Using a Parr Bomb to Measure Enthalpy

I. Introduction

The term enthalpy is used to describe the thermodynamic potential of a system. Heat of reaction, one form of enthalpy, is the energy given off by a substance upon combustion. It can be measured using a Parr bomb, which is an adiabatic container, held at a constant volume that ignites a substance. In this case, the heat of reaction is equal to the enthalpy because the reaction takes place at a constant volume and the change in pressure is negligible. The heat of reaction is also equal to the internal energy because of the constant pressure.

The substance to be measured is placed in the bomb, and the bomb is filled with oxygen and then sealed. The bomb is placed in a vessel of circulating water. The substance is ignited through wires touching it which are connected to a fuse box. The energy given off from combustion can then be measured from the changes in temperature of the water surrounding the bomb using a thermistor. The thermistor can be linked to a computer and the temperate as a function of time can be monitored and recorded. A substance of known enthalpy, or heat of reaction, is first used to create a constant for mass times C_p, so that a substance of unknown enthalpy can then be calculated from that information. The change in mass from each experimental run is negligible, making this method valid. The thermistor itself must also first be calibrated, by measuring a constant temperature bath and then changes in the temperature bath as a function of resistance.

II. Method

First, water that was a slightly above room temperature was placed in a vessel. The vessel was placed into the calorimeter and allowed to cool to room temperature. This change in temperature was measured and recorded with a thermistor and graphed on the computer as a function of resistance. This part of the experiment was performed without the actual bomb, as it was used for calibration purposes.

Next, the enthalpy of benzoic acid was measured. First, about 2000 g of water at room temperature was poured into the vessel. The exact weight of the water and vessel were recorded. Then a sample of about 1 g of benzoic acid was made into a pellet, and placed into the combustion cup. The exact weights of the benzoic acid pellet and combustion cup were also recorded. Fuse wire was attached to the electrodes, and it was made sure that the fuse wire securely touched the benzoic acid pellet. The lid of the bomb was securely attached, the bomb was purged with oxygen, and about 25 atmospheres of oxygen was pumped into the bomb.

The vessel was then placed into the calorimeter and bomb was placed into the vessel. The fuse connections were plugged into the bomb right before it was submerged in water. The lid of the calorimeter was placed on as to make sure the thermistor was in place in the water and so that the propeller could spin properly. The propeller was turned on and data was recorded on the computer. Once a stable temperature was reached, the bomb was ignited and again data was collected until a stable temperature was reached. This process was repeated for naphthalene and a food source (rice cakes).

III. Results

Calculations:

a. Heat capacity (mC_p) from benzoic acid

q_b.a. = 26.43 kJ/g (theoretical)

q = [mC_p] ΔT
q = K ΔT

K = q / ΔT
K = (26.43 kJ/g) (0.9488 g) / (3.69 K)
K = 6.80 kJ / K

 

b. Heat of reaction of naphthalene and % error

q_nap = 47.86 kJ/g (theoretical)
q_nap = (47.86 kJ/g) x (0.9068 g)
q_nap = 43.42 kJ

q_nap = K ΔT
q_nap = (6.80 kJ / K) x (5.09 K)
q_nap = 34.6 kJ (actual)

Percent Error:
(34.6 kJ – 43.42 kJ) / 43.42 kJ x 100% = 20.31%

 

c. Heat of reaction of rice cake and % error.

16 Cal / 4 g (looked up) = x / 0.5097 g
x = 2.0388 Cal (for the 0.5097 g pellet)
2.0388 Cal x (4.184 kJ / Cal) = 8.53 kJ

q_{rice cake} = 8.53 (theoretical)

q_{rice cake} = (6.80 kJ / K) x (1.36 K)
q_{rice cake} = 9.25 kJ (actual)

Percent Error:
(9.25 kJ – 8.53 kJ) / 8.53 kJ x 100% = 8.44%

 

IV. Conclusion

The difference in heat of reaction between the actual and theoretical values for naphthalene was 20.1%. The measured value was lower than the theoretical value. This means the change in water temperature was lower than it should have been. This error could have come from a few sources. The water used may have been warmer than room temperature, meaning that difference between the temperatures that the water was at and the temperature of room temperature water would have not been accounted for in the temperature difference. Another problem could have been that bomb was not flushed correctly with oxygen. This would cause an uneven ignition of the naphthalene, and the temperature might not have gone up as much as it should.

The difference in the heat of reaction between the actual and theoretical values for the rice cake was 8.44%. This seems like a fairly reasonable result. The actual value was high than that of the theoretical value. This could be accounted for in a few ways. The water used may have been colder than room temperature, causing a bigger change in temperature than there should have been. The bomb may also have been dirty, causing debris to ignite along with the rice cake, which would add to the temperature difference. The rice cake may also have gained some water vapor, causing water to ignite along with the rice cake, also causing a bigger change in temperature. Overall, the measured values as somewhat close to the theoretical values, showing there is some merit to the theories we have learned.