Equilibrium

[anbuitachi]

Hi just a few questions.

When writing equilibrium equations, why is solid and liquid considered 1 and left out even when the solid participates in the reaction?

Is Kc for liquid and solids and Kp for gasses?

Why is it that if you add He to the Haber process (N2 + 3H2 --> 2H3 + Heat), the total pressure increases but equilibrium is not shifted?

Thanks

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[ksmi117]

First, the solids and liquids are not included because their concentrations do not change. Yes, they react, but Keq depends on concentrations and those are constants for solids and liquids. For example, liquid water ALWAYS has a concentration of 55.5 M. Since they don't change, we just ignore them.

Kc is the constant in terms of concentrations. This is used for aqueous systems or aqueous and gas systems. Kp is the constant in terms of partial pressures. This is used for all gas systems.

Adding an inert gas won't shift the system because it doesn't change the parital pressures of each gas in the reaction. It changes the overall pressure, but the partial pressures stay the same. Keep in mind that this only holds if volume is kept constant. If the volume changes, the partial pressures will change, and the system will shift.

Hope this helps.

 

[anbuitachi]

Hey thanks for the explanation but just have few questions.

is the partial pressure from reactants from collisions? is that why helium doesn't change the partial pressure of the others? if you add other gases how do those affect the partial pressure?

thanks

 

[ksmi117]

From Wiki:

The partial pressure of an ideal gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature.

So technically adding other gases wouldn't initially affect the partial pressures either (unless you changed the volume) BUT other gases would react with the gases already in the system and mess up the system that way. That's why He is an example, because it's an inert gas (it won't react).

Make sense, kinda?

 

[anbuitachi]

ohhh yes, thanks