Question 8.87- Breaking phase changes up into steps  [ENDORSED]

Moderators: Chem_Mod, Chem_Admin

Vicky Wang 2C
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Question 8.87- Breaking phase changes up into steps

Postby Vicky Wang 2C » Mon Jan 11, 2016 11:09 pm

"How much heat is required to convert a 42.30 g block of ice at -5.042 degrees C into water vapor at 150.35 degrees C?"

I'm still a little confused as to why we have to break this up into the different stages (raising temp of ice, melting ice, raising temp of water, etc.). Also, the answer key says that for the first step:
Delta H = (42.30g) (2.03 J/degrees C * g) (0.00 degrees C - (5.042 degrees C)) = 0.433 kJ

Where did the (2.03 J/degrees C * g) come from? Thank you and sorry

JasmineAlberto4J
Posts: 73
Joined: Fri Sep 25, 2015 3:00 am

Re: Question 8.87- Breaking phase changes up into steps

Postby JasmineAlberto4J » Mon Jan 11, 2016 11:15 pm

I don't know the answer to your other questions:(, but I do know that 2.03 is the specific heat capacity of ice

carrie_shih_3L
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Question 8.87- Breaking phase changes up into steps

Postby carrie_shih_3L » Tue Jan 12, 2016 12:35 am

You have to split this into multiple parts because each phase requires a different equation.
When you graph out the heating curve, the sections where it is the slope (change of temperature, but no change in phase) you use the H = m * C * T formula. For the sections where it is flat (change of phase, but no change of temperature) you get multiply the moles by the heat of fusion or vaporization. You would look at table 7.3 for the numbers for this.

Jason Muljadi 2C
Posts: 62
Joined: Thu Jul 27, 2017 3:01 am

Re: Question 8.87- Breaking phase changes up into steps

Postby Jason Muljadi 2C » Sun Jan 21, 2018 12:06 am

For step 2, I realized that we had to use dHfus= 6.01 kJ.mol^-1. Do we always use this value whenever we are melting ice and only when we are melting ice at 0 degrees C?

Jessica Benitez 1K
Posts: 50
Joined: Fri Sep 29, 2017 7:04 am

Re: Question 8.87- Breaking phase changes up into steps  [ENDORSED]

Postby Jessica Benitez 1K » Sun Jan 21, 2018 7:53 pm

Doing a phase change and raising the temperature of the system require different equations and constant values. Breaking up the problem into different steps keeps everything organized because of the different equations and constant values you need to use for each step. It is easier to do the steps separately and add everything at the end rather than trying to combine every step especially if it is a longer problem.


Return to “Phase Changes & Related Calculations”

Who is online

Users browsing this forum: No registered users and 1 guest