6 Reversible ProcessesWhen 1 mol of water is frozen at 1 atm at 0C to form 1 mol of ice, q = Hvap of heat is removed.To reverse the process, q = Hvap must be added to the 1 mol of ice at 0C and 1 atm to form 1 mol of water at 0C.Converting between 1 mol of ice and 1 mol of water at 0C is a reversible process.
7 Irreversible Processes An irreversible process cannot be reversed to restore the system and surroundings back to their original state.A different path with different values of q and w are needed. Surroundings are not returned to original conditions.
9 Spontaneous Expansion of a Gas Why does the gas expand?Why is the reverse process nonspontaneous?
10 Spontaneous Expansion of a Gas Consider 2 gas molecules in one flask (a).Once the stopcock is open, there is a higher probability that one molecule will be in each flask than both molecules being in the same flask (b).
12 In ice, molecules are very well ordered because of the H-bonds. Ice has a low entropy.As ice melts, intermolecular forces are broken , order is interrupted.Water is more random than ice, has higher entropy.Ice spontaneously melts at room temperature.
13 There is a balance between energy and entropy considerations. When an ionic solid is placed in water two things happen:the water organizes into hydrates about the ions (so the entropy decreases), andthe ions in the crystal dissociate (the hydrated ions are less ordered than the crystal, so the entropy increases).A balance between energy and entropy considerations.When an ionic solid is placed in water two things happen:Hydrates form (entropy decreases)Ions dissociate (entropy increases)
17 Entropy and the Second Law of Thermodynamics For a reversible process: Suniv = 0.For a spontaneous process (i.e. irreversible): Suniv > 0. Suniv< 0 means spontaneous is opposite direction.Second law states that the entropy of the universe must increase in a spontaneous process. Entropy of a system can decrease as long as the entropy of the surroundings increases.For an isolated system, Ssys = 0 for a reversible process and Ssys > 0 for a spontaneous process.
19 Example Packet p 7The element mercury, Hg, is a silvery liquid at room temperature. The normal freezing point of mercury is -38.9C and its molar enthalpy of fusion is ∆H = kJ/mol. Calculate the entropy change of the system when 50.0 g liquid mercury freezes at the normal freezing point.Remember: fusion = meltingUse:
27 Atomic Modes of MotionMovement of molecules is related to energy and entropy.Three atomic modes of motion:translation (moving from one point in space to another),vibration (shortening and lengthening of bonds, including the change in bond angles),rotation (spinning around an axis).
29 Third Law of Thermodynamics Third Law of Thermodynamics: the entropy of a perfect crystal at 0 K is zero.As we heat a substance from absolute zero, the entropy must increase.Entropy changes dramatically at a phase change.Entropy increases when--Liquids or solutions are formed from solids--Gases are formed from solids or liquids--The number of gas molecules increases.
30 Heat and EntropyAs we heat a substance from absolute zero, the entropy must increase.
31 Boiling corresponds to a much greater change in entropy than melting.
34 Entropy Changes in Chemical Reactions Absolute entropy can be determined from complicated measurements.Standard molar entropy, S: entropy of a substance in its standard state. Similar in concept to H.Units: J/mol-K. Note units of H: kJ/mol.Standard molar entropies of elements are not zero.For a chemical reaction which produces n moles of products from m moles of reactants: