1 st Law of Thermodynamics Energy is conserved. E = q + w
SPONTANEOUS: occur without any outside intervention Example: drop an egg The REVERSE is not spontaneous!!
REVERSIBLE PROCESS: change can be restored to its’ original state by exactly reversing the change. Example: ice water at 0 o C
IRREVERSIBLE PROCESS: cannot simply be reversed to original state. Example: gas expanding
Processes in which the disorder of the system increases tend to occur spontaneously. Ex: gas expanding, ice melting, salt dissolving
ENTROPY: (S) the change in disorder. (Change in randomness) The more disorder, the larger the entropy. S = S final - S initial
S = > 0 when the final state is in more disorder S = < 0 when the final state is more ordered than original
H 2 O (l) H 2 O (s) Ag + (aq) + Cl - (aq) AgCl (s)
- a solid melts - a liquid vaporizes - a solid dissolves in water - a gas liquefies
For a process at constant temperature, the entropy change is the value of q rev divided by the absolute temperature. S = q rev /T
Example: Calculate the entropy change when 1 mol of water is converted into 1 mol of steam at 1 atm pressure. ( H vap = 40.67 kJ/mol)
(1 mole)(40.67 kJ/ mol)(1000 J/1 kJ) 373 K S = 109 J/K
The normal freezing point of mercury is -38.9 o C, an its molar enthalpy of fusion is H fus = 2.331 kJ/mol. What is the entropy change when 50.0 g of Hg (l) freezes at the normal freezing point?
-2.48 J/K The answer is negative because the process brings more order
The normal boiling point of ethanol, is 78.3 o C and its molar enthalpy of vaporization is 38.56 kJ/mol. What is the change in entropy when 25.8 g of C 2 H 5 OH (g) condenses to liquid at the normal boiling point?
3 rd Law of Thermodynamics: the entropy of a pure crystalline substance at absolute zero is zero. S(0K) = 0
In general, the entropy increases when: Liquids or solutions are formed from solids Gases are formed from either solids or liquids The number of molecules of gas increases during a chemical reaction.
CaCO 3(s) CaO (s) + CO 2(g) N 2(g) + 3H 2(g) 2NH 3(g)
Standard molar entropies: (S o ) absolute entropies for substances in their standard state. (J/mol-K) 1. Unlike enthalpies of formation, the standard molar entropies of elements are not zero. 2. The S o of gases are greater than those of liquids and solids. 3. The S o generally increases with increasing molar mass. 4. The S o generally increase with the number of atoms in the formula.
S o = nS o (products) - mS o (reactants) Calculate S o for the synthesis of ammonia: N 2(g) + 3H 2(g) 2NH 3(g)
S o = (2 mol)(192.5 J/mol-K) - [(1 mol)(191.5 J/mol-K) + (3 mol)(130.6 J/mol-K)] = -198.3 J/K
Using Appendix C, calculate the standard entropy change, for the following reaction: Al 2 O 3(s) + 3H 2(g) 2Al (s) + 3H 2 O (g)
GIBBS FREE ENERGY The spontaneity of a reaction involves both enthalpy and entropy. The relationship is known as free energy. G = H - T S
1. If G is negative, the reaction is spontaneous in the forward direction. 2. If G is zero, the reaction is at equilibrium. 3. If G is positive, the reaction in the forward direction is nonspontaneous; work must be supplied from the surroundings to make it occur. However, the reverse reaction will be spontaneous.