Question

Why so2 can act as oxidising as well as reducing agent but H2s act as only reducing agent

Answer 1

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SO2 can act as both oxidizing as well as reducing agent since it has +4 oxidation state which is exactly between its highest oxidation state +6 and lowest oxidation state +2 ,Thus  it can change its oxidation number either from +4 to-2(reduction) and +4 to+6 (oxidation) .

Where as in H2S the oxidation state of sulphur is -2 so it can lose electrons to attain +4 and +6 oxidation state but it can not gain the electrons. Therefore, H2S act as reducing agent only. 
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Answer 2

In H2S the oxidation state of Sulphur is -2 so it can loss electron to attain +4 and +6 oxidation state but it can not gain the electron. Therefore H2S act as reducing agent.

In SO2 the oxidation state of sulphur is +4, it can lose its two more electron to attain +6 oxidation state. Moreover it can gain electrons to reach the oxidation state of -2 (lowest oxidation state observed in its compound). Since it has a capability to lose and gain electron therefore  it acts  as oxidizing as well as reducing agents.