CP4700 Assignment 9 E ° Enthalpy (Chapter 5) , Enthalpy Crash Course( × x × pche

Question

CP4700 Assignment 9 E ° Enthalpy (Chapter 5) , Enthalpy Crash Course( × x × pchemweb.files wordpress.com/2017/09/exam2fall2017 pdf The provisions of the College's Honor Code apply, as always. 1. Explain why it was necessary to create the state function that we call enthalpy. Use equations in your 2. What is the heat capacity of a substance? Why does its value depend on whether a process takes place 3. Explain why it was necessary to create the state function we call entropy. Derive the Second Law of explanation as appropriate at constant-pressure or constant volume Thermodynamies as part of your explanation. 4. Explain what is meant by each of the following terms: system surroundings universe 5. Model the processes of the melting of ice and the freezing of water in the following manner to show that entropy increases for both of these processes. Set up a simplified model with two systems at different temperatures separated by a heat-conducting wall. The volumes of the systems are constant but not necessarily equal. The two systems are isolated from the surroundings. Derive an equation for the entropy dS of the two systems and use it to show that whether ice melts or water freezes, the entropy will increase. 6. Explain why, for a reversible prodess, dS-0 7. Calculate the entropy change for the isothermal, reversible expansion of one mole of an ideal gas from 10.0 L to 22.5L DOLL F4 F5 F6 F7 F8 F9 F11 F12 :

Explanation / Answer

Q1

the state funciton of Enthalpy (H) is used in lots of problems in engineering/science

For instance "heating" process at constant pressure:

this is exaclty equal to the chang of enthalpy

One could claculate it otherwise:

Q = dU + d(PV)

clacualteing d(PV) can result in waste of time/clacualtions

so we use

dH = dU + d(PV)

at constant P --> dU + P*dV, if dV is negelgible, then DH= dU

Q2

Heat capacity --> amount of heat which is required to change 1 °C

if heat cpacity is large, then we require lots of energy to change 1 °C ( water for instance)

it depends mainly on the definitions of heat under constant P and V

at constant P, there is change in volume, which must be accounted in the heat capcity

at constna V, there is nochange in V, therefore, the change is smaller

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