Half-Cell aka Ion-Electron Method

Different than the oxidation number method.

Say what?

Different than the oxidation number method.

"But, but, but.... how will I know which method to use?" 


The question will ask you to use one method or the other method.
You will be told!



The method:
-if the reactionis acidic or neutral (neutral = no mention of either acidic or basic conditions)
divide the reaction into two half reactions (an oxidation half and a reduction half) - [at this stage it should be quite obvious about the halves]
Working with one half reaction at a time:
  -balance species except for H and O
  -balance O by adding H2O
  -balance H by adding H+
  -determine charge on reactant/product side of half reaction
  -add electrons (e-) to balance charges
-at this point one reaction should have electrons on the reactant side (reduction half cell - gained electrons) and the other half reaction should have electrons on the product side (oxidation half cell - lost electrons)
   -make electrons equal by multiplying entire half reactions by necessary coefficient
   -add up the two halves, simplify (cancel) as needed
Check the finished reaction by ensuring that the charge is equal on both sides and that the atoms are balanced.

If the reaction is basic, add OH- to both sides equal to the number of H+ in the reaction
Simplify the water: OH- + H+ = H2O (cancel/reduce water that lies on both sides of the reaction)
Check the finished reaction

The above outlined procedure follows along with the Nelson text. There are variations of the procedure out there that also work. Worked out examples can also be found in the Nelson text.

Practice quiz/test type questions?
Try the following Waterloo links. There is a lesson presented and some practice questions that follow - with answers. Some of the questions are are at greater level of difficulty than expected for this course. But everyone likes a challenge, right?

Oxidation States: http://www.science.uwaterloo.ca/~cchieh/cact/c123/oxidstat.html
(Again some variations  of the oxidation rules have been presented. The oxidation answers do not change.)

Half Reactions: http://www.science.uwaterloo.ca/~cchieh/cact/c123/halfreac.html

Balancing Half Reactions: http://www.science.uwaterloo.ca/~cchieh/cact/c123/balance.html

Not that you are expected to be doing a lot of Chem*Is*Trying over the Winter Break, but....

The final answers to all of the reactions (oxidations, oxidation under A/B conditions and half reaction balancing) have all been moodlized.

The the next lesson (Galvanic Cells) and unit review have also been moodlized. Please note, if you decide to work ahead on the Galvanci cell stuff - the Nelson text does something a wee bit different than the representation style than will be followed in the classroom lessons. The text opted for their own version of "Cell notation", it is not Standard Cell Notation as many others follow. The following link provides examples of the classroom lesson format: pages.towson.edu/ladon/electrochem.doc

EChem in the New Year
-Galvanic Cell
-Galavanic Cell lab/Corrosion Lab
-Corrosion and Corrosion Prevention

All of the organic reaction handouts have also been moodlized.

If someone gets really organized, exam review will also get moodlized.

The moodle may get a bit of revamp over the Winter Break. The ask a question forum may shift to the top of the page so that it becomes an ask a question about any unit. The forum for question question asking is checked on a regular basis. Take advantage of the opportunity. During the break, answers may not appear immediately, but they will still appear. [Unless technical gremlins get in the way.]

May each and everyone of us enjoy our respective Winter Break.

Oxidation Numbering Balancing Day 2

By now
-you know the rules for assigning oxidation numbers
   -the always (elemental state =0;alkali = +1; alkaline earth = +2; fluorine=-1)
   -the usual (O=-2, H=+1)
   -the rest
-you know how to balance a redox reaction using oxidation numbers
    -assign oxidaton numbers to all
    -decide what changed
    -temporary balance of what changed
    -determine electron movement & balance redox portion
    -balance the rest

This lesson will follow all of the above rules but with a few added steps.
Theses redox equations will be under acidic or basic conditions. Whether the equation is under acidic or basic conditions must be stated along with the unbalanced equation.

Acidic Conditions:
-follow all of the steps up to the redox portion being balanced, then
    -determine charge on each side
    -balance charges by adding H+ to one side
    -balance H atoms using H2O
 Basic Conditions:
-follow all of the steps up to the redox portion being balanced, then
     -determine charge on each side
     -balance charges by adding OH- to one side
     -balance H by using H2O

Examples can be found on pages 666-667

The follow-up to this lesson are found on the balancing equations handout

Next up: Half Cell Method (aka ion-electron method)
-balancing equations using this 3rd method are different than using oxidation numbers to balance
       -on a test/quiz the type of balancing to be used will be stated
       -it is expected that equation will be balanced using the prescribed method
       -if the prescribed method is not followed, a mark penalty will result

Is a tree petrified because it is scared? Do librarians use bookworms for bait when they go fishing? When cows play cards, do they pay for big stakes? Is a plumber's favorite vegetable a leek? Is the prettiest vegetable a cute-cumber? When vegetables get married do they wear onion rings?

Balancing Redox Reactions & 3rd Time's The Charm

Follow the rules!
Follow the rules!
Follow the rules!

Rules to Balancing Equations Using Oxidation Numbers:
(1) Assign oxidation numbers to all species
(2) Identify those two reactant species that change
(3) Temporarily balance the species that change [Miss this step and the balancing could be off!]
(4) Determine electron movement. (Oxdn numbers up - lost electrons; oxdn numbers down, gained electrons)
(4) Balance electrons. [make total gained = total lost]
(5) Balance the remainder.

Example in the text - page 664-665

Followup from this lesson:
handout: Balancing Equations Using Oxidation Numbers

Next lesson: Balancing Redox Reactions in Acidic and/or Basic Conditions Using Oxidation Numbers

If you were determining the molar mass of a solid acid.   Basic Instructions have been moodlized.
What concentration of NaOH might you use? ~01.M-0.2M Solid acids can have larger molar masses. 50g/mol to 250g/mol
What volume of that 0.1~0.2M NaOH solution would you prepare....maybe 500mL (or more....)

If you opt to carry out the 3rd Time's The Charm Titration - have all of the calculations completely mapped out. In the lab the only thing that you would then need to do is (1) the titration and (2) finishing the calculations usign the titration values.
Wed Dec 16th = make and standardize base (~30 minutes)
Thurs Dec 17th= determine molar mass of acid day (~30 minutes)

This is not a 'big to do'. There are lots of procedures out there on the www that give instructions for determining the molar mass of a solid acid sample.

A few other tidbits.
(1) You may make and standardize the base with a partner BUT you will each be given your own separate acid for molar mass determination. Partners require more base than individuals.
(2) The complete calculation is submitted before leaving on Thursday. See moodle for expectations of submission.
(3) Not everyone is carrying out this lab, it is not necessary to use your lab book for this 'extra'. You can if you want to, but it is optional. [Adherence to safety rules is not optional :<]

Oxidation Numbers

LEO says GER
Loss of Electons Oxidation Gain of Electrons Reduction

Assigning Oxidation Numbers
Always Oxidation Numbers
-elementary state = 0  [H2(g) H=0)
-alkali metal ion = +1 [Na of NaCl; Na = +1]
-alkaliine earth metal ion = +2 [Ca of CaCl2; Ca = +2]
-flourine ion = -1 [F of NaF; F=-1]

Usually Oxidation Numbers
-hydrogen = +1 [except in hydrides, NaH, where H is -1]
-oxygen = -2 [except in peroxides, H202 where O is -1]

Workout everything else
-the sum of all oxidation numbers must equal the total charrge
(if no charge is indicated then the total charge is zero)

example: Na3PO4   Na = +1 (alkali metal)  O = -2 (oxygen is usually -2)
3(Na) + P + 4(O) = 0
3(+1)  +P + 4(-2) = 0
P= +5

example NO3 1- (nitrate ion)
N + 3(O) = -1
N + 3(-2) = -1  [oxygen usually -2]
N = +5

OIL RIG

Oxidation Is Loss Reduction Is Gain

From the Oxidation Number lesson:
p659 (12-16)
p662 (19)

In honour of LEO (GER!!!!!!!!!), the following:
Why do Lions eat raw meat?  [Because they don't know how to cook.]  Why did the lion loose at the card game?  [Because he was playing with a cheetah.]
A lion spots a monkey walking through the jungle. He grabs him by the neck and roars "Who's the king of the jungle?” The frightened monkey says, "You are, of course, your majesty." The lion does this to several other animals, with the same results. Then the lion goes up to an elephant, grabs him by the trunk and roars, "Who's the king of the jungle?" The elephant picks the lion up with his trunk, bounces him a few times on the ground, grabs his tail, twirls the lion around over his head, and then lets him go flying into a mud puddle. The lion looks up at the elephant and says, "Well, if you don't know the answer, just say so!”

A couple of zoologists decided to give a lion a cell phone in order to keep track of his whereabouts. Unfortunately, whenever they tried to call, the lion was busy!
A hungry lion was roaming through the jungle looking for something to eat. He came across two men. One was sitting under a tree and reading a book the other was typing away on his typewriter. The lion quickly pounced on the man reading the book and devoured him. Even the king of the jungle knows that readers digest and writers cramp.

[Lion jokes from : http://mojolion.com/Lion_Jokes.html]