In this particualr question:
"Assume that in order to determine the heat of formation of Carbon dioxide the folowing data was obtained:
(i)Upon thee combustion of 0.6g of graphite to CO2 with excess of oxygen at 298K in a bomb caloriemeter (a const volume adiabatic system),a temperature increase of 235K was found.
(ii) Uon electrically heating the same caloriemeter ,filled with the reaction products an adiabatic increase of 0.80K was found.The resistance of the heating coil placed inside the caloriemeter was 30 ohms the current 1.67 A and the time 80s.
Calculate ΔU⁰ and ΔH⁰(at 298K) for the reaction
C(graphite)+O2 (g)=CO2(g)
Assume the heat capacities of CO2 and O2 are constant and independent of temperature.Given ρC(s)=2.26 g/cc."I have been told to break this into multiple steps and particularly consider two steps.
(i)[0.05C+nO2]at 298K---->[0.05CO2+(n-0.05)O2]at 298K
(ii)[0.05CO2+(n-0.05)O2]at 298K---->[0.05CO2+(n-0.05)O2]at 533K.
I do not understand why it was assumed that the temperature would remain constant till graphite was completely converted to CO2 and also why we chose these two steps in particular. Please give the answer in as much detail as possible.
Dear Rajorshi,
the temperature is not considered constant troughout the process.
The key point is that you need to calculate the enthalpy and internal energy variations. Since these are state functions (their integral over a closed curve is zero) , their value depends only on their initial and final state, if you consider them as a function of the temperature only, you will just need the initial and final temperature, the specific heat of the products and their mass.
The two steps are due to following reasons:
1) The first step is done to get the value of the enthalpy of the products in the standard state (ambient temperature, from tables).
2) Since these products are given by an exothermal reaction, there is a production of energy, which is accounted by the raise of temperature, so if you have tables of the enthalpy of the products at different temperatures, you can just calculate the difference between the enthalpy of products at 533 K and the products at 298 K.
That said, I would suggest another approach, that is calculating the enthalpy variation as the heat capacity of every product, multiplied by the variation of temperature...
Since the temperature during the whole process is not constant, it is necessary to assume the same baseline of temperature, to make sure all the measurements are based on the same temperature. Actually the heat capacity also depends on temperature. To simplify this scenario, it is reasonable to assume the heat capacities are not dependent of T.
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