Percentage purity of calcium carbide — a quick lab

In Ontario Grade 11 Chemistry, students study percentage yield of a chemical reaction as well as percentage purity of a reagent. I use a nice, quick lab on percentage yield¹, followed by a determination of the percentage purity of calcium carbide, CaC₂.

The substance in a can of calcium carbide has the appearance of little “rocks” and is not pure CaC₂.

Give students the following unbalanced chemical equation and ask how they could determine the percentage purity of solid calcium carbide, CaC₂.

Equation 1:

CaC₂(s)  +  H2O(l)  →  C₂H₂(g)  +  Ca(OH)₂(aq) 

You may get a suggestion to collect the ethyne by downward displacement of water and measure its volume. Assuming SATP (24.8 L•mol^–1) students can convert volume of C₂H₂ to mols and then to grams and then calculate percentage purity. On paper, this looks great, but my Chemistry Education Research Group showed this to be less accurate and less straightforward than a simpler method. (See post-lab questions.)

Through judicious questioning, you’ll be able to elicit the idea of simply measuring the combined mass of water and of impure (unreacted) CaC₂ before the reaction and by measuring the mass of the mixture post-reaction — after the ethyne has been liberated. Students can use approximately 1 g of impure CaC₂ and a centigram balance.

Safety:  No open flames or sparks. Students should work in a fume hood or in a well ventilated area; make sure that the fume hood is on before the reaction; when motors are activated, there can be a spark, which could ignite the mixture of ethyne and air.

Pre-lab questions (answers)

1. Balance equation 1 above.
2. If 1.00 g of pure calcium carbide reacts with excess water, calculate
   1. mass of ethyne expected.
   2. volume of C₂H₂ expected, at SATP.
3. If 1.00 g of impure CaC₂ reacts with excess water, 0.33 g of C₂H₂ is collected.
   1. Calculate the percentage purity of the CaC₂.

   2. What assumption(s) must be made regarding

      1. the reactivity of any impurity (or impurities) in the calcium carbide, if your calculations in part (a) are to be considered valid? (unreactive impurities)

      2. the solubility of C₂H₂ in water? (C₂H₂ not water soluble)

      3. the reactivity of C₂H₂ with water? (C₂H₂ not reactive with water)

Post-lab questions (answers)

1. Use your data:
   1. to determine the % purity of the CaC₂.
   2. What must you assume about the percentage yield in this reaction? (100%)
2. Draw the Lewis structures of acetylene (ethyne).
   1. In C₂H₂ the atoms are attached linearly as HCCH. Include an electron dot structure, using different symbols to denote electrons from neighbouring atoms, and a second structure showing single, double, triple bonds and lone pairs of electrons as appropriate.
   2. Repeat part (a) for the carbide ion, C₂^2–.
3. Redraw the Lewis structure for ethyne. Indicate each bond as ionic, polar covalent or non-polar covalent; indicate the molecule as polar or non-polar.
4. Design an alternate experiment to determine the percentage purity of CaC₂ by collection of the ethyne gas produced by downward displacement of water. Your answer should include:
   1. a labelled diagram of the apparatus.
   2. required assumptions. (Ethyne is insoluble in water and unreactive with water; 100% yield of reaction —see above; SATP.)
   3. what needs to be recorded before and after the reaction. (before: mass of impure CaC₂; after: volume of C₂H₂ collected)
   4. an explanation of the calculation required to determine percentage purity of the CaC₂. (volume of C₂H₂ in L→ mol → grams, and on to the percentage purity calculation via balanced chemical equation)

References

1. C Diehl, Chem 13 News, December 2006, page 2