Thermochemistry
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Thermochemistry. Renee Y. Becker Valencia Community College CHM 1046. Enthalpies of Chemical Change: Often called heats of reaction ( D H ) units kJ Endothermic: Heat flows into the system from the surroundings and D H has a positive sign .

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Thermochemistry

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Thermochemistry

Thermochemistry

Renee Y. Becker

Valencia Community College

CHM 1046


Thermochemistry

  • Enthalpies of Chemical Change:Often called heats of reaction (DH) units kJ

    Endothermic:Heat flows into the system from the surroundings and DH has a positive sign.

    Exothermic:Heat flows out of the system into the surroundings and DH has a negative sign.


Thermochemistry

Are the following reactions endothermic or exothermic? Favorable or unfavorable?

C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) DH = –2219 kJ

CH3Cl(g) + HCl(g)  CH4(g) + Cl2(g) DH = 98.3 kJ


Entropy

Entropy

  • The measure of molecular disorder in a system is called the system’s entropy; this is denoted S.

  • Entropy has units of J/K (Joules per Kelvin).

    DS = Sfinal – Sinitial

    Positive value of DS indicates increased disorder.

    Negative value of DS indicates decreased disorder.


Entropy1

Entropy

  • Second Law of Thermodynamics: Reactions proceed in the direction that increases the entropy of the system plus surroundings.

  • A spontaneous process is one that proceeds on its own without any continuous external influence.

  • A nonspontaneous processtakes place only in the presence of a continuous external influence.


Thermochemistry

Predict whether DS is likely to be positive or negative for each of the following reactions.

  • a. 2 CO(g) + O2(g)  2 CO2(g)

  • b. 2 NaHCO3(s)  Na2CO3(s) + H2O(l) + CO2(g)

  • c. C2H4(g) + Br2(g)  CH2BrCH2Br(l)

  • d. 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(g)


Thermochemistry

  • To decide whether a process is spontaneous, both enthalpy and entropy changes must be considered:

    Spontaneous process:Decrease in enthalpy (–DH).Increase in entropy (+DS).

    Nonspontaneous:Increase in enthalpy (+DH).Decrease in entropy (–DS).


Free energy

Free Energy

  • Gibbs Free Energy Change (DG): Weighs the relative contributions of enthalpy and entropy to the overall spontaneity of a process.

    DG = DH – TDS

    • DG < 0Process is spontaneous

    • DG = 0Process is at equilibrium

    • DG > 0Process is nonspontaneous


Thermochemistry

Which of the following reactions are spontaneous under standard conditions at 25°C?

a.AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) DG° = –55.7 kJ

b.2 C(s) + 2 H2(g)  C2H4(g)DG° = 68.1 kJ

c.N2(g) + 3 H2(g)  2 NH3(g) DH° = –92 kJ; DS° = –199 J/K


Thermochemistry

  • Equilibrium (DG = 0):

    Equilibrium is the point where DG = DH – TDS = 0

  • Estimate of the temperature at which a reaction becomes spontaneous (set DG =0)

    T = DH / DS


Thermochemistry

2 NH3(g)  N2(g) + 3 H2(g)

DH = 92.0 kJ DS = 199 J/K

Is this reaction spontaneous @ 25C? If not, at what temperature does this reaction become spontaneous?


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