llbm Calculations - Day 2

The day 2 lesson of llbm calculations involved the quadratic equation.

The quadratic equation approach was necessary as
the K expression was no longer a perfect square.

A complete solution for these questions will include:
 - completed ICE chart
 - llbm law (the K expression)
 - substitution into the K expression
 - necessary work to get to the quadratic equation
 - the quadratic equation itself
 - both possible values of "x"
[indicate which value is 'invalid' - sub into the llbm expression on the ICE chart]
 - final answer - where various llbm conc have been determined.

 Three tedious questions to complete for homework.
p480(9) and p481(4,5)

The text also works through some examples. Check out the text too.

Have you complete the clock reaction lab yet?

Soon, very soon the lab will be marked and returned.
You may want to be one of those people who gets the lab returned to them.

Soon to be loaded on moodle. The next lab - Quantitative Equilibrium.
There is some pre-lab calculating to do. If you wish to work ahead. The lab
document has three parts to it. Two of the three parts will be loaded up in a
few minutes. The last part rquires scanning - and that will not happen until later
this evening.

Solutions to old homework can also be found on moodle. At this point,
the recent llbm calculations have not been revealed. Something about
having the kiddies work through it first, before seeing the solutions.

Why are gold fish orange? [Ans: The water makes them rusty.] Who held the baby octopus ransome? {Ans: Squidnappers]. Okay, off to upload a few things to moodle.

llbm Calculations Day 1 - a revisit

The lesson was all about calculating llbm concentrations.
(Finding 'x')

The handouts from the day's lesson are up on moodle (llbm calcn's page 1 & page 2)

If you visit moodle, you may also see the homework solutions to the Le Chatelier stuff [pages 457 - 460]

From the day's lesson - homework:
page 466(3,4)  page 472 (5,6)  page480(10) and page 481(3abc)

Most (if not all) of the questions require ICE charts.

Consider the generic reaction A (g)  +  B (g)  <=> 2C (g)  Kc = 37.9
A typical question may be: Determine the llbm concentration of C if 1.80 mol of A and 1.80 mol of B are placed in a 2.00L container and allowed to reach llbm.
 - complete an Initial Change Equilibrium chart and write the llbm law (K= expression)
 - when completing the chart, express values as concentrations
 - since no amount of product is mentioned, it will = 0
 - once you know one change, use the mole ratios of the balanced chemical equation to determine all other changes.
       A      +       B    <=>   2C
I    0.900M    0.900M        0
C    -X              -X           +2X
E  0.900-X   0.900-X        2X

Kc = [C]2 / [A][B] = 37.9 = (2X)2 /(0.900-X)(0.900-X)  
- notice that both numerator and denominator involve 'perfect squares'
- square root both sides

6.156 = 2X / (0.900-X)
5.540 - 6.156X = 2X
5.540 = 8.156X
0.679 = X

Do not leave the answer at 'X',
the question asked for [C]llbm = 2X = 1.36M

Demos were today- :<)

Have you submitted the clock reaction lab yet?

Finally, something (sort of) in honour of the 40th anniversary of Sesame Street. A monster of a cookie joke. Why did the cookie go to the doctor? [Ans: It was feeling crumby.]

Le Chatelier

Le Chatelier Principle:
Ref: pp450-456
When an llbm system is upset, the system responds in such a way as to oppose the change.

Ways to Upset llbm
-change the concentration (add/remove stuff)
   add - llbm shifts to the opposite side to try to 'use up' what was added
   remove - llbm shifts to same side to try to 'replace' what was removed
-change the temperature
  -it sometimes helpful to include the heat term as part of the equation [not deltaH notation], then think of 'heat' like any other substance
   exothermic:
    reactant -->  product + heat
    increase temp (like adding heat), reaction shifts to reactant side(reverse reaction requires heat)
    decreae temp (Like remoivng heat), reaction shifts to product side (forward reaction produces heat)
  endothermic - the opposite of exothermic
-change the pressure/volume
   increase volume (decr pressure) - llbm shifts to side with more moles of gas
        more moles of gas = more pressure, thus trying to increase pressure to be close to previous pressure
   decrease volume (incr pressure) - llbm shifts to side with fewer moles of gas
        fewer moles of gas = less pressure, thus trying to decrease pressure to be close to previous pressure

Variables that DO NOT affect llbm position
-addition of a catalyst - just reach llbm sooner
-addition of an inert gas (gas that does not react with any aspect of the llbm system) - interferes equally affects the probability of collision between both reactant and product particles

Hmwrk: p457(1,4,5,6)    p458(8)   p459(2*,3,5,6)  p460(10,12)  *just predict the shift, no sketch

Why did the chicken cross the road?  According to Le Chatelier: The chicken crossed the road because there were too many moles of chicken on the reactants side of the road equilibrium.

llbm Qualitative Lab

The investigation revolved around the llbm reaction of Fe/SCN and FeSCN and shifting the llbm position of the reaction to either the reactant and/or product side

the reaction:
Fe3+  +  SCN-     <=>     FeSCN2+
clear/colourless            reddish-brown

the addition of the KSCN (added SCN-) pushed the llbm to the FeSCN side
thus using up the added SCN ion
- evidence of llbm shift: solution became darker

the addition of the Fe(NO3)3 (added Fe3+) pushed the llbm to the FeSCN side
thus using up the added Fe3+ ion
 - evidence of llbm shift: solution became darker

the addition of the Na2HPO4 pushed the llbm to the Fe 3+ /SCN- side
 - evidence of llbm shift: soution became colourless
-the HPO4 binds to the Fe3+ removing it from solution, the FeSCN then breaks down as the reaction shifts back to the reactant side trying to replace the lost Fe3+

A couple of small* jokes to end this post. Why did the scientist install a knocker on the door? [Ans: In order to win the No-Bell prize.] Why did the atoms cross the road? [Ans: It was time to split.] *Small jokes - font size. :<)