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You are normally used to 2 chemicals reacting together to make a number of different products.
For example.....
Hydrochloric acid + sodium hydroxide --> sodium chloride + water
2 reactants have gone to make 2 products!
All we do is mix the 2 chemicals together at room temperature here and the reaction just happens...
But sometimes, a chemical reaction can take place where only 1 reactant is used and it does not react with something else. This sounds a little bit strange. Think about it carefully though.... what if you could start with a chemical that was made of fairly big particles and somehow break that particle apart into a number of smaller particles?
Well that is possible... but what could we use to do the 'breaking apart'?
HEAT!
A thermal decomposition reaction, whilst it might sound quite a mouthful, is quite a simple idea... you need to look at each word in turn.....
Thermal: Relating to heat energy
Decomposition: To break something down
So a thermal decomposition reaction is one where a chemical compound is broken down by heat energy! Ta da, simple!
The most common type of substances that thermally decompose are the metal carbonates....
e.g.
calcium carbonate (CaCO3)
Sodium carbonate (Na2CO3)
Copper (II) carbonate (CuCO3)
Lead (II) carbonate (PbCO3)
When a metal carbonate thermally decomposes, it makes 2 different products....
Metal carbonate ------> metal oxide + carbon dioxide
Notice that the metal carbonate is not reacting with anything else.... it is just breaking apart when we heat up the compound. We don't write "+ heat" in the equation because heat is not a chemical. We can show the heat in a different place though....
Look above the arrow here. The little triangle is the symbol we use to show 'heat' being added for a chemical reaction. We can also just write "HEAT" above the arrow if we don't want to use the triangle.
Because a thermal decomposition reaction needs a lot of heating to occur, it is a very good example of an endothermic reaction, which you saw earlier in the term. Remember, endothermic reactions are ones which take in heat energy from the surroundings.
We can prove that carbon dioxide gas is released when a metal carbonate is thermally decomposed, by using a very simple test that we saw lower down school.
CARBON DIOXIDE GAS WILL MAKE LIMEWATER GO FROM COLOURLESS TO MILKY
Have a look at the experimental setup above. We have put the metal carbonate into a test-tube and then connected a bung and delivery tube to the test-tube. The delivery tube then goes into another container which contains limewater.
When we start heating the metal carbonate with the Bunsen burner, it starts to thermally decompose... the carbon dioxide that is released goes own the delivery tube and is forced to bubble through the limewater. This then makes the limewater go milky!
Different metal carbonates will thermally decompose at different rates (speeds). So if you heated CuCO3, you might expect it to 'break down' faster or slower than if you heated up, say, MgCO3. It will probably take a different amount of energy for each one to break down...that would make sense...the bonds in each of these compounds are probably of a different strength as they contain a different element.
We could perform an investigation to look at this.... that would be the intelligent thing to do!
Our hypothesis might be something like.....
Different metal carbonates will thermally decompose at different speeds
To make this a fair test we can only change one thing and measure one thing... so....
Independent variable (the one I change): The type of metal carbonate in each experiment
Dependent variable (the one I measure): The time taken for the limewater to go milky
Control variables (all other things I MUST keep the same to make it a fair test):
1. The mass of metal carbonate I use in each experiment
2. The surface area of each metal carbonate I use (make sure they are all powders!)
3. The volume of limewater I use in each experiment
4. The intensity of the Bunsen burner flame in each experiment
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