## Equilibrium Constant for Solids/Gases

alicechien_4F
Posts: 104
Joined: Sat Jul 20, 2019 12:15 am

### Equilibrium Constant for Solids/Gases

For the modules, if we were given an equation like A (g) + B (g) --> C (s), why do we write the equilibrium constant as 1/[A][B] instead of just [A][B]? I thought we were supposed to just take solids and liquids out of the equilibrium constant.

Charisse Vu 1H
Posts: 101
Joined: Thu Jul 25, 2019 12:17 am

### Re: Equilibrium Constant for Solids/Gases

You do take solids and liquids out of the equilibrium constant equation, but not including them does not eliminate the numerator. A quick google search indicated that the activity (I believe Professor Lavelle briefly mentioned that we would be considering concentrations to be the same as the activity of a compound) of solids and liquids in any equilibrium reaction is equal to 1, hence the need to keep the numerator in your example.

Mitchell Koss 4G
Posts: 128
Joined: Sat Jul 20, 2019 12:17 am

### Re: Equilibrium Constant for Solids/Gases

Equilibrium constants are ratios and you cannot just delete the numerator, it needs a place holder.

Minh Ngo 4G
Posts: 137
Joined: Thu Jul 25, 2019 12:17 am

### Re: Equilibrium Constant for Solids/Gases

Taking them out doesn’t mean there’s nothing there. Since K is Product over Reactant, you have to indicate that there is Product, so you put 1 there. If you just put [A][B], it would mean that those are the concentration of your products

KaleenaJezycki_1I
Posts: 127
Joined: Sat Aug 17, 2019 12:18 am
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### Re: Equilibrium Constant for Solids/Gases

alicechien_4F wrote:For the modules, if we were given an equation like A (g) + B (g) --> C (s), why do we write the equilibrium constant as 1/[A][B] instead of just [A][B]? I thought we were supposed to just take solids and liquids out of the equilibrium constant.

It is still written as 1/[A][B] because A & B are in the reactants and when you find equilibrium constants it will always be concentration of products over concentration of reactants. Therefore single solids and liquids are not included, you fill in the Kc but still have to put the reactants in the denominator, making it 1/[A][B] , keeping the ratio consistent to the Kc equation.

Michelle Xie 2B
Posts: 99
Joined: Sat Aug 17, 2019 12:18 am

### Re: Equilibrium Constant for Solids/Gases

Solids and liquids are not included in the equation.