the metal, the harder it is for the metal’s compounds to decompose as more heat is required for it to decompose. Therefore Group I compounds are more stable to heat than the corresponding compounds in Group II, as Group II lie below Group I metals in the reactivity table, therefore their compounds require lesser heat to overcome the chemical bonds existing in their compounds compared to Group I metals’ compounds. Moderately reactive metals such Zinc and Iron’s compounds require even lesser heat than Group II metals’ compounds to overcome the chemical bonds and to decompose. Thermally stability decreases down the metals’ compounds according to the metals in the reactivity table.
Thermal Decomposition of Solid Carbonates
Metal carbonates are also salts which are also compounds, where they all also decompose at a certain temperature, where there is sufficient heat energy for them to decompose. Most metal carbonates would decompose into its metal oxide and carbon dioxide gas is liberated. For silver carbonate, where silver is a very unreactive metal, once silver carbonate decomposes into silver oxide, as very little amount of energy is used to overcome the silver and oxygen ions, it will continue to decompose into silver and release oxygen gas
EXPERIMENT to FIND OUT which Metal Carbonates decompose upon heating when heated by Bunsen Burner
Apparatus:
2 Test-tubes (per carbonate)
Delivery Tube
Spatula
Bunsen Burner
Clamp and Retort Stand
Wooden Tongs
Chemicals Required:
Limewater (calcium hydroxide solution)
Potassium Carbonate, anhydrous
Sodium Carbonate, anhydrous
Calcium Carbonate
Zinc Carbonate
Iron(II) Carbonate
Copper(II) Carbonate
Silver Carbonate
Procedure:
- Put a spatula of the first carbonate to be tested in a test-tube
- In another test-tube, fill it with limewater, calcium hydroxide, to about half of the test-tube
- Fit the delivery tube and clamp the test-tube so that the delivery tube dips into the second test-tube containing limewater
- Heat the solid gently at first, then more strongly (controlled heating)
- Lift the delivery tube from the limewater once the experiment concludes, when the carbonate mostly decomposes, but leave the Bunsen burner still on and remove the delivery tube from the limewater before turning off the gas to support the Bunsen burner’s burning (to avoid “suck back”)
- Take down if the carbonate decomposed and whether a gas is released. Additionally. Take down whether a white precipitate formed if the gas released bubbles through the limewater
- Use the wooden tongs to remove the heated test-tube or wait until the test-tube has cooled before removing it
- Repeat the experiment with the other metal carbonates and in each case, take down the things that were taken down in step (preferably by using a table)
In conditions of a Bunsen Burner:
| ***Metal Carbonate | Whether thermal decomposition took place when heated | Colour of substance before heating | Colour of substance after heating | Gas evolved | Whether a white precipitate was formed if the gas produced bubbled through it |
| Potassium Carbonate | No | White | White | No | No |
| Sodium Carbonate | No | White | White | No | No |
| Calcium Carbonate | Yes | White | White (Calcium Oxide) | Yes (Carbon Dioxide) | Yes |
| Zinc Carbonate | Yes | White | Yellow when hot, white when cooled (Zinc Oxide) | Yes (Carbon Dioxide) | Yes |
| Iron(II) Carbonate | Yes | Green/Teal | Green (Iron(II) oxide) | Yes (Carbon Dioxide) | Yes |
| Copper(II) Carbonate | Yes | Green | Black (Copper(II) oxide) | Yes (Carbon Dioxide) | Yes |
| Silver Carbonate | Yes | Pale Yellow-Green | Grayish-white (Silver) | Yes (Carbon Dioxide and oxygen) | Yes |
-
For silver
carbonate, once it decomposes to produce silver, carbon dioxide and oxygen, to
test for oxygen, the glowing splint will rekindle/relight
Chemical Equations:
K2CO3 ---> no thermal decomposition
Na2CO3 ---> no thermal decomposition
CaCO3 ---> CaO + CO2
ZnCO3 ---> ZnO + CO2
CuCO3 ---> CuO + CO2
Ag2CO3 ---> Ag + CO2 + O2
1.
Heating Calcium(Reactive Metal) Carbonate with a Bunsen
Burner
(Set Up for this is completely different, as their motive is just to show that it is difficult to observe using our eyes whether calcium carbonate has decomposed as both calcium carbonate and calcium oxide are both white in colour, but other ways are to use the limewater test to test for carbon dioxide or using mass to calculate that carbon dioxide has escaped(mole calculation) but the video did not show those)
2.
Heating Zinc(Moderately Reactive Metal)
Carbonate with a Bunsen Burner
3. Heating Copper(II)(Not really very Reactive Metal) Carbonate with a Bunsen Burner
(The limewater test is not efficient, as not only might the limewater drop and wet/dirty the place, the white precipitate cannot be seen clearly)
3. Heating Copper(II)(Not really very Reactive Metal) Carbonate with a Bunsen Burner
(Only difference between the set up used for this video & the set up used for the above experiment is that the video's set up uses wooden tongs to hold the test tube when it is being heated instead of using a clamp in the above set up)
