Let's
try another example of balancing reactions.
Hg(NO3)2
+ AlCl3 → Al(NO3)3 + HgCl2
If
we count elements and ions, we find:
Left
side
|
Right
side
|
Hg
= 1
|
Hg
= 1
|
NO3
= 2
|
NO3
= 3
|
Al
= 1
|
Al
= 1
|
Cl
= 3
|
Cl
= 2
|
This
is obviously not balanced, so let's see what we can do. Starting at
the left, we see that the mercury's are balanced, but the nitrates
are not.
We
have three nitrates on the left and two on the right. There is,
unfortunately, no single number I can put in as a coefficient to make
these match. There is, however a relatively easy solution. The
nitrates on the left come in pairs. That means that I must always
have 2 or 4 or 6, or 8, etc. nitrates on the left side as I increase
the coefficient. On the right side, the nitrates come in trios. That
means that as I add coefficients, I will have 3 or 6 or 9, etc.
nitrates. The connection? ...6. In other words I need to get 6
nitrates on each side to balance them.
Another
way of arriving at this number (for those who are mathematically
minded) is to look for the least common multiple (the lowest number
that each original value can be multiplied to form). The least common
multiple of 2 and 3 is 6. That means that I need to have 6 nitrates
on both sides to balance the equation.
To
achieve these matching 6's, I need a coefficient of 3 on the left and
a coefficient of 2 on the right:
3
Hg(NO3)2 + AlCl3 → 2 Al(NO3)3
+ HgCl2
Now
if I count elements and ions, I get:
Left
side
|
Right
side
|
Hg
= 3
|
Hg
= 1
|
NO3
= 6
|
NO3
= 6
|
Al
= 1
|
Al
= 2
|
Cl
= 3
|
Cl
= 2
|
Now
my nitrates match, but my mercury's no longer do. However, I can
solve that by adding a coefficient of 3 to the mercury compound on
the right:
3
Hg(NO3)2 + AlCl3 → 2 Al(NO3)3
+ 3 HgCl2
Left
side
|
Right
side
|
Hg
= 3
|
Hg
= 3
|
NO3
= 6
|
NO3
= 6
|
Al
= 1
|
Al
= 2
|
Cl
= 3
|
Cl
= 6
|
Getting
closer. Now both the nitrates and the mercury's match, but the
aluminum's and chlorides are still a problem. Focusing just on the
aluminum (1 on the left and 2 on the right) it seems clear that I
should add a coefficient of 2 on the left side before the aluminum
compound.
3
Hg(NO3)2 + 2 AlCl3 → 2 Al(NO3)3
+ 3 HgCl2
Left
side
|
Right
side
|
Hg
= 3
|
Hg
= 3
|
NO3
= 6
|
NO3
= 6
|
Al
= 2
|
Al
= 2
|
Cl
= 6
|
Cl
= 6
|
So,
now my mercury's, nitrates and aluminum's all match and...wait a
minute...the chlorides do as well.
The
reaction is balanced.
No comments:
Post a Comment