Why are ∆H and ∆S temperature dependent
Moderators: Chem_Mod, Chem_Admin
-
- Posts: 53
- Joined: Tue Feb 02, 2021 12:16 am
Why are ∆H and ∆S temperature dependent
During lecture, the professor said that K should be calculated at two different temperatures for Van't Hoff equation and that ∆H and ∆S are not temperature dependent. However, I was curious why we know that these values are not temperature dependent. Can this be identified from the way they are derived?
-
- Posts: 102
- Joined: Fri Sep 24, 2021 5:12 am
- Been upvoted: 1 time
Re: Why are ∆H and ∆S temperature dependent
I think you miss understood slightly. You can use the equation when ΔH and ΔS do not significantly change due to temperature. They do change when temperature changes, but if the temperature change is very slight, such as a couple degrees, they don't vary significantly and you can use the equation.
Think of it this way, entropy is a measure of disorder. A gas at 25C in 1L volume and 1atm will have less disorder than the gas at 2500C in 1L volume and 1atm because as the gas moves faster and with more energy it will be more disordered. However, a gas at 25C will have a very similar entropy to a gas 27C and because the entropy is so close, we can assume that the entropy is the same for the sake of the calculation.
Think of it this way, entropy is a measure of disorder. A gas at 25C in 1L volume and 1atm will have less disorder than the gas at 2500C in 1L volume and 1atm because as the gas moves faster and with more energy it will be more disordered. However, a gas at 25C will have a very similar entropy to a gas 27C and because the entropy is so close, we can assume that the entropy is the same for the sake of the calculation.
-
- Posts: 99
- Joined: Fri Sep 24, 2021 5:26 am
Re: Why are ∆H and ∆S temperature dependent
My understanding is that delta S and delta H are both independent of temperature, but the translation between the two measurement involves temperature. thus for K, which is a measurement of the favorability of reaction, we need to unify change in S and H to see if the total entropy change is positive Or if the change in free energy is negative. Thus K would be temperature dependent.
-
- Posts: 103
- Joined: Fri Sep 24, 2021 6:19 am
Re: Why are ∆H and ∆S temperature dependent
I agree with Lucas! Delta H and Delta S are held relatively constant for small temperature changes, but they will change by larger amounts when temperature changes by larger amounts. Even for smaller temperature changes, the numerical values of H and S change; it is the difference delta H and delta S that do not change by much.
Re: Why are ∆H and ∆S temperature dependent
One way you can see that they are temperature-dependent is through the equation Delta G=Delta H-T Delta S.
Re: Why are ∆H and ∆S temperature dependent
Reading through these explanations confused me a little bit. If we were asked straight up: "Are H and S temperature dependent or independent" would you say they are temperature dependent since they do vary with large temperature changes?
-
- Posts: 34
- Joined: Mon Jan 09, 2023 2:32 am
Re: Why are ∆H and ∆S temperature dependent
Basically ∆H and ∆S are standard values (either molar or specific) based on the identity of the substances, so they have nothing to do with the temperature of the substance. These values are based on how many bonds are being formed/broken and if the total entropy of the system is being increased, both of which are going to change the same amount regardless of the temperature you begin at.
-
- Posts: 35
- Joined: Mon Jan 09, 2023 2:43 am
Re: Why are ∆H and ∆S temperature dependent
305607822 wrote:Reading through these explanations confused me a little bit. If we were asked straight up: "Are H and S temperature dependent or independent" would you say they are temperature dependent since they do vary with large temperature changes?
Yes because H and S are essentially standard values based on the identity of the substances; they are unrelated to the substance's temperature. These values depend on the number of bonds that are being created or broken and whether the total entropy of the system is increasing. Regardless of the initial temperature, both of these values will vary by the same amount.
Return to “Van't Hoff Equation”
Who is online
Users browsing this forum: No registered users and 3 guests