1 / 14

ENTROPY

ENTROPY. Measure of: Level of disorder in a system or Number of Microscopic Energy Levels Available to a Molecule (i.e. microstates ). SPONTANEOUS CHEMICAL PROCESSES. Spontaneity = Tendency for process to occur naturally e.g . Iron tends to rust, diamond tends to turn to graphite,

idania
Télécharger la présentation

ENTROPY

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. ENTROPY Measure of: Level of disorder in a system or Number of Microscopic Energy Levels Available to a Molecule (i.e. microstates)

  2. SPONTANEOUS CHEMICAL PROCESSES Spontaneity = Tendency for process to occur naturally e.g. Iron tends to rust, diamond tends to turn to graphite, (fortunately, over a very long time period - relax, ladies & DeBeers!) dead plant material tends to decay, ice tends to melt at room temperature, brain cells tend to decay with time!

  3. Spontaneity • Are all spontaneous changes exothermic?

  4. Spontaneity • No, but most exothermic processes are spontaneous. • Some reactions may be spontaneous under one set of conditions, but non-spontaneous under other conditions. • e.g. Formation of Lime from Chalk: • CaCO3 (s) CaO (s) + CO2 (g) H = +178.3 kJ • Reaction becomes spontaneous > 800 0C, even though it is endothermic!

  5. Absolute Entropy (S) • With increasing temperature, the entropy of a molecule increases (more microscopic energy levels beome available, e.g. vibrations, rotations etc.) • As the temperature of a molecule approaches absolute zero, microscopic energy levels also approach zero, so its entropy also approaches zero.

  6. Absolute Entropy (S) • S = k lnW • W = No. of microscopic energy levels • In perfectly ordered solid, at 0K, W = 1 • S = k ln 1 = 0

  7. this leads to:

  8. the 3rd Law of Thermodynamics

  9. 3rd Law of Thermodynamics • "the Entropy of a perfect crystalline substance approaches zero as the absolute temperature appraches zero"

  10. Entropy Changes ( S) • Reaction Entropy ( Sr) • Sr = Sproducts - Sreactants

  11. Standard Entropy of Reaction ( S0r) • ∆ S0r = (Entropy of products in standard states at temp. T) - (Entropy of reactants in standard states at temp. T) • for : aA + bB cC + dD • ∆ S0r = cS0(C) + dS0 (D) - aS0 (A) - bS0(B)

  12. Standard Entropy Change Calculation • Calculate S0r at 25 0C for: • N2H4 (l) + 3 O2 (g) 2NO2 (g) + 2 H2O(l) • S0r = 2 x S0(H2O(l))+ 2S0 (NO2 (g))- S0 (N2H4 (l)) - 3S0(O2 (g)) • = 2 mol (69.91 JK-1mol-1)+ 2 mol (239.95 JK-1mol-1) - 1 mol (121.21 JK-1mol-1) - 3 mol (205.03 JK-1mol-1) • = -116.58 JK-1

  13. So what happened to the 2nd Law of Thermodynamics?

  14. 2nd Law of Thermodynamics • In any spontaneous process, the Entropy of the Universe (i.e. system + surroundings) must increase: • so • Ssys+ Ssurr >0

More Related