Reversible chemical reactions
Apparatus and materials: Lime water (Ca(OH)2); calcium carbonate (CaCO3); hydrochloric acid; 2 test tubes with rubber stoppers; delivery tube; retort stand and clamp; bunsen burner.
Method and observations
- Half-ﬁll a test tube with clear lime water (Ca(OH)2).
- In another test tube, place a few pieces of calcium carbonate (CaCO3) and cover the pieces with dilute hydrochloric acid. Seal the test tube with a rubber stopper and delivery tube.
- Place the other end of the delivery tube into the test tube containing the lime water so that the carbon dioxide that is produced from the reaction between calcium carbonate and hydrochloric acid passes through the lime water. Observe what happens to the appearance of the lime water. The equation for the reaction that takes place is: Ca(OH)2 + CO2 → CaCO3 + H2O CaCO3 is insoluble and it turns the limewater milky.
- Allow the reaction to proceed for a while so that carbon dioxide continues to pass through the limewater. What do you notice? The equation for the reaction that takes place is: CaCO3(s) + H2O + CO2 → Ca(HCO3)2 In this reaction, calcium carbonate becomes one of the reactants to produce hydrogen carbonate (Ca(HCO3)2) and so the solution becomes clear again.
- Heat the solution in the test tube over a bunsen burner. What do you observe? You should see bubbles of carbon dioxide appear and the limewater turns milky again. The reaction that has taken place is: Ca(HCO3)2 → CaCO3(s) + H2O + CO2
- If you look at the last two equations you will see that the one is the reverse of the other. In other words, this is a reversible reaction and can be written as follows: CaCO3(s) + H2O + CO2 ⇔ Ca(HCO3)2
- Is the forward reaction endothermic or exothermic? Is the reverse reaction endothermic or exothermic? You should have noticed that the reverse reaction only took place when the solution was heated. Sometimes, changing the temperature of a reaction can change its direction.