## System and surroundings

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Jasmin Argueta 1K
Posts: 69
Joined: Fri Sep 28, 2018 12:16 am

### System and surroundings

Can anyone explain why the change in the system= the negative change of the surrounding in regards to reversible reactions. I can't seem to understand this conceptually.

Vincent Li 4L
Posts: 48
Joined: Fri Sep 28, 2018 12:19 am

### Re: System and surroundings

If this is specifically regarding entropy, that is because in reversible reactions, the total change in entropy (in the universe) is equal to zero. As a result, any change in entropy in the surroundings must be equal and opposite to the change in entropy of the system because they add up together to equal the total change in entropy.

Another case you might be thinking of is when q = -w. The reason why this works is because reversible gas expansions are typically isothermal. Isothermal means the temperature is unchanging, and if the temperature is unchanging, so is the total energy of the system. Since deltaU = q + w, if deltaU = 0, 0 = q + w, or q = -w. Hope this helps.

Karina Jiayu Xu 4E
Posts: 58
Joined: Fri Sep 28, 2018 12:29 am

### Re: System and surroundings

Recall that the change in entropy of the universe is equal to the sum of the change in entropy of the surrounding and the system. And in a reversible reaction, the total change in entropy in the universe is 0, and thus the change in entropy of the surrounding and the system have to equal.

Nicole Elhosni 2I
Posts: 62
Joined: Fri Sep 28, 2018 12:28 am

### Re: System and surroundings

It is similar to work and how work affects the system and surroundings. If work is put into the system, it is because the surroundings have released it. And if the surroundings are worked on, it is because the system has released it. So work of the surroundings = work of the system. I think it is the same idea for entropy.

KimGiang2F
Posts: 31
Joined: Fri Sep 28, 2018 12:19 am

### Re: System and surroundings

The change in the system= the negative change of the surrounding in regards to reversible reactions is conceptually based on the fact that the amount lost and gained in both systems must be equal to 0. Essentially, if a system loses 10 units, the surrounding must gain 10 units so that the amount of work transferred in between systems equals to the total change in entropy in the universe, which is 0.