## Q2 : Week 2 – 11/21- 11/25

If you have not already please join the REMIND for this class.

_______________________________________________________________________________________________________________________________________
11/21 – Monday – A Day – 2/3a Lab, 4                                       Jump toMonday Homework

Main focus –

a) To review the weak acid /strong base Titration

b) To identify buffer solutions

c) To use the Henderson – Hasselbach to answer point 3 questions

Every Titration Curve Has 5 points that need to identified or calculated AND every ACID/BASE problem represents every one of these points. They include:

Point 1 = initial pH
Point 2 = half equivalence – obtain Ka of acid through Henderson-Hasselbach equation
Point 3 = buffer position (when any ml of titrant has been added but is less than equivalence)
Point 4 = equivalence point
Point 5 = final pH.

Period 2/3a: Collect Titration quiz (from the form).

1. Review Lab 11 and the homework form (another weak acid/base titration).

2. Review Past 2 Acid Base Homeworks – YOU MUST BE ABLE TO IDENTIFY which Points on the titration graph ( 1 – 5) you are working with.

Acid Bases Pre-Titration key p.pdf

Titration Graph Lab questions Key p .pdf

3.  Lab activity – Not a Lab – Practice skills learned

a) Determine the concentration of the weak acid with a Standard Base using a chemical indicator.

b) Determine the Ka of the acid.

Period 4: Collect Titration quiz (from the form).

1. Review Lab 11 and the homework form (another weak acid/base titration).

2. Review Past 2 Acid Base Homeworks – YOU MUST BE ABLE TO IDENTIFY which Points on the titration graph ( 1 – 5) you are working with.

Acid Bases Pre-Titration key p.pdf

Titration Graph Lab questions Key p .pdf

_________________________________________________

ACID and Base NOTES – Acidic/Base Titration Notes

The new skills  are identifying the parts of a Titration curve.  The striking part of the Titration graph is the asymptotic line that occurs as the pH changes becomes exponential changes as the pH nears the equivalence point.

We learned the endpoint in an Acid/Base titration is really an approximation of the equivalence point.  The endpoint refers to the volume of titrant added (Standard Base ) and pH that results when the chemical indicator changes color.  This color change will never be exactly at the equivalence point but it approximates the volume of titrant added if the indicator has a color change on the asymptotic line.

Consider the titration in the lecture (Strong Acid/Strong Base).  We used phenolphthalein which has a color change (turns from colorless to pink at pH changes of 8 – 9).  What if we used Thymol blue or Methy red?

 Table M Do not forget that these indicators are themselves conjugate acid base pairs in equilibrium. In the case of Thymol blue  Notice in the first Titration which you should recognize as a strong acid / strong base titration, with a pH of the equivalence pH of 7, can utilize both Thymol blue and Methyl red as indicators as the endpoints will be on the asymptotic line.  They would have endpoints with approximately the same volume as the equivalence point. The titration on the right is a Strong Base/weak acid concentration that we will study and do in lab next week. NOTICE THE ENDPOINT IS NOT AT A pH of 7 in this case!

Weak acids have an incredibly small Keq because virtually all of the acid remains undissociated and thus there are less products and more reactants. Weak acids dissociate very UNspontaneously!

HC2H3O2  —–>  H+   +   C2H3O2

Keq   =    [H+][C2H3O2] / HC2H3O2

Keq  =   Very Small

Now because these reactions are very similar  (HA   —->  H+  +   A– we call them Ka!
Keq  = Ka when we deal with acids dissociating.

We can find the Ka graphically because of the relationship between Ka and what it equals: Using this relationship we know that when we have equal amounts of conjugate Base (A) and conjugate acid (HA) the log [A] / [HA] will go to zero because the log of 1 = 0 !!

Thus when [A]  =  [HA]   the pH = pKa

and we can get the pKa of acid which is just the -log of the Ka.  Once we have the pKa we just

perform the 10-x calculation to attain the Ka  of the acid.

How do we find the point where the conjugate Base (A) and conjugate acid (HA) are equal

We find the half equivalence point!!! (point 2!) So once we establish the equivalence point which is in the middle of the asymptote we attain the volume at that point which in the example to the left is 20 ml.  Now we take the equivalence volume and halve it: 20 ml / 2 = 10 ml 10 ml represents the volume of titrant (base) added to neutralize half of the acid.  This point is where half of the weak acid has been converted to the conjugate base.  So if we had 1 mole of weak acid, .5 moles of the weak acid remains and  .5 moles of conjugate base has been created. This is the point where the acid = conjugate base !                              pH of this point = pKA
So in the above example the pH at the Half – equivalence point is equal to 3.  Pka of the acid = 3.

The Ka of the acid is thus:    10-3 =    1 x 10-3

This is a Ka of a weak acid because it is less than 1 which means the reactants or the undissociated acid ( HA ) is a larger quantity because the reverse reaction is more spontaneous.
Instead of pH ranges for the color changes for chemical indicators we can use pKa’s
They are also acid/base equilibrium systems

HIn    <—>   H+    +     In

and thus they also have Ka’s and pKa’s

What do you think the pKa can tell us about the acid/base indicators? What is significant about the pKa of the acid base indicators in terms of of the pH ranges?

Today’s Notes:

From this weekends homework:

Reaction during the titration – Driven by the strong Acid to completion to the equivalence point

H3O+      +     C6H7O2–           – –>       HC6H7O         +      H2

strong acid      Weak Conjugate Base      —>    stronger conjugate acid
Reaction that occurs at the equivalence point – Equilibrium reached (NOT DRIVEN)

HC6H7O2     +    H2O    <—>     C6H7O2-1       +       H3O+

conjugate acid ionizes water                                 Lowers pH at equivalence

 Strong Acid Strong Base  ~Lab 10 pH = 7 @           equivalence only spectators that DO NOT ionize water Weak Acid  Strong Base ~Lab 11, pH  >7 @ equivalence Conjugate Base produced reacts with water                                                              to produce OH- Strong Acid  Weak Base ~Last nights HW pH  <7 @ equivalence Conjugate Acid produced reacts with water to produce H3O+  The  Blood buffers the blood pH so that you survive!!!  There are so many organic molecules that have acidic or basic properties that would cause death in our aqueous plasma (fluid part) of our blood. The main buffer in our blood that prevents large pH changes if the following conjugate acid base pair:    Conjugate Acid                      Conjugate Base             H2CO3            <—>          H+  +  HCO3-1    Neutralizes Bases                  Neutralizes Acids

The pKa of  H2CO3 = 6.1   The Ka of this week acid = 10 3.8   = 1.58 x 10-4

Blood is buffered at pH of about 7.4.  What is the ratio of conjugate base to conjugate acid in this buffered solution?

pH = pKa + log [A]/[HA]

7.4 =  6.1  + log [HCO3-1]/[H2CO3]

1.3 = log [HCO3-1]/[H2CO3]

10X to both sides of the equation

101.3 = [HCO3-1]/[H2CO3]

=    19.9     :      1        ratio

*Normal human blood has about a 20 : 1 ratio!

What would this look like on a titration curve?

Sodium Carbonate / HCl (weak base / strong acid Titration): The blood buffer!

End Of Thursday!

Side by side comparison of the 2 types of titrations:

 Strong Acid/ Strong Base play at 1.75 speed Weak Acid/ Strong Base play at .75 speed

_____________________________________________________

11/21 – Monday’s Homework: –

1. Please complete with me the Determining pKa and Indicators.pdf worksheet. Please review with key and or the posted video that reviews some parts of the worksheet worksheet.
Determining pKa and Indicators.pdf

Determine Ka and Indicator key p.pdf
New Skills
pKa = pH at half equivalence point
a) determining Ka of Acid Graphically
b) determining indicator based on pKa
c) Henderson – Hasselblach equation derivation                                                                                                                                                                                                                                                                              2. Please predict the following the 5 points on piece of graph paper given the following titration.  Show all work for each point. This will be collected tomorrow. There is no form for this assignment.

Point 1 = initial pH = 11.4 (given)
Point 2 = half equivalence
Point 3 = buffer position (when 8.0 ml  of titrant has been added )
Point 4 = equivalence point
Point 5 = final pH.

Your graph will predict the titration curve for the following titration:                                                                                                                                                                                                                                                                                                                                                                             25.0 ml of a 0.30 M NH3 solution is titrated with a
total of 38 ml of a 0.25 M solution of HCl

You will need the following additional data to complete the prediction:                                                                                                                                                                                                                                                                                                                                               The Ka of NH4+ = 5.6 x 10-10
The initial pH of the analyte solution is pH = 11.4
The pH at the equivalence point = 4.2
Please predict point 3 when 8.0 ml  of titrant has been added

Determining Ka, Indicator worksheet review:

End of Monday.

11/22 – Tuesday – B Day – 2, 3b Lab/4

Main focus –                                                                                                                                                                                                                                                                                                                                                   a) To review a weak base strong acid prediction titration curve of 5 points.

b) To perform a weak base strong acid titration (Lab 12)

Period 2

1. Review the Weak base strong acid titration prediction graph
2. Perform Lab 12 – (Weak base Strong Acid Titration)
BUFFER PROBLEMs new worksheet  was actually given out

Period 3b/4
1. Same as above.

2. BUFFER PROBLEMs (point 3) new worksheet  was actually given out and reviewed here. Next Tuesday I have it posted with key. May need to add it here if it fits.

If Last nights homework was a disaster please do not hand it in.  I will have everyone do another prediction graph next week . Please review the key when you get a chance. The titration we did today we will do in class next Monday!

______________________

Lab 1 2 – Weak Base / Strong Acid Titration

In this Lab we were trying to determine the concentration of a the weak base solution using a KNOWN concentration of a STRONG Acid.  I tried to make a 0.30 M solution of the weak base but because my stock solution was diluted from time our actual titration will be a little different.

Thus 25.0 ml of a unknown M NH3 solution is titrated with a total of 38 ml of a 0.25 M solution of HCl

1: Determine the equivalence point of the titration. (Identify of the graph)
2: Determine the concentration of the acid.
3: Determine the end point of the titration if phenolphthalein was used. (Identify on the graph)
4: Determine the halfway equivalence point. (Identify on the graph)
5: Confirm the final pH of the titration.
6: Confirm the initial pH of the titration. YOU CANNOT DO THIS STEP!!!!!!!(In the spring)
7: On the first 4 points of the titration curve (1 = initial pH, 2 = halfway equiv., 3 = buffer position, 4 = equivalence               point,) determine the percentage of the Conjugate Acid/ Conjugate Base / OH
8: Write the net-ion reaction.
9: Determine the Ka of the Acid.

10. Confirm the pH of point 3 when _____ml of acid was added to the weak base.

_____________________________________________________

11/22 – Tuesday’s Homework: –

Tryptophan is an essential amino acid: It is found in all foods that have complete set of amino acids (that out body cannot make) ! Based on the above chart you can see that turkey does not have the greatest amount of proteins with tryptophan.

AND NO tryptophan does not make you sleepy!!!  If it did vegetarians would always be asleep because they love tofu which is made of soybeans!

Also if that myth was true turkey sandwiches would have warning labels like,

“Do not operate heavy machinery while digesting turkey!”

How many tryptophan related accidents would we have on Thanksgiving?

Acidic or Basic Foods: _______________________________________________________________________________________________________________________

11/23 – Wednesday – A Day – THanksgiving Eve – Gobble

_________________________________________________________________________________________________________________________

11/24 – Thursday – B Day –  Happy Thanksgiving!!!

_________________________________________________________________________________________________________________________

11/25 – Friday – A Day – Post Thanksgiving.