Carissa Schroll

Physical Chemistry

Expt. 6 Heats of Combustion

Lab #7

Oct. 23, 1997

Theory

This experiment is used for the determination of the heat of combustion of naphthalene, C10H8. The heat of combustion is -D H for the reaction at constant pressure and temperature.

C10H8(s) + 12O2(g) = 10CO2(g) + 4H2O(l)

The energy and enthalpy are independent of the pressure. It is also independent of the path. There is a path which consists of two steps:

1.) This is the change of state which occurs adiabatically to yield the products, but in another temperature.
A(To) +B(To) +S(To) = C(To) + D(T1) + S(T1)
    1. Heat is added to the products of step one to bring them to the initial temperature.
C(T1) + D(T1) + S(T1) = C(T0) + D(T0) + S(T0)

Adding step one and two equals the D H.

D H=D H1 +D H2

The heat for step 1 is zero, and the heat for step 2 is calculated or measured. It is measured by the addition of measurable quantity of heat or electrical energy. Also, calculated by the change in the temperature from the adiabatic step of 1 if the heat capacity of the system is known.

If the system is constant volume process the D H is calculated by using the formula:

D H= D E_+ D (pV)
D H=D E + RTD ngas

However, it is more readily calculated using heat and the equation from step two.

D H2=ò Cp(C + D+ S)dT
D H=-Cp(C + D + S)(T1-T0)

The heat capacity can be determined by running another experiment with knowns. Then the heat capacity can be calculated. For example in this lab the equation was

g(B.A.)· 6318 cal/g + g(Fe)· 1400cal/g = C D T

Then, conduct the run for napthalene. The D H is calculated at a constant volume. With these results the D H at constant pressure can be calculated and compared to the literature values.

The D T can be calculated by plotting the temperature vs. the time. Then with this graph the (dT/dt)f can be calculated. This can be used to obtain a more precise D T by using the equation

D T = (Tf – Ti) -(dT/dt)I(td-ti)- (dT/dt)f(tf – td)

 

 


 
Heats of Combustion 
Trial 1 w/ Benzoic Acid
 
 
 
 
 
 
 
 
Time (min.)
Temp. ( C)
 
 
 
 
 
 
 
0.00
22.54
 
 
 
 
 
 
 
0.25
22.54
 
 
 
 
 
 
 
0.50
22.54
 
 
 
 
 
 
 
0.75
22.54
 
 
 
 
 
 
 
1.00
22.54
 
 
 
 
 
 
 
1.25
22.54
 
 
 
 
 
 
 
1.50
22.54
 
 
 
 
 
 
 
1.75
22.59
 
 
 
 
 
 
 
2.00
22.93
 
 
 
 
 
 
 
2.25
23.43
 
 
 
 
 
 
 
2.50
23.82
 
 
 
 
 
 
 
2.75
24.12
 
 
 
 
 
 
 
3.00
24.37
 
 
 
 
 
 
 
3.00
24.54
 
 
 
 
 
 
 
3.25
24.67
 
 
 
 
 
 
 
3.50
24.77
 
 
 
 
 
 
 
3.75
24.85
 
 
 
 
 
 
 
4.00
24.9
 
 
 
 
 
 
 
4.25
24.95
 
 
 
 
 
 
 
4.50
24.98
 
 
 
 
 
 
 
4.75
25.00
 
 
 
 
 
 
 
5.00
25.02
 
 
 
 
 
 
 
5.25
25.03
 
 
 
 
 
 
 
5.50
25.05
 
 
 
 
 
 
 
5.75
25.05
 
 
 
 
 
 
 
6.00
25.06
 
 
 
 
 
 
 
6.25
25.06
 
 
 
 
 
 
 
6.50
25.06
Time (min.)
Temp. ( C)
 
 
 
 
 
6.75
25.07
8.25
25.06
 
 
 
 
 
7.00
25.07
8.50
25.06
 
 
 
 
 
7.25
25.07
8.75
25.06
 
 
 
 
 
7.50
25.07
9.00
25.06
 
 
 
 
 
7.75
25.07
9.25
25.06
 
 
 
 
 
8.00
25.06
9.50
25.06
 
 
 
 
 
ChartObject Temp vs. Time


Last Updated on 5/6/98
By CARISSA SCHROLL