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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Diamonds on February 20, 2013, 09:42:04 PM

Title: Clausius-Clapeyron: Ea or dH?
Post by: Diamonds on February 20, 2013, 09:42:04 PM
Hi guys,

I have seen the Clausius-Clapeyron equation expressed as:

ln(P1/P2) = ΔH / R  (1/T2 - 1/T1)
ln(K1/K2) = ΔH / R  (1/T2 - 1/T1)
ln(K1/K2) = Ea /  R  (1/T2 - 1/T1)

which is right? If they are all right (because all have worked in different cases), doesn't that mean that ΔH = Ea?
Title: Re: Clausius-Clapeyron: Ea or dH?
Post by: formaldehyde23 on February 20, 2013, 11:56:58 PM
The only one of the three equations that is the Clausius-Clapeyron equation is the first one, which relates pressure and temperature.
I do not think the second equation exists.
The third equation is the Van't hoff's equation, which relates the rate constant and temperature.
But, no dH is definitely not equal to Ea. Think about a simple reaction. dH is the change in enthalpy of the products and reactants. However, Ea is the minimum amount of energy needed to convert a reactant into a product.
Title: Re: Clausius-Clapeyron: Ea or dH?
Post by: Diamonds on February 21, 2013, 12:03:47 AM
So if a question involves pressures, I use the Clausius-Clapeyron equation to find out ΔHvap, and if it asks about activation energy, I use the Van't Hoff equation to find out the Ea?
Title: Re: Clausius-Clapeyron: Ea or dH?
Post by: formaldehyde23 on February 21, 2013, 12:34:44 AM
Yes, exactly.
Title: Re: Clausius-Clapeyron: Ea or dH?
Post by: Sophia7X on February 21, 2013, 05:39:02 AM
No, the second equation definitely does exist and that is the Van't Hoff equation.

The third equation is the Arrhenius equation which does not involve K but k, the rate constant.