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Archive – Q3 –  week 9 – 18-19

week 9

     Week of  4/1 – 4/5 – REFRESH

 
4/1 – Monday –  period 2
 
1.  Lab:  Titration of weak Acid:  30.o ml KHC8H5O5 potassium hydrogen phthlalate (KHP) 
      with 0.06 M Ca(OH)2
 
2. Verify the 5 points on the graph as per the guidelines for the lab below.
                               period 3/4
 
1.  Lab:  Titration of weak Acid:  30.o ml KHC8H5O5 potassium hydrogen phthlalate (KHP) 
      with 0.06 M Ca(OH)2
 
2. Verify the 5 points on the graph as per the guidelines for the lab below.
 
3.  Start the Acid Base Blank graph for Titration Prediction that will be practice for tomorrow test.
Today’s Titration: 
 
KHC8H5O5 (aq  +   Ca(OH)2 (aq)  —–>    H2(l)   +   ?
 
This is not a net ion reaction so you must determine the Net Ion Reaction.
 

 KHP or potassium hydrogen phthlalate was the acid we used to standardize the strong base earlier in the year.

We use KHP to standardize strong bases like NaOH or Ca(OH)2 because
1) KHP is not very reactive (stable)
2) KHP is a solid with a high molecular mass (204.22 g/mol)
3) The volume of water that the KHP is dissolved in does not need to
be precisely measured as long as we have moles of the acid

 

 

 Potassium hydrogen phthalate (KHP) lab requirements:
 
Weak Acid – Strong base Titration Lab activity- 
25.o ml KHC8H4O4 potassium hydrogen phthalate (KHP) with 0.05 M Ca(OH)2
Included on the graph of pH and Volume of titrant from this lab that you printed.
 
A) Write the Net Ion Reaction of the titration
 
B) Graphically identify the 5 points.
 
C) Write the net ion reaction for the titration.
D) Determine the concentration of the KHP
 
D)- Verify with calculations the following points in the graph (PLEASE NUMBER THEM on your graph!)
        1) initial pH
        2)  half equivalence point – Determine the Ka of the weak acid.
        3) a pH value  in between max buffering position and below the equivalence point @ 20 ml(this new!)
        4) pH at the equivalence point.
        5) final pH
E)- determine the appropriate indicator(s) if an ENDPOINT was to be reached from the list below.
     *Remember that an Equivalence Point (moles of H+ = moles of OH) is not the same as the Endpoint!
       The Endpoint is point in the titration that you estimate the equivalence point by using chemical acid base           indicators that are acid and conjugate bases themselves with Ka’s and pKa’s.  
 
Chemical acid/base indicators which are weak acid base equilibrium reaction are written the following way:
 
HI          <—–>     H+        +           I
 
                                                           Conjugate acid                                           Conjugate Base
    
    If Litmus is the chemical acid/base indicator used the conjugate acid is RED while the Conjugate Base is Blue. These color changes are due to the changes in the chromophore that we have exhaustively covered.  The point here is that we can use this color change to stop a titration (ENDPOINT) If the color change occurs near the Equivalence point.   
 
When would litmus have a color change?  
 
It would occur when there is 50% conjugate acid and 50% conjugate base.  This would be at ITS HALF EQUIVALENCE POINT!   At the half equivalence point the pH = pKa!  So all you need to do is choose an appropriate chemical indicator that has a pKa at +1 or -1 of the Equivalence point.In the case of the litmus we would use this indicator if the titrations equivalence point was about 5.5 to 7.5.  
 
*Remember if the color change is occurring near the equivalence point it is occurring ON the asymptote and thus the volume determined by stopping the titration (Endpoint) will be very similar to the volume determined on the Equivalence Point. 
 
If you want more on this please view my notes on this topic
Please use this chart to determine best indicator to use with this titration.
Show all work on graph.

Acid Base Indicators

4/9 – Monday -Homework  Acid/Base Titration test Tuesday – 
 
1. Complete Lab with video below if you did not complete in class.
2. Complete the practice tests.
   
    You will be given a piece of blank graph paper and will be asked to sketch the titration cureve with limited data.  You will be calculating 5 points in a weak acid and strong base titration.
These points are:
 
1: initial ph of acid in a beaker
2. max buffering position (and volume).
3. a point between max buffering position and equivalence point.
4. Equivalence point.
5. final ph of solution (after a known amount of base has been added).
 
These 5 points need to be placed on the graph and you will sketch the graph with the supporting calculations for the 5 positions above.
 
If you get all 5 points correct with calculations I will give you a 100 on this test and a 100 on your lowest test grade.
 
In the hard copy I gave you in class, I left out that the concentration of the acid is 0.10 M.
This test is really the complete opposite of how you would complete the lab as you are verifying the same points in the lab you did last Thursday and Today.
Here is an example:
Blank:
Acid Base Blank graph for Titration Prediction 1617 p.pdf
View Download
 
Key: (I made  small error in this key as I stopped the titration at 75 ml instead of 70 ml of base – Thanks Paige!)
Acid Base Titration Prediction KEY.pdf
View Download
 
Example 2: 
Blank:
Acid Base Blank graph for Titration Prediction 2 p.pdf
View Download
 
Key:
Acid Base Blank graph for Titration Prediction 2 KEY p .pdf
View Download
Lecture on how to complete this example 1 :

 

Lecture on how to complete this example 2 :
Lecture on how to complete the Lab:  
Play in Youtube and look under the video for the links (time code in blue) to take you where you need go in the video to complete. Most of you need points 3 and 5.

 

 


          
 4/1 – Tuesday    period 2/3
 
1.  Acid /Base prediction graph test/ walk out with key
 
2.  Started a classwork problem (finished most of it)
 
Acid Base 6 – 2003 B.pdf
View Download
Acid Base 6 2003A – key AP Central .pdf
View Download
 
                                period 4 – 
 
1.  Acid /Base prediction graph test / walk out with key
 
 
4/2 – Tuesday Homework
 
A) Review today’s Titration Curve Test with the key I gave you after the test.
 
B) Take a Part 2 Acid Base question – (under timed conditions with complete set of AP reference  
                                                                             Tables)
 
Period 2/3 please complete the question we started in class. – Period 4 will do this class tomorrow.
 
C) Take Another that moves in the other direction
 
A. Complete Acid Base 3 – 2007.pdf worksheet. It is a part 2 question from 2007 that could be very like a question you could get in the first three question in part 2.  Please complete this question under  timed conditions!  Give yourself 20 – 25 minutes tops for this question.  
 
 Grade it with the college board key below to see how many of the 9 point (which would be scaled to 10 points) you would of earned.   
 
Please View my Grodski key (which the last page of the key) to read my comments and SEE HOW I would do these problems.
*After taking this question and reviewing MY Answer key Can you see my reasoning for spending so much time with titration curve calculations? If so you are getting it!!!!!!
 
If you really struggled then I bet you struggled on today’s test.  Please view the review videos
     below that will review how to complete every question.
 
 Acid Base 3 – 2007 .pdf
Acid Base 3 – 2007 Key grodski & college board keys p.pdf
View Download
 
Lecture that reviews Acid Base 3 -2007.pdf – (last question used equilibrium way to solve)
 
Lecture that reviews Acid Base 3 -2007.pdf  – specifically the last question using the Henderson Hasselbalch Equation (buffer equation).
 

5.  Period 2/3 – Please complete the part 2 question that we started in class and review with the key.

Acid Base 6 – 2003 B.pdf
View Download
Acid Base 6 2003A – key AP Central .pdf
View Download
 
End of Tuesday..

4/3 – Wednesday –  
 
1. review of Hw- Did you visualize the points on the titration curve that each problem presented yet the question                                     did not utilize or present any titration curves?
 
2. Buffer discussion –   What are buffer solutions?
 
A buffer solution is one where a conjugate acid and a conjugate base ARE both present in a solution AND they resist pH changes in the solution.
 
*Remember that conjugate acid/base pairs are the SAME chemical plus or minus a proton (H+).  
 
Having an acid (conjugate acid) and a base (conjugate base) will “buffer” OR maintain the pH of a solution BECAUSE the addition of an acid to “buffered solution” will neutralize the addition of an acid as the conjugate base will react with the added acid AND prevent the pH from changing.  If a base is added to a “buffered” solution the conjugate acid will neutralize added base AND prevent the pH from changing.
 
Lets look at ammonia NH3 (weak base) in water:
 
reaction 1:                              NH3      +       H2O    <—–>       NH4+      +      OH
                                             conjugate base                          conjugate acid
Because NH3 is  weak base it will reach equilibrium and both conjugate base  and  conjugate acid  wil be present in solution.  A conjugate acid /base pair will never neutralize each other because their ability to donate a proton (acid) and ability to accept a proton (base) are inversely related to each other in terms of Kw (1 x 10-14).   Lets revisit a Derivation that we have done:
 
Lets start with the NH3 in aqueous solution and view its corresponding equilibrium expression
Now lets view the the conjugate acid,  NH4in aqueous solution and its equilibrium expression
 
These 2 chemical reaction are competing when they are in water, remember that water is the chemical that is in largest quantity and that these 2 reaction are really the forward and reverse reaction of reaction 1 above.
 
So lets add them together to get a perspective of how the forward reaction and the reverse are related:

                          Wait?  Isn’t this the auto ionization of water that has a Kw = 1 x 10-14  ???

 
             YES!!! And if we are adding the reactions we are also multiplying the Ka and Kb:
 

   So what this means that as the conjugate acid strength increases its conjugate base strength decreases!! The ability of conjugate acid to act like an acid is inversely related to the ability of the conjugate base to act like a base. 

 
“Stronger the conjugate acid (higher Ka) the weaker its conjugate base (lower Kb)!”
“Weaker the conjugate acid the stronger the conjugate base!”
 
Given the following Ka’s for 2 weak acids, which conjugate base is the strongest?
 
                                                HNO2   —–>    H+    +    NO2–        Ka = 4.6 x 10-4
 
                                                     HF     —–>    H+    +       F           Ka = 3.5 x 10-4
 
answer: F    – This means that F–   ionize water (make into OH) better than NO2– because it is a stronger base than NO2– because its conjugate acid (HF) is weaker than the conjugate acid (HNO2) of NO2 .
 

So now it should be clear why A conjugate acid /base pair will never neutralize each other because their ability to donate a proton (acid) and ability to accept a proton (base) are inversely related to each other in terms of Kw (1 x 10-14)!

 
” A conjugate 
 
Because the conjugate acid and base do not react with each other they are available to react with both acid or base that are added to a buffer solution. As long as there is enough of the conjugate acid or the conjugate base the buffer solution will not change it pH if an acid or a base is added.  That is what buffers do!!!!!
 
So if I have the conjugate acid base pair from reaction 1 above the conjugate base will neutralize the acid that is added:  
 
NH3   +   Acid added to buffered solution     —–>    NH4 +   +    conjugate base of acid added.
 
And if I add a base to the buffered solution from reaction 1 above, the the conjugate acid will neutralize the base added :
 
 NH4 +  Base added to buffered solution   —–>   NH3    +  conjugate acid of base added.
 
                  So a base or an acid added to this buffered solution will maintain this pH!
 
We calibrate our pH probes with a buffered solution at a pH of 7.  I use the following 

  The conjugate acid is delivered by the salt:

potassium phosphate monobasic:

          KH2PO —->  K+  +  H2PO4-1

   The conjugate base is delivered by the salt:

sodium phosphate Dibasic:

         Na2HPO —–>   2Na+  +  HPO4-2
 
This product contains a mixture of these 2 salts in a plastic capsule that is to be dissolved in 100 ml of water.

Lets look at some titration curves of weak acids that we have titrated or will titrate:

 

-Notice that the points labeled the “Buffered Region” is where a Buffer solution is being made as this is the place in the graphs that both the conjugate acid AND conjugate base are both present.

Notice that the half equivalent points determine the pKa of the acid WHICH also tells us the maximum buffer pH or the median pH range of that particular buffer.  We determine appropriate based on the pKa’s or Ka’s of the conjugate acid!  This means if I need a buffer to maintain a pH at 9 I would use a buffer solution with approximately equal amount of  NH4+ and NH3 .

If we titrate “partially” meaning we add enough base to drive the reaction forward enough to produce some conjugate base but do not add enough to reach the equivalence point we create a buffer and we are in the “Buffer Region”!


The best Buffer solution has 

                                    1. Equal amounts of Conjugate Pairs  This would allow for a greater range                                                           buffering ability. Think about the half- equivalence point.  At that position the                                                                   solution can equally buffer an acidic or basic additions.

                                           2.  Has the largest concentration of Conjugate Pairs – This would allow     
                                                the buffer to RESIST pH changes the most. The greater the moles of the conjugate  
                                                pairs,  the greater amount of acid or base the buffer solution can neutralize before it  
                                                runs out of one the conjugate pairs and pH will drastically change.
 
                                           3.  The pKa of the conjugate acid approximates the pH region you  
                                          want to buffer. – The half equivalence point will be the that region!!!

So we make a buffered solution 3 ways:

                1:  Making an aqueous solution of a conjugate acid or conjugate base .
 
                    It the reaction will go to equilibrium quick making the the other conjugate partner.
 
Step 1 is the Not a Preferred method because the reaction will not make much of the other conjugate ion since these acid and base are weak.
 
              2:  Partial titration – Stoichiometrically drive the reaction to a point that is below the equivalence point so that there is a significant amount of both the conjugate base and conjugate acid.
 
            *3:  Deliver the conjugate base or conjugate acid via a salt!! (students often do not recognize  
                    this way as they forget to look for the ion in the formula that is the conjugate partner).


Period 2 –   Buffer Lecture – used notes above

Period 3 – Buffer Lecture – used notes above

Period 4 – classwork – 20 – 25 minutes of part 2 question – 2003 B and reviewed with key

Acid Base 6 – 2003 B.pdf
View Download
Acid Base 6 2003A – key AP Central .pdf
View Download
 
4/3 – Wednesday –  Homework
 
1. Complete Acid Base 4 – 2005.pdf worksheet. It is a part 2 question from 2005 that could be very like a question you could get in the first three question in part 2.  Please complete this question under timed conditions!  Give yourself 20 – 25 minutes tops for this question.  
 
2. Grade it with the college board key below to see how many of the 9 point (which would be scaled to 10 points) you would of earned.   
 
3.  Please View my Grodski key (which the last page of the key) or the lecture below!
Acid Base 4 – 2005.pdf
View Download
Acid Base 4 – 2005 Key grodski and college board keys p.pdf
View Download
Lecture on Acid Base 4 worksheet: 
 

4. Please complete the 

Acid Base 9 – 2000 D.pdf worksheet below and review with the key below.
 
Acid Base 9 – 2000 D.pdf
 
Acid Base 9 2000 D 1819 Key.pdf
View Download
 
5. Please complete the form If you did not complete in class:
 

Acid Base MC Form

 

End of Wednesday….

4/4 – Thursday –  period 2/3 – DO NOT COMPLETE THE LAB – WE WILL FINISH IN CLASS!
 
1.  Passed out weekend homework, new reference tables, and explained the version.

2.  

Weak Base/ Strong Acid Titration: We finished in class!
Titrating unknown concentration of 40.0 ml of Na2CO3 with a total 50 ml of 1.0M HCl
                                    period 4
 
1.  Passed out weekend homework, new reference tables, and explained the version.

2.

Weak Base/ Strong Acid Titration: Collected data and started the write-up
Titrating unknown concentration of 40.0 ml of Na2CO3 with a total 50 ml of 1.0M HCl
 
4/4 – Thursday –  Homework
 
DO NOT COMPLETE THE LAB – WE WILL FINISH IN CLASS!
 
1.  Make another submission to last nights form if you have not already done so.
 

2. Study for the make-up titration test of the century.  I would review 

Acid Base 9 – 2000 D.pdf worksheet as I will be giving you a weak base titrated with a strong acid. (given in extra – help only)
 
3. Please Watch the Electrochemistry Review lecture- in its entirety !
                               We did all of this already except the 1 new equation for ∆G0 !
 
Please complete the following worksheets along with me.  I will start with:
 
Voltaic cells – Review with thermodynamics     
2010 AP voltaic cell question with scoring guide.pdf – I gave this one out!
View Download
 
The second half of the video I go over 
Electrolytic cells
2007 – Electrolytic cell question with scoring guide.pdf – I did not give this one out! 🙁
View Download
 
Electrolytic cells
            1) electrolysis of fused salts
            2) electroplating – calculating with amperage
                  Faraday’s constant, New delta G Formula that ties voltage with Delta G.
 
Complete Electrochemistry Review lecture-
                                      
From your Reference Tables:

Additional Electrochemistry practice:
 
2013 – Electrochemistry Question with scoring guide.pdf
View Download
 
2002 – Acid Base Part 2 question with Grodski and College B keys.pdf
View Download

 

 Electrolytic – (electroplating/electrolysis)
Non-spontaneous
volts = negative

∆G = POSITIVE


Endothermic:  Electricity  +  Reactants –> Products
Anode = Place of oxidation
Cathode = Place of reduction
Electron flow from Anode to Cathode

 Voltaic/Galvanic Cell –
Spontaneous
volts = positive

∆G = NEGATIVE


Exothermic:    Reactants –> Products   +   Electricity
Anode = place of oxidation
Cathode = Place of reduction
Electron flow from Anode to Cathode

 

Weak Base/ Strong Acid Titration:  –
Titrating unknown concentration of 40.0 ml of Na2CO3 with a total 50 ml of 1.0M HCl
YOU do not have to do this for Homework!
In this titration we are using a salt (ionic compound) Na2CO3 that is basic due to the anion CO3-2 having the ability to accept 2 protons (H+) thus the salt is dibasic.
 
After we titrate the  Na2CO3 (weak base) with HCl (strong acid) please complete the following on the graph.
1. Ka1 and Ka2 of Carbonic acid and hydrogen carbonate.                             
2. Initial concentration of the salt: Na2CO3
3. Verify the initial pH.
4. Verify the final pH.
5. Verify the equivalence points (both of them) with their pH’s!
6. Write the 2 net ion equations.
7. Mark on the graph the percent of all predominant species.
8. Mark on graph where the bubbles occurred.
 

Lecture for the  

Weak Base/ Strong Acid Titration:

  

  I realize that I did not show you how to calculate the pH’s  at the equivalence points in this video but its the same way that we have done in the last 40 titrations that we have done, including last nights homework. 

 


4/5 – Friday –  period 2
I will finish the course today!
 
1. Collect weak base strong acid Titration
    a)Dibasic carbonate – buffer pH of HCO3 – Blood, pool water
 
 
2.  Strong Base (NaOH) – weak Polyprotic Acid Titration (H3PO4)
                                        period 3/4
 
1. Collect weak base strong acid Titration
    a)Dibasic carbonate – buffer pH of HCO3 – Blood, pool water
 
2.  Strong Base (NaOH) – weak Polyprotic Acid Titration (H3PO4)
 
3.  Strength of acids.
 
 
Logger Pro File for the Today’s Titration:
Table 6 PolyP Acid SB Titration good example.cmbl
Download
 
4/5 – Friday – Homework – We will start taking full AP tests in class next week.  
 
1: Complete the H3PO4 on the graph of the titration. Requirements below. Most of you will almost complete this in class.
2:  Complete the Part 2 section of  of the 2017 test.  Pretend that you have just sat for 90 minutes finishing the multiple choice section of the test that you did not have a calculator for.  You will be given a 10 minute break and then Part 2 (Free Response Section Begins). 
 
Part 1: Multiple Choice – 50 % of the test score
Part 2 : Free Response – 50 % of the test score
 
Now you will need to take out lined paper that will be provided for you on test day in either a booklet form or the lined paper to respond will be directly on the test in between each question.  You will not have any space to answer the questions in between the questions.  I have been making space in between these question for you in our AP tests but you need to practice writing your answers on a separate paper.  Make sure you label appropriately!!
 
Example: Question 1:
 
                    a) (i)  Your work
 
                         (ii)  Your silly work
 
Do not make it hard for the AP reader to FIND your answer and do not make it hard for them to Read Your answers (make it legible!!)
 
   a) Please give yourself 1 hour and 45 minutes to complete this section. You will have the use of      
        full reference tables and your graphing calculator!
 
* Your are to omit question 7b (ii) – We will learn this last piece of redox Monday.
 
   b) Once you complete this part 2 section please review to see how many point your earned WITH          the AP Central Key then Review with my Key to understand how complete each question.  I  
        wrote my key days after the AP questions were released last year.
            Question 1:     10 points
            Question 2:     10 points
            Question 3:     10 points
            Question 4:       4 points
            Question 5:       4 points
            Question 6:       4 points
            Question 7:       4 points
 
2017 Free Response Questions:
AP Chemistry 2017 Free Response (part 2 questions).pdf
View Download
AP Central Key and Rubric:  
AP Chemistry 2017 Free Response – College Board key.pdf
View Download
 
Grodski Key:
AP Chemistry Free Response 2017 – Grodski Solutions p.pdf
View Download
 
Write up requirements: 40.0ml of H3PO4 is titrated with 100 ml of 0.10 M NaOH
a) determine the concentration of the Phosphoric acid solution.
b) determine the Ka1 of  H3PO4,  , Ka2 of H2PO4, Ka3 of HPO4-2
c) Write All predominant reaction in different parts of the curve.
        1) Mark the percent that the chemical exists at each equivalent point.
d) Mark the equivalence points and max buffer positions on graph.
e) validate initial and final pH.

 

 

 
 
 

end of week 9!
End of quarter 3.