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### Writing Equilibrium Constant Labels

Posted: Mon Jan 06, 2020 5:41 pm
When writing the products and reactants for the equilibrium constant, why do we add P before the formula that is part of the chemical equation? Does this represent the partial pressure?

### Re: Writing Equilibrium Constant Labels

Posted: Mon Jan 06, 2020 5:45 pm
Bryce Ramirez 1J wrote:When writing the products and reactants for the equilibrium constant, why do we add P before the formula that is part of the chemical equation? Does this represent the partial pressure?

I believe that you are correct. If all of the substances in the equilibrium reaction are in the gas phase, that means that to determine when the system is at equilibrium we will use P, the partial pressure to calculate K. If the products and reactants are all in an aqueous solution, then concentration (the estimate for activity) would be used in the equilibrium expression.

### Re: Writing Equilibrium Constant Labels

Posted: Mon Jan 06, 2020 5:56 pm
P does means partial pressure because you are using a gas throughout the entire equilibrium. The equilibrium constant would also be denoted as Kp. You can often use the equation, PV = nRT in order to get the concentration of the whole equilibrium if you were asked to find it.

Both Kp and Hc have the same system where products are over reactants in a ratio.

### Re: Writing Equilibrium Constant Labels

Posted: Mon Jan 06, 2020 5:57 pm
Little clarification, even when all the products and reactants are given in the gas phase you don't always have to use partial pressures to calculate the K value. If you are given concentrations, then using those and calculating K_c is fine as well.

### Re: Writing Equilibrium Constant Labels

Posted: Mon Jan 06, 2020 6:24 pm
Kc is used for concentrations (M), which can be gases or aqueous molecules, Kp uses partial pressures of gases. Question should be clear on which one to use.