5.3 a: Equilibrium Constant

Equilibrium Constant

Y12 Chem

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​5.3 a deduce the equilibrium expression (in terms of Keq) for homogeneous reactions occurring in solution
5.3 b perform calculations to find the value of Keq and concentrations of substances within an equilibrium system, and use these values to make predictions on the direction in which a reaction may proceed

​What we will learn about

• ​what is the equilibrium constant and the reaction quotient

• ​relationship between the above

• ​equilibrium law

• ​equilibrium expression

• ​size of the equilibrium constant and the extent of a reaction + amounts of reactants and products

• ​effect of temperature on an equilibrium constant

• ​Next lesson: calculating with equilibrium

​Equilibrium Law

• ​K_eq = concentration of products divided by concentration of reactants at equilibrium, where the index for each concentration is the coefficient in the balanced equation

• Solid and liquid species are not included

Multiple Choice

What are [A] and [B]?

• concentration of reactants
• concentration of products
• energy of reactants
• energy of products

Multiple Choice

N₂O₄(g) ↔ 2 NO₂(g)

What is the equilibrium constant expression?

• K_(eq) = [NO₂]²/[N₂O₄]

• K_(eq) = [N₂O₄]/[NO₂]²

• K_(eq) = [N₂O₄]²/[NO₂]

• K_(eq) = [NO₂]/[N₂O₄]²

Multiple Choice

This is the equilibrium expression for which of the following reactions?

• 2 N + 2 H3 <----> 3 NH2

• 2 NH3 <----> N2 + 3 H2

• N2 + 3 H2 <----> 2 NH3

• N2 + H3 <----> NH2

Multiple Choice

What is the equilibrium expression for:

Fe₃O₄(s) + 4H₂(g) <=> 3Fe(s) + 4H₂O(g)

K_(eq) =

• [Fe]³ [H₂O]⁴ / [Fe₃O₄] [H₂]⁴

• [Fe₃O₄] [H₂]⁴ / [Fe]³ [H₂O]⁴

• [H₂O]⁴ / [H₂]⁴

• [Fe] [H₂O] / [Fe₃O₄] [H₂]

​Reaction Quotient and K_eq

• ​Q = reaction quotient

• K_eq = equilibrium constant

• ​measures the relative amounts of products and reactants present during a reaction at a particular point in time

• ​at equilibrium, Q = K_eq

• ​for a rxn, K_eq is constant for all equilibrium mixtures at a fixed temperature - even if concentrations are different

• Q is used when not at equilibrium

​Relationship between Q and Keq

• ​Q is smaller than K_eq

  • ​shifts to right

  • ​more product

• ​Q = K_eq

  • ​equilibrium

  • ​no shifting

• ​Q is bigger than K_eq

  • ​shifts to left

  • ​more reactants

​Working with K_eq

• ​can determine the extent of a reaction and the relative amounts of reactants and products

​Value of K_eq

• ​will be asked to calculate the K_eq

• ​and then could be asked about the value of it and what this means for the extent of the reaction

• ​or asked to calculate the concentration of a specific species given the K_eq

• ​or to calculate using stoichiometry

Multiple Choice

N₂(g) + 3H₂(g) ↔ 2 NH₃(g) + heat

Increasing the temperature will cause the equilibrium to:

• Shift right

• Shift left

• Have no change

• Speed up

Multiple Choice

Consider the following equation 
H_{2 (g)} + I_{2 (g)} <===> 2 HI_(g)  
Calculate Keq if at 300^oK the concentrations are [H₂] = 0.40 M   [I₂] = 0.45 M ; [HI] = 0.30 M

• 0.42
• 0.5
• 1.67
• 0.6

​Next lesson

​How to do all calculations involving Keq