Post Test Equilibrium

1
Post Test - Equilibrium

• 1. Define equilibrium
• 2. Write Keq for
• a) H2 (g) I2 (g) ? HI (g)
• b) Cu2 (aq) (SO4)2- (aq) ? CuSO4(s)
• c) O3 (g) ? O2 (g)

2
Chapter 14 Equilibrium

• Equilibrium is the condition in which the rate of
  formation of products equals the rate of
  formation of reactants

3
Equilibrium

• We have seen reactions proceed from reactants to
  products
• A reversible reaction occurs when enough of the
  product has been formed that the product begins
  to break down and form reactants

4
Equilibrium

• A reversible reaction is symbolized by double
  arrows

2 SO2
O2
2 SO3
Equilibrium occurs when the rate of the forward
reaction equals the rate of the reverse reaction
5
Equilibrium

• Equilibrium is defined in terms of concentration
• Once at equilibrium, the concentration of
  reactants and products does not change

6
Equilibrium Continued

• If one of the double arrows is drawn larger than
  the other one, the longer arrow indicate the
  favored side of the reaction
• Equilibrium is a balance between the rate of the
  forward reaction and the reverse reaction

7
Equilibrium Expressions

• Equilibrium expressions (Keq) equal products over
  reactants
• Kc can be used in place of Keq
• Kc specifies concentration
• Kp specifies pressure (used for gases)

8
Equilibrium Expressions

• Ka weak acid (weak electrolytes)
• Kb weak base (weak electrolytes)
• Ksp solubility of substances (slightly soluble)

9
Equilibrium Expressions

• Keq expressions are written only for reactants
  and products in aqueous or gas form
• Solids and liquids are left out

10
Setting Up Equilibrium Expressions

• mA nB ? sC rD
• The equilibrium expression is
• Keq products / reactants

Cs Dr
Keq
Am Bn
11
Interpreting Equilibrium Expressions

• The symbol means molar concentration
• The exponents in the expressions are the
  coefficients for the balanced equation
• The units for Keq are not generally used

12
Write Equilibrium Expressions

• 1. N2O4 ? 2NO2
• 2. H2 I2 ? 2HI
• 3. N2 H2 ? NH3
• 4. BrCl ? Br2 Cl2

13
Solving Equilibrium Problems

• 5) What is the value for the Keq when 0.1 M H2
  reacts with 0.5 M O2 to produce 0.5 M H2O?
• First, write balanced equation
• Second, write Keq expression
• Third, substitute values in the expression and
  solve
• Ans) Keq 50

14
Solve

• 6) 4 M N2 reacts with 6 M H2 to produce 10 M
  NH3
• 7) 5 M CO2 breaks down to produce 6 M CO and 3 M
  O2 in an equilibrium reaction
• Ans 2) 0.116
• Ans 3) 4.32

15
More About Kp

• Kp expressions are written for gases
• 2 NO (g) O2 (g) ? 2 NO2 (g)
• Kp (PNO2)2 / ( ( PNO ) 2 PO2)

16
Ideal Gas Law Kp Values

• Ideal Gas Law PV nRT
• P nRT/V, n/V concentration, c
• P cRT, R 0.0821, T in Kelvin
• Substitute cRT in place of P in Kp

17
Substitution for Kp

• Kp (cRTNO22 ) / ((cRTNO)2 cRTO2)
• Through complex substitutions,
• Kp KcRT?ngas
• ?ngas mole products - mole reactants

18
Substitution for Kp

• Kp Kc(RT?ngas)
• Example Consider the following equilibrium
  N2O4 (g) ? 2 NO2 (g)
• The reaction occurs at 319 K and Kp is found to
  be 0.660. Determine Kc.

19
More About Kp

• Kp KcRT?ngas
• Given Kc 1.8 x 10-6 for
• 2 NO (g) O2 (g) ? 2 NO2 (g) _at_257K
• Derive Kp

20
Reverse Kc and Kp

• If the Kc or Kp is known, the inverse (Kc or
  Kp) can be determined by taking the inverse
  value
• If Kc 1.8 x 10-3, what is Kc?

21
Values for Kc and Kp

• When actual numerical values for Kp and Kc are
  determined, the magnitude of the value infers
• a. The reaction goes to completion if K is large
  (mainly products)
• b. The reaction proceeds slowly if K is small
  (mainly reactants)

22
Examples

• Is the reaction CaO (s) CO2 (g) ? CaCO3 (s)
  likely to occur to any appreciable extent at 298
  K? The kP is found to 1.9 x 10-23 for CaCO3 (s) ?
  CaO (s) CO2 (g)
• Ans) Yes, because Kp is large (5.3 x 1022)

23
Using Equilibrium Expressions

• 8) A mixture of 5.00 x 10-3 moles of H2 (g) and
  1.00 x 10-2 moles I2 (g) are placed in a 5.00 L
  container at 448 K and allowed to react
  equilibrium. At equilibrium, the HI 1.87 x
  10-3 M. Calculate Kc.

ICE
24
Solution

• Use an ICE Chart (Initial, Change, Equilibrium)
• To determine change, consider the stoichiometry
  in the balanced equation
• Ans) 50.5

25
Pop Quiz

• 1. Define equilibrium
• 2. Write equilibrium expressions
• a) Ag (aq) Cl- (aq) ? AgCl (s)
• b) CaCO3 (s) ? CaO (s) CO2 (g)

26
Example 9

• The initial concentrations of N2 and O2 _at_2300 K
  are both 1.4 M. What are the equilibrium
  concentrations of all three components if Kc
  1.7 x 10-3?
• N2 (g) O2 (g) ? 2 NO (g)
• Use the ICE Chart

27
Example 10

• A 0.100 M Acetic Acid (HC2H3O2) reacts with water
  to form hydronium (H3O) and Acetate Ion
  (C2H3O2-). Determine the equilibrium
  concentrations of all reactants and products (as
  applicable) if Kc 1.77 x 10-5

28
Example 11

• The reaction below has a Kc of 4.06 at 500C. If
  0.100 mol of CO and 0.100 mol H2O are placed in
  a 1.00 liter container, calculate the equilibrium
  concentrations.
• CO (g) H2O (g) ? CO2 (g) H2 (g)

29
Example Problem 12

• At 35C, K 1.6 x 10-5 for the reaction
• 2 NOCl (g) ? 2 NO (g) Cl2 (g)
• Calculate the concentrations of all species at
  equilibrium

30
Quiz - ICE Chart Problems

• The Keq for the following reaction is 1.0 x 102
  at a certain temperature. If 2.0 moles each of
  H2 F2 are introduced into a 0.50 L container,
  determine the equilibrium concentrations.
• H2 (g) F2 (g) ? HF (g)

31
LeChâteliers Principle

• LeChâteliers Principle states that when a change
  is imposed on a system at equilibrium, the system
  will shift in a manner that will reduce the
  impact of the stress

32
LeChâteliers Principle

• In order to determine shift in equilibrium, the
  stress must be considered good or bad
• If the stress is good, the system shifts in the
  opposite direction of the stress
• If the stress is bad, the system shifts in the
  same direction of the stress

33
Possible Stresses LeChâteliers Principle

• 1) The addition of more reactants this is
  good and will shift equilibrium toward the
  products
• 2) The removal of reactants this is bad and
  will shift equilibrium toward the reactants

34
Possible Stresses LeChâteliers Principle

• Example H2 (g) O2 (g) ? H2O (g)
• Adding more H2 or O2 will shift toward products
• Removing H2 or O2 will shift toward reactants

35
Possible Stresses LeChâteliers Principle

• 3) Additional Products will shift toward
  reactants side removal of product will shift
  toward products side
• 4) Heat To determine the shift in equilibrium
  in heat, heat must be a reactant or product in
  the equilibrium
• Example N2 (g) O2 (g) ? 2 NO (g) ?H 180.5
  kJ

36
Possible Stresses LeChâteliers Principle

• 5) Catalysts have no overall effect on
  equilibrium
• 6) Inhibitors have no overall effect on
  equilibrium

37
Possible Stresses LeChâteliers Principle

• 7) Pressure Changes
• If pressure increases, volume decreases
• If pressure decreases, volume increases
• To determine volume, consider coefficients for
  the substances that are gases

38
13) Predict the following shifts

• 2 SO3 (g) ? 2 SO2 (g) O2 (g) ?H 510 kJ
• A) Additional SO3 is added
• B) SO2 is removed
• C) The catalyst MnO4 is added
• D) O2 is added
• E) The temperature is decreased
• F) Pressure is increased

39
14) Review - Predict Changes

• PCl3 (g) 3 NH3 (g) ? P(NH2)3 (g) 3 HCl (g),
  ?H - 198 kJ
• a. Addition of PCl3
• b. Addition of NaOH
• c. Reducing Pressure
• d. Reducing Temperature