1. Chapter 18 Lesson Starter
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Lesson Starter
List two everyday processes that can easily be reversed and two that cannot.
The freezing of water and the melting of ice can be reversed
The cooking of an egg or the lighting of a match cannot be reversed.
For the reversible processes, describe the conditions that favor the process going in a particular direction.
Low temperature favors freezing, and high temperature favors melting.

2. Chapter 18 Reversible Reactions
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Reversible Reactions
Most (but not all) chemical reactions are considered to be reversible under suitable conditions.
A chemical reaction in which the products can react to re-form the reactants is called a reversible reaction.

3. Reversible Reactions, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Reversible Reactions, continued
A reversible chemical reaction is in chemical equilibrium when the rate of its forward reaction equals the rate of its reverse reaction and the concentrations of its products and reactants remain unchanged.
A state of dynamic equilibrium has been reached when the amounts of products and reactants remain constant.
Both reactions continue, but there is no net
change in the composition of the system.

4. Reversible Reactions, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Reversible Reactions, continued
The chemical equation for the reaction at equilibrium is written using double arrows to indicate the overall reversibility of the reaction.

5. Equilibrium, a Dynamic State
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Equilibrium, a Dynamic State
Many chemical reactions are reversible under ordinary conditions of temperature and concentration.
They will reach a state of equilibrium unless at least one of the substances involved escapes or is removed from the reaction system.
When the products of the forward reaction are favored, there is a higher concentration of products than of reactants at equilibrium.
The equilibrium “lies to the right”

6. Equilibrium, a Dynamic State, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Equilibrium, a Dynamic State, continued
When the products of the reverse reaction are favored, there is a higher concentration of reactants than of products at equilibrium.
the equilibrium “lies to the left”

7. The Equilibrium Expression
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression
Initially, the concentrations of C and D are zero and those of A and B are maximum.
Over time the rate of the forward reaction decreases as A and B are used up.
The rate of the reverse reaction increases as C and D are formed.
When these two reaction rates become equal, equilibrium is established.

8. Check yourself – answer the following questions then check your answers with your neighbor
1. Do reactions start out at equilibrium?
2. What is equal after equilibrium is reached?
3. Does the reaction stop at equilibrium?
4. Equilibrium will only be reached if the reaction is _________.
True or false - If a reaction forms only a small amount of the products it could be at equilibrium
If a reactions forms a large amount of product before reaching equilibrium the reaction is said to be ______.
When the reaction starts what are the concentrations of the products?

9. Reaction Rate Over Time for an Equilibrium System
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Reaction Rate Over Time for an Equilibrium System

10. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
After equilibrium is reached, the individual concentrations of A, B, C, and D undergo no further change if conditions remain the same.
A ratio of their concentrations should also remain constant.
The equilibrium constant is designated by the letter K.

11. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
The constant K is independent of the initial concentrations.
K is dependent on the temperature of the system.
If K is equal to 1 at equilibrium, there are roughly equal concentrations of reactants and products.

12. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
The Equilibrium Constant, continued
If the value of K is small, the reactants are favored.
A large value of K indicates that the products are favored.
Only the concentrations of substances that can actually change are included in K.
Pure solids and liquids are omitted because their concentrations cannot change.

13. Determining Keq for Reaction at Chemical Equilibrium
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Determining Keq for Reaction at Chemical Equilibrium

14. Equilibrium Constants
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Equilibrium Constants

15. Check yourself – answer the following questions then check your answers with your neighbor
1. What is the reverse reaction rate at time = 0 ?
2. When is the forward reaction rate the highest?
3. The equilibrium constant for a given reaction can only change if ___.
4. A reaction with a large amount of reactants left at equilibrium would have a _______ K.
What does a large K for a reaction tell you?
Write the expression for K for the following reaction 2KClO3 (s)  2KCl (s) + 3O2 (g)

16. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
The H2, I2, HI Equilibrium System
The rate of the reaction between H2 and I2 vapor in a sealed flask at an elevated temperature can be followed by observing the rate at which the violet color of the iodine vapor diminishes.
The color fades to a constant intensity but does not disappear completely because the reaction is reversible.
Hydrogen iodide decomposes to re-form hydrogen and iodine.
The constant color achieved indicates that equilibrium exists among hydrogen, iodine, and hydrogen iodide.

17. Rate Comparison for H2(g) + I2(g) 2HI(g)
Section 1 The Nature of Chemical Equilibrium
Chapter 18
Rate Comparison for H2(g) + I2(g) 2HI(g)

18. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
The H2, I2, HI Equilibrium System, continued
The net chemical equation for the reaction is
The following chemical equilibrium expression is?
The value for K is constant for any system of H2, I2, and HI at equilibrium at a given temperature.

19. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
The H2, I2, HI Equilibrium System, continued
At 425°C, the equilibrium constant for this equilibrium reaction system has the average value of
Choose 1 experiment and calculate K for that experiment. Confirm your answer with the answer given in the chart.

20. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
The H2, I2, HI Equilibrium System, continued
The balanced chemical equation for an equilibrium system is necessary to write the expression for the equilibrium constant.
Once the value of the equilibrium constant is known, the equilibrium constant expression can be used to calculate concentrations of reactants or products at equilibrium.

21. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
Sample Problem A
An equilibrium mixture of N2, O2 , and NO gases at
1500 K is determined to consist of 6.4  10–3 mol/L of
N2, 1.7  10–3 mol/L of O2, and 1.1  10–5 mol/L of NO.
What is the equilibrium constant for the system at
this temperature?

22. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
Sample Problem A Solution
Given: [N2] = 6.4  10–3 mol/L
[O2] = 1.7  10–3 mol/L
[NO] = 1.1  10–5 mol/L
Unknown: K
Solution:
The balanced chemical equation is
The chemical equilibrium expression is

23. The Equilibrium Expression, continued
Section 1 The Nature of Chemical Equilibrium
Chapter 18
The Equilibrium Expression, continued
Sample Problem A Solution, continued

24. Chapter 18 Lesson Starter Imagine children playing on a seesaw.
Section 2 Shifting Equilibrium
Chapter 18
Lesson Starter
Imagine children playing on a seesaw.
Five boys are sitting on one side and five girls on the other, and the seesaw is just balanced.
Then, one girl gets off, and the system is no longer at equilibrium.
One way to get the seesaw in balance again is for one of the boys to move toward the girls’ side.

25. Lesson Starter, continued
Section 2 Shifting Equilibrium
Chapter 18
Lesson Starter, continued
When he gets to the middle, the seesaw is again at equilibrium.
The stress of one girl getting off is relieved by having one of the boys move his position.
How would a chemical system in equilibrium respond to removing one of the products?

26. Predicting the Direction of Shift
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift
Changes in pressure, concentration, or temperature can alter the equilibrium position and thereby change the relative amounts of reactants and products.
Le Châtelier’s principle states that if a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress.

27. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Pressure
A change in pressure affects only equilibrium systems in which gases are involved, but only if you have different numbers of moles of gas in the products and reactants

28. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Pressure, continued
the Haber process for the synthesis of ammonia
4 molecules of gas molecules of gas
When pressure is applied (increased), the equilibrium will shift to the right, and produce more NH3.
By shifting to the right, the system can reduce the total number of molecules. This leads to a decrease in pressure.

29. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Pressure, continued
Even though changes in pressure may shift the equilibrium position, they do not affect the value of the equilibrium constant.
What would happen in the reaction on the previous slide if you had decreased the pressure instead of increased it?

30. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Concentration
An increase in the concentration of a reactant is a stress on the equilibrium system.
An increase in the concentration of A creates a stress.
To relieve the stress, some of the added A reacts with B to form products C and D.
What will happen to the concentrations of A,B,C, and D if you add A to this system at equilibrium?

31. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Concentration, continued
Changes in concentration have no effect on the value of the equilibrium constant.
Such changes have an equal effect on the numerator and the denominator of the chemical equilibrium expression.

32. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Concentration, continued
High pressure favors the reverse reaction.
Low pressure favors the formation of CO2.
Because both CaO and CaCO3 are solids, changing their amounts will not change the equilibrium concentration of CO2.

33. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Temperature
Reversible reactions are exothermic in one direction and endothermic in the other.
The effect of changing the temperature of an equilibrium mixture depends on which of the opposing reactions is endothermic and which is exothermic.

34. Predicting the Direction of Shift, continued
Section 2 Shifting Equilibrium
Chapter 18
Predicting the Direction of Shift, continued
Changes in Temperature, continued
The synthesis of ammonia by the Haber process is exothermic.
Which way will the reaction shift if you increase the temperature (add energy)
What if you decrease the temperature?

35. Temperature Changes Affect an Equilibrium System
Section 2 Shifting Equilibrium
Chapter 18
Temperature Changes Affect an Equilibrium System