Understanding the Impact of Pressure on Reaction Energy

[PPonte]

The heat of reaction is:

The amount of heat absorbed or evolved when specified amounts of compounds react under constant pressure.

The change in the enthalpy of the system that occurs when a reaction is run at constant pressure.

My question is: what does the change in pressure do to the energy involved in a reaction?

 

[Hootenanny]

To increase the pressure you have to do work on the system, thereby increasing the energy. To decrease the pressure (aslong as it is above 1 atm) you have to allow the system to do work. This a bit of a rough and ready explanation I'll admit.

 

[PPonte]

Ok. Rough but good.
So, the heat of reaction is the change in the internal energy of the systhem. Right?

 

[Hootenanny]

Temperature is the mean kinetic energy of the particles in that system. The average kinetic energy of a system can said to be
$$E_k = \frac{3}{2}kT$$
The ideal gas law links, pressure volume and temperature:
$$pV = nRT = NkT$$
$n$ is number of moles; $R$is molar gas constant; $N$ is number of molecules/particles; $k$ is the Boltzmann constant $= \frac{R}{N_A}$ where $N_A$ is Avagadro's constant.

For more information on heat see http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html#heacon . But be aware that this is heat from a physics perspective and chemistry.