THURSDAY MAY 19, 2011

ELECTROCHEMISTRY

We started the day with a goal. That goal was to be able to improve and perfect our skills in balancing redox equations. We were reminded to always keep the halfsheet in mind when attempting to balance the redox equations. The halfsheet states six steps which are needed to balance those equations. (listed below)

STEP 1: Use oxidation numbers to identify which species is oxidized and which is reduced. STEP 2: Write separate equations for the two half-reactions.
STEP 3: Balance each half-reaction.
a) Balance the elements other than H and O.

b) Balance O by adding H2O.

c) Balance the H by adding H+.

d) Balance charge by adding electrons (e-) to the side that is realtively more positive.

STEP 4: Multiply each half-reaction by an integer so that the electrons (e-) in each equation balance.
STEP 5: Add the two half-reactions, canceling identical specis wherever possible.
STEP 6: CHECK THE EQUATION, making sure both the elements and the charge are balanced.
Using this half sheet, we went over packet page number 4.

Example, letter C:
Zn + [NO3]- --> [NH4]+ + [Zn]2+

First, we have to observe and find out “what” is turning into “what.” For this problem, the Zn would be turning into [Zn]2+ and the [NO3]- would be turning into [NH4]+. After that is decided, you need to separate those elements in to two half-equations. You have now completed steps 1&2 from above. To accomplish the third step above, you need to balance everything including the charges of the two half-equations. For the first half equation (Zn-->[Zn]2+), you need to balance the charge. By doing this, you would add two electrons to the right side which makes both Zinc elements have a charge of 0.

The balancing of the other half-equation ([NO3]- -->[NH4]+) is more challenging. Since there is Oxygen molecules present on the left side, but not the right, you have to add H2O to the right side to balance out of the O molecules. In this case, you would add 3 H2O molecules, because there are 3 O molecules in NO3-. Also, since there are H molecules in NH4+ and H2O, you need to balance by adding H+ molecules to the left side. The 6 H molecules in H2O and the 4 molecules combine to a total of 10 H molecules on the right side, meaning you would balance the left side by adding 10 H+ molecules. The last thing that needs to be done to the second half-equation is balancing out the charge. You would need to add 8 electrons to the left side in order to balance the charge. All of these steps to balance the equation just cover step 3 from the above steps.

Lastly, we need to make sure the top reaction has an equal number of electrons to the bottom reaction. This requires us to multiply the entire top equation by four. After you multiply the top equation by four, you combine both of the equations together. The combined equation should be: 8e- + 10[H+] + 4Zn + [NO3]- --> [NH4]+ + 3H2O + 4 [Zn]2+ + 8e-. If there are “like terms” on opposite sides, you are able to cancel them out. For this instance, you cancel out the electrons, 8e-.

This leaves the equation as: 10[H+] + 4Zn + [NO3]- --> [NH4]+ + 3H2O + 4 [Zn]2+ as your answer for the complete balanced equation.

We finished more problems on this subject, but also moved on to packet page 6 which we were introduced to the topic of salt bridges. Salt bridges are added to provide a permeable membrane which allows a flow of ions between the half cells to prevent a buildup of net charge. Without salt bridges, the Zn in the example would diminish and buildup on the side of the Cu. This would make the battery go dead, becaues there would be no more reactant left (Zn) to keep the ions flowing. A picture of a salt bridge is below: