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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

Thermochemistry

Renee Y. Becker

Valencia Community College

CHM 1046

slide2
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.

slide3
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.
slide7
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)
slide8
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 < 0 Process is spontaneous
    • DG = 0 Process is at equilibrium
    • DG > 0 Process is nonspontaneous
slide10
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

slide11
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

slide12
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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