Thermal Decomposition Of Solid Carbonate
What is thermal decomposition?
Thermal decomposition, also known as thermolysis, is a chemical decomposition caused by heat.
The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition.
If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion.
What is the equation for thermal decomposition of solid carbonate?
Metal carbonate(s) àmetal oxide(s) + carbon dioxide(g)
Examples:
Thermal decomposition of copper(II) carbonate :
Calcium carbonate à Calcium oxide + Carbon dioxide
CuCO3 à CuO + CO2
Thermal decomposition of lead(II) carbonate:
Lead(II) carbonate à Lead(II) oxide + Carbon dioxide
PbCO3 à PbO2 + CO2
-Bunsen burner
-Lab goggles
-Test tubes
-Delivery tubes
-Spatula
-Retort stand with clamp
Procedure
First, place a large spatula measure of the carbonate to be tested in a test-tube.
Second, fit a delivery tube and then clamp the test-tube so that the delivery tube dips into a second test-tube containing 2-3 cm of limewater.
Third, heat the solid gently at first, then more strongly.
Fourth, lift the delivery tube from the limewater before, or as soon as, the heating is stopped. This is to avoid "suck-back".
Fifth, take note of all observations. Notice what happens to the limewater and how long it takes to turn milky (if at all). Notice whether any melting occurs in the heated test-tube and any colour changes taking place. Write your results in tabular form.
Sixth, repeat the experiment with the other metal carbonates supplied, and in each case write down your observations.
Thermal decomposition of copper(II) carbonate
Metals which are higher up in the reactivity series, such as calcium, have carbonates that need a lot of energy to decompose them.
Metals lower down in the reactivity series, like copper, have carbonates that decompose more readily.
Therefore, copper carbonate is often used at school to show these reactions, as it can easily be decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to notice.
The thermal decomposition of copper(II) carbonate is demonstrated above
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Conclusion
Metals that are lower in the reactivity series take lesser time to decompose, as they are less thermally stable. The bonds holding the atoms together are weaker and hence are more prone to being broken apart when heated.
Metals that are higher in the reactivity series tend to take longer to decompose as they are more thermally stable. The bonds holding the atoms together are stronger, hence they take a longer time of heating to break the bonds. HOWEVER, sodium carbonate and potassium carbonate are TWO carbonates which are THERMALLY STABLE and therefore CANNOT decompose when heated.
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