Identifying Half Reactions

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Hussain Chharawalla 1G
Posts: 100
Joined: Sat Jul 20, 2019 12:15 am

Identifying Half Reactions

Postby Hussain Chharawalla 1G » Sun Feb 23, 2020 3:14 pm

In our discussion, we did an example of balancing redox reactions for:
MnO4- (aq) + H2SO3 (aq) --> Mn2+ (aq) + HSO4-

In section, it was said that MnO4- was reduced and H2SO3 was oxidized. I didn't quite understand this because to me it seemed like Mn2+ means that two electrons were lost meaning oxidized, and the negative charge on HSO4- means it gained an electron and reduced. Can someone explain this please


Christine Honda 2I
Posts: 116
Joined: Sat Sep 14, 2019 12:17 am

Re: Identifying Half Reactions

Postby Christine Honda 2I » Sun Feb 23, 2020 3:51 pm

MnO4- is being reduced because when we calculate the oxidation of state of Mn in this compound it is +7 (do this knowing that O is -2, multiplying it by 4 and knowing that the entire molecule has a charge of -1). But in the products, Mn has a charge of +2. This shows that Mn has gained 5 electrons and has been reduced.
Last edited by Christine Honda 2I on Sun Feb 23, 2020 3:52 pm, edited 1 time in total.

Riya Sood 4G
Posts: 160
Joined: Sat Aug 24, 2019 12:18 am

Re: Identifying Half Reactions

Postby Riya Sood 4G » Sun Feb 23, 2020 3:51 pm

The best way to recognize which element is oxidized and reduced is to look at the oxidation number.
The general rule is that:
1. If oxidation number increases= element is oxidized (reducing agent)
2. If oxidation number decreases= element is reduced (oxidizing agent).

Matt F
Posts: 100
Joined: Sat Aug 17, 2019 12:17 am

Re: Identifying Half Reactions

Postby Matt F » Sun Feb 23, 2020 5:44 pm

A little bit of cheat too is that MnO4- lost its Oxygen, while H2SO3 gained an Oxygen. Therefore, the MnO4- is reduced while the H2SO3 is oxidized. I'm not sure if this is true in every reaction, but it makes sense

Emil Velasco 1H
Posts: 96
Joined: Wed Nov 21, 2018 12:19 am

Re: Identifying Half Reactions

Postby Emil Velasco 1H » Sun Feb 23, 2020 5:58 pm

Gaining electrons is reduction (going for negative) and losing electrons is oxidation (going for positive)

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