Question

When the reaction Al (s) + Cr2O72- (aq) → Cr2+ (aq) + Al3+ (aq) is correctly balanced in acid, Which one is it? Please explain.

A. 7 protons (H+) are consumed

B. 3 protons (H+) are consumed

C. 8 protons (H+) are consumed

D. 42 protons (H+) are consumed

E. 12 protons (H+) are consumed

Answer 1

It is 42 H+ consumed. Here is the step by step:

1) break into Ox/Red reactions.
Ox: Al(s) -> Al3+(aq)
Red: C2O7 2- (aq) -> Cr2+

Balance Redox Reaction (Ox is balanced other than charge)
1st Balance Oxygen by adding H2O
C2O7 2- -> Cr2+ +7H2O

Next, balance Hydrogens by adding H+
(in acidic solution so no need to neutralize with OH-)
C2O7 2- +14H+ -> Cr2+ +7H2O

Finally, Balance charges in Red and Ox
Red: C2O7 2- +14H+ +8e- -> Cr2+ + 7H2O
Ox: Al -> Al3+ +3e-

Balance electrons by multiplying each reaction so that electrons cancel.
Red (x3) = 3C2O7 2- +42H+ +24e- -> 3Cr2+ +7H2O
Ox (x8) = 8Al -> 8Al3+ + 24e-

Finally, add equations to get the total reaction (and so electrons cancel).
3C2O7 2- + 8Al + 42H+ -> 8Al3+ + 3Cr2+ + 7H2O

Therefore, 42 H+ are consumed in the reaction.

Sorry for the bad formatting, hope this helps!