Experiments for better teaching

Carbon Dioxide Chemistry

P

time requ.: 20 minutes.
objective: Setting free of carbon dioxide. The hydrocarbonate equilibrium.
material:
  • snap top vial 5ml or plastic bottle cap
  • syringe needle
chemicals:
  • calcium carbonate (limestone) small pieces
  • calcium carbonate (limestone) powder
  • calcium hydroxide solution (saturated)
  • hydrochloric acid c~2mol/l

     

preparation: Prepare saturated calcium hydroxide solution in a 1 liter bottle from distilled water and 50g of calcium oxide. Shake from time to time with new distilled water. When required filter the necessary volume for use.
procedure 1: Insert a small amount of limestone powser into the test tube and add 2ml of hydrochloric acid.
observation: Gas bubbles are emerging. The limestone disappears in about 3-5 min.
interpretation: Limestone dissolves in acids forming carbon dioxide:
     CaCO3        +    2HCl(aq)   ---> Ca2+(aq) + 2Cl-(aq) +  CO2    + H2O
calcium carbonate  hydrochloric acid          calcium chloride    carbon dioxide  water
procedure 2:

Use the gas generator. Use hydrochloric acid with small pieces of limestone (d=2-3mm) to obtain carbon dioxide. Transfer 2-3 loads of gas with syringe 2 of the gas generator and the extra needle in 1-1,5ml of saturated calcium hydroxide solution in the extra snap top vial.

observation 2: Soon a white precipitation appears, but disappears after more than 5ml carbon dioxide has been inserted.
interpretation 2:      1 CO2      +   Ca2+(aq) + 2 OH-(aq)  --->    CaCO3       +   H2O
carbon dioxide  calcium hydroxide                   calcium carbonate     water
      CaCO3       +     CO2       + H2O ---> Ca2+(aq) + 2 HCO32-(aq)
calcium carbonate carbon dioxide  water           calcium hydrocarbonate

Sum reaction:
  2 CO2      +   Ca2+(aq) + 2 OH-(aq)  ---> Ca2+(aq) + 2 HCO32-(aq)
At high carbon dioxide input the soluable hydrocarbonate is obtained.

source: Barke, H.-D. et al.: One Hundred Chemistry-Experiments to Avoid Chalk and Talk; University of Muenster, 2004.

© Walter.Wagner ät uni-bayreuth.de, Stand: 31.05.11

Didaktik der Chemie
Universität Bayreuth