Upload
omichan13
View
234
Download
1
Embed Size (px)
Citation preview
8/12/2019 pKa Titration IAr
1/26
Investigating the pH of Weak Acid and Strong Base Titration
with Ethanoic Acid and Sodium Hydroxide
IB HL Chemistry Internal AssessmentKevin Omana - Mendoza
April 29, 2013
8/12/2019 pKa Titration IAr
2/26
8/12/2019 pKa Titration IAr
3/26
3
Once I have graphed the points from the titration, I will find the first derivative line in
order to find the equivalence point. That is, the point at which the amount of acid is
equal to amount of base. At this point, a rapid increase in pH occurs because the acid in
the beaker has been neutralized and as such, the excess of base remains and any
additional NaOH will increase the pH once the reaction reaches equivalence point. I will
also calculate the second derivative in order to confirm the values returned by the first
derivative.
In order to find the equivalence point using the first derivative, I will find the peak of the
first derivative, which indicates the greatest slope of the titration curve, where the
equivalence point occurs. This value can be confirmed by checking if the maximum point
on the first derivative curve corresponds to the point at which the second derivative has
a negative value. Once I find the equivalence point, I will divide the volume by two in
order to find the half equivalence point.
For a visual aid in identifying when the equivalence point is reached, I will use the
indicator Phenolphthalein, which has a pH range of 8.3-10.0 and is therefore the most
suitable. The suitability of Phenolphthalein is because the experiment is a weak acid-strong base titration, and as such, due to salt hydrolysis, the equivalence point will be at
a pH greater than 7. The salt in the reaction, CH3COONa, is made up of the weak acid
CH3COO- and the strong base Na+. Thus, the salt resulting from the titration will be
basic, causing the equivalence point pH to be greater than 7.
The Ka and pKa values are found using the expression:
KaH
A HA
moldm-3
8/12/2019 pKa Titration IAr
4/26
4
Which can be adjusted, using the general equation for this reaction, to:
KaH CH3COO
CH3COOH
moldm-3
Here is where the importance of the half equivalence point comes in. At the half
equivalence point, pH is equal to pKa and exactly half of the weak acid will have been
converted into its conjugate base and thus the concentrations of the weak acid and
conjugate base will be the same. In this reaction that would mean
CH3COO= CH3COO
At which point they would cancel out leaving the expression to read:
Ka H At this point, I would take the log of both sides in order to get the equation:
log(Ka) log(H)
This step is important because at the half equivalence point, pKa is equal to pH and the
pH equation is:
pH log(H
)
And thus, if pH=pKa at the half equivalence point, we can substitute pKa in for pH. Doing
this, we would then need to adjust the equation: log(Ka) log(H)
To read:
Ka
8/12/2019 pKa Titration IAr
5/26
5
Which can also be read as :
The literature values for Ka and pKa are2:
Variables:
Independent Variable: The volume of 0.1M NaOH added to a 25 ml solution of 0.1M
CH3COOH
Dependent Variables: The Recorded p
Aspect 2: Controlling the vairables:
Control: Temperature, Pressure and Molarity of NaOH and CH3COOH (0.1M)
2IB HL Chemistry Data Booklet
Ka 4.76
Ka 1.74 10 5
Ka 10 pKa
8/12/2019 pKa Titration IAr
6/26
6
Materials:
Safety goggles NaOH (0.1M) CH3COOH (0.1M) CBL Unit pH probe (0.01pH) Graphing Calculator (Ti-84) 50 cm3 Burette (0.05 cm3) 25 cm3Pipette (0.06 cm3) Phenolphthalein Indicator 1 pH probe holder 1 pipette filler
Magnetic stirrer Stirring magnet Buffer solution
with pH of 4
Buffer solutionwith pH of 7
Buffer solutionwith pH of 10
1 Burette Stand 1 Burette Clamp 4 250mL
beakers
8/12/2019 pKa Titration IAr
7/26
7
Diagram of materials:
8/12/2019 pKa Titration IAr
8/26
8
Aspect 3: Developing a method for data collecting
PROCEDURE
1. Put on safety goggles2. Set up the CBL unit, making sure the pH probe is in the correct channel and the
channel is set up to look for a pH probe.
3. Calibrate the CBL unit using the 4 and 10 pH buffer solutions to createbenchmarks and check calibration using the pH 7 buffer.
4. Condition burette by rinsing with 0.1 M NaOH 3 times, making sure that thereare no air bubbles trapped anywhere in the burette.
5. Fill burette to 50.00 cm3 (0.05cm3) and place securely in the burette clamp onthe stand.
6. Using the pipette filler, fill the pipette to 25.00 cm3 (0.06 cm3) with 0.1 MCH3COOH.
7. Empty Pipette into a clean, 250ml beaker.8. Place beaker underneath the burette on top of the magnetic stirrer unit. (which
at this time should be plugged in)
9. Place Stirring magnet in CH3COOH solution.10.Place the pH probe in the CBL probe stand and Position the pH probe so that it is
inside the beaker of CH3COOH.
11.Add deionized water to the beaker of CH3COOH such that the pH probe is in thesolution but without touching the sides of the beaker or the stirring magnet.
12.Carefully add two drops of phenolphthalein indicator to solution.13.Switch on the magnetic stirrer such that the magnet stirs the solution gently
without splashing.
14.Conduct one quick titration as a benchmark by Opening the burette to allowNaOH to be added to the beaker. When the solution begins to turn pink, slow the
flow of NaOH. Once the solution remains pink for 30 seconds, record the volume
8/12/2019 pKa Titration IAr
9/26
9
of NaoH.
15. Fill burette to 50.00 cm3 (0.05cm3) and place securely in the burette clamp onthe stand.
16.Using the pipette filler, fill the pipette to 25.00 cm3 (0.06 cm3) with 0.1 MCH3COOH.
17.Empty Pipette into a clean, 250ml beaker.18.Place beaker underneath the burette on top of the magnetic stirrer unit.19.Place Stirring magnet in CH3COOH solution.20.Place the pH probe in the CBL probe stand and Position the pH probe so that it is
inside the beaker of CH3COOH.
21.Add deionized water to the beaker of CH3COOH such that the pH probe is in thesolution but without touching the sides of the beaker or the stirring magnet.
22.Carefully add two drops of phenolphthalein indicator to solution.23.Switch on the magnetic stirrer such that the magnet stirs the solution gently
without splashing.
24.Using the CBL unit, record the initial pH of the CH3COOH.25.Begin a slow titration, taking pH measurements every 1.00 cm3 (o.o5 cm3) until
20.00 cm3
(o.o5 cm3
) has been added.
26.At this point begin to take pH measurements ever 0.50 cm3(0.05 cm3) until thesolution turns purple for 30 seconds and for 2.00 (0.05 cm
3) cm
3following. Then
return to taking pH measurements every 1.00 cm3
(0.05 cm3) until 30.00 cm
3
(0.05 cm3) has been added.
27.Repeat steps 15-26 for three trials.Chemical Safety:
-Wear Safety Goggles at all times during the experiment
-NaOH is highly corrosive, and can cause severe burns to areas of skin contact.3
-Ethanoic Acid can cause severe eye damage and potentially cause burns to areas of
contact.4
3http://www.jtbaker.com/msds/englishhtml/s4034.htm
4http://cartwright.chem.ox.ac.uk/hsci/chemicals/ethanoic_acid.html
8/12/2019 pKa Titration IAr
10/26
10
Data Collection and Processing:Aspect 1: Recording Raw Data:
The software on the CBL unit and the graphing calculator allowed for theelectronic recording of data. These data were then imported directly
from the graphing calculator onto the computer using the program
Logger Pro. Hence, the Raw Data tables appear in computer type, with
the exception of qualitative observations, which are handwritten in.
We made the decision not to do an entire curve for the quick titration dueto the fact that the quick titration was done in order to get a benchmark
as to where the equivalence point would occur, not as a trial. Conducting
the quick titration as a trial would skew results because it was done less
carefully that the other three trials. End pH and end Volume are
measured from the point at which the indicator turned pink for 30
seconds.
First and Second Derivative values were added to the raw data table afterthe experiment had been completed. They are present for ease of
understanding and lack of redundancy of data.
CONDITIONS:
Room Temperature (20.4C 0.1C
8/12/2019 pKa Titration IAr
11/26
11
Slow Titration Trial One
Volume of 0.1
M NaOH
added (0.05cm3)
pH (0.01) First Derivative SecondDerivative
0.00 2.65 0.287 -0.037302469
1.00 2.96 0.253 -0.04045216
2.00 3.19 0.212 -0.043143519
3.00 3.41 0.152 -0.033907407
4.00 3.45 0.129 -0.011888889
5.00 3.63 0.144 -0.002666667
6.00 3.77 0.136 -0.004552469
7.00 3.90 0.128 -0.004421296
8.00 4.01 0.131 -0.008256173
9.00 4.18 0.119 -0.017214506
10.00 4.27 0.087 -0.018580247
11.00 4.32 0.079 -0.01654321
12.00 4.45 0.058 -0.014205247
13.00 4.45 0.032 0.004094136
14.00 4.45 0.061 0.025476852
15.00 4.54 0.115 0.022585859
16.00 4.77 0.104 0.017913925
17.00 4.72 0.114 0.036127739
18.00 4.90 0.236 0.006798321
19.00 5.08 0.137 -0.047075429
20.00 5.22 0.074 -0.028891906
20.50 5.17 0.035 0.035599434
CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
8/12/2019 pKa Titration IAr
12/26
12
Volume of 0.1
M NaOH
added (0.05cm3)
pH (0.01) First Derivative SecondDerivative
21.00 5.20 0.133 0.113640285
21.50 5.34 0.194 0.106243901
22.00 5.39 0.251 0.072613536
22.50 5.62 0.257 0.073302469
23.00 5.66 0.255 0.23212963
23.50 5.80 0.410 0.586419753
24.00 6.02 0.769 1.066851852
24.50 6.43 1.44 1.48383157
25.00 7.11 2.63 1.096857363
25.50 9.45 3.06 -0.260353836
26.00 10.81 2.21 -1.334483277
26.50 11.69 1.21 -1.332266468
27.00 12.03 0.504 -0.647237948
28.00 12.30 0.265 -0.238925794
29.00 12.48 0.185 -0.058544974
30.00 12.57 0.219 0.017795194
Slow Titration Trial One (Cont) CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
8/12/2019 pKa Titration IAr
13/26
13
Slow Titration: Trial Two
Volume of 0.1
M NaOH
added (0.05cm3)
pH (0.01) First Derivative Second Derivative
0.00 2.60 0.277 -0.0188595679012
1.00 2.87 0.271 -0.0311520061728
2.00 3.23 0.214 -0.0362893518519
3.00 3.27 0.179 -0.0256921296296
4.00 3.55 0.171 -0.0213341049383
5.00 3.64 0.145 -0.0226296296296
6.00 3.86 0.111 -0.0129398148148
7.00 3.82 0.110 0.00229166666667
8.00 4.04 0.144 -0.00621141975309
9.00 4.19 0.110 -0.0244444444444
10.00 4.27 0.066 -0.0185185185185
11.00 4.27 0.064 -0.00364197530864
12.00 4.40 0.067 0.00316358024691
13.00 4.41 0.075 0.00529475308642
14.00 4.54 0.088 0.00113888888889
15.00 4.63 0.066 0.00823709315376
16.00 4.63 0.087 0.0273279461279
17.00 4.77 0.131 0.0350587355032
18.00 4.9 0.206 -0.00786744428411
19.00 5.07 0.111 -0.0266020230651
20.00 5.08 0.0954 0.000266824327935
20.5 5.26 0.120 0.0445620189995
CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
8/12/2019 pKa Titration IAr
14/26
14
Volume
of 0.1 M NaOH
added (0.05cm
3)
pH (0.01) First Derivative Second Derivative
21.00 5.21 0.155 0.0936204438566
21.50 5.39 0.243 0.0794427542622
22.00 5.49 0.255 0.0327744708995
22.50 5.66 0.248 0.0500823045267
23.00 5.75 0.240 0.187283950617
23.50 5.8 0.408 0.39024691358
24.00 6.12 0.612 0.496697530864
24.50 6.57 0.865 0.645617283951
25.00 6.79 1.37 0.705237654321
25.50 8.06 1.69 0.488727366255
26.00 8.51 1.89 0.14447197027
26.50 9.90 2.04 -0.503833994709
27.00 11.12 1.23 -0.67616872428
28.00 11.89 0.681 -0.456538359788
29.00 12.25 0.384 -0.313806363904
30.00 12.39 0.223 -0.213043062904
Slow Titration Trial two (Cont) CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
8/12/2019 pKa Titration IAr
15/26
15
Slow Titration: Trial Three
Volume of 0.1
M NaOH
added (0.05cm3)
pH (0.01) First Derivative Second Derivative
0.00 2.56 0.288 -0.0345185185185
1.00 2.87 0.258 -0.0360138888889
2.00 3.14 0.205 -0.0308148148148
3.00 3.23 0.189 -0.0197731481481
4.00 3.50 0.188 -0.0258888888889
5.00 3.68 0.132 -0.0280586419753
6.00 3.72 0.111 -0.0141280864198
7.00 3.86 0.124 -0.0153317901235
8.00 4.04 0.083 -0.0183333333333
9.00 4.00 0.066 -0.00422067901235
10.00 4.14 0.080 0.00611882716049
11.00 4.18 0.082 0.0128549382716
12.00 4.27 0.115 0.00907407407407
13.00 4.45 0.116 -0.00533950617284
14.00 4.49 0.103 -0.0195910493827
15.00 4.67 0.072 -0.0291217732884
16.00 4.67 0.025 -0.0145821188071
17.00 4.72 -0.015 0.0473912538079
18.00 4.46 0.139 0.133896103896
19.00 4.90 0.218 0.0422805148996
20.00 5.03 0.197 0.0192196916252
20.50 5.31 0.216 0.0192015563801
CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
8/12/2019 pKa Titration IAr
16/26
16
Volume of 0.1
M NaOH
added (0.05cm3)
pH (0.01)First Derivative Second Derivative
21.00 5.26 0.203 0.0683933715461
21.50 5.44 0.322 0.0491801146384
22.00 5.67 0.290 -0.019608686067
22.50 5.76 0.218 0.0742181069959
23.00 5.80 0.298 0.330956790123
23.50 6.03 0.514 0.682222222222
24.00 6.26 0.889 1.14456790123
24.50 6.61 1.82 1.22917989418
25.00 8.24 2.44 0.634010361552
25.50 9.23 2.62 -0.320664388595
26.00 11.17 2.11 -1.21445537289
26.50 11.67 0.999 -1.23195965608
27.00 11.98 0.492 -0.582060405644
28.00 12.39 0.221 -0.283509920635
29.00 12.39 0.079 -0.148482069371
30.00 12.39 0.022 -0.0878203997648
Slow Titration Trial three (Cont) CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
8/12/2019 pKa Titration IAr
17/26
17
Aspect 2: Processing Raw Data
Overview: Graphing the results from each trial, I took the first and second derivative in
order to find the equivalence point, which occurs at the maximum point of the first
derivative and the point where the second derivative crosses the X-axis and becomes
negative. I then half the X value of the equivalence point to find the half equivalence
point which is equivalent to the pKa value. Furthermore, the Ka value can be calculated
via the pKa value according to the following formula
I will also do analysis of uncertainties in the experiment according to the following data
and equations
Uncertainty values:
Pipette- 0.06 cm3
Burette- 0.05 cm3
pH Probe- 0.01pH
Temperature -0.1C
%Uncertainty =uncertainty
observed valuex100
Ka 10pKa
Ka
8/12/2019 pKa Titration IAr
18/26
18
re%Temperatu+Probe%pH+%Burette+%Pipette=tyUncertain%Total
I will go through sample calculations for trial one, including pKa, Ka, and uncertainty,
although all data will be analyzed and presented.
Trial one, sample calculations
Equivalence point: 25.50 cm30.06 cm
3NaOH
Half equivalence point: 12.75 cm30.05 cm
3NaOH
pH at half equivalence point = 4.48 0.01
Because at the half equivalence point, pH = pKa,
pKa = 4.48 0.01
Ka 104.48
Ka3.31105
Calcuations of error:
Pipette uncertainty: 0.06 cm3/ 25 cm
3(100%) = 0.24%
Burette uncertainty: 0.05 cm3 / 25.50 cm
3(100%) = 0.19%
a 10
pKa
8/12/2019 pKa Titration IAr
19/26
19
pH probe uncertainty: 0.01 / 4.48 (100%) = 0.22%
Temperature uncertainty: 0.1 / 20.4 (100%) = 0.49%
Total % Uncertainty = 0.24% + 0.19% + 0.22% + 0.49%
Total % Uncertainty = 1.14 %
Uncertainty of pKa = 1.14% x 4.48 = 0.05
Uncertainty of Ka = 1.14% x 3.31105= 3.77 x 10
-7
Experimental values of pKa and Ka with uncertainties
Equivalence
Point
0.05 cm3
Half
Equivalence
Point (cm3)
pH at
Equivalence
point
0.01
pH at Half-
Equivalence
Point 0.05
pKa
value
0.05
Ka value
3.77 x 10-7
Trial 1 25.50 cm3
NaOH
12.75 cm3
NaOH
9.45 4.48 4.48 3.31105
Trial 2 26.48 cm3
NaOH
13.24 cm3
NaOH
9.90 4.43 4.43 3.72 x 10-5
Trial 3 25.51 cm3
NaOH
12.75 cm3
NaOH
9.23 4.40 4.40 3.91 x 10-5
8/12/2019 pKa Titration IAr
20/26
20
Aspect 3: Presenting Processed Data
CH3COOH(aq)+NaOH(aq)CH3COONa+H2O
Half Equivilance PointpH= 4.73
Equivilance PointpH = 9.45
Base Hydrolysis
8/12/2019 pKa Titration IAr
21/26
21
CH3COOH(aq)+NaOH(aq)CH3COONa+H2OPH
NaOH
8/12/2019 pKa Titration IAr
22/26
22
CH3COOH(aq)+NaOH(aq)CH3COONa+H2OPH
8/12/2019 pKa Titration IAr
23/26
23
AVERAGE Ka and pKa Values:
Average Ka=Trial1 + Trial 2 + Trial 3
3
Average Ka=3.3110
-5+ 3.7210-5+ 3.9810
-5
3
Average Ka= 3.67 x 10-53.77 x 10
-7
Average pKa=Trial1 + Trial 2 + Trial 3
3
Average pKa=4.48 + 4.43 + 4.40
3
Average pKa= 4.44 0.05
Given Literature Values:
pKa= 4.76
Ka= 1.74 x 10-5
I will calculate the percent error and then compare it to the total percent uncertainty to
establish whether or not any discrepancy was due to random or systematic error.
Percent Error:
% error=|experimental value- literature value|
literature value100
% error Ka=| 3.6710
-5 - 1.7410-5 |
|1.74
10
-5
|
100
% error Ka= 110.00%
% error pKa=| 4.44 - 4.76 |
|4.76|100
8/12/2019 pKa Titration IAr
24/26
24
% error pKa= 6.72
Conclusion and Evaluation
Aspect 1: Concluding
I was able to successfully titrate 0.1 M ethanoic acid and 0.1 M sodium hydroxide. Also,
both the readings from the phenolphthalein indicator and the pH probe show
characteristics of a weak acid-strong base titration. As I explained in my hypothesis, the
salt in the reaction, CH3COONa, which has a stron conjugate base component of CH-
3COO-according to the equation:
CH3COO-(aq)+ H2O(l)CH3COOH(aq)+ OH
-
Thus, the salt resulting from the titration will be basic, causing the equivalence point pH
to be greater than 7. This characteristic was consistently demonstrated. I also was able
to gather data for three titration curves with a starting pH of ethanoic acid around 3.00
0.01 and an ending pH of approximately 130.01. As per my hypothesis, we were able
to use this lab to calculate Ka from pH =pKa, Ka= 10-pka
. We took our hypothesis and
through calculations, we were able to not only find the pKa, but apply it and find the Ka
of all the trials. We had an Average Kaof 3.67 x 10-5
which is 110% off the literature
value of 1.74 x 10-5
and an average pKa value of 4.44 which is 6.72% off of the literature
value of 4.76. The % error can be offset slightly by 1.14% total percent uncertainty
within the experiment. In the titration, we hypothesized that even the smallest value
could have offset the Ka, and as it is proven with our data, the 6.72% resulted in a 110%
difference from the literature value. Where this is especially significant is with the Ka
value, which was the only value derived experimentally. Although the 1.14% uncertainty
does not completely offset the 6.72% error in regards to Ka, it does offer a partial
explanation as to the error is both Ka and pKa. It must be noted that because of the use
of a logarithmic function to calculate Ka from pKa, even a tiny error in pKa can have a
significant effect on Ka. Thus, the more important percent error is in relation to pKa
8/12/2019 pKa Titration IAr
25/26
25
because any error in pKa will have such a significant impact on Ka. Even a discrepancy
of 0.05 can have a significant effect on final results when involved in a logarithmic
function. The rest of the errors were either random or systematic.
Aspect 2: Evaluating Procedure
1. The temperature during our experiment was 20.4C, whereas the temperature atwhich the literature values were calculated was 24.85C, and temperature affects
the Ka and pKa meaning that there would be some discrepancy between our
values and the literature values.
2. We used deionized water during our experiment as opposed to distilled water.Deonized water contains more impurities than distilled water and thus could
have affected our results.
3. To be more ready, we should have taken more measurements, that way therewould be a smaller window for room error. This room for error could be a factor
in to our 110% window of error.
4. Another factor in this was the burette and pipette which could have easilydripped too much or too little of the NaOH into the BaOH, causing a room for
error. More preparation could have been done to avoid this. This effect wouldinfact cause the 110% difference as even one +/- 0.05 of NaOH and BaOH would
affect not only the equivilance point but also affect the base hydrolysis and the
overall pKa and Ka.
5. Our equivalence point volumes did not divide to find the half-equivalence pointsin a manner that would reflect accurate measurements taken. Because data was
only taken ever 1cm3during the times when the half-equivalence points would
occur, the pH values of the half equivalence points are only approximationsbased on the trends in the graph and hence there is a large room for error.
Aspect 3: Improving the Investigation
1. While there is no certain way to account for the temperature discrepancy, itis a factor that should have taken into account when looking at percent
8/12/2019 pKa Titration IAr
26/26
uncertainty, which could explain the discrepancies between experimental
values and literature values.
2. Using distilled instead of deionized water would eliminate or greatly reduceany error associated with the impurities in deionized water.
3.References:
Green, John, and Sadru Damji. "Acids and Bases." Chemistry. Melton: IBID,1998. 218.
IB HL Chemistry Data Booklet http://www.jtbaker.com/msds/englishhtml/s4034.htm http://cartwright.chem.ox.ac.uk/hsci/chemicals/ethanoic_acid.html
http://www.jtbaker.com/msds/englishhtml/s4034.htmhttp://www.jtbaker.com/msds/englishhtml/s4034.htmhttp://www.jtbaker.com/msds/englishhtml/s4034.htm