Different enthalpy of vaporizations for water

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Vera Ong 3H
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Joined: Fri Jun 17, 2016 11:28 am

Different enthalpy of vaporizations for water

Postby Vera Ong 3H » Mon Jan 16, 2017 5:27 pm

On page 284 in the textbook, there are two listed enthalpy of vaporizations for water. Why is this the case? If this is already the standard enthalpy of physical change, shouldn't 25 degrees Celsius already by assumed? When should the 40.7 degrees Celsius value be used?

Yi Ying Chen 3N
Posts: 13
Joined: Wed Sep 21, 2016 2:55 pm

Re: Different enthalpy of vaporizations for water

Postby Yi Ying Chen 3N » Mon Jan 16, 2017 7:44 pm

The 40.7 degrees celsius value would be used when we're dealing with water at its boiling point (100 degrees celsius), as mentioned at the bottom of pg 283.

The standard reaction enthalpy only indicates that the reactants and products are in their standard state (pure form at 1 bar/atm). Since temperature is not part of the definition of standard states, we can't assume that the water is at 25 degrees celsius.

NinaSheridan
Posts: 30
Joined: Wed Sep 21, 2016 2:58 pm

Re: Different enthalpy of vaporizations for water

Postby NinaSheridan » Thu Jan 19, 2017 11:28 am

I'm a little confused about the idea of "standard states" to begin with. For example, a gas is only in its standard state at 1 atm. So what if it isn't at 1 atm? What does that make it? Does that just mean the reaction enthalpy isn't the standard reaction enthalpy?

Jessie_Chen_2L
Posts: 20
Joined: Wed Sep 21, 2016 2:59 pm

Re: Different enthalpy of vaporizations for water

Postby Jessie_Chen_2L » Thu Jan 19, 2017 11:54 am

The standard state just means what phase the substance is at 1atm and at 25 degrees Celsius. For example, the standard state of water under these conditions would be liquid. If it's not at 1atm, then it's not in its standard state, so you would find the reaction enthalpy.


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