Stalactites and stalagmites in most caves are made of limestone (calcium carbonate). The presence of calcium sulfate is explained by the following sequence of reactions. a) H2S (aq) + 2 O2 (g) → H2SO4(aq) b) H2SO4 (aq) + CaCO3(s) → CaSO4 (s) + H2O (L) +CO2 (g) Which (if either) of these reactions is a reduction oxidation reaction? Identify the substance oxidized and substance reduced, and the oxidizing agent and the reducing agent.

Explanation:

a) H2S (aq) + 2 O2 (g) → H2SO4(aq)

b) H2SO4 (aq) + CaCO3(s) → CaSO4 (s) + H2O (L) +CO2 (g)

To identify a redox reaction, we have to monitor the oxidation number of the elements to see if there is change.

In reaction (a);

The oxidation number of S increases from -2 (H2S) to + 6 (H2SO4). Also the oxidation number of O decreases from 0 (O2) to -2 (H2SO4). This is a redox reaction.

The oxidizing agent is O2 because it was reduced and the reducing agest is H2S.

In reaction (b);

There is no change in oxidation number in any f the elements. This is not a redox reaction.

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