1. Chemistry 18.2

2. Reversible Reactions and Equilibrium
18.2
In the early 1900s, German chemists refined the process of making ammonia from elemental nitrogen and hydrogen.This process allows the manufacture of nitrogen fertilizers. You will learn how reaction conditions can influence the yield of a chemical reaction.

3. 18.2
Reversible Reactions
Reversible Reactions
How do the amounts of reactants and products change in a chemical system at equilibrium?

4. 18.2
Reversible Reactions
At chemical equilibrium, no net change occurs in the actual amounts of the components of the system.

5. 18.2
Reversible Reactions
If the rate of the shoppers going up the escalator is equal to the rate of the shoppers going down, then the number of shoppers on each floor remains constant, and there is an equilibrium.
If the rate at which shoppers move from the first floor to the second is equal to the rate at which shoppers move from the second floor to the first, then the number of shoppers on each floor remains constant. Applying Concepts Is it necessary that an equal number of shoppers be on each floor? Explain.

6. 18.2
Reversible Reactions
A reversible reaction is one in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.

7. SO3 decomposes to SO2 and O2
18.2
Reversible Reactions
SO3 decomposes to SO2 and O2
SO2 and O2
react to give SO3
At equilibrium, all three types of molecules are present.
Molecules of SO2 and O2 react to give SO3 . Molecules of SO3 decompose to give SO2 and O2 . At equilibrium, all three types of molecules are present in the mixture.

8. 18.2
Reversible Reactions
When the rates of the forward and reverse reactions are equal, the reaction has reached a state of balance called chemical equilibrium.
The relative concentrations of the reactants and products at equilibrium constitute the equilibrium position of a reaction.

9. 18.2
Reversible Reactions
These graphs show how the concentrations of O2, SO2, and SO3 vary with time. Left: Initially, SO2 and O2 are present. Right: Initially, only SO3 is present. INTERPRETING GRAPHS a. Navigate Where on the graphs can you find the initial concentrations of the reactants and products? The equilibrium concentrations? b. Read Which gas is most abundant at equilibrium? c. Interpret How do the equilibrium concentrations of O2, SO2, and SO3 compare?

10. Take a close look at a generalized reversible reaction.
Reversible Reactions
Animation 23
Take a close look at a generalized reversible reaction.

11. Factors Affecting Equilibrium: Le Châtelier’s Principle
18.2
Factors Affecting Equilibrium: Le Châtelier’s Principle
Factors Affecting Equilibrium: Le Châtelier’s Principle
What three stresses can cause a change in the equilibrium position of a chemical system?

12. Factors Affecting Equilibrium: Le Châtelier’s Principle
18.2
Factors Affecting Equilibrium: Le Châtelier’s Principle
Stresses that upset the equilibrium of a chemical system include changes in the concentration of reactants or products, changes in temperature, and changes in pressure.

13. Factors Affecting Equilibrium: Le Châtelier’s Principle
18.2
Factors Affecting Equilibrium: Le Châtelier’s Principle
The French chemist Le Châtelier proposed what has come to be called Le Châtelier’s principle: If a stress is applied to a system in dynamic equilibrium, the system changes in a way that relieves the stress.

14. Factors Affecting Equilibrium: Le Châtelier’s Principle
Simulation 24
Simulate Le Châtelier’s principle for the synthesis of ammonia.

15. Factors Affecting Equilibrium: Le Châtelier’s Principle
18.2
Factors Affecting Equilibrium: Le Châtelier’s Principle
Concentration
Rapid breathing during and after vigorous exercise helps reestablish the body’s correct CO2:H2CO3 equilibrium, keeping the acid concentration in the blood within a safe range.
The rapid exhalation of CO2 during and after vigorous exercise helps reestablish the body’s correct CO2:H2CO3 equilibrium. This keeps the acid concentration in the blood within a safe range.

16. Factors Affecting Equilibrium: Le Châtelier’s Principle
18.2
Factors Affecting Equilibrium: Le Châtelier’s Principle
Temperature
Dinitrogen tetroxide is a colorless gas; nitrogen dioxide is a brown gas. The flask on the left is in a dish of hot water; the flask on the right is in ice.
Dinitrogen tetroxide is a colorless gas; nitrogen dioxide is a brown gas. The flask on the left is in a dish of hot water; the flask on the right is in ice. Interpreting Illustrations How does an increase in temperature affect the equilibrium of a mixture of these gases?

17. Factors Affecting Equilibrium: Le Châtelier’s Principle
18.2
Factors Affecting Equilibrium: Le Châtelier’s Principle
Pressure
Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium
Pressure affects a mixture of nitrogen, hydrogen, and ammonia at equilibrium. a) The system is at equilibrium. b) Equilibrium is disturbed by an increase in pressure. c) A new equilibrium position is established with fewer gas molecules. Interpreting Diagrams What effect does a decrease in volume have on the number of gas molecules?

18. Conceptual Problem 18.1

19. Conceptual Problem 18.1

20. Conceptual Problem 18.1

21. for Conceptual Problem 18.1
Problem Solving 18.6 Solve a similar problem with the help of an interactive guided tutorial.

22. Equilibrium Constants
18.2
Equilibrium Constants
Equilibrium Constants
What does the value of Keq indicate about the equilibrium position of a reaction?

23. Equilibrium Constants
18.2
Equilibrium Constants
The equilibrium constant (Keq) is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation.

24. Equilibrium Constants
18.2
Equilibrium Constants
A value of Keq greater than 1 means that products are favored over reactants; a value of Keq less than 1 means that reactants are favored over products.

25. 18.1

26. 18.1

27. 18.1

28. 18.1

29. for Sample Problem 18.1
Problem Solving 18.7 Solve Problem 7 with the help of an interactive guided tutorial.

30. 18.2

31. 18.2

32. 18.2

33. 18.2

34. for Sample Problem 18.2
Problem Solving 18.9 Solve Problem 9 with the help of an interactive guided tutorial.

35. 18.2 Section Quiz.
18.2.

36. 18.2 Section Quiz.
1. In a reaction at equilibrium, reactants and products
decrease in concentration.
form at equal rates.
have equal concentrations.
have stopped reacting.

37. 18.2 Section Quiz.
2. In the reaction 2NO2(g)  2NO(g) + O2(g), increasing the pressure on the reaction would cause
the amount of NO to increase.
the amount of NO2 to increase.
nothing to happen.
the amount of O2 to increase.

38. 18.2 Section Quiz.
3. Changing which of the following would NOT affect the equilibrium position of a chemical reaction?
concentration of a reactant only
concentration of a product only
temperature only
volume only

39. 18.2 Section Quiz.
4. For the following reaction, Keq = 1.
A(g) + B(g)  C(g) + D(g) Therefore, at equilibrium
[C] = [A].
[A][B] = 0.
[AB] = [CD] = 1.
[A][B] = [C][D].

40. END OF SHOW