Ramy Chaoul
BOMB CALORIMETRY: DETERMINATION OF THE HEAT OF COMBUSTION AND THE STANDARD ENTHALPY OF FORMATION OF GLUCOSE
Course: Chem 220 Name: Ramy Chaoul Instructor: Dr. Samar Sadek Group (C)
Date: 25/02/2013 1
Ramy Chaoul
Abstract: The purpose of this experiment is to measure the heat of combustion of glucose using a constant –volume bomb calorimeter; where heat causes an increase or rise of the temperature by the calorimeter is which is evaluated and used to calculate the c hange of energy during combustion. Thus we can use this data to and apply thermodynamic relations to calculate the enthalpy of combustion and the enthalpy of formation of glucose. To dete rmine the specific heat capacity (Cv) of the bomb calorimeter we standardize it by the use of benzoic acid pellets of which the heat capacity (Cv Benzoic acid) is known to and thus enables the dete rmination of the calorimeter heat capacity. We also use the benzoic acid for spiking and triggering glucose combustion. After calibration, we add a cert ain defined mass of benzoic acid to glucose, and thus glucose combustion is studied. Two sources of heat that cause a rise in temperature are considered as an error to be corrected; the first is the fuse combustion while the second is due to the presence of N2 in the bomb calorimeter. We determine the increase in temperature due to the fuse combustion by weighing the fuse before and after each run; while the nitric oxide in the calorimeter is converted to nitric acid when it reacts with water, and thus to correct for N2 present as part of entrapped air, a titration with standardized base of sodium carbonate is o
conducted. Our obtained values for ΔHcombustion and ΔH
formation
are -3855.854 KJ /mol and -
respectively, with % errors of (13.45%) and (29.79%)respectively. 609.1805 KJ/m ol
.
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Ramy Chaoul (2)
Introduction :
(1)
f ig 1: bomb calor imeter
During the combustion of glucose the heat transfer is monitored and measured using a constant volume bomb calorimeter. Each material has a specific enthalpy of combustion, which is the enthalpy change accompanying a complete oxidation of the material to form (1) H2O and CO2 .The combustion reaction is: C6H12O6 + 6O2→ 6CO2+ 6H2O An enthalpy change that accompanies an isothermal process is not measured directly (i.e., the change of state is not carried out isothermally). Instead the process is carried out (2) adiabatically, using a path composed of 2 steps :
To start, we measure the experimental process of enthalp y change isothermally by the formula: R 1(T0)+R 2(T0)+S(T0) P1(T0)+P2(T0)+S(T0) Where
Uisothermal= UT0
“Where S refers to all parts of the system that is in contact with the reaction medium other than the reactants and products by that we mean that S refers to the stirrer, the calorimeter walls, the water bucket ...etc” The determination step is not carried in a single isothermal step because the fall and rise of heat causes it variation, thus the determination is carried adiabatically by 2 steps, where U is a state function that is independent of the used path: 3
Ramy Chaoul
(1) Adiabatic Process: (from T0 to T1) R 1(T0) +R 2 (T0) + S (T0) →P1(T1)+P2(T1)+S (T1) Ucal (2) Imaginary Process back to the initial temperature(from T1 to T0) P1(T1)+P2(T1)+S(T1) with
But
Ucal =
P1(T0)+P2(T0)+S(T0)
Uimag
Uisothermal = Ucal + Uimag
qv + W = 0
where we have a constant volume: ∆V =0 then W=p∆V = 0 where the heat flow in an adiabatic process qv is also = 0. 0
Then,
Uisothermal
= Uimag = qv + w = Cv ∆T= Cv (T0- T1) = -Cv (T1-T0) where Cv = Cv (S) + Cv (H2O) + Cv (CO2)
To determine the heat capacity Cv (of the whole bomb), standardization is applied, where the known heat of combustion of benzoic acid is used in the standardization reaction. To calculate Cv, ∆T (the calorimeter measures the temperature rise), the following equation is used: m benzoic acid U benzoic acid + mfuse Ufuse + meq U N2= -Cv (S) T with: mfuse = mass of fuse combusted (g) meq = milliequivalence = mmol of N2 present at equivalence (mmol)= N(base) x V(base at eq.) Ufuse =
-1400.0 cal/g
U benzoic acid = T
U N2 = -14.1
cal/meq
-6318.0 cal/g
= temperature rise measured by the calorimeter (°C)
Since the number of moles produced from the combustion of standard material of carbon dioxide and water is different from the number of moles produced upon the combustion of the material under study, we will encounter an error that we will consider as a minimal error, because of the very small mass of the sample used which is £ 1g, compared with the mass of water in the bucket which is 2000 g, where the masses of the bomb and bucket > 500 g. CV can thus be approximated as being equal to CV (S), and thus the equation becomes: Uisothermal = -Cv (S) (T1-T0) Ideally the situation would be when a change in temperature is solely due to a change in the energy of combustion of glucose. However, the ideal situation is not present in this experiment. A correction is needed for these errors. 4
Ramy Chaoul
The first correction we account for is the change in energy due to the combustion of the fuse used, where two measures of the fuse are conducted one before and one after a calorimeter run in order for us to calculate the mass of the burned wire. The heat correction for fuse combustion is given as – 1400 cal/g. The second correction is for the air that enters the bomb and causes a temperature rise, where the N2 present in air is converted to nitric oxide upon reacting with oxygen then into nitric acid upon reacting with the water inside the bomb. We calculate the concentration of nitric oxide by a base titration of the nitric acid HNO3 with standard sodium carbonate NaHCO3 (0.0709 M) and thus we will be able to know the amount of nitrogen gas trapped in the bomb. Methyl orange is used as an indicator. The heat released from this reaction of nitrogen and ox ygen is – 14.1 cal/meq of HNO3. (1)
The third and final correction is for the calculated standard enthalpy of formation from the enthalpy of the combustion reaction, where we neglect the pressure effect on the values of energy and enthalpy. In reality, the effect of pressure changing on the energy and enthalpy is very small relative to the accompanied chemical changes. Thus the equation for glucose becomes: msugar Usugar + m BA ∆UBA + mfuse Ufuse + meq U(N2) = -Cv (s) T
where: mBA = mass of benzoic acid used as spiking material (g) mfuse = mass of fuse combusted (g) meq = milliequivalence = mmol of N2 present at equivalence (mmol) Ufuse =
-1400.0 cal/g
U N2 = -14.1
cal/meq
U benzoic acid = T
-6318.0 cal/g
= the measured temperature rise by the calorimeter (°C)
After we determine Uglucose, its enthalpy of combustion is calculated using th e following thermodynamic relation: ΔHglucose = ΔUglucose + Δ (PV) ΔHglucose = ΔUglucose + ΔngasRT with: ngas= change in number of moles of gases (mol) = Σn Σnreactants products R = gas constant 8.314 J/mol.K
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T = T0 = temperature of the adiabatic process (°C) 0
The standard enthalpy of formation of glucose ΔH f is determined using: ∆H reaction = Σ∆H formation of products - Σ∆H formation of reactants
0
ΔH glucose = 6ΔH
f (CO2) +
0
6 ΔH
0 f (O2) - ΔH f (glucose)
Experimental Plan: 1- The calorimeter should be warmed for 20 min until the jacket temperature is constant and stabilized at 35 °C. 2- Cut a 10 cm of Ni alloy wire and weigh it on e ach run, the wire must be twisted it in a bell-shaped loop with no sharp edges to avoid the cutting of the wire, so that we will attach the loop to the openings at the end of the grip electrodes of the bomb. 3- Weigh a benzoic acid pellet to know its mass, and then transfer it to the sample capsule on the electrode loop, where it is fixed in position using the Ni alloy fuse wire. 4- Add 1 mL of distilled water to the bomb, where it will act as an absorbing agent and thus absorb nitric oxide entrapped in the air and convert it to nitric acid. The sealing ring must be moistened. 5- Use the pressurized oxygen gas to fill the bomb with oxygen at a pressure of 30 atm that should be maintained, release the valve two co nsecutive times to expel unwanted N2 entrapped in the air. 0
6- Fill the bucket with 2000 grams of water (at constant temperature 30 C) and then place the bucket in the calorimeter so that the bomb calorimeter will be placed in the bucket. 7- Using the specific clamp, lower t he bomb and place it in the bucket after attaching the two ignition wires into the sockets on the bomb’s head and make sure of the complete
immersion of the bomb into the bucket. 8- Now we close the calorimeter and press “shift start” then enter “sample ID” # followed by the “weight” of the sample under study.
9- At the process end we press “DONE” bottom to store the results, where we open the cover to disconnect, and remove t he bomb be using the clamp once again.
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10- More water is added to the 1 ml already present in order to wash the bomb where the washings are titrated with 0.0709 N S odium carbonate for acid correction. 11- The procedure is repeated twice for benzoic acid (standardization), and another two times for glucose (determination), and finally a last time for the unknown sugar determination.
Data: 7
ID number
8
Mass benzoic acid pellet ±0.0001g
1.0125
1.0119
Mass 10 cm wire
±0.0001g
0.0169
0.01603
Mass remaining fuse
±0.0001g
0.0034
0.0031
Volume Na2CO3
3.80
±0.02 mL
5.10
Initial Temperature
±0.00001°C
27.7049
27.1285
ΔT temperature rise
±0.00001°C
2.65477
2.65087
Tabl e 1: Standardi zati on by combustion of Benzoic Aci d pel lets (sampl e 7 and 8) ID number
9
10
Mass benzoic acid
±0.0001g
0.2946
0. 2985
Mass sugar in pellet
±0.0001g
0.6940
0.6965
Mass pellet
±0.0001g
0.9696
0.9874
Mass 10cm wire
±0.0001g
0.0165
0.0165
Mass remaining fuse
±0.0001g
0.0029
0.0039
Volume Na2CO3
±0.02 mL
1.90
4.40
27.3024
28.7004
1.79823
1.84897
Initial Temperature
±0.00001°C
ΔT temperature rise
±0.00001 C
o
Tabl e 2: deter mi nati on by combusti on of B enzoi c Acid i n M i xed pel l ets (sampl e 9and 10)
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ID number Mass benzoic acid
0.2957
±0.0001g
Mass unknown powder
0.6979
±0.0001g
Mass pellet
±0.0001g
0.9656
Mass 10cm wire
±0.0001g
0.0162
Mass remaining fuse
±0.0001g
0.0014
Volume Na2CO3
±0.02 mL
2.00
Initial Temperature
±0.00001°C
ΔT temperature rise
±0.00001 C
28.2060
o
1.78447
Tabl e 3: combustion of unk nown powder mi xed wi th benzoic acid (sample11)
N.B.
Instruments Burette Analytical balance Bomb calorimeter
Uncertainties ±0.02ml ±0.0001 g ±0.00001 0C
Table 4: uncert ainty
Calculation: CV of the calor imeter
() ()
m fuse = m fuse before firing – m fuse after firing. m eq (milliequivalence) = NHNO3 × VHNO3 = N Na2CO3 × V Na2CO3 = number of millimoles of HNO3 at equilibrium
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given:
∆U benzoic acid = -6318.0 cal/g ∆U wire = - 1400.0 cal/g ∆U (N2) = -14.1 cal/meq “ (reference 2)
Sample #7 ( standardization ):
Cv = (-1/ΔT) (mΔU benzoic acid+mfuseΔUfuse+meqΔU(N2)) = (-1/2.65477) ((0.9985g*-6318cal/g) + (0.0135g*-1400cal/g)+(3.80*0.0709N*-14.1) 0
Then, C v= 2421.20 cal/ C
Sample # 8 :
Cv = (-1/ΔT) (mΔU benzoic acid+mfuseΔUfuse+meqΔU(N2)) = (-1/2.65087) ((0.9941*-6318cal/g) + (0.0129g*-1400cal/g)+(5.10*0.0709N*-14.1) 0
Then, C v= 2436.26 cal/ C →Average Cv : Cvavg= (Cv71+ Cv74 )/2= 2428.73 cal/0C
Determining the ∆U combustion of gl ucose: ∆Uglucose = [ - Cv∆T – (mBA ∆UBA + m burned fuse ∆U fuse + meq ∆U(N2) ] / m glucose
Sample #9(determ inati on): Mass of glucose=0.6940g Mass of benzoic acid=0.2946g Mass of the pellet= 0.9696 g 9
Ramy Chaoul
%glucose = (0.6940/(0.6940+0.2946))*100=70.20% %benzoic acid = (0.2946/(0.2946+0.6940))*100=29.80% Then, the mass of benzoic acid in the pellet is: m b= m pellet* percentage of benzoic acid= 0.9696*29.80%=0.2889g And the mass of glucose in the pellet is: mg= m pellet* percentage of glucose= 0.9696*70.20%= 0.6807 g using the equation: ΔUglucose=(1/ mglucose) (-CvΔT - mΔU benzoic acid-mfuseΔUfuse-meqΔU(N2) ) =(1/0.6807)(2422.905*1.79823 - 0.2889*-6318 – 0.0136*-1400 – 1.90*0.0709*14.1)= -3694.035cal/g
→ ΔU glucose ( f or sample 9 ) = -3694.035cal/g Total mass ± 0.0001 g
0.9696
% glucose ± 0.8 %
70.20%
% benzoic acid ± 1%
29.80%
M glucose
0.6807 g
M benzoic acid
0.2889g
∆U glucose
-3694.035cal/g
Tabl e 5: sampl e 9
Then using the formula: ΔH=ΔU+Δ (pV)= ΔU+ RTΔn but, Δn= 0 so ΔHcombustion=ΔUglucose Thus ΔHcombustion=ΔUglucose= -3694.035cal/g Then, conversion of the unit of the combustion into joule units: ΔHm=ΔH ×M ×4.184= -3694.035×180.16 ×4.184= -2781.8626KJ/mol Moreover, we can calculate the standard enthalpy of combustion of glucose, the used formula:
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Ramy Chaoul 0
ΔHcombustion= ∑(product)v Hf - ∑(reactants) v H i
0
Where v is the stochiometric coefficient of the reaction And using the following given informations: 0
Δ Hf ( H2O)=-285.83KJ/mol 0
Δ Hf (CO2)= -393.509KJ/mol 0
0
0
0
Thus, ΔHf (glucose)=6 Δ H f ( H2O)+ 6 Δ Hf (CO2)- 6 Δ Hf (O2)- ΔHcombustion = -648.394 KJ/mol. Sample #10:
Doing the same calculations for sample 10 we get: Total mass ± 0.0001 g
0.9487
% glucose ± 0.8 %
70.00%
% benzoic acid ± 1%
30.00%
M glucose
0.6641 g
M benzoic acid
0.2846g
∆U glucose
-4017.6396cal/g
Tabl e 6: sample 10
ΔUglucose=(1/ mglucose) (-CvΔT - mΔU benzoic acid-mfuseΔUfuse-meqΔU(N2) ) =(1/0.6641)(2422.905*1.84897- 0.2846*-6318 – 0.0129*-1400 – 4.40*0.0709*14.1)=-4017.6396cal/g
→ ΔU glucose ( for sample 10) = -4017.6396cal/g
Then, ΔHcombustion=ΔUglucose=-4017.6396cal/g ΔHm(76)= ΔH ×M ×4.184= -4017.6396cal/g ×180.16 ×4.184= -3028.454 KJ/mol Next, the standard enthalpy of combustion of glucose is calculated: 11
Ramy Chaoul 0
0
0
0
ΔHf (glucose)=6 Δ Hf ( H2O)+ 6 Δ Hf (CO2)- 6 Δ Hf (O2)- ΔHcombustion -568.667 KJ/mol
=
0
: The average value of ΔH m and ΔH f of glu cose The average value of enthalpy of combustion: ΔH m = -3855.854 KJ /mol The average value of standard enthalpy of combustion of glucose: 0
ΔH f = -609.1805 KJ /mol.
Sample #11 (un known): Mass of the unknown=0.6979g Mass of benzoic acid= 0.2957 g Mass of pellet= 0.9656 g % unknown = (0.6979/ (0.6979+0.2957))*100=70.24% %benzoic acid = (0.2957/ (0.6979+0.2957))*100=29.76%
→mass of benzoic acid in the pellet = 0.9656×29.76%= 0.2873g →mass of unknown in the pellet= 0.6782g ΔUunknown =(1/ munknown ) (-CvΔT - mΔU benzoic acid-mfuseΔUfuse-meqΔU(N2) ) = -3656.628 cal/g → ΔU unkown ( f or u nkn own sample) = -3656.628 cal/g Total mass ± 0.0001 g
0.9656
% glucose ± 0.8 %
70.24%
% benzoic acid ± 1%
29.76%
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M glucose
0.6782 g
M benzoic acid
0.2873 g
∆U glucose
-3656.628cal/g
Table 7:sample 11 (un known )
Discussion: Compar ison to literatur e value A constant volume oxygen bomb calorimeter was used for the determinantion of the enthalpy of combustion of glucose of an unknown sugar.
Function
| | | |
Experimental
Theoretical
[1]
% error
ΔHcombustion
-3855.854
-3508.00
13.45%
ΔHoformation
-609.1805
-1068.04
29.79%
Tabl e 8: exper im ental and theoretical valu es
There are errors obtained in our result due to many factors: The titration of nitric acid where the added volume of Na2CO3 could hold an error in addition from the burette and an error in reading the value obtained, where the change in color of the methyl orange as indicator from orange to yellow is hard to be noticed directly which might contribute to an excess addition of Na2CO3. 2. Errors due to the uncertainty of instruments used like the analytical balance (±0.0001 0 g) and thermostat uncertainty ((±0.00001 C), where we might also encounter reading errors. 3. The fact that we neglect the pressure effect on the values of energy and enthalpy, where in reality the effect of pressure changing on the energy and enthalpy is very 1.
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small relative to the accompanied chemical changes, which causes a certain (2) error . 4. Another value neglected was the contribution of Water and carbon dioxide on the heat of combustion because we used the same Cv for calculations for benzoic acid and for glucose and the unknown sugar. 5. Still there would be certain errors encountered that would affect our readings and values. 6. Other problems may occur in transferring material and various substance from a place to another or from a vessel to another which may lead to some loss of our substance.
Aerobic and Anaerobic Respiration:(3) “Aerobic respiration is the process in which glucose is co nverted into CO2 and H2O in (3)” the presence of oxygen, releasing large amounts of ATP . The process of an aerobic respiration is represented in this equation: Glucose + Oxygen →Energy + Carbon dioxide + Water During the process of aerobic respiration 38 molecules of ATP are produced for every (3) molecule of glucose that is utilized . “Anaerobic respiration is the cellular respiration which takes place in the absence of oxygen. The process of anaerobic respiration is relatively less energy yielding as compared to the aerobic respiration process.” The process of anaerobic respiration for production of energy can occur in either of the ways represented below : Glucose →Energy (ATP) + Ethanol + Carbon dioxide Glucose →Energy (ATP) + Lactic acid The enthalpy of the combustion of glucose is -2808.00KJ/mol via aerobic respiration while the enthalpy of combustion of anaerobic fermentation is -1344.00KJ/mol then the total will be 2688.00KJ/mol. The difference between the aerobic respiration and anaerobic fermentation is around 120KJ/mol.
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Calcul ation of the str ain energy of cyclopr opane : Strain energy is the energy associated with ever y compound, corresponding to the bending or stretching of bond from their normal state as a result of geometric (4)” requirement. “
The standard enthalpy difference of the dissociation of cyclopropane is only as a function of strain energy S. The standard enthalpy of dissociation ΔHdis is the sum of the bond 3 energies B. B values are positive and for stable compounds, ΔHdis is positive . ΔHdis= ∑Bi-S+R; Bi is the bond energy, S strain energy, R is resonance energy, result from aromatic character. “Cyclohexane carboxylate has a lower strain energy than n-butyl cyclopropane because 0 0 the angle of cyclohexane is around 109 while n-butylcyclopropane is approximately 60 . Bomb calorimetry experiment is done for the two compounds so ΔHcombustion will be measured and the strain energy can be deduced.” In this case the strain energy of the (4) cyclopropane is 112.887KJ/mol . O
O
OCH3
OCH2CH2CH2CH2
cyclopropanecarboxylic acid
cyclohexanecarboxylic acid
Resonance energy of benzene: The resonance energy of a compound is a measure of the extra stability of the conjugated (5) system compared to the corresponding number of isolated double bonds .
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In order to identify the resonance energy of benzene a comparaison must be done between its enthalpy of combustion and that of its conjugated system. Benzene has two resonance structures. its stabilization energy(resonance energy) is determined by the bomb calorimetry. we consider the energy change for the following reaction as the resonance energy: B * (g) → B (g)
[5]
B* represents the Kekule form of benzene (cyclohexatriene, not resonance stabilized) and B represents the actual benzene (resonance stabilized). The combustion of Kekule benzene and actual benzene, the difference in the combustion energies corresponds to the resonance energy: B *( g )
∆U resonance = ∆U comb
B ( g )
[5]
- ∆U comb
We need to combust a molecule that has similar structure as a Kekule benzene structure (i.e. something with 3 C=C, 3 C-C, and 6 C-H bonds), in condition that the molecule does not have any contributions to its internal energy (such as steric strain). We use a combination of molecules that are combined equal to a Kekule benzene structure (there is no one molecules equal to Kekule structure): TTCC
CHX
3 C=C
B* 0 C=C
9 C – C
6 C – C
18 C – H
12 C – H
3 C=C 3 C – C 6 C – H
where: TTCC = 1,5,9-trans,trans,cis-cyclododecatriene and CHX = cyclohexane, both strainfree. Accordingly,
B* ( g )
∆U comb
= ∆U comb
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T TCC ( g )
- ∆U comb
C HX ( g )
Ramy Chaoul
The enthalpy:
∆ H =∆U +∆ ( PV ) = ∆U + RT ∆ngas
the reaction of combustion of benzene: C6H6 (g) +15/2O2 (g) 6 CO2 (g) + 3 H2O (g) that ∆n gas= (9-17/2) = 1/2. Substituting ∆U =∆ H - RT ∆n gas into B *( g )
∆U resonance = (∆ H comb
B ( g )
- 1/2RT)- (∆ H comb
B *( g )
∆U resonance = ∆ H comb
B *( g )
where: ∆ H comb
= ∆ H comb
T TCC ( g )
T TCC ( g )
∆U resonance = ∆ H comb
- 1/2RT)
B ( g )
- ∆ H comb
C HX ( g ) [3]
- ∆ H comb
- ∆ H comb
C HX ( g )
B ( g )
- ∆ H comb
This equation gives the resonance energy of Benzene
(3)
.
Conclusion: To accomplish this experiment, we used a constant volume bomb calorimeter, to measure the heat of combustion of glucose. In order to calculate the energy change associated with the combustion of glucose we observe and calculate the temperature change. Thus we will be able to calculate and determine the enthalpy of combustion and the enthalpy of formation of glucose using some thermodynamic functions. We standardize the calorimeter two times by the use of a benzoic acid of known heat of combustion, and thus we can obtain Cv of the bomb. We apply the necessary calculations to determine the relative enthalpies of glucose and of the unknown sugar (ID#78). Our results contain an error which is 13.45 % and 29.79% for enthalpy of combustion and formation relatively. This error is due to many factors explained above.
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References: 1- Bomb Calorimetry Handout.Dr. Halaoui (spring 2013)
2- Wikipedia: http://en.wikipedia.org/wiki/Benzene
3- http://www.chemistryguide.org/
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