When the gases sulphur dioxide and hydrogen sulphide mix in the presence of water, the reaction #"SO"_2 + 2"H"_2"S" -> 2"H"_2"O" + 3"S"# occurs. Here hydrogen sulphide is acting as?

#a)# an oxidizing agent
#b)# a reducing agent
#c)# a dehydrating agent
#d)# a catalyst

1 Answer
Peter M.
Jun 28, 2018

Hydrogen sulphide is a reducing agent.

Explanation:

A good starting point for any reaction is to look at oxidation numbers. If they change, a redox reaction has occurred.

Here are some thoughts.

A dehydrating agent will extract water from another reactant. I would say that you need a hydroxyl (OH) group for this to occur. A dehydrating agent could be an acid and protonate the OH group, which can then leave as water. A catalyst is not used up in the reaction, so is hydrogen sulphide used up?

Hydrogen is usually in a +1 oxidation state and oxygen a -2 state.

Reactants

#H_2^color(red)(+1)S^color(blue)(-2)#

#S^color(red)(+4)O_2^color(blue)(-2)#

Products

#S^color(red)0#

#H_2^color(red)(+1)O^color(blue)(-2)#

Look at #H_2S#, the oxidation number of sulphur has gone from -2 to 0. This means that it has been oxidised and is therefore a reducing agent.

For #SO_2#, the oxidation number of sulphur has gone from +4 to 0. This is a reduction and makes #SO_2# an oxidising agent. We can essentially ignore H and O in this case as they don't change.

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