Balancing Equations: Al + H2SO4 to Al3+ + SO42- + SO2 + H2O

[mohlam12]

balancin an equation!

hello everybody,
i just got back from my chemistry class, with some homework...
it s about balancin equations (hard ones) and they are relating to the lesson of oxidation-reduction...
here is one of them that i have to balance--
if someone can show me the steps to take, that would be awesome :) because i have like 5 more to do...

Al + H2SO4 --> Al 3+ + SO4 2- + SO2 + H2O

thank you :)

oh yeah, the problem in this equation is that i have SO4 2- and SO2
so to determine the number of oxydation for S
should it be -8 or -4 ==> should i use SO4 -2 or SO2

when i know what to use, i ll see the change of this number, then what ?

 

[andrewchang]

use both ${SO}^{-2}_4$ and $SO_2$
gah sorry i can't get the LaTeX to work ...

 

[mohlam12]

hmm
so would i use the oxydation number of -8 or -4

 

[andrewchang]

for ${SO}^{-2}_4$ the oxidation number of sulfur is +6 (O is -2, the total charge is -2, so $-2 \times 4 + S = -2$ and $S = +6$

the oxidation number of S in $SO_2$ is +4

 

[mohlam12]

okay, so when i want to see the change of the number of oxidation S, to find the coefficient for balancin the reaction; would i do:
(+6) - (+6) = 0
or
(+4) - (+6) = -2

??
thanks

 

[andrewchang]

yeah, that's right.

 

[mohlam12]

ummm... i think i asked for WHICH ONE TO USE!
...

 

[andrewchang]

oh haha sorry. you need to use both of them

so write a redox half reaction for both

dont forget the Al

 

[mohlam12]

still don't get how to really do it... if anyone can give me some steps for this one!
thanks