Thermodynamics

1
Thermodynamics

• Will a reaction happen?

2
Energy

• Substances tend to react to achieve the lowest
  energy state.
• Most chemical reactions are exothermic.
• Doesnt work for things like ice melting.
• An ice cube must absorb heat to melt, but it
  melts anyway. Why?

3
Entropy

• The degree of randomness or disorder.
• S
• Drop a box of marbles.
• Watch your room for a week.

4
Entropy
Entropy of a solid
Entropy of a liquid
Entropy of a gas

• A solid has an orderly arrangement.
• A liquid has the molecules next to each other.
• A gas has molecules moving all over the place.

5
Entropy increases when...

• Reactions of solids produce gases or liquids, or
  liquids produce gases.
• A substance is divided into parts -so reactions
  with more reactants than products have an
  increase in entropy.
• the temperature is raised -because the random
  motion of the molecules is increased.
• a substance is dissolved.

6
Entropy calculations

• There are tables of standard entropy.
• Standard entropy is the entropy at 25ºC and 1
  atm pressure.
• Abbreviated Sº, measure in J/K.

7
Spontaneity

• Will the reaction happen, and how can we make it?

8
Spontaneous reaction

• spontaneous reactions will happen.
• nonspontaneous reactions dont.
• Two factors influence.
• Enthalpy (heat) and entropy(disorder).

9
Two Factors

• Exothermic reactions tend to be spontaneous.
• -negative DH.
• -Reactions where the entropy of the products is
  greater than reactants tend to be spontaneous.
• Positive DS.
• -A change with positive DS and negative DH is
  always spontaneous.
• -A change with negative DS and positive DH is
  never spontaneous.

10
Other Possibilities

• Temperature affects entropy.
• Higher temperature, higher entropy.
• For an exothermic reaction with a decrease in
  entropy (like rusting).
• Spontaneous at low temperature.
• Nonspontaneous at high temperature.
• Entropy driven.

11
Other Possibilities

• An endothermic reaction with an increase in
  entropy like melting ice.
• Spontaneous at high temperature.
• Nonspontaneous at low temperature.
• Enthalpy driven.

12
Gibbs Free Energy

• The energy free to do work is the change in Gibbs
  free energy.
• DGº DHº - TDSº (T must be in Kelvin)
• All spontaneous reactions release free energy.
• If DG lt0 for a spontaneous reaction.
• DG 0 reaction is at equilbrium
• DG gt0 for a non spontaneous reaction.

13
DGDH-TDS
Spontaneous?
DH
DS
DG
At all Temperatures
At high temperatures, entropy driven
At low temperatures, enthalpy driven
Not at any temperature, Reverse is spontaneous
14
2H2S(g) O2(g) 2H2O(l) 2 S

• Find ?G if
• ?H is 531.4 kJ and ?S is 412 J/K
• at 25 C.
• Change the temperature to Kelvin
• 25 273 298

15
2H2S(g) O2(g) 2H2O(l) 2 S

• ?G ?H - T ?S
• ?G -531.4 kJ - 298K (-412.5 J/K)
• ?G -531.4 kJ 123000 J
• ?G -531.4 kJ 123 kJ
• ?G -408.4 kJ
• Spontaneous
• Exergonic- it releases free energy.

16

• Find ?G if ?H is 366 kJ and ? S is 340 kJ/K at
  a temp of 38C.
• Change temp to Kelvin
• 38 273 311 K
• ?G ?H -T ?S
• -366 kJ (311K) ( 340kJ/K)
• -106106 kJ
• - 110000 kJ

17

• Assignment
• p 948 488, 489,491
• P 548 24,25
• P 549 31