Question

The Equilibrium-Constant Expression

Learning Goal:

To understand the form and meaning of the equilibrium-constant expression.

Chemical reactions are usually not one-way trips from reactant to product. In fact, most chemical reactions are reversible to at least some extent. Products formed by the forward reaction may react with each other to regenerate the reactants. When reactants are mixed, they will begin to react at a forward reaction rate particular to that chemical reaction. As reactants are depleted and products are formed, however, the rate of the forward reaction begins to slow, and the rate of the reverse reaction begins to increase.

Eventually, the forward and reverse reaction rates will be identical. Equilibrium is dynamic, meaning that it is a balance of continuous forward and reverse reactions. To indicate this dynamic nature of equilibrium, chemical equations of reactions with measurable reverse reaction rate are written using a double-headed arrow:

aA+bB⇌cC+dD

The equilibrium-constant expression The equilibrium-constant expression is used to describe the concentration of reactants and products for a reaction in dynamic equilibrium. For ideal gases and ideal solutions in homogeneous equilibria, where all reactants and products are in the same phase, the extent to which a particular chemical reaction proceeds to products is given by the equilibrium equation aA+bB⇌cC+dD ,      K=[C]c[D]d[A]a[B]b where K is the equilibrium constant and the right-hand side of the equation is known as the equilibrium-constant expression. The concentration of each product raised to its coefficient is divided by the concentration of each reagent raised to its coefficient according the the balanced chemical equation. Therefore, the higher the concentration of products, the larger the value of K will be. Part A Identify the proper form of the equilibrium-constant expression for the equation N2(g)+O2(g)⇌2NO(g) Hints Identify the proper form of the equilibrium-constant expression for the equation \rm N_2(g) + O_2(g) \rightleftharpoons 2 NO(g) SubmitMy AnswersGive Up Correct Understanding the magnitude of K By analyzing the equilibrium-constant expression we can determine that a larger concentration of products in the numerator of the expression will result in larger values for K . Greater values of K are associated with reactions running toward completion, whereas smaller values of K are associated with a larger concentration of reactants. K=[NO][N2][O2] K=[NO]2[N2][O2] K=[N2][O2][NO]2 K=2[NO][N2][O2] Value of K Reaction favors Reaction lies to K⋘1 reactants left K∼1 neither reactants nor products center K⋙1 products right Note that reaction stoichiometries with a large difference between the number of moles of reactants and products may not strictly follow these general guidelines. Part B The equilibrium-constant of the reaction NO2(g)+NO3(g)⇌N2O5(g) is K=2.1×10−20 . What can be said about this reaction? Hints The equilibrium-constant of the reaction \rm NO_2(g) + NO_3(g) \rightleftharpoons N_2O_5(g) is K = 2.1 \times 10^{-20}. What can be said about this reaction? At equilibrium the concentration of products and reactants is about the same. At equilibrium the concentration of products is much greater than the concentration of reactants. At equilibrium the concentration of reactants is much greater than that of products. There are no reactants left over once the reaction reaches equilibrium.

Answer 1

For a reaction,

aA + bB ⇌ cC + dD ,

the equilibrium constant is

K= [concentration of product]^{​no. of moles of product} / [Concentration of reactant]^{​no. of moles of reactant}

K = [C]^c [D]^d / [A]^a [B]^b

For part A

N2(g) + O2(g) ⇌ 2NO(g)

Hence here,

K = [NO]² / [N2]^​1 [O2]^​1

K =[NO]² / [N2]^​ [O2]^​

part B

NO2(g) + NO3(g) ⇌ N2O5(g)

The equilibrium-constant of the reaction K=2.1×10⁻²⁰

we know that,

Value of K	Reaction favors	Reaction lies to
K⋘1	reactants	left
K∼1	neither reactants nor products	center
K⋙1	products	right

Here,

The value of K is 2.1×10⁻²⁰ which is very much less than 1.

i.e. K<<<1

and at K<<<1, the reaction favours reactants, i.e. reactants dominate the mixture. which means At equilibrium the concentration of reactants is much greater than that of products.