Question

During the power stroke of an internal combustion engine, the air-fuel mixture is ignited and the expanding hot gases push on the piston. Fuel efficiency is maximized in this process when the ignited gas is as hot as possible, the gas expands allowing a maximum amount of work to be done, and cooled exhaust gas is released at the end of the cycle. Assuming the engine exhibits the highest efficiency possible, which of the following statements concerning the exhaust gas must be true to avoid violating the second law of thermodynamics?

The exhaust gas must be cooled to the same temperature as the outside air.

The exhaust gas must be at the same pressure as the outside air.

The exhaust gas must be hotter than the outside air temperature.

.

The exhaust gas must be at the same volume as the outside air.

A tray of water is placed into a freezer. As the water cools, its entropy decreases and eventually it turns to ice. Why doesn’t this process violate the second law of thermodynamics?

The entropy of the water decreases, but upon freezing it increases to its previous value.

The process as described does violate the second law of thermodynamics.

The overall entropy increases due to the refrigerator chilling and eventually freezing the water.

When the ice is later taken out and melted, the entropy will increase back to what it was before the tray was put into the freezer

Answer 1

1) correct option is C - The exhaust gas must be hotter than the outside air temperature.

for any heat engine to produce work, there must be two different temperature source and sink, where high temperature exhaust gas is released in the air, which is cooler than the exhaust gas. according to law of thermodynamics heat is transferred from region of high temperature to low temperature region( i.e air).

if the air where exhaust gas is released is of same temperature as the gas ,then there will be no pressure difference and eventually the internal combustion could not operate properly.

2) correct option is C - The overall entropy increases due to the refrigerator chilling and eventually freezing the water.

here, the refrigerator and freezing water is considered to be a system, we realize that water is becoming ice and after solidifying its entropy decreases, ( as in solids, molecules arrangements are more ordered unlike fluids), but relatively the entropy of the chilling refrigerator is increasingly very high.  

so overall the entropy of the system increases only due to refrigerator chilling,

and also without violating the second law of thermodynamics.