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### Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 5:32 pm
In lecture, I wrote down that Standard Reaction Enthalpy is defined at 1 atm if it's in a gas state (1 M if in a solution) and of elements in their most stable forms.

I wasn't sure if I misheard Dr. Lavelle when he discussed the temperature of 298 Kelvin / 25 degrees Celsius; does standard reaction enthalpy have to be at this temperature?

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 5:42 pm
No the definition of standard states doesn't include temperature so a standard state can exist at any temperature.

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 5:44 pm
I don't think that it has to always be at that temperature. A majority of the times, it is at 25 degrees Celsius/298 Kelvin. However, the temperature can vary if the reaction is stated at a specified temperature of interest.

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 7:23 pm
So it can be at any temperature, so long as the substances are existing in their standard state? Thanks!

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 7:30 pm
McKenna disc 1C wrote:So it can be at any temperature, so long as the substances are existing in their standard state? Thanks!

Yes, that is correct. However, most of the time, the reported data is for 25 degrees celsius.

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 7:31 pm
Also, Dr. Lavelle said that standard reaction enthalpy is at 1 atm, but my TA said it was at 1 bar. Which is it?

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 9:29 pm
For the standard reaction enthalpy definition, it does not matter what the pressure or temperature is. It refers to a specific reaction enthalpy value at a constant pressure and constant temperature. Naturally, this implies that the values change with temperature and volume. However, most people just use 25 C and 1 atm.

### Re: Standard Reaction Enthalpy Definition

Posted: Sun Jan 14, 2018 9:50 pm
No it does not have to be at this temperature but most reactions happens at 25 degrees Celsius