HW From textbook “CHEMISTRY a Contemporary Approach”

• Read “Factors that Control Chemical Equilibrium” p345-347

• Do exercise 11.2  p347

• Read “Effects of Changes in Pressure” p350

• Read “Effects of Changes in Temperature” p352-359

• Do Exercises 11.4, 11.5, 11.6, 11.7 p356-359

• Do "Questions for Review" 1, 5, 6, 8, 13, 15, 16, 20, 23-27, 29, 37-40 p371-377

LeChatelier's principle can be used to predict the effect of stress (change in pressure, volume, concentration, and temperature) on a system at equilibrium.

Le Chatelier's Principle states that when a system at equilibrium is subjected to a stress it will adjust by changing the equilibrium to relieve the stress.

Pressure and Volume

If the pressure of a system at equilibrium increases (or volume decreases) the system will relieve the stress by favoring the side of the reaction that has the smallest volume.

If the pressure of a system at equilibrium decreases (or volume increases) the system will relieve the stress by favoring the side of the reaction that has the largest volume.

Pressure changes have virtually no effect on solids and liquids. 

Example

2 H₂(g) + O₂(g) <--> 2 H₂O(g)

The reactants side contains 3 moles of gas. The product side contains 2 moles of gas. Therefore, the reactants side has a larger volume than the product side. 

Problem

Describe the possible outcome of changing the pressure in the following systems.

1.  2 SO₂(g) + O₂(g) <--> 2 SO₃(g)

2. N₂O₄(g) <--> 2 NO₂(g)

3. At equilibrium the system in question 2 is light brown. What will be the color of the system when pressure increases and when pressure decreases? Hint, N₂O₄(g) is colorless and NO₂(g) is brown.

Concentration

If the concentration of a substance in the system at equilibrium changes the system will relieve the stress by adjusting it to reach a new equilibrium

Example

Consider the reaction CaCl₂(s) + H₂O(g) <--> CaCl₂.H₂O(s)

The forward reaction is CaCl₂(s) + H₂O(g) --> CaCl₂.H₂O(s)

The reverse reaction is CaCl₂.H₂O(s) --> CaCl₂(s) + H₂O(g)

Temperature

If the temperature increases the reaction absorbing heat (endothermic) is favored.

If the temperature decreases the reaction releasing heat (exothermic) is favored.

Example

N₂(g) + O₂(g) <--> 2 NO(g) ΔH = +182.6 KJ

The forward reaction is endothermic N₂(g) + O₂(g) + 182.6 KJ --> 2 NO(g)

The reverse reaction is exothermic 2 NO(g) --> N₂(g) + O₂(g) + 182.6 KJ

Problem 

Indicate the change imparted by each stress introduced in this system.

N₂(g) + 3 H₂(g) <--> 2 NH₃(g) + 22.0 kcal

TEST YOUR UNDERSTANDING

1/07

44 Given the system at equilibrium: 2POCl₃(g) + energy <--> 2PCl₃(g) + O₂(g)

Which changes occur when O₂(g) is added to this system?

(1) The equilibrium shifts to the right and the concentration of PCl₃(g) increases.

(2) The equilibrium shifts to the right and the concentration of PCl₃(g) decreases.

(3) The equilibrium shifts to the left and the concentration of PCl₃(g) increases.

(4) The equilibrium shifts to the left and the concentration of PCl₃(g) decreases.

6/07

44 Given the equation representing a reaction at equilibrium: N₂(g) + 3H₂(g)<-->2NH₃(g) + energy

Which change causes the equilibrium to shift to the right?

(1) decreasing the concentration of H₂(g)   (2) decreasing the pressure  (3) increasing the concentration of N₂(g)    (4) increasing the temperature

8/07

Base your answers to questions 76 and 77 on the information below.

The equilibrium equation below is related to the manufacture of a bleaching solution. In this equation, Cl^–(aq) means that chloride ions are surrounded by water molecules.

Cl₂(g) + 2OH^–(aq) <--> OCl^–(aq) + Cl^–(aq) + H₂O(l)

76 Draw two water molecules showing the correct orientation of each water molecule toward the chloride ion.

77 Explain, in terms of collision theory, why increasing the concentration of Cl₂(g) increases the concentration of OCl^–(aq) in this equilibrium system.

1/06

6/06

8/06

1/05

46 Given the reaction: Ba(OH)₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2 H₂O(l) + energy

As the barium hydroxide solution is added to the solution of sulfuric acid, the electrical conductivity of the acid solution decreases because the

                (1) volume of the reaction mixture increases (2) temperature of the reaction mixture decreases

                (3) concentration of ions increases (4) concentration of ions decreases

6/04

43 Given the reaction: N₂(g) + O₂(g) + 182.6 kJ <--->2 NO(g)  

Which change would cause an immediate increase in the rate of the forward reaction?

        (1) increasing the concentration of NO(g) (2) increasing the concentration of N₂(g)
        (3) decreasing the reaction temperature (4) decreasing the reaction pressure

Base your answers to questions 79 and 80 on the information and equation below.
                Human blood contains dissolved carbonic acid, H2CO3, in equilibrium with carbon dioxide and water. The equilibrium system is shown below.

                H₂CO₃(aq) <---> CO₂(aq) + H₂O(l)

79 Explain, using LeChatelier's principle, why decreasing the concentration of CO₂ decreases the concentration of H₂CO₃.

80 What is the oxidation number of carbon in H₂CO₃(aq)? 

8/04

45 Given the reaction at equilibrium: N₂(g) + O₂(g) + energy <---> 2 NO(g) Which change will result in a decrease in the amount of NO(g) formed?

                1) decreasing the pressure  2) decreasing the concentration of N₂(g)     3) increasing the concentration of O₂(g)    4) increasing the temperature

Base your answers to questions 63 and 64 on the information below.

Given the equilibrium equation at 298 K:

63 Describe, in terms of LeChatelier’s principle, why an increase in temperature increases the solubility of KNO₃.

64 The equation indicates that KNO₃ has formed a saturated solution. Explain, in terms of equilibrium, why the solution is saturated.

1/03

Given the system at equilibrium: N₂O₄(g)  + 58.1 kJ <--> 2 NO₂(g) What will be the result of an increase in temperature at constant pressure?

(1) The equilibrium will shift to the left, and the concentration of NO₂(g) will decrease.

(2) The equilibrium will shift to the left, and the concentration of NO₂(g) will increase.

(3) The equilibrium will shift to the right, and the concentration of NO (g) will decrease.

(4) The equilibrium will shift' to the right, and the concentration of NO₂(g) will increase.

6/03

Base your answers to questions 74 through 76 on the information below.

A student wishes to investigate how the reaction rate changes with a change in concentration of HCl(aq).

Given the reaction: Zn(s) + HCl(aq) ---> H₂(g) + ZnCl₂(aq)

74.  Identify the independent variable in this investigation. 

75.  Identify one other variable that might affect the rate and should be held constant during this investigation. 

76.  Describe the effect of increasing the concentration of HCl(aq) on the reaction rate and justify your response in terms of collision theory.

8/03

46 Given the reaction at equilibrium: A(g) + B(g) <--> AB( g) + heat The concentration of A(g) can be increased by
    (1) lowering the temperature (2) adding a catalyst (3) increasing the concentration of AB(g) (4) increasing the concentration of B(g)

6/02

Given the reaction at equilibrium:  N₂(g) + 3 H₂(g) <--> 2 NH₃(g) + 92.05 kj

    a)  State the effect on the number of moles of N₂(g) if the temperature of the system is increased.

    b)  State the effect on the number of moles of H₂(g) if the pressure on the system is increased.   

    c)  State the effect on the number of moles of NH₃(g) if a catalyst is introduced into the reaction system. Explain why this occurs.

8/02

Given the equilibrium reaction in a closed system: H₂(g) + I₂(g) + heat  <--> 2 HI(g) What will be the result of an increase in temperature?

(1) The equilibrium will shift to the left and [H₂] will increase. (3) The equilibrium will shift to the right and [HI] will increase.

(2) The equilibrium will shift to the left and [H₂] will decrease. (4) The equilibrium will shift to the right and [HI] will decrease.

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