<p>When solving Kc (Chemical Equilibrium constant), I understand that the the general formula is: {(Conc.Product1)(Conc.Product2)}/{(Conc.Reactant1)(Conc.Reactant2)}</p>
<p>However, when is it necessary to calculate the reciprocal (1/Kc)? Is it when the first reaction is a reverse reaction? For example, if there is no concentration of products given in an equilibrium equation, the equilibrium must shift toward the right (forward reaction). Likewise, it is correct to say that if there is no concentration of reactants given, the equilibrium will shift to the left (reverse reaction). If this is the case, do you calculate the Kc with the formula above, or the reciprocal?</p>
<p>Thanks in advance.</p>
<p>You also have to raise each concentration to the cooresponding coefficient. Clearly if you have Kc for the forward reaction, then the constant for the reverse reaction will be 1/Kc. If Kc>1, then your concentration of products is higher than the concentration of the reactants. I may be misinterpreting your question, but are you asking about starting with certain reactants when no reaction has taken place? You mentioned only having products, which in that case usually involves strong electrolytes where there is no equilibrium (since the reaction is entirely forward).</p>
<p>If there is no inital [reactants], the reaction must shift to the left to reach equilibrium. Therefore, is the equilibrium constant = 1/Kc or still Kc?</p>
<p>Example: PCl3(g) + Cl2(g) <----> PCl5(g)</p>
<p>If there is an initial [PCl5], but none for [PCl3] and [Cl2]], the reaction must shift toward the left, in order to establish an equilibrium.(CORRECT?) Provided that is correct, is the equilbirum constant calculated as such: {[PCl3][Cl2]}/{[PCl5]} (which is the reciprocal) OR {[PCl5]}/{[PCl3][Cl2]}?</p>
<p>If there is excess PCL5, then the reaction will shift to the left to reach equilibrium. The equilibrium constant is something that is calculated when the reaction is at equilibrium. Therefore you will have a constant for the forward reaction, and one for the reverse reaction. It seems you are looking for the reverse reaction, which would be Kc (reverse) = [PCl3][Cl2]/[PCl5].</p>
<p>Thanks for the help :)</p>