What is spontaneity? What is disorder?

15.2 Entropy and spontaneity Entropy (S) refers to the distribution of available energy among the particles. The more ways the energy can be distributed the higher the entropy. Gibbs free energy (G) relates the energy that can be obtained from a chemical reaction to the change in enthalpy (ΔH), change in entropy (ΔS), and absolute temperature.

Entropy is a more complete direction of change Everything in the universe will naturally progress toward a more disordered state Coffee naturally cools, your room becomes cluttered “not knowing the second law of thermodynamics is like never having read a work of Shakespeare” (249)

Spontaneous change- occur naturally without the need to do work Entropy (S)- distribution of available energy Matter and energy tend to disperse and become more disordered (second law of thermodynamics) Predicting entropy changes (p.250)

Increase entropy (+ΔS) decrease entropy (-ΔS) decrease entropy (-ΔS) No significant change (0≈ΔS) Decrease (-ΔS) No significant change (0≈ΔS)

Absolute entropy A perfectly ordered solid at absolute zero has an entropy of zero (this never occurs) Chart on p. 252 + section 12 of data booklet Use this formula!

ΔS° = (230) – (220+131) ΔS° = -121 J K-1 mol-1

Entropy changes of the surroundings So far, we have only considered the entropy of the substances in the system, but how does the entropy of the system relate to the entropy of the universe? Adding heat to the surroundings results in a general dispersal of heat in the universe The change in entropy of the surroundings, ΔS(surroundings) can be calculated from the enthalpy change in the system ΔH(system) and the absolute temperature, T.

The change in the entropy of the surroundings is proportional to -ΔH(system)

Endothermic reactions can occur if the change in entropy of the system can compensate for the negative entropy change of the surroundings For chemical reactions, neither ΔH(system) nor ΔS(system) alone can reliably be used to predict the feasibility of a reaction

Gibb’s free energy!! ΔG must be negative for a spontaneous process

Using ΔG(system) to predict the feasibility of a change Temperature has a large impact on whether or not a change will happen spontaneously

+ΔS° (increase in moles of gas, increase in disorder) H2O(l)  H2O(g) +ΔS° (increase in moles of gas, increase in disorder) ΔG= ΔH- TΔS° Well- this is endothermic (+ΔH) so at a low temp: ΔG= +ΔH - (0) But at a low temp: ΔG= - TΔS° So, water boils at a high temperature (100°C) where ΔG is negative.

H2O(l)  H2O(g) ΔH° =(products)-(reactants) ΔH° =(-242)-(-286) ΔH° =+44 kJ mol-1 We know the boiling point, but we can show it using: ΔG= ΔH- TΔS° H2O(l)  H2O(g) 0= ΔH- TΔS° ΔS° =(products)-(reactants) T= ΔH/ΔS° ΔS° =(+188.8)-(70) T= +44/+0.1188 =+118.8 J K-1 mol-1 T= 370 K T= 97 °C So kinda close.

It is better to understand the formula than to memorize this table. The effect of enthalpy, entropy and temperature on the spontaneity of a reaction It is better to understand the formula than to memorize this table.

Calculating ΔG values

Gibb’s Free Energy and Equilibrium We are pretty much going to skip this for now and come back to it in topic 7