Physical Chemistry Questions - NEED HELP

  • #1

Homework Statement

1. Ideally, the handle of a frying pan should not get hot while you are cooking with it. Based on the information listed in Table 2.8 of your textbook (or 2.7 in 8th, 2.6 in 7th ed.) choose the substance below that would be the best frying pan handle material. Assume that the same number of moles of substance would be used for each material. (We will ignore other physical properties that one would normally consider in the manufacture of a good frying pan handle, such as strength, weight, cost etc...).
a. Aluminum
b. Graphite.
c. Iron
d. Silver
e. Gold

2. Suppose 1.80 moles of an ideal gas [C(V,m) = 11.31 J K-1 mol-1] undergoes a process that changes its state. Calculate the change in temperature if during this process 628.6 J of heat flows into the gas, while the gas performs 628.6 J of work. Write your answer in Kelvin accurate to the first decimal place.

3. A system in state A undergoes a series of reversible processes shown in the p-V graph below that brings it to state E. The system is then returned to its original state through a single reversible process. We are told that 30 J of work was done on the system to bring it back to state A from state E and in this process the system lost 60 J of heat. Based on this information and the appearance of the p-V graph below, which of the following statements would be TRUE? More than one answer can be chosen but marks are deducted from incorrectly chosen answers. Make sure you check that the direction of the process matches the sign of the given energy values.

a. The internal energy change going from state A to state E (ΔUAE) is -30 J. (Take care that you do the calculation for the correct direction - from A to E!)
b. For the process following path ABCDE the system is doing work.
c. If wABCDE = -20 J then the absolute value of (wBC + wCD + wDE) must be less than 20 J.
d. ΔHABCD=CpΔT, where ΔT is the TD-TA.

4. Which of the following are true? Choose all that apply. (p.s. I lose marks for choosing a wrong answer)
a. It is not possible to use ΔU=CVΔT to calculate the change in internal energy of a fixed quantity of gas that undergoes an irreversible compression.
b. The heat generated by a chemical reaction run in a container that is open to the air is equal to the enthalpy of that reaction.
c. For a constant pressure process, ΔH=ΔU+pΔV.
d. Heat capacity is a path function.

The Attempt at a Solution


Here's what I have thus far:
1) I searched up the specific heat capacity for each element, since I figured that the element with the highest heat capacity will be the one that gets the least hot. Thus, I think Gold would be the answer.
Aluminum = 24.35
Graphite = 8.53
Iron = 25.10
Silver = 25.351
Gold = 25.42

2) This is where I'm stuck, so maybe somebody could help me out! =)
I know that (delta)U = q(subscript V) = n * C(subscript V,M) * (Delta)T
I know q, but I'm also given "w" (work). Assuming that I don't use "W"
628.6 = 1.8 * 11.31 * (delta)T
(delta)T = 30.9 K

I'm really stuck on this problem! =(

(A) can't be true. (delta)U = q + w. If from E->A we do 30 J of work ON the system, while it loses 60 J of heat FROM the system, we have -60 + 30 = -30.
HOWEVER, from A->E, the system needs to DO 30 J of work, while RECEIVING 60 J of work, meaning that (delta)U = 60 - 30 = 30 J.

(B) should be true. We can calculate the work done from A to E by finding the area under the lines that connect B-C, and D-E.

(C) seems false. From A-B, the volume stays constant, meaning (delta)V= 0, and thus V = 0. As mentioned in (B), it's from B-C and D-E that work is done.

(D) - I have this gut feeling that it's true since "H" (enthalpy) is a state function. Help?

(A) is tricky for me. I'm guessing false because U is a state function, but I don't know why.

(B) I'm guessing true. My logic: a container open in air is subject to constant pressure, and thus heat transfer can be used to measure enthalpy.

(C) False. H = H2 - H1. H2 = U2 -p2V2, H1 = U1 - p1V1

(D) True. We need to know conditions under which heat capacity for a material was measured, and thus we must know the path.

Answers and Replies

  • #2
during this process 628.6 J of heat flows into the gas, while the gas performs 628.6 J of work

If you put in 628.6 J and you draw 628.6 J out, how much is left to change the temperature?
  • #3
Would it be 0.0 since (delta)U = 628.6 - 628.6 = 0?
Because I know that q + w =/= nC(v,m)*(delta)T.

Do you have any comments for my other questions? They would really help! =)
  • #4
1st sounds OK, especially if you are asked to use table containing heat capacities.
  • #5
So then I have #1-2 right? What about the last two questions?
  • #6

I have to hand this in tomorrow and I won't have any time tomorrow morning to do any work on it! Any help whatsoever for #2-4 will MAKE A HUGE DIFFERENCE! =D =D =D

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