Handwrite everything in the picture THEORY. The standard enthalpy of combustion for a substance is defined as the enthalpy change AH° which accompanies a process in which the given substance undergoes reaction with oxygen gas to form specified combustion products [such as CO2(g), H2O(1), N2(g), SO2(g)), all reactants and products being in their respective standard states at the given temperature T. Thus the standard enthalpy of combustion of benzoic acid at 298.15°K is AH° 298. 15 for the process CeHsCO2H(s) + 15/202(g) = 7CO2(g) + 3H20(1) (1) with reactants and products in their standard states for this temperature. As will be shown below, the enthalpy of combustion can be calculated from the temperature rise, which results when the combustion reaction occurs under adiabatic conditions in a calorimeter. It is important that the reaction in the calorimeter take place rapidly and completely. To this end, the material is burned in a steel bomb with oxygen under a pressure of about 25 atm. A special acid- resistant alloy is used for the construction of the bomb because water and acids are produced in the reaction. In the adiabatic-jacket bomb calorimeter the bomb is immersed in a can of water fitted with a precise thermometer. This assembly is placed within an outer water- filled jacket. Both before and after the combustion occurs, the jacket temperature is maintained (by external means) at the same value as that of the water in the can. If the temperatures are matched with sufficient accuracy, the can and contents do not gain or lose energy by radiation or conduction and the process is therefore adiabatic. This method affords convenience in work of moderate accuracy, but there is inevitably some error due to time lag in adjustment of the outer-jacket temperature. For the most exacting research measurements, an isothermal jacket is used and accurate cooling corrections are made.[1-3a,4] It should be recognized that the process, which actually takes place in the calorimeter, does not correspond exactly to one of the type of Eq. (1) in the actual calorimeter process, the final and initial temperatures are not equal, and the reactants and products are not in their standard states. Products at pressure P

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Chapter9: Energy And Chemistry
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Handwrite everything in
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THEORY. The standard enthalpy of combustion for a substance is defined as the
enthalpy change AH,° which accompanies a process in which the given
substance undergoes reaction with oxygen gas to form specified combustion
products [such as CO2(g), H20(1), N2(g), SO2(g)), all reactants and products
being in their respective standard states at the given temperature T. Thus the
standard enthalpy of combustion of benzoic acid at 298.15°K is AH° 298. 15
for the
process
CeHsCO2H(s) + 15/202(g) = 7CO2(g) + 3H20(1)
(1)
with reactants and products in their standard states for this temperature.
As will be shown below, the enthalpy of combustion can be calculated from the
temperature rise, which results when the combustion reaction occurs under
adiabatic conditions in a calorimeter. It is important that the reaction in the
calorimeter take place rapidly and completely. To this end, the material is burned
in a steel bomb with oxygen under a pressure of about 25 atm. A special acid-
resistant alloy is used for the construction of the bomb because water and acids
are produced in the reaction.
In the adiabatic-jacket bomb calorimeter the bomb is immersed in a can of water
fitted with a precise thermometer. This assembly is placed within an outer water-
filled jacket. Both before and after the combustion occurs, the jacket temperature
is maintained (by external means) at the same value as that of the water in the
can. If the temperatures are matched with sufficient accuracy, the can and
contents do not gain or lose energy by radiation or conduction and the process is
therefore adiabatic.
This method affords convenience in work of moderate accuracy, but there is
inevitably some error due to time lag in adjustment of the outer-jacket
temperature. For the most exacting research measurements, an isothermal
jacket is used and accurate cooling corrections are made.[1-3a,4]
It should be recognized that the process, which actually takes place in the
calorimeter, does not correspond exactly to one of the type of Eq. (1) in the
actual calorimeter process, the final and initial temperatures are not equal, and
the reactants and products are not in their standard states.
Products at pressure P3
temperature T2
AU.
JCaT
Reactants at pressure P1
temperature T,
Products at pressure P2
temperature T,
AUT
Reactants in standard
states at temperature T,
Products in standard
states at temperature T,
AUT
Figure 1. Relationship between pertinent states of the calorimeter system
(comprising of the can and its entire contents).
The relationship of the calorimeter process to the isothermal standard-state
process may be clarified by reference to Figure 1. The initial and final
temperatures in the calorimeter process are T; and T2, respectively. The various
states of interest are shown, and on each arrow is written the energy change for
the can and
contents in
going from one state to another in the direction indicated.
Here AU, is the energy change for the actual calorimeter process, while AUT is
the energy change for an imagined process in which the final state is at T, rather
than T2. The heat capacity C is that for the can and its contents under the
conditions of the experiment. The work of expansion of water in the can is
entirely negligible.
Transcribed Image Text:Handwrite everything in the picture THEORY. The standard enthalpy of combustion for a substance is defined as the enthalpy change AH,° which accompanies a process in which the given substance undergoes reaction with oxygen gas to form specified combustion products [such as CO2(g), H20(1), N2(g), SO2(g)), all reactants and products being in their respective standard states at the given temperature T. Thus the standard enthalpy of combustion of benzoic acid at 298.15°K is AH° 298. 15 for the process CeHsCO2H(s) + 15/202(g) = 7CO2(g) + 3H20(1) (1) with reactants and products in their standard states for this temperature. As will be shown below, the enthalpy of combustion can be calculated from the temperature rise, which results when the combustion reaction occurs under adiabatic conditions in a calorimeter. It is important that the reaction in the calorimeter take place rapidly and completely. To this end, the material is burned in a steel bomb with oxygen under a pressure of about 25 atm. A special acid- resistant alloy is used for the construction of the bomb because water and acids are produced in the reaction. In the adiabatic-jacket bomb calorimeter the bomb is immersed in a can of water fitted with a precise thermometer. This assembly is placed within an outer water- filled jacket. Both before and after the combustion occurs, the jacket temperature is maintained (by external means) at the same value as that of the water in the can. If the temperatures are matched with sufficient accuracy, the can and contents do not gain or lose energy by radiation or conduction and the process is therefore adiabatic. This method affords convenience in work of moderate accuracy, but there is inevitably some error due to time lag in adjustment of the outer-jacket temperature. For the most exacting research measurements, an isothermal jacket is used and accurate cooling corrections are made.[1-3a,4] It should be recognized that the process, which actually takes place in the calorimeter, does not correspond exactly to one of the type of Eq. (1) in the actual calorimeter process, the final and initial temperatures are not equal, and the reactants and products are not in their standard states. Products at pressure P3 temperature T2 AU. JCaT Reactants at pressure P1 temperature T, Products at pressure P2 temperature T, AUT Reactants in standard states at temperature T, Products in standard states at temperature T, AUT Figure 1. Relationship between pertinent states of the calorimeter system (comprising of the can and its entire contents). The relationship of the calorimeter process to the isothermal standard-state process may be clarified by reference to Figure 1. The initial and final temperatures in the calorimeter process are T; and T2, respectively. The various states of interest are shown, and on each arrow is written the energy change for the can and contents in going from one state to another in the direction indicated. Here AU, is the energy change for the actual calorimeter process, while AUT is the energy change for an imagined process in which the final state is at T, rather than T2. The heat capacity C is that for the can and its contents under the conditions of the experiment. The work of expansion of water in the can is entirely negligible.
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