### Heat and Work relation

Posted:

**Mon Jan 22, 2018 3:25 pm**Based on the deltaU=q+w equation, if a system is endothermic and q is positive, would the work always be negative? If not, when does it become positive?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=75&t=26113

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Posted: **Mon Jan 22, 2018 3:25 pm**

Based on the deltaU=q+w equation, if a system is endothermic and q is positive, would the work always be negative? If not, when does it become positive?

Posted: **Mon Jan 22, 2018 4:16 pm**

I don't think the work needs to be negative if the given q is positive; I believe it is not a causal relationship ("if q is positive, then w must be negative", etc.). If you take a look at 8.13, you can see that this problem gives a negative deltaU value and a negative (but not as large) deltaH value, so therefore a large negative w value is needed.

Posted: **Thu Jan 25, 2018 9:55 am**

In that equation q and w are separate so they don't directly affect each other since they are calculated separately.

Posted: **Thu Jan 25, 2018 9:59 pm**

Not necessarily for this equation.

Posted: **Fri Jan 26, 2018 10:42 am**

Normally when solving delta U = q(heat) + w(work) q and w are calculated separately. The work could have a negative or positive value depending on the delta V in the equation.

Posted: **Fri Jan 26, 2018 11:02 am**

The value of work is not necessarily negative.

According to what the textbook states in CH8: First Law of thermodynamics, △U=q+w

1. the q and w is determined by the realistic circumstance

2. the positive and negative sign is assigned to represent that if the work is done to the system and the system does work to the surroundings (or if the heat is transferred from the system to the surroundings or from the surroundings to the system)

3. the values of q and w are actually really random

Hope this helps!

According to what the textbook states in CH8: First Law of thermodynamics, △U=q+w

1. the q and w is determined by the realistic circumstance

2. the positive and negative sign is assigned to represent that if the work is done to the system and the system does work to the surroundings (or if the heat is transferred from the system to the surroundings or from the surroundings to the system)

3. the values of q and w are actually really random

Hope this helps!

Posted: **Sun Jan 28, 2018 2:53 pm**

No, the work doesn't necessarily have to be negative in this case.