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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,

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ENTROPY

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  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

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