The heat transfer inside the fridge are almost completely based on convection. So the air moves around, changing it's temperature - there is not much separation of cold and warm air normally.
That is intended, of course, and to distribute cold air through the fridge, it's best to put the cooling element at top end - I assume that's the case in your fridge. Now, warm air can go up to the cooling element, and cold air comes down from there.
In normal operation, the only other relevant heat flow is the leaks through the walls and door.
In comparison, outside of the salad tray, there is some heat leaking from the walls, and some convection up and down to the cooling element.
Inside the salad tray, there is less leak from the walls because of the additional isolation from the tray itself, but very few convection transporting away the heat that leaked in.
Because of this, it gets somewhat warmer in the tray. The heat conduction through the tray wall and through the fridge wall (plus minor convection leaking out of the tray into the fridge, added to tray wall conduction) will find a balance, with the tray at a slightly higher temperature.
(The effect is much stronger if the bottom of the fridge is also an outside wall, leaking heat in. But it may also be the top of a colder compartment of the fridge, so I ignore it for simplicity.)
The Second Law of Thermodynamics states that the entropy of the universe always increases or stays constant. This means that we can reduce the entropy of the gas in the box (gas compressed from the whole box to half the box at constant temperature), only if we increase the entropy somewhere else. For example, we can compress the gas, doing work on it, and thus heating it. Then we have to allow the gas to lose this heat to the environment so that it ends at the original temperature, in half the box. The result is that by heating the environment I have increased the entropy of the environment by at least as much as I have decreased the entropy of the gas in the box.
Since most systems are not isolated (they can exchange energy with the environment), I always have to include both the system (box of gas etc) and the environment.
2) -> 1): This is more probable, and energy is set free.
If we allow the gas to expand suddenly into the whole box by removing the partition in the middle, the entropy increases, but there is no change in the energy. This is an irreversible process - entropy increases.
ΔQ=TΔS
This is not generally true. It is only correct when the change is reversible - in other words the change occurs slowly through a series of equilibrium states. You also have to interpret it correctly. ΔQ is the heat added to the system from the outside. ΔS is the increase in entropy of the system. If the process of taking this heat from the outside world is also reversible, the outside world loses ΔQ of heat, and loses ΔS in entropy, so the entropy of the universe is unchanged.
The case above (sudden expansion of the gas) is an example where this equation does not hold. There is no transfer of heat, but the entropy of the gas increases.
To summarise. The temperature of a gas is a fairly easy concept - it is proportional to the average random kinetic energy of the molecules. The entropy is more complicated. Entropy is increased by allowing the gas to occupy more volume, because the molecules can be arranged in space in a larger number of ways (more "states" as you describe it). Entropy is also increased by raising the temperature of the gas, because this gives more ways for the kinetic energy to be distributed.
When does a change in a system occur spontaneously (for example a chemical reaction, or the expansion of a gas)? It occurs either because the change increases the entropy of the system, or because it decreases the total energy of the system. The spare energy leaves the system, heating and thus increasing the entropy of the environment.
Of course, if the course you are taking is not in physics or physical chemistry, it is likely to include a great deal of nonsense when it comes to entropy. The subject can only be understood by disentangling the concepts as I have tried to do.
Always ask,
1. Energy or entropy? Energy is conserved, entropy is not.
2. Energy, heat, or temperature - all different concepts.
2. Is the change reversible or irreversible?
3. When the system changes, does the outside world also change?
Best Answer
The laws of thermodynamics are the basic principles that form the foundation of the subject, but that doesn't mean they are the best starting point for analysing systems.
In this case frozen chicken breasts defrost faster in water than in air of the same temperature because:
water has a higher specific heat than air
water has a higher thermal conductivity than air
The higher specific heat means that for every 1 degree drop in temperature water delivers more heat to the chicken than air does, and the higher thermal conductivity means water delivers that heat faster than air does.
The laws of thermodynamics are involved. For example the first law tells that that because energy is conserved the energy that heats up the chicken must come from the energy released by cooling down the water/air. The second law tells us that the cold chicken/warm water system will equilibrate to a uniform temperature. but neither are useful in determining the kinetics of the process.