Moad 9

Question 1

Expanding a gas slowly

Answer 1

When we expanded a gas infinitely slowly, the system was able to absorb heat from the surroundings allowing it to do maximum work. The process could be reversed, compressing the gas infinitely slowly, and that same amount of heat (let’s call this qrev) would then be transferred back to the surroundings. Thus, this process was fully reversible.

Question 2

Expanding a gas quickly

Answer 2

But when we did the same expansion but in 1 or 2 steps, the expansion was done so quickly that the surroundings didn’t have chance to equilibrate with the system, and so not all the available heat was transferred, i.e. q < qrev. Thus, less energy was available to do work.

Question 3

General law for heat expansion

Answer 3

qrev-q> 0 for an irreversible process to occur spontaneously

Question 4

what is a spontaneous change?

Answer 4

Spontaneous changes are ones that will happen without any intervention by us. Note, there is no timescale mentioned (that’s kinetics), so we don’t know if the spontaneous process will start in a second, or a day, or 1000 years, just that it will eventually start.

Question 5

Spontaneous changes are always accompanied

Answer 5

Spontaneous changes are always accompanied by a reduction in the ‘quality’ of the energy, in the sense that it is degraded into a more dispersed, chaotic form.

Question 6

All spontaneous processes are

Answer 6

All spontaneous processes are irreversible, because they lose energy by giving it to the surroundings in a chaotic form of heat that cannot subsequently be retrieved.