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Edexcel IAL Chemistry AS 3.P6 Evaluating experimental methods

Edexcel IAL Chemistry AS 3.P6 Evaluating experimental methods
Pearson Edexcel IAL Chemistry syllabusChemistry YCH11First assessment 2019

Name the purpose of each step, connect it to the chemistry or measurement and propose a specific change whose effect on the result is clear.

How this is tested

  • Explain apparatus choices such as a loose lid that retains solid while releasing gas.
  • Use heating to constant mass as evidence that reaction or water removal is complete.
  • Suggest a method improvement and state how it changes loss, completion or measurement bias.

Question 22(d)(ii)

Potassium chlorate(V), KClO3, is a crystalline solid used in fireworks.
It is produced by the Liebig Process in two stages.
Stage 1 Chlorine gas is passed through hot calcium hydroxide solution forming calcium chlorate(V), Ca(ClO3)2\mathrm{Ca}\left(\mathrm{ClO}_{3}\right)_{2}.

6Ca(OH)2(aq)+6Cl2( g)→Ca(ClO3)2(aq)+5CaCl2(aq)+6H2O(l)6 \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})+6 \mathrm{Cl}_{2}(\mathrm{~g}) \rightarrow \mathrm{Ca}\left(\mathrm{ClO}_{3}\right)_{2}(\mathrm{aq})+5 \mathrm{CaCl}_{2}(\mathrm{aq})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

Stage 2 Potassium chloride solution is then added to form potassium chlorate(V).

Ca(ClO3)2(aq)+2KCl(aq)→2KClO3(aq)+CaCl2(aq)\mathrm{Ca}\left(\mathrm{ClO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KCl}(\mathrm{aq}) \rightarrow 2 \mathrm{KClO}_{3}(\mathrm{aq})+\mathrm{CaCl}_{2}(\mathrm{aq})

The solution is heated to reduce its volume and then allowed to crystallise.
The crystals are filtered off.
The remaining filtrate is evaporated further to obtain more crystals.

In fireworks, potassium chlorate(V) decomposes. This thermal decomposition takes place in two stages with a solid catalyst.

4KClO3→KCl+3KClO4KClO4→KCl+2O2\begin{gathered} 4 \mathrm{KClO}_{3} \rightarrow \mathrm{KCl}+3 \mathrm{KClO}_{4} \\ \mathrm{KClO}_{4} \rightarrow \mathrm{KCl}+2 \mathrm{O}_{2} \end{gathered}

A student investigated the role of the catalyst in this reaction.

Procedure

Step 1 Heat a sample of KClO3\mathrm{KClO}_{3}, in a test tube, with a known mass of insoluble catalyst until the mass remains constant.

Step 2 Mix the contents of the test tube with water.
Step 3 Filter the mixture and rinse with deionised water.
Step 4 Dry the remaining solid.
Step 5 Measure the mass of the dry solid.
Explain how each of the steps in this procedure is needed to show that the catalyst is not used up in this reaction.