Equilibrium Concentrations

[Tinisha 1G]

Hi!
When we calculate K, are we using equilibrium concentrations or initial concentrations?
Thanks guys!

[Nicklas_Wright_1A]

Use equilibrium concentrations to calculate K. Use initial concentrations to calculate Q.

[Ibrahim Malik 1H]

K is a constant referring to the equilibrium constant whereas Q refers to the equilibrium at a certain point in time.

[Stevin1H]

When we are calculating the equilibrium constant, K, we would use the equilibrium concentrations. If we were to use the initial concentrations, the molar concentration ratio of products over reactants would not be equal to the correct value for the equilibrium constant. Using the initial concentrations would give us the quotient reactant, Q, to determine which direction the reaction will favor (either favor products or reactants depending on the Q value relative to K).

[Ethan Baurle 1A]

K is the value of the concentrations of the products over the reactants at equilibrium, thus we would only ever use equilibrium concentrations to find K. Any other concentrations we are given at a particular point in the reaction should be put into the same equation we have been practicing in class. If we find a different value, this is the value of Q and the reaction is not at equilibrium. The problem would most likely tell you whether the given concentrations were at equilibrium or not, or it would give you a value for K and ask if the reaction is at equilibrium (which you would solve using the equation).
Hope this helps!