Questions about 23, 25, 26 and 27 on miditerm 3

[robert zellmer]

I've received a number of questions about questions 23, 25 and 27
on the 3rd midterm.  I also noticed several people making the same
mistake on #26.

#23:

You needed to look at the balanced eqn.  A lot of people seemed to just
have assumed this was a voltaic cell and treated the Cr electrode as the 
cathode
and the Mn electrode as the anode.  If you look at the eqn. it is the Mn
electrode which is acting as the cathode (the std reduction electrode 
potential shown
below the balanced eqn. is used as it appears in the rxn meaning it is 
the reduction rxn
and thus the cathode).

This is also one of the cases in which it is difficult to get the exact 
answer given
because it depends quite a bit on when you round and which eqns you 
use.  However,
there's only one answer with the correct power of 10 and your answer 
should have
a coefficient relatively close to the answer given.

# 25:

This one turned out to more "tricky" then intended.  It wasn't meant to 
be tricky.
You have to read it carefully and understand a voltaic cell.

A voltaic cell is a spontaneous redox rxn.  If the two electrodes were 
both placed in
the same beaker the electrons would travel through the soln from anode 
to cathode.

However, in a voltaic cell the reactants (electrodes) are separated in 
separate beakers
so they don't react directly.  In order for the rxn to occur the 
electrodes have to be
connected by a wire and the electrons flow through the wire from anode 
to cathode.
The electrons are not in the soln.

#26:

I know of at least five people I've spoken to who made the same 
mistake.  They
almost wrote the correct eqn for the problem.  However, they forgot the 
negative
sign in the eqn when copying it from the back page or thought they knew 
it well
enough to not look at it.  Use what's given and be careful.  An extra 
second or two
of diligence can assure you getting a question correct.

# 27:

In this problem many people failed to actually get the balanced eqn for 
the rxn.  You
must write the correct balanced eqn in order to get Q, the reaction 
quotient.  In the
correct expression for Q you would have [V^3+]^4/[Hf^4+]^3.

Dr. Zellmer