Hess S Law Lab

Joshua McMahon
IB Chemistry
Matt Chase 3A
11/5/14

Finding the Molar Enthalpy Change of sodium bicarbonate by using Hess Law

Research Question

By using Hess’ Law, can the Molar Enthalpy Change of sodium bicarbonate be calculated?

Hypothesis

If we are attempting to determine the enthalpy change of the thermal decomposition of Sodium Bicarbonate, then Hess’s Law will be will be the most effective.

Introduction

Sodium bicarbonate, more commonly known as baking soda, has many uses in todays world, its main purposes are for cleaning, baking, and medicine. Some of the most common uses for sodium bicarbonate, or baking soda, are relieving ulcer pain, removing splinters, sunburn remedy, deodorant, enhanced sports performance, insect bites, teeth whitener, foot soak, exfoliation, detox bath, and an antacid (Mercola)

In baking, sodium bicarbonate or baking soda is combined with moisture and an acid (like yogurt, chocolate, buttermilk, honey, etc.) and this results in a chemical reaction that produces carbon dioxide, or CO2 , bubbles that start to expand under higher temperatures, like inside an oven, causing baked goods to rise. This process is an almost instant reaction when the ingredients are mixed together. If a Baking recipe calls for baking soda, it needs to be baked immediately, or else they will fall flat!

Exploration

Hess ' Law states that the heat released or absorbed in a chemical process is the same whether the process takes place in one or in several steps. This law is also known as the law of constant heat summation. Molar Enthalpy is the transfer of heat in a reaction of one mol. Heat capacity is an important property and it is used in heat transfer and other calculations.
The equations used in this experiment to determine molar enthalpy change are 1) Molar enthalpy = ΔH/n, ΔH is the change in thermodynamic potential divided by the number of moles; Molar enthalpy is expressed in KJmol. 2) ΔH = -Q, The change in thermodynamic potential, ΔH, is the negative amount of heat transferred, Q; ΔH is expressed in KJ. 3) Q = mcΔT. Where Q is the amount of heat transferred, m is the Mass (g), c is the Specific Heat Capacity of a material (Jg ℃), and ΔT is the change in temperature(℃); Q is expressed in J (EMSB).

Procedures

1. Make sure to have safety garments and gear on, goggles and aprons, before starting the experiment
2. Calculate how much of a 3M HCl solution and water need to be mixed to dilute the acid to a 2M HCl solution
3. Pour 50 ml of the 2M acid into a styrofoam cup
4. Record the temperature of the HCl
5. Place a mass boat on an electronic scale and zero the scale, measure out 8.4 g of solid NaHCO3
6. While recording the temperature of the acid every 30 seconds, slowly pour the NaHCO3 into the acid in the styrofoam cup, avoid overflows or spillage of substances
7. Record the temperature until a maximum or minimum is reached, either 4 consecutive measurements or until the temperature starts to drop (maximum) or rise (minimum)
8. Repeat steps 1-7 four more times to have a total of 5 trials, after every trial completely rinse out the styrofoam cup and the thermometer to remove any left over substances that could affect the experiment and create invalid results causing the experimenter to redo the trial
9. Repeat steps 1-8 for 5.3 g of NaCO3

Raw Data

Mass of NaHCO3
± .01g
Mass of NaCO3
± .01g ml of HCl ± .5ml *
Initial Temp. of HCl ± .5℃
Trial 1
8.40
5.3
50
24.0
Trial 2
8.40
5.3
50
23.8
Trial 3
8.40
5.3
50
23.8
Trial 4
8.40
5.3
50
23.7
Trial 5
8.40
5.3
50
23.8

*assumed that 2M HCl has the same density and specific heat as water, meaning that there is a 1:1 ratio between mass and volume, or 1 ml of HCl = 1 g of HCl

Time ± .01s
Trial 1 NaHCO3
Temp. ± .5℃
Trial 2 NaHCO3
Temp. ± .5℃
Trial 3 NaHCO3
Temp. ± .5℃
Trial 4 NaHCO3
Temp. ± .5℃
Trial 5 NaHCO3
Temp. ± .5℃
30
13.1
24.0
26.3
23.8
25.2
60
25.0
26.2
26.7
24.2
25.9
90
25.0
26.1
27.0
24.5
26.0
120
25.0
26.0
26.9
24.7
26.0
150
25.0
26.3
26.9
24.8
26.0
180
25.0
26.5

24.9
26.0
210
24.9
26.4

25.0

Time ± .01s
Trial 1 NaCO3
Temp. ± .5℃
Trial 2 NaCO3
Temp. ± .5℃
Trial 3 NaCO3
Temp. ± .5℃
Trial 4 NaCO3
Temp. ± .5℃
Trial 5 NaCO3
Temp. ± .5℃
30
13.1
14.0
13.0
13.1
13.1
60
12.5
12.5
12.6
12.6
12.7
90
12.5
12.5
12.6
12.6
12.7
120
12.5
12.5
12.6
12.6
12.7
150
12.5
12.5
12.6
12.6
12.7

Temperature change (ΔT) of HCl + NaHCO3 (Tf-Ti)

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
ΔT ± 1℃
-11.5
-11.3
-11.2
-11.1
-11.1

Temperature change (ΔT) of HCl+ NaCO3 (Tf-Ti)

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
ΔT ± 1℃
+4.5
+4.4
+4.5
+4.2
+4.2

Q of NaHCO3 (rounded to 3 sig figs)

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Q
-2810.00± 270.00 J
-2760.00± 270.00 J
-2740.00± 270.00 J
-2710.00± 270.00 J
-2710.00± 270.00 J

Q of NaCO3 (rounded to 2 sig figs)

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Q
1.0 x 103 ±
2.4 x 102 J
1.0 x 103 ±
2.4 x 102 J
1.0 x 103 ±
2.4 x 102 J
9.7 x 102 ±
2.4 x 102 J
9.7 x 102 ±
2.4 x 102 J

ΔH of NaHCO3 (rounded to 3 sig figs)

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
-Q or ΔH
2.81± 0.27 KJ
2.76± 0.27 KJ
2.74± 0.27 KJ
2.71± 0.27 KJ
2.71± 0.27 KJ

ΔH of NaCO3 (rounded to 2 sig figs)

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
- Q or ΔH
-1.0 x 100 ±
2.4 x 10-1 KJ
-1.0 x 100 ±
2.4 x 10-1 KJ
-1.0 x 100 ±
2.4 x 10-1 KJ
-1.0 x 100 ±
2.4 x 10-1 KJ
-1.0 x 100 ±
2.4 x 10-1 KJ

molar enthalpy of NaHCO3 (rounded to 3 sig figs)

.1 mols

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
ΔH/mol
28.1± 2.7 KJmol
27.6± 2.7 KJmol
27.4± 2.7 KJmol
27.1± 2.7 KJmol
27.1± 2.7 KJmol

Molar enthalpy of NaCO3 (rounded to 2 sig figs)

.05 mols

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
ΔH/mol
2.1 x 101 ±
4.8 x 100 KJmol
2.0 x 101 ±
4.8 x 100 KJmol
2.1 x 101 ±
4.8 x 100 KJmol
1.9 x 101 ±
4.8 x 100 KJmol
1.9 x 101 ±
4.8 x 100 KJmol

Molar enthalpy averages

NaHCO3
NaCO3
Average enthalpies
27.5 ± 2.7 KJmol
2.0 x 101 ± 4.8 x 100 KJmol

Quantitative Data
Qualitative Data
HCl mass
50.0 ± .5g
HCl mass volume
50.0 ± .5ml
NaHCO3 mass
8.40 ± .01g
NaCO3 mass
5.3 ± .1g
NaHCO3 is a fine white powder
NaHCO3 reaction made the cup colder
NaCO3 reaction made the cup warmer
NaHCO3 reaction made the liquid fiz up 3/4 of the cup, and caused lots of liquid to spring upwards (none fell out of the cup)
NaCO3 reaction made the liquid barely fiz

Calculations of trial 1
Q = mcΔT m= 8.40g ± .01 (3 sig figs) c= 4.184
ΔT= 12.5 ± .5 - 24.0 ± .5 = -11.5 ± 1 (3 sig figs)

Q =8.40g ± .01 (4.184) (-11.5 ± 1)

Q= -2810.00± 270.00 J (calculated to 3 sig figs)
ΔH = -Q
ΔH = -(-2810.00± 270.00 J)
ΔH = 2810.00± 270.00 J
ΔH = 2.81± 0.27 KJ

Molar enthalpy = ΔH/n
ΔH = 2.81± 0.27 KJ n= mass/ molar mass n= 8.4/83.97 n=.1 mol
Molar enthalpy = 2.81± 0.27 KJ/.1 mol
Molar enthalpy = 28.1 ± 2.7 KJmol
2NaHCO3 → NaCO3 + H2O +

Percent error
Actual = 85KJmol theoretical= 75KJmol
85-7585x 100= 35.897%

Conclusion and Evaluation

The results were calculated to give an average specific heat of .25± .12Jg℃ for copper, while the actual is .39Jg℃. The experiment gave a calculated average that has a percent error of 35.897%. The only trial with a specific heat close to copper is trial 1 which was only off by .03Jg℃, all other trials were off by .1 to .2Jg℃ off. The data collected during experimentation resulted with trial 2, 3, and 4 being relatively similar in specific heat and trial 1 and 5 to be higher outliers that were actually more accurate than the 3 precise trials. With a percent error so high the experiment had many flaws that affected the results, though those flaws seemed to be less prevalent in trial 1, which had the most accurate result.
Weaknesses
The usage of a different scale during trials 4 and 5
Use of analog measuring devices resulted in high uncertainties
Experimenting on 2 different days, change of classroom environment
Misreading the temperature of the thermometer during the experiment

Improvements

Experiment in 1 day rather than 2
Use the same scale every time
The use of digital tools and not analog, analog creates higher uncertainty
Cool off all tools, try to recreate the conditions as close as possible to trial 1; cooled cup, and cooled thermometer

Strengths

consistent mass of metal through whole experiment the use of some digital equipment

The experiments weaknesses were greater than the strengths, thus leading to a higher percent error. If the experiment were to be redone there are steps that can be taken to make the experiment even more accurate. If the experiment is done with vernier probes instead of alcohol thermometers, the uncertainty would have been greatly reduced. The thermometer had an uncertainty of 1℃ and the temperature changed between 1.7 to 3.2 ℃, this creates an uncertainty of 59% to 31% just for one of the uncertainties. If a vanier probe was used the uncertainty would have been between 12% and 6%, which is about 5 times less. Also the calorimeter was an open system meaning the system was in contact with the surroundings, if a lid is put on top then the calorimeter would be more accurate because it is now a closed system.

The hypothesis is correct, an experimenter can find the specific heat using calorimetry, but not with perfect accuracy. If a few adjustments are made then the experiment could be more accurate.

The knowledge of copper’s specific heat is greatly valued for many reasons, copper is a very commonly used metal, it is in cars, planes, computers, phones, etc. Many cars have copper wiring and have copper in their radiators, and recently with the rising production of electric cars more and more copper is being used in engines as wiring for electrical current. Copper is also used for electric currents is computers, with the knowledge of specific heat, a manufacturer can know how powerful a cooling agent needs to be to make sure the copper wiring does not overheat. Specific heat has allowed for the varied usage of copper and other metals in daily lives, specific heat helps create materials and machines that work more effectively and efficiently.

Bibliography

"Calorimeters and Calorimetry." The Physics Classroom. The Physics Classroom,, n.d.
Web. 08 Oct. 2014.

"Definition of Calorimetry." Chegg. Chegg Inc, n.d. Web. 10 Oct. 2014.

Leon, Nelson De. "Specific Heat." IUN. N.p., n.d. Web. 07 Oct. 2014.

Mycheme. "MyChemE." MyChemE. Wordpress, 31 Aug. 2013. Web. 08 Oct. 2014.

Bibliography: "Calorimeters and Calorimetry." The Physics Classroom. The Physics Classroom,, n.d. Web. 08 Oct. 2014. "Definition of Calorimetry." Chegg. Chegg Inc, n.d. Web. 10 Oct. 2014. Leon, Nelson De. "Specific Heat." IUN. N.p., n.d. Web. 07 Oct. 2014. Mycheme. "MyChemE." MyChemE. Wordpress, 31 Aug. 2013. Web. 08 Oct. 2014.