Nothing Special   »   [go: up one dir, main page]
Scribd Logo
Upload

Welcome to Scribd!

  • 1 views

    Thermokey

    Uploaded by

    lshskhemani

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd

    Thermokey

    Uploaded by

    lshskhemani
    1 views1 page

    Original Title

    thermokey

    Copyright

    © © All Rights Reserved

    Available Formats

    PDF, TXT or read online from Scribd

    Did you find this document useful?

    Is this content inappropriate?

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    1 views1 page

    Thermokey

    Uploaded by

    lshskhemani

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 1

    Thermodynamics Key

    1. Predict the sign of ΔSrxn (positive or negative) for the following reactions:
    a. 2 Mg (s) + O2 (g) → 2 MgO (s) ΔS__-__
    b. C6H12O6(s) J 2C2H5OH(l) + 2CO2(g) ΔS__+__
    c. 2NH3(g) + CO2(g) J H2O(l) + NH2CONH2(aq) ΔS___-__
    d. Cu(s) (100 C) J Cu(s) (25 C)
    o o
    ΔS__-_

    2. Which two variables can indicate if a reaction is spontaneous or not? ____ΔG and ΔStot__

    3. The second law of thermodynamics tells us that:


    a. Energy is conserved.
    b. The entropy of a pure solid crystal at 0 Kelvin is zero.
    c. The total enthalpy of the universe is increasing.
    d. The total entropy of a spontaneous process is increasing.
    e. The free energy of a spontaneous process is increasing.

    4. Which combination of ΔH and ΔS for a reaction is not spontaneous at low temperatures and becomes
    spontaneous at higher temperatures? Answer: ΔH _positive_ and ΔS _positive

    5. The process of a liquid boiling has a:


    a. Positive enthalpy change and a positive entropy change.
    b. Positive enthalpy change and a negative entropy change.
    c. Negative enthalpy change and a positive entropy change.
    d. Negative enthalpy change and a negative entropy change.
    e. Not enough information is given.

    6. Calculate the temperature at which this reaction changes from being spontaneous to non spontaneous: Mg(s)
    + O2(g) ' MgO(s). Is the reaction spontaneous above or below this temperature? (Hint – balance the rxn
    and use data from Appendix B)

    ΔHorxn = [ 2(-601.7)] – [0] = -1203 kJ


    ΔSorxn = [ 2(26.9)] – [ 1(32.7) + 1(205.0) ] = -216.6 J/K

    0 = (-1203 kJ) – T (-0.2166 kJ/K)


    T = -1203 kJ / -0.2166 kJ/K
    T = 5550 Kelvin or 5280oC spont below this temp

    7. Calculate ΔGorxn for this reaction: 2 CH3OH (l) + 3 O2 (g) → 2 CO2 (g) + 4 H2O (l)
    Given: ΔHfo CH3OH (l) = -238.6 kJ/mol ΔHfo H2O (l) = -285.83 kJ/mol
    o
    S CH3OH (l) = 126.8 J/molK ΔHfo CO2 (g) = -393.5 kJ/mol
    So O2 (g) = 205.0 J/molK So CO2 (g) = 213.6 J/molK
    o
    S H2O (l) = 69.91 J/molK (only use this given data for your solution)

    ΔHorxn = [ 2(-393.5) + 4(-285.83)] - [ 2(-238.6) + 0] = -1453.12 kJ


    ΔSorxn = [ 2(213.6) + 4(69.91)] - [ 2(126.8) + 3(205)] = -161.76 J/K = -0.16176 kJ/K
    ΔGorxn = -1453.12 kJ/mol - (298 K)-0.16176 kJ/molK = -1405 kJ

    You might also like