The Heat of Combustion of an Organic Compound PDF

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Required Lab for Physical Chemistry at FIU. Complete paper graded with an A...

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The Heat of Combustion of an Organic Compound Michael J. Blanco

Abstract Introduction A calorimeter involves a reaction vessel in a bucket of water that is then placed inside an insulated jacket. The temperature is measured with a thermometer and a motor driven propeller mixes the water. The propeller removes the temperature gradient in the bucket of water. Finally, a current is passed through a fuse wire to initiated combustion. The heat evolved from the combustion of the sample caused an increase in the temperature of the water in the surrounding. A constant-volume (isochoric) bomb calorimeter can measure the heat, which is equivalent to the change in internal energy. The change in temperature times the heat capacity of the calorimeter can be used to measure the heat of the system.

q=−C B ∆ T

The internal energy of the combustion can be known if the heat of the system and the mass of the combusted compound is given. The molar internal energy and molar enthalpy of combustion is related by the following equation: ∆ H comb =∆ U comb +(∆ n g) RT Using simple mathematic from the change in temperature from the combustion and the heat capacity of the calorimeter then it is easy to calculate the internal energy of the combustion and enthalpy of combustion of the reaction. Procedure

Refer to Procedure on page 20-21 Results Benzoic acid Pallet (g) Wire (g) Pallet+ Wire (g) Wire after combustion

Trial 1 1.001 g 0.029 1.03 0.025

Trial 2 1.004 g 0.029 1.029 0.024

Trial 3 1.000 g 0.029 1.029 0.023

Time

Temperature (

Temperature (

Temperature ( ° ∁ ¿

0 :30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00 6:30

°∁ ¿

°∁ ¿

24.70 24.62 24.55 24.55 24.90 25.84 26.94 27.02 27.08 27.10 27.11 27.11

22.45 22.45 22.45 22.69 23.35 24.01 24.45 24.70 24.86 24.92 24.99 25.03 25.04 25.04

24.59 24.43 24.41 24.41 24.79 25.62 26.27 26.58 26.70 26.79 26.82 26.83 26.84 26.84

Time vs. Temperaure of Benzoic Acid 27

Temperature (C)

26 25 Trial 1 Trial 2 Trial 3

24 23 22 21 20 0

50

100

150

200

250

300

350

400

450

Time (s)

Example (Trial 1) calculations for CB: Benzoic Acid q=m C H 6

5

COOH

∆ U °comb( C 6 H 5 COOH ) +m w ∆ U °comb (w)

J J q=(1.001 g )(−26410 )+( .004 g )(−5850 ) g g q=¿ -26459.8 J q=−C B ∆ T −q =C B ∆T

Benzoic Acid Trial 1 q (J ) -26459.8 2.56 ∆T (

℃¿ CB

10,335.86

Trial 2 -26544.9 2.59

Trial 3 -26445.1 2.43

Average

Standard Deviation

10,248.99

10,882.76

10,489.20

343.59

(

J ¿ ℃ −(−26459.8 J ) (27.11 −24.55)℃

C B=¿ 10,335.86

J ℃ Average ( Mean)=x=

∑ xi i

n

=10,489,20

J ℃

√

Standard Deviation=S N =

N

1 (x i−x )2 =343.59 ∑ N−1 i=1

Confidence Interval at 95% Degree of Freedom = 2 *T95 C.I = 4.30 95 %C . I .=x ±

(4.30)(343.59 ) tS =10,489.20± =10,489.20 ±853.00 √n √3

Naphthalene Trial 1

Trial 2

Trial 3

Pallet (g)

1.001

1.002

1.000

Wire (g)

.020

.020

.020

1.021

1.020

1.020

.016

.017

.015

Pallet+ Wire (g) Wire after combustion

Time

Temperature (

°∁ ¿

Temperature (

°∁ ¿

Temperature (

°∁ ¿

0 :30 1:00

21.80 21.90 22.60

23.39 23.39 24.51

23.17 23.19 23.20

1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00 6:30

22.90 23.12 24.05 24.56 24.79 24.93 25.00 25.04 25.04

25.87 26.91 27.68 27.97 28.12 28.19 28.24 28.24

23.30 24.18 25.23 26.09 26.54 26.80 26.92 26.99 27.02 27.04 27.04

Temperature (C)

Time vs Temperature of Naphthalene 29 28 27 26 25 24 23 22 21 20

Trial 1 Trial 2 Trial 3

0

50

100

150

200

250

300

350

400

450

Time (s)

Example (Trial 1) Calculation for CB: Naphthalene q=−C B ∆ T q=−( 10,489.20) ( 3.24)=−33985.008 J q=m C

10

H8

∆ U °comb( C 10 H 8 ) +m w ∆ U °comb (w)

∆ U °comb ( C10 H 8 )=

q−m w ∆ U °comb ( w ) mC H 10

∆ U °comb ( C10 H 8 ) =

8

−33985.008 – (.004 )(−5850 ) 1.001

∆ U °comb ( C 10 H 8 )=¿ -33927.68

−33927.68

J g

kJ g J x 10−3 kJ=−4348.53 x 128.1705 mol mol g

Combustion Reaction C10H8(s) + 12O2(g)  10CO2(g) + 4H2O(l) ∆ H comb =∆ U comb + ∆ ng RT

(

∆ H comb =−4348.53 + ( −2 ) .00831

)

kJ kJ ( 21.80 + 273.15 )=−4353.62 mol − K mol

∆ H form =[ 10 ∆ H form ( CO2 )+4 ∆ H form ( H 2 O ) ]−[∆ H form ( C 10 H 8) −12 ∆ H form ( O 2) ] ∆ H form ( C 10 H 8 )=10 ∆ H form

(

) (

)(

)

∆ H form( C 10 H 8) =10 −393.5 kJ + 4 −258.5 kJ − −4343.625681 kJ =615.57 kJ mol mol mol mol

Confidence Interval at 95% Degree of Freedom = 2 Naphthalene (experimental results) Trial 1 Trial 2 Trial 3 -33985.008 -50872.62 -40593.204 q (J) 3.24 4.85 3.87 Average ∆ T (°C) -33927.68 -50753.56 -40563.954 ∆ U comb (J/g) -41782.23 -6505.11 -5199.10 -5355.25 −4348.53 ∆ U comb (kJ/mol) -6510.11 -5204.00 -5355.91 −4353.62 ∆ H comb (kJ/mol) 1541.01 235.00 615.37 797.13 ∆ H form (kJ/mol) *T95 C.I = 4.30 ∆ H comb (kJ/mol) (4.30 ) ( 1086.24 ) tS 95 % C . I .=x ± =−5355.91± =−5355.9 ± 2696.71 √n √3

Std. dev. 8529.20 1093.19 1086.24 671.71

∆ H form (kJ/mol) ( 4.30 ) (671.71 ) tS 95 %C . I .=x ± =797.13± =797.13 ± 1667.59 √n √3 Relative error = |value measured – actual value| / actual value ∆ H

comb

(kJ/mol) = -5355.91– (-5150.09)| / -5150.09 = 0.040 or 4.00 %

error ∆ H form (kJ/mol) = |797.13– (78.53)| / 78.53 = 9.15 or 915% error

Discussion The purpose of the experiment was to determine the standard of combustion and formation of naphthalene using a bomb calorimeter. The heat capacity of the calorimeter was determined by combusting benzoic acid, since its internal energy of combustion is known.

The heat capacity of the calorimeter was determined to be

10,489.20 ± 853.00

at a 95% confidence limit. The heat capacity of the calorimeter is used to calculate the standard of combustion and formation of naphthalene. The calculated standard of combustion and formation of naphthalene −5355.9 ± 2696.71 and 797.13 ±1667.59 at a 95% confidence limit, respectfully. The high confidence limit could have resulted from having done only three trials. Additional trials would lead to a higher precision and higher confidence at the 95% between trials....