The Reaction Quotient, “Q”

Presentation on theme: "The Reaction Quotient, “Q”"— Presentation transcript:

The Reaction Quotient, “Q”
Learning Goal: I will understand that the reaction quotient is the same as K, just not at equilibrium. I will understand how to compare the reaction quotient (Q) to the equilibrium constant (K) to determine it a system is at equilibrium, or what direction a shift will occur.

What is “Q”? Q is a value we can use to determine if a reaction is at equilibrium. If a reaction is NOT at equilibrium, we can predict which direction (LEFT or RIGHT) the reaction will shift in order to reach equilibrium by comparing the value of “Q” to the value of “Keq”

Comparing Q and Keq If Q<Keq: system must shift right toward products to reach equilibrium because product-to-reactant ratio is too low If Q=Keq: system is at equilibrium If Q>Keq: system must shift left toward reactants to reach equilibrium because product-to-reactant ratio is too high

How Do We Get the Value of “Q”?
It is calculated exactly like Keq, EXCEPT the concentrations we plug in are NOT equilibrium concentrations. They are just the concentrations of reactants and products at whatever time Q is calculated.

Example #1: at 472oC, Keq = .105 N2(g) + 3H2(g)  2NH3(g)
2 minutes after this reaction starts, you want to know if it’s at equilibrium so you measure the concentrations and find: What is the value of Q? In order to reach equilibrium, will this reaction shift left or right? [N2] = .0020M [H2] = .10M [NH3] = .15M These are not necessarily Equilibrium concentrations

You Try # 1: Keq = 170 for CoCl2(g)  Co(g) + Cl2(g)
If [Co] = [Cl2] = .15M and [CoCl2] = 1.1x10-3, is the reaction at equilibrium? If not, which way will it shift to reach equilibrium?