Heinemann questions i had trouble with

[Mao]

yea i get that....so ur allowed to use S204-2 twice?....i thought i had to use the water

notice the oxidation numbers in , the number for sulfur is +3, +2 and +5.
since the latter two sourced from the same compound (the first one), that means some of it must have gone through oxidation, and some gone through reduction.

[lanvins]

Sorry to bother u, but i'm still a little confused about Q3 like what are hydronium ions and how did u know to balance it with O2- (where did it say in the question)? Also, how did u do Q3 b, like where did Al2O3 come from?

[Mao]

in the thermite reaction, everything happens under that mold, and the question did not say "there are also hydronium ions in the paste", so you cannot assume that there are H⁺ ions available for this reaction.
as a result, you'd balance the oxides with oxide ions, O^2-


in a reaction, say, involving the reduction of permanganate (which involves H⁺ ions), the question would say that it is an "acidified solution", meaning you can follow the KOHES rule

if you had written the other half of the equation:

and combined the two half equation together, it becomes apparent that Al₂O₃ is formed from the two aluminium ions and the three oxide ions.

[lanvins]

i promise this is the last time i'm gonna bother u.
So tell me if this is right:
1. So whenever it says its a thermite reaction, you balance it with  O2-
(r there a list of these reactions i should noe?)

if you had written the other half of the equation:

and combined the two half equation together, it becomes apparent that Al₂O₃ is formed from the two aluminium ions and the three oxide ions.

2. Two Half equations
Al(s) = Al3+(s) + 3e-
FeO3(s) + 6e- = 2Fe(s) 3O2-

Getting rid of the e-
2Al = 2Al3+ + 6e-
Fe2O3 + 6e- = 2Fe + 3O2-

Overall equation
2Al + Fe2O3 = 2Al 3+ + 3O 2-+ 2Fe

[Mao]

yes, kind of, not really

1: you can only use when there are some. In a question that you will be using it, the question will tell you that the solution is "acidified" so you can use it. Those are generally only applicable to Mn and V compounds.

2. The reaction is actually a lot more spontaneous than that, like a "swap" of metal ions. Half equations is useful to determine what is happening, but it is not absolute.

[lanvins]

r u sure they didn't just simplify the H in     Fe2O3      +6e-   +6 H+         -->  2 Fe    +  3 H2O to get Fe2O3(s) + 6e- -->  2Fe(l) + 3O 2- (s)?

(this question is so retarded)

[Mao]

r u sure they didn't just simplify the H in     Fe2O3      +6e-   +6 H+         -->  2 Fe    +  3 H2O to get Fe2O3(s) + 6e- -->  2Fe(l) + 3O 2- (s)?

(this question is so retarded)

unless you can explain how some hydronium ions miraculously appeared inside the mold, then yes

[lanvins]

what do you think the chances are of a question of that kind being in the exam (cause i would be f...ked)?

It would be nice if the textbok mentioned something about these sepcial exceptions for balancing half equations.

[Mao]

I have not seen thermite reactions in exams, but just to be safe, read the questions carefully and determine whether there will be H⁺ available
The key is to not "create" something the question has not given you. That can be a deciding factor between a high achiever and a very high achiever.

and maybe if you are unsure about that, memorise the thermite reaction formula.