Redox agents?

[andafoo]

---

Members don't see this ad.

How do you know what is what?

For ex. from CBT10

2HCl + H2O2 + MnO2 -> O2 + MnCl2 + 2H2O

What do you look at to determine the agents?

What happens when you look at Cl?
What happens when you look at Mn?

Moreso, what happens when you look at O2? There are so many Os!

I really don't know where to start with complicated redox reactions - anyone know a good solid method to get through these?

 

[Phlame217]

//This should be inherent knowlege that immediately clicks:

The most solid is label the oxygen's, hydrogens, and halogens.
Those always remain constant. Oxygen is 2-(except in peroxides where its 1-), halogens 1-(in most all cases), and hydrogens 1+ (except its occasionally 1-).

Now you should know a Reducing agent gets oxidized.
An oxidizing agent is reduced.

Also, Know "OIL RIG", Oxidize is lose [electrons], Reduce is Gain [electrons]"
//end inherent knowlege

Now lets review
Reactants:
HCl [H:1+ Cl:1-] in this reaction is neither oxidized nor reduced.
H2O2 [H:1+ H:1+ O:1- O:1-] Oxygen is 1- in peroxides due to the O-O bond
MnO2 [Mn:4+ O:2- O:2-] Notice Mn is 4+

Products:
O2 [Shifts to 0] Oxygen is oxidized, loses 1 electron as seen from peroxide.
MnCL2 [Mn:2+ Cl:1- Cl:1-] Mn shifts to 2+, therefore has been reduced.
H2O [H:+ H:+ O:2-] Hydrogen and Oxygen maintain natural state.

Analysis:
Mn shifted from 4+ to 2+ therefore being reduced, since it is reduced, it must be the oxidizing agent. (MnO2 is the oxidizing agent)

Oxygen shifted from a 1- to a 0 state, therefore was oxidized. Oxidized means it was the reducing agent. (H2O2 is the reducing agent)

Overview:
Always start by analyzing the transition metal. Most all MCAT problems will use one of these and these will give you one answer which will be able to knock out some of the multiple choice. If you don't immediately narrow it down to one, go back and finish the problem. It took me 10 seconds to look at this and derive the answers.

 

[andafoo]

//This should be inherent knowlege that immediately clicks:

The most solid is label the oxygen's, hydrogens, and halogens.
Those always remain constant. Oxygen is 2-(except in peroxides where its 1-), halogens 1-(in most all cases), and hydrogens 1+ (except its occasionally 1-).

Now you should know a Reducing agent gets oxidized.
An oxidizing agent is reduced.

Also, Know "OIL RIG", Oxidize is lose [electrons], Reduce is Gain [electrons]"
//end inherent knowlege

Now lets review
Reactants:
HCl [H:1+ Cl:1-] in this reaction is neither oxidized nor reduced.
H2O2 [H:1+ H:1+ O:1- O:1-] Oxygen is 1- in peroxides due to the O-O bond
MnO2 [Mn:4+ O:2- O:2-] Notice Mn is 4+

Products:
O2 [Shifts to 0] Oxygen is oxidized, loses 1 electron as seen from peroxide.
MnCL2 [Mn:2+ Cl:1- Cl:1-] Mn shifts to 2+, therefore has been reduced.
H2O [H:+ H:+ O:2-] Hydrogen and Oxygen maintain natural state.

Analysis:
Mn shifted from 4+ to 2+ therefore being reduced, since it is reduced, it must be the oxidizing agent. (MnO2 is the oxidizing agent)

Oxygen shifted from a 1- to a 0 state, therefore was oxidized. Oxidized means it was the reducing agent. (H2O2 is the reducing agent)

Overview:
Always start by analyzing the transition metal. Most all MCAT problems will use one of these and these will give you one answer which will be able to knock out some of the multiple choice. If you don't immediately narrow it down to one, go back and finish the problem. It took me 10 seconds to look at this and derive the answers.

Thanks, that helped.

But what confused me was the oxygen. There is one initially from H2O2 and another from MnO2 - how do you know which product forms starting from which reactant?

I guess I assumed that H2O2 -> H2O and MnO2 -> O2

In such a case where you have a common atom in several species - how do you keep track of the oxidation state of such atoms?

 

[Phlame217]

Well you have to remember that in reality, its very random so an oxygen from one molecule can end up anywhere. In this reaction, you just use elimination though. Since Mn and O are the only two that change, and one set of oxygen remains 2-, while another set goes from 1- to 0, it has to be oxygen.

But... Thats also why i said start with the metal(s), as they will indicate the majority of the time what you're looking for.