Questions about CHEM1220_MT2_Practice_SK exam

[robert zellmer]

I've received a couple of questions about a few of the problems on this 
practice
exam.  Since more than one of you has asked about the same problems I'm
sending this to everyone, particularly since the explanation on one of 
them is
incorrect.

#27
The solution given is not stated correctly.  For metal cations the 
acidity (acid
strength) is the following:

The LARGER the CHARGE and the SMALLER the CATION the more acidic
the metal cation (the larger the K_a ).  It's about charge density 
(charge/size ratio).

Remember, the reason a cation is acidic is it first undergoes a Lewis 
acid-base
reaction with water where the cation is the LA and the water is the LB.  It
forms a hydrated cation.  Then the water molecules associated with the 
cation
lose H+ ions to the surrounding bulk water.  This occurs because the 
metal cation
pulls electron density away from the waters of hydration making the OH bonds
in the water molecules more polarized and weakens the bonds so a 
hydrogen can
break off as an H+, making the solution acidic.  The greater the 
attraction of the
metal cation for the water molecules the more it pulls electron density 
toward it
from the water molecules.  This means the greater the charge density of 
the metal
cation the greater this pull.

A smaller cation has a more concentrated positive charge.  This makes for a
stronger attraction between it and the water molecules around it.

The greater the positive charge on the cation the stronger the 
attraction between
it and the surrounding water molecules.

Thus, acidity increases as charge density increases, as described above.

# 28
Is sodium bicarbonate (baking soda), NaHCO_3 , acidic or basic?

This is actually an example I did in lecture.  When put in water you get 
Na^+
and HCO_3 ^- .  The Na^+ is neutral. The HCO_3 ^- is an amphoteric anion 
(sometimes
referred to as an "acidic" salt).  It can act as acid or a base. The 
question is in
water which will it be?

You have to look at it acting as a base or an acid.  You need to look at 
the K_a
for it acting as an acid and the K_b for it acting as a base.  I 
understand myself
why the answer key labels the K_b as K_b2 but I can see what that label 
might
be confusing.  It's the K_b for HCO_3 ^- .  I've annotated the page from 
my notes
and attached it.

You need to do this type of analysis for amphoteric anions.

The one major exception is HSO_4 ^- , bisulfate. This is the conj. base 
of H_2 SO_4 ,
a strong acid.  Thus, HSO_4 ^- will not act as a base.  However, it's 
not neutral like
the other conj. bases of strong acids.  Bisulfate is a relatively strong 
acid, with
a K_a of 1.2 x 10^-2 .

Dr. Zellmer
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