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### Kc and Qc

Posted: Wed Jan 09, 2019 11:40 pm
What is the difference between Kc and Qc? I know K has the brackets while Q uses partial pressures, however, what is the distinction?

### Re: Kc and Qc

Posted: Wed Jan 09, 2019 11:51 pm
Kc is the equilibrium constant and uses the ratio of the concentrations of products over reactants at equilibrium. It can also be expressed as Kp when using partial pressures. Qc and Qp are the ratio of the concentrations or partial pressures of a system at a specific point at which the forward and reverse reactions are occurring. It has not yet reached equilibrium, so it will be different from K. If Q is equivalent to K, then that "point" is equilibrium.

### Re: Kc and Qc

Posted: Wed Jan 09, 2019 11:53 pm
Qc and Kc are calculate the same way, but Qc is used to determine which direction a reaction will proceed, while Kc is the equilibrium constant (the ratio of the concentrations of products and reactants when the reaction is at equilibrium). So, Qc could be = to Kc, but it may not be.

When Q < K, there is more reactant than when in EQ so the forward reaction is favored
If Q = K, then the reaction would be in equilibrium and neither the forward nor reverse reaction would be favored
When Q > K, there is more product than when in EQ so the reverse reaction is favored

Let me know if you need more clarification!

### Re: Kc and Qc

Posted: Thu Jan 10, 2019 10:09 am
K can be used to determine the equilibrium constant using either concentration (Kc) or partial pressure (Kp). K determines the ratio of product over reactant at equilibrium and the value should be the same for each reaction under the same conditions and temperature. Q is very similar to K as they are calculated the same way with using products over reactants of the reaction. However, the Q value is not at equilibrium and is used to determine whether the reaction will favor reactant or product formation. If Q<K, then the reaction will favor product formation to reach equilibrium. if Q>K, then the reaction will favor reactant formation to reach equilibrium.

### Re: Kc and Qc

Posted: Thu Jan 10, 2019 1:02 pm
Kc and Kp both find the equilibrium constant for the reaction, with Kc being calculated using equilibrium concentration and Kp being calculated using equilibrium partial pressures. Qc and Qp can be calculated at any time during a reaction in order to determine which direction the reaction will proceed in, with Qc being calculated using the concentration at that time and Qp being calculated using the partial pressure at that time. If Q < K, the forward reaction is favored. If Q > K, the reverse reaction is favored.