A student wants to prepare chlorine gas in the laboratory. Which combination of reagents and collection method is appropriate?
Show the answer and explanation
Correct answer
B. Add concentrated hydrochloric acid to manganese dioxide and warm, collect by upward delivery.
Principle or equation
Chlorine gas is prepared by oxidizing concentrated hydrochloric acid with manganese dioxide: MnO2 + 4HCl(conc) → MnCl2 + Cl2 + 2H2O. Chlorine is denser than air and soluble in water, so it is collected by upward delivery (gas jar inverted) or by downward displacement of air.
Why this answer is correct
Concentrated HCl is required; dilute HCl does not react with MnO2. The reaction is heated. Chlorine is about 2.5 times denser than air, so it can be collected by upward delivery (the gas jar is inverted, and the gas enters from the bottom, displacing air upward). It is not collected over water because it dissolves significantly.
Example
In a fume cupboard, mix MnO2 with concentrated HCl in a flask, warm gently, and collect the greenish-yellow gas in an inverted gas jar.
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