Dry Cell Question (1 Viewer)

[SomeDownie]

Im kinda confused as to how the reactions at the cathode occur. Most textbooks say something like:

2MnO2 + 2NH4+ + 2e- -> Mn2O3 + H2O + NH3

This is pretty hard to memorise and i did hear from someone that you can actually simplify it down into 2Mn4+ + 2e- -> 2Mn3+. Not sure if this is legit or not so itd be great if someone could clarify why/why not...

Ive uploaded the working out here: http://i60.tinypic.com/j8f690.jpg

The reaction 2NH4+ <-> 2NH3 + 2H+ comes from chem contexts

Thanks in advance!

 

[anomalousdecay]

The use of the NH4+ is to show that it is part of the electrolyte NH4Cl.

Hence, the way to think of this is to put the electrolyte in the equation as to show what the total reaction occurring is.

If you just put down: 2MN4+ + 2e- --> 2MN3+ ,

then you are not actually making it clear that the electrolyte is also being converted into different products (the electrolyte reacts here!!!).

As a result the part which gains electrons is placed in the cathode equation (the part of the electrolyte and the actual cathode).

I think the Chlorine ions combine and turn into gas when releasing the excess electrons.

 

[bengosha60]

The use of the NH4+ is to show that it is part of the electrolyte NH4Cl.

Hence, the way to think of this is to put the electrolyte in the equation as to show what the total reaction occurring is.

If you just put down: 2MN4+ + 2e- --> 2MN3+ ,

then you are not actually making it clear that the electrolyte is also being converted into different products (the electrolyte reacts here!!!).

As a result the part which gains electrons is placed in the cathode equation (the part of the electrolyte and the actual cathode).

I think the Chlorine ions combine and turn into gas when releasing the excess electrons.

Does that also mean that the equation:

Zn + 2Mn4+ + 4O2- + 2H+ ---> Zn2+ + 2Mn3+ + 3O2- + H2O

is also wrong? I just happened to have ahmad shah's notes in front of me and just quickly checked. He didnt mention the NH4+ but got rid of it. Oh and he broke everything up into its ions. Good or bad thing to do in an exam?

 

[anomalousdecay]

Does that also mean that the equation:

Zn + 2Mn4+ + 4O2- + 2H+ ---> Zn2+ + 2Mn3+ + 3O2- + H2O

is also wrong? I just happened to have ahmad shah's notes in front of me and just quickly checked. He didnt mention the NH4+ but got rid of it. Oh and he broke everything up into its ions. Good or bad thing to do in an exam?

There are many ways to show the reaction occurring.

I can't say its wrong or not. Tbh, I was just telling OP why the ammonium ion is shown in the equation.

I can't really say that one is wrong or not because in the end they all represent the same thing happening overall. Even if you do use the net ionic equation, you can probably get away with it if you acknowledge that the electrolyte does react (say if you forget). However, you don't need to memorise the equation. If you think logically or clearly about it, then you can come to the conclusion involving the electrolyte reacting.

In an exam, I'm not too sure, but as I said above you would probably get away with it. However, if you wanted to really show your understanding, then you would want to show the electrolyte taking part in the overall reaction and also show the net ionic equation.

Its been 9 months since I last touched this stuff, so I am rusty with Chemistry now. I'm just explaining why the electrolyte is shown which is all because they are included as the actual cathode/anode reaction occurring as a whole (Oxidation is Loss, Reduction is gain, etc).

 

[pheelx3]

No, you must write out the whole equation

Anode: Zn(s) -> Zn2+ + 2e-
Cathode: 2MnO2(s) + 2NH4+ + 2e- -> Mn2O3(s) + H2O(l) + NH3(g)

There's actually a way of deriving these:
(That's why I decided to learn both the lead-acid and the dry cell)

Acidic:
1. You start with the backbone. I.e.
MnO2(s) -> Mn2O3(s)

2. Balance non oxygen atoms first. I.e.
2MnO2(s) -> Mn2O3(s)

3. Balance oxygen atoms with water. I.e.
2MnO2(s) -> Mn2O3(s) + H2O

4. Balance hydrogen atoms with H+. I.e.
2MnO2(s) + 2H+ -> Mn2O3(s) + H2O

5. Balance charges with electrons. I.e.
2MnO2(s) + 2H+ + 2e-> Mn2O3(s) + H2O(l)

6. Other features. We understand that ammonia is in the electrolyte and is converted to NH3(g) I.e.
2MnO2(s) + 2NH4+ + 2e-> Mn2O3(s) + H2O(l) + NH3(g)