19.3 a) The diagram represents a solid turning into a gas. Because gases are more disordered than solids, S is positive. H is also positive because

Transcription

1 19.1a) 19.1b) S is positive because the gases become more random as they spread out to fill the containers. H is zero, assuming the gases do not exchange heat with the surroundings. c) This process is not reversible d) As established in (b), gases do not exchange heat with the surrounds. Therefore, the entropy of the surroundings does not change.

2 19.3 a) The diagram represents a solid turning into a gas. Because gases are more disordered than solids, S is positive. H is also positive because melting and boiling are endothermic processes. b) Because S is positive and G = H -T S, G will be negative and high temperatures (those above the boiling point of the substance) and will be positive at lower temperatures (those below the boiling point of the substance. c) Since S for the system is positive, S for the surrounds must be negative. If the process is spontaneous, we know that Suniv will be positive. As such, the Ssurr will be a smaller negative value than Ssys. If the system is at equilibrium, then Ssurr will be equal in magnitude to Ssys, but will still be negative Both reactants and products are gases. However, there are twice as many moles of product, indicating that entropy increases. As such, S will be positive.

3 19.6a) Since isomers have the same chemical formulas, the balanced equations representing their combustions will be identical. As such, we would expect the bonds being broken and formed to be equal for both isomers, suggesting very similar H s. b) We would expect n-pentane to have great molar entropy because the elongated molecule is more likely to vibrate and rotate a) Ripening banana - spontaneous b)dissolving sugar in coffee - spontaneous c)nitrogen atoms forming N2 - spontaneous d)lightning - spontaneous e)formation of CH4 and O2 from CO2 and H2O - nonspontaneous 19.13a) Ammonium nitrate dissolving & ice cream melting at RT. b) Melting a solid is spontaneous at temperatures above it s melting point and not spontaneous at temperatures below its melting point.

4 19.15 a) Boiling water is an endothermic process b) This process is spontaneous at T > 100 C at 1 atm. c) This process is nonspontaneous at T < 100 C at 1 atm. d) The two phases in equilibrium at T = 100 C at 1 atm a) A reversible process is unique because it can be reversed through the same pathway that is occurred with no change in q or w. b) When a reversible process is reversed the surroundings (and the system) are returned to their original state. c) Water vaporizing is reversible at the boiling point of water. d) No. None of the processes which we witness in the world around us are reversible because they would not occur. Instead, the processes we witness are spontaneous and therefore irreversible.

5 19.18 a) An irreversible process is one which cannot be undone by simply reversing the process through the same pathway. b) When the system is returned to its original state, we know it must have returned through a different pathway. As such, qf and wf are not the same as qr and wr, meaning there is a net change in the surroundings. c) The condensation of a liquid is irreversible at any temperature other than the boiling point of the liquid a) If a gas cools from 300K to 200K without a change in volume, either its pressure will drop or some gas must escape. b) This change would be possible at constant pressure if the volume is allowed to decrease or gas is allowed to escape. c) No, E does not depend on the path taken.

6 19.21a) An ice cube at 0 C can melt reversibly. b) We know that melting is an endothermic process. Even though temperature does not change during melting, H is not zero a) S for an isothermal process is calculated by knowing the amount of heat transferred if the process were reversible. b) S does not depend on path a) When Br2 boiling it creates a gas with greater entropy. As such entropy increases.

7 19.27a) The 2nd law of thermo says that any spontaneous process increases the entropy of the universe; any equilibrium process does not effect the entropy of the universe. b) In an reversible process (equilibrium), Suniv = 0. If the entropy of the system increases, the entropy of the surroundings must decease equally. c) Because the process is spontaneous, we know the entropy of the universe is increasing. If the entropy change of the system is 42j/K, the know the decrease in entropy of the surroundings must be some value less an 42j/K a) Whenever a gas expands isothermally we expect the entropy to increase, making S positive. c) The temperature change is not needed as long as the change is isothermal.