Textbook 5G.11, Q vs K

[VanessaZhu2L]

Write the reaction quotient Q for

(a) 2BCl3(g) + 2Hg(l) → B2Cl4(s) + Hg2Cl2(s)

The answer in the solutions for this is 1 / BCl3^2. Can someone explain how they got this? I was also wondering if someone could explain what the difference is between Q and K and how to solve problems that ask for that. Thank you!

[Faith St Amant 3D]

Notice that BCl3 is the only gas in the reaction, so it's concentration is the only one included in the calculation of Q. Since Q = [products]/[reactants], and we're only considering BCl3, Q= 1/[BCl3]^2. As for the difference between K and Q: K is the equilibrium constant and denotes the ratio of equilibrium product concentration to equilibrium reactant concentration. Q is the reaction quotient, which is the ratio of product concentration to reactant concentration at any point in the reaction, not just at equilibrium--that's why we compare the Q and K values to determine how the reaction will proceed toward equilibrium.

[Manseej Khatri 2B]

Hi. The reaction quotient q is the products over reactants raised to its coefficients. The difference between q and k is that k is the reaction quotient at equilibrium.
Mercury is a liquid and I am assuming is in high excess and is thus left out of the reaction quotient. B2Cl4 and Hg2Cl2 are solids and thus are left out of the quotient. The products over reactants is then 1/[BCl3]^2 = q.

If asked for Q, take the concentrations that are present in solution at the moment in time it is asked, and if you are asked for K, take the concentrations that are present at equilibrium.

[Natalie Do 3F]

Only gases are included in the reaction quotient so only BCl3 is needed. It is in the denominator because it is a reactant and it is squared because it has a coefficient of two.

[Astha Patel 2J]

Reaction quotients don't include solids and liquids and BCl3 is the only one in the gas phase so Q would equal 1/BCl3^s