PAG 1- Heating in a crucible

Question 1

What can this method be used for?

Answer 1

• Measuring mass loss in various thermal decomposition reactions
• Measuring mass gain when reacting magnesium in oxygen

Question 2

Method for measuring mass gain by reacting magnesium in oxygen (oxidation reaction)
Mg(s) + O2(g) -> MgO(s)

Answer 2

• Weigh an empty, clean, dry crucible with the lid.
• Add 2g of coiled magnesium ribbon (that has been cleaned) to the crucible and weigh again.
• Heat strongly with a Bunsen burner for a few minutes.
• Occasionally lift lid with tongs to allow in some oxygen to react, but take care not to let magnesium oxide powder escape.
• Allow to cool.
• Weigh the crucible and its contents again.
• Heat again until constant mass is reached.

Question 3

Method for the removing the water of crystallisation (from calcium sulfate crystals)- decomposition reaction
CaSO4. XH2O(s) -> CaSO4(s) + XH2O (g)

Answer 3

• Weigh an empty, clean, dry crucible with the lid.
• Add 2g of hydrated calcium sulfate to the crucible and weigh again.
• Heat strongly with a Bunsen for a few minutes.
• Allow to cool
• Weigh the crucible and its contents again.
• Heat crucible again and reweigh until constant mass is reached.

Question 4

Key details of experiment (list)

Answer 4

• Heating with the lid
• Not using large amounts of reactants
• Dry crucible
• Not using very small amounts

Question 5

How does heating with the lid improve accuracy?

Answer 5

• Prevents loss of solid from the crucible
  -However should be loose fitting to allow gas to escape/enter

Question 6

Why should large amounts of substances not be used?

Answer 6

The reactions (oxidation and decomposition) will be unlikely to go to completion

Question 7

Why does the crucible need to be dry?

Answer 7

• A wet crucible would provide an inaccurate result
• Because it would cause mass loss to be too large as water would be lost when heating

Question 8

Why does the crucible need to be dry?

Answer 8

• A wet crucible would provide an inaccurate result
• Because it would cause mass loss to be too large as water would be lost when heating

Question 9

Why should small amounts not be used?

Answer 9

• The percentage uncertainties in weighing will be too high, making the results less accurate