Chemistry 104 Study Groups
EQUILIBRIUM
 

CONCEPTS

  Understanding the formulation and meaning of the reaction quotient is critical for much of the material this semester.

Students often are confused between the reaction quotient and the equilibrium constant. This confusion is understandable because "equilibrium constant" is often used to mean reaction quotient. Once you are comfortable with the concepts, you will use them interchangably too! But until then remember this:

    The equilibrium constant is a number.

    The reaction quotient is an expression.

    Consider the reaction A2 + B2 2AB which has an equilbrium constant of 3.78 at 25 oC.

 
 
  Reaction Quotient (Qc or Qp) Equilbrium Constant(Kc or Kc)
 
 
 

3.78
at
25oC
 
 
  • The reaction quotient is always equivalent to this expression.

  • At any point in the reaction, the concentrations of A2, B2 and AB may be inserted into this expression to find a numerical representation of the extent of the reaction, relative to equilibrium.

  • When the numerical value of the reaction quotient is equal to the numerical value of the equilibrium constant the reaction is at equilibrium.

  • The equilibrium constant for this reaction always has the numerical value of 3.78 as long as the temperature is 25 oC.

 
 

Exercises

  In the reversible reaction A2 + B2 2AB, an inital mixture contains 2 mol A2 and 1 mol B2.
Which of the following is the amount of AB expected at equilibrium? Explain your answer.
  1. 1 mol
  2. 2 mol
  3. more than 2 but less than 4 mol
  4. less than 2 mol

 
  An equilibrium mixture of SO2, SO3 and O2 gases are maintained in a 2.05 L flask at a temperature at which Kc = 35.5 for the reaction:
2SO2(g) + O2(g) 2SO3
  1. If the numbers of moles of SO2 and SO3 in the flask are equal, how many moles of O2 are present?
  2. If the number of moles of SO3 in the flask is twice the number of moles of SO2, how many moles of O2 are present?

 
  Pure NOCl gas was heated at 240 oC in a 1.00 L container. At equilibrium the total pressure was 1.00 atm and the NOCl pressure was 0.64 atm.
2 NOCl(g) 2NO(g) + Cl2(g)
  1. Calculate the partial pressures of NO and Cl2 in the system.
  2. Calculate the equilibrium constant Kp

  The reaction H2(g) + I2(g) <––> 2HI(g) has Kc=160 at 500K.

  1. If 0.05 mol H2(g) and 0.05 mol I2(g) are put into a 5.00 L flask and allowed to react, what is the equilibrium composition of the system?
  2. If 0.03 mol H2(g), 0.01 mol I2(g) and 0.02 mol HI(g) are present in a 5.00 L flask at 500K at a certain time, is the reaction at equilibrium? If not, in which direction will the reaction proceed? What would be the equilibrium composition of the mixture?
  3. Determine the equilibrium composition of the system if 0.05 mol HI(g) is placed into a 5.00 L flask and allowed to come to equilibrium.

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