Hi,
I am trying to do a prelab and I am really stuck... I have no idea how to go about this, I don't knwo if its worded funny or waht, so any help would be greatly apprecaited. The question is:
(This is for a bomb calorimeter of constant volume)
Benzoic acid is often used to calibrate calorimeters as its thermodynamic properties have been well characterized. The change in inter energy upon combusting benzoic acid is DeltaU 25degrees = -26.43kJ/g. (If T1 is within a few degrees of 25 degrees then the difference between deltaU and deltaU 25 degrees is negliglblie). Use the data below to calculate the calorimeter constant C. Remember to include a correction for the heat produced in combusting the fuse wire. The heat equivalent for the combustion of fuse wire is -9.6 J/cm.
Benzoic acid= 0.7386g
Fuse wire consumed = 7.2cm
Chnage in temp = 1.940 degrees C
I know that the equations for constant volume are work = 0, delta U = (Cv)deltaT, deltaU=Q=CdeltaT, deltaU(total)=deltaU(fusewire) + deltaU(sample)
I just don't know waht to do and how this gives me anything to solve it with. Thanks for taking the time to read this.
The molar enthalpy of formation of water vapor is -241.82 kJ/mol at 298 K. Calculate the molar enthalpies of formation of water vapor and of liquid water at 100 C and at -0.1 C (four values).
To calculate the calorimeter constant C, you will need to use the given information and the equations you mentioned. Here are the steps to follow:
1. Start by calculating the energy change (ΔU) for both the fuse wire and the benzoic acid.
For the fuse wire:
- The heat equivalent for the combustion of the fuse wire is given as -9.6 J/cm.
- The length of fuse wire consumed is 7.2 cm.
- Since the heat equivalent is given per centimeter, multiply the length consumed by the heat equivalent to get the heat produced by the fuse wire: (-9.6 J/cm) * (7.2 cm) = -69.12 J.
For the benzoic acid:
- The change in internal energy upon combusting benzoic acid at 25 degrees Celsius is given as ΔU 25degrees = -26.43 kJ/g.
- The mass of benzoic acid used is 0.7386 g.
- Convert the given value of ΔU to J, and then multiply it by the mass of benzoic acid: (-26.43 kJ/g) * (0.7386 g) = -19.490238 J.
2. Next, calculate the total change in internal energy (ΔU) for the system, which would be the sum of the changes in energy for the fuse wire and the benzoic acid: ΔU(total) = ΔU(fuse wire) + ΔU(sample).
ΔU(total) = (-69.12 J) + (-19.490238 J) = -88.610238 J.
3. Now, use the equation ΔU = CΔT to find the calorimeter constant C. Rearrange the equation to solve for C:
C = ΔU(total) / ΔT.
- The change in temperature is given as 1.940 degrees Celsius. Convert it to Kelvin by adding 273.15: ΔT = 1.940 + 273.15 = 275.09 K.
- Substitute the values into the equation to find C: C = (-88.610238 J) / (275.09 K) = -0.322 J/K.
Note: The negative sign indicates that heat is being lost from the system, which is typical for an exothermic reaction like combustion.
So, the calculated calorimeter constant C is -0.322 J/K.