jovan
Uploaded by
12 SLIDES
228 VIEWS
120LIKES

Understanding Thermochemistry: Enthalpies, Entropy, and Gibbs Free Energy

DESCRIPTION

Explore the concepts of enthalpies, entropy, and Gibbs free energy to predict spontaneity of reactions. Learn about endothermic, exothermic reactions and the second law of thermodynamics.

1 / 12

Download Presentation

Understanding Thermochemistry: Enthalpies, Entropy, and Gibbs Free Energy

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Thermochemistry Renee Y. Becker Valencia Community College CHM 1046

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. 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)

  7. 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).

  8. 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

  9. 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

  10. 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

  11. 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?

More Related