Unit 7: Topic 3 - Reaction Quotient and Equilibrium Constant Flashcards
What does the reaction quotient Q represent and how do we calculate it?
The reaction quotient Q represents the relative amounts of products and reactants present during a reaction at a particular point in time. Q can be represented by Qc, which uses concentrations, whereas in gas phase reactions, Q can also be represented by Qp, which uses partial pressures.
For the reversible reaction aA + bB <–> cC + dD where a, b, c, and d are coefficients and A, B, C, and D are substances, Qc is equal to [C]^c[D]^d/[A]^a[B]^b. Similarly, Qp would be equal to [PC)^c(PD)^d/(PA)^a(PB)^b, where P of each substance is equal to the partial pressure of that substance.
What is the value of reaction quotient Qc of the reaction N2O4(g) <–> 2NO2(g) when [N2O4] = 0.00150M and [NO2] = 0.571M?
Since NO2 is the product and N2O4 is the reactant, Qc = [NO2]^2/[N2O4]. If we plug in the concentration values, we determine that Qc = (0.571)^2/(0.00150)^1 = 217.
What does the equilibrium constant Keq represent and what is its relationship with the reaction quotient Q?
The equilibrium constant Keq represents the relationship between products and reactants of a reaction at equilibrium, or when the rate of the forward reaction is equal to that of the reverse reaction (and concentrations are constant). Keq, similar to Q, can be represented by Kc, which uses concentrations, whereas in gas phase reactions, K can also be represented by Kp, which uses partial pressures.
At equilibrium, for the reversible reaction aA + bB <–> cC + dD where a, b, c, and d are coefficients and A, B, C, and D are substances, Kc is equal to [C]^c[D]^d/[A]^a[B]^b. Similarly, Kp would be equal to (PC)^c(PD)^d/(PA)^a(PB)^b, where P of each substance is equal to the partial pressure of that substance.
The only difference between K and Q is that K is only calculated with concentrations or partial pressures when the reaction is at equilibrium. Q can use values that apply at a different point in a reaction. It is also important to note that Q tends toward K. When a reaction is at equilibrium, the value of Q will be equal to the value of K such that Kc = Qc and Kp = Qp.
When the reaction 2SO2(g) + O2(g) <–> 2SO3(g) is at equilibrium, the concentrations are measured to be [SO2] = 0.90M, [O2] = 0.35M, and [SO3] = 1.1M. What is the value of the equilibrium constant Kc?
We can find Kc because we have the concentrations of each substance when the reaction is at equilibrium. Since SO3 is the product and O2 and SO2 are the reactants, Kc = [SO3]^2/[SO2]^2[O2]^1. If we plug in the equilibrium concentration values, we determine that Kc = (1.1)^2/(0.90)^2(0.35)^1 = 4.3.
What does it mean when Q < K, Q > K, and Q = K? How does the reaction proceed as a result?
If Q < K, the reaction will proceed in the forward direction to convert reactants into products. This is because a greater K indicates that at equilibrium, there will be a greater concentration of products. As a result, the reaction tends to create more products to reach equilibrium.
If Q > K, the reaction will proceed in the reverse direction to convert products into reactants. At equilibrium, there will be a greater concentration of reactants than is present in its current state, so the reaction tends to create more reactants to reach equilibrium.
If Q = K, the reaction is at equilibrium. At this point, the concentrations of reactants and products stay constant.
Which types of substances in a reaction are excluded from the expressions for Qc and Kc? Why?
Solids and pure liquids are excluded from calculations of the reaction quotient and equilibrium constant because their concentrations do not change throughout a reaction. Therefore, only gases and aqueous solutions are included.
What is the only type of substance that is included in the expressions for Qp and Kp?
Since Qp and Kp use partial pressures, we only include gases in their expressions. For example, in the reaction FeO(s) + CO(g) <–> Fe(s) + CO2(g), the expressions for Qp and Kp would be (PCO2)/(PCO) because CO and CO2 are the only gases involved.
Write the expression for the reaction quotient Qc for the reaction CH4(g) + 2O2(g) <–> CO2(g) + 2H2O(l).
Since solids are excluded from the expression for Qc, we do not include H2O(l). Thus, the correct expression for Qc would be [CO2]^1/[CH4]^1[O2]^2.
