Does the Value of K Depend on Initial Reactant and Product Amounts?

[i_love_science]

Here is the question:
Is it true that the value of K depends on the amounts of reactants and/or products that are mixed together initially? Explain.

This is my explanation:
The reaction always shifts left or right given any initial reactant and product amounts to attain equilibrium. Thus, the ratio of the equilibrium concentrations (or partial pressures), which is represented by the value of K, is also not affected by the initial reactant and product amounts. The statement is false.

I need help on writing clear and concise explanations. Could anyone correct my solution or recommend a better answer? Are there any strategies for writing clearly and concisely? Thank you!

 

[mjc123]

You are correct that the statement is false. The concept of an equilibrium constant would be meaningless if it were true.
What may confuse people is that the actual concentrations at equilibrium may be different, depending on the initial concentrations of reagents. Thus in a reaction A + B ↔ C + D, you will get different equilibrium concentrations of the various species if you start with an A:B ratio of, say, 1:1 or 2:1 or 1:3. But the equilibrium concentrations will always satisfy [C][D]/[A][B ] = K.
The best advice I can suggest for writing clearly is to understand the subject clearly. Possibly that might not be very helpful.

 

[Borek]

Other reason why people often seem to misinterpret K is that they often confuse K with Q - reaction quotient. Reaction quotient is that thing on the right side of the K definition, and it is perfectly OK for the reaction quotient to take any value - it just means reaction is not at equilibrium and it will proceed till Q = K.

Yes, it doesn't address your main concern.

 

[Mayhem]

An equilibrium constant (K) relates only the concentrations/partial pressures of dissolved or gaseous substances when the reaction is at equilibrium. The concentrations/partial pressures at any other state can be quantified using the reaction quotient (Q) and we can compare K and Q to see how far we are from an equilibrium. If initial concentrations/partial pressured affected K, then K would not be constant - then of what use is it?