Chemical Reactions: Energy, Rates, and Equilibrium

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Chapter 7

• Chemical Reactions Energy, Rates, and Equilibrium

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Energy and Chemical Bonds

• Chapter 6
• Kept a careful accounting of atoms as they
  rearranged themselves
• Reactions also involve a transfer of energy

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Energy and Chemical Bonds

• Two fundamental kinds of energy.
• Potential energy is stored energy.
• Kinetic energy is the energy of motion.
• Law of Conservation of Energy
• Energy can be converted from one kind to another
  but never destroyed

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Energy and Chemical Bonds

• A chemical
• Potential - attractive forces in an ionic
  compound or sharing of electrons covalent
  compound
• Kinetic (often in form of heat) occurs when
  bonds are broken and particles allowed to move

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Heat Changes during Chemical Reactions

• Bond dissociation energy The amount of energy
  that must be supplied to break a bond and
  separate the atoms in an isolated gaseous
  molecule.

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Heat Changes during Chemical Reactions

• Bond breakage requires energy to be added to the
  system ( energy)
• Bond formation gives off energy as the bonds form
  (- energy)

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Heat Changes during Chemical Reactions

• Heat of reaction (Enthalpy)
• Represented by DH
• is the difference between the energy absorbed in
  breaking bonds and that released in forming bonds
• Endothermic
• More energy is required than released.
• DH is positive
• Exothermic
• More energy is released than required
• DH is negative

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Exothermic and Endothermic Reactions
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Problem

• Br2 (l) ? Br2 (g) ?H 7.4 kcal/mol
• Is this reaction endothermic or exothermic
• Is this reaction spontaneous with respect to
  enthalpy?
• 2C8H18 25O2 ? 16CO2 18H2O 239.5 kcal
• Is this reaction endothermic or exothermic?
• What is the sign of ?H?

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Why do Chemical Reactions Occur? Free Energy

• Events that lead to the system having less energy
  are said to be spontaneous with respect to
  enthalpy
• Exothermic reactions are spontaneous
• Why would endothermic reactions occur?
• Free Energy (?G)
• Enthalpy ?H heat of reaction
• Entropy (S)

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Entropy

• Entropy measures the spreading out of energy
  universe moves toward disorder
• Entropy favored system is one that goes from a
  concentrated area of energy to the energy being
  more spread out
• ?S is positive
• Unfavorable process involves concentrating the
  energy into less area
• ?S is negative

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Why do Chemical Reactions Occur? Free Energy
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Problem

• Identify each of the following as entropy favored
  or disfavored. For each state the sign of the
  ?S.
• Assembling a jig-saw puzzle
• I2 (g) 3F2 (g) ? 2 IF3 (g)
• A precipitate forming when two solutions are
  mixed
• Demolition of a building
• CS2(g) 4H2(g) ?  CH4(g) 2H2S(g)
• 2HBr(g)  ? H2(g) Br2(g)

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Why do Chemical Reactions Occur? Free Energy

• Free Energy

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Why do Chemical Reactions Occur? Free Energy
?H ?S ?G
(-) favorable () favorable (-) spontaneous always
() unfavorable (-) unfavorable () nonspontaneous always
(-) favorable (-) unfavorable (-) spontaneous _at_ Low T () nonspontaneous _at_ High T
() unfavorable () favorable () nonspontaneous _at_ Low T (-) spontaneous _at_ High T
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Problem

• H2 (g) Br2 (l) ? 2 HBr (g)
• Is this reaction spontaneous with respect to
  entropy?
• If the ?H -17.4 kcal/mol is the reaction
  spontaneous with respect to enthalpy?
• If the ?H -17.4 kcal/mol and ?S 27.2 cal/mol
  K, is the reaction spontaneous with respect to
  free energy?
• What is the value of ?G at 300 K?

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Problem

• Given the reaction
• 8 Al(s) 3 Fe3O4(s) --gt 4 Al2O3(s) 9 Fe(s)
  3350 kJ
• Is the reaction endothermic or exothermic?
• The sign of ?H should be positive or negative?
• According to enthalpy, is the reaction favored or
  not favored?
• According to entropy, is the reaction favored or
  not favored?
• The sign of ?S should be positive or negative?
• Calculate Gibbs free energy for this reaction at
  25oC if ?S215.1 J/K and has the sign you
  determined in part e.
• Is the reaction favored according to free energy?

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How do Chemical Reactions Occur? Reaction Rates

• DG indicates whether a reaction will occur
• But how fast will it occur?
• To what extent does the reaction occur?

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Rates of Reaction

• Rate of Reaction
• How fast does a reaction go?
• Properly oriented collisions
• Sufficient energy to break the bonds of the
  reactants
• Factors affecting collisions and energy
• Concentration of reactants
• Temperature of system

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How do Chemical Reactions Occur? Reaction Rates

• Orientation

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How do Chemical Reactions Occur? Reaction Rates

• Sufficient energy
• Energy of activation

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Effects of Temperature, Concentration, and
Catalysts on Reaction Rates
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Effects of Temperature, Concentration, and
Catalysts on Reaction Rates

• A third way to speed up a reaction is to add a
  catalysta substance that accelerates a chemical
  reaction but is itself unchanged in the process.
• A catalyzed reaction has a lower activation
  energy.

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Problem
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Chemical Equilibrium

• Equilibrium
• To what extent a reaction occurs

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Reversible Reactions and Chemical Equilibrium

• Many reactions result in complete conversion from
  reactant to product.
• Many however do not

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Chemical Equilibrium
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Equilibrium Equations and Equilibrium Constants

• Consider the following general equilibrium
  reaction
• aA bB ? mM nN
• Where A, B, are the reactants
• M, N, . are the products
• a, b, .m, n, . are coefficients in the balanced
  equation.
• At equilibrium, the composition of the reaction
  mixture obeys an equilibrium equation.

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Equilibrium Equations and Equilibrium Constants

• The value of K varies with temperature.

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Problem

• Write an equilibrium constant equation for
• N2(g) 3H2(g) ? 2NH3(g)
• FeCl3(aq) 3NaOH(aq) ? Fe(OH)3(s)
  3NaCl(aq)

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Equilibrium Equations and Equilibrium Constants

• K larger than 1000 Reaction goes essentially to
  completion.
• K between 1 and 1000 More products than
  reactants are present at equilibrium.
• K between 1 and 0.001 More reactants than
  products are present at equilibrium.
• K smaller than 0.001 Essentially no reaction
  occurs.

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Problem

• Indicate the primary substance or substances in
  the reaction vessel given the K values of the
  reactions
• 2CO(g) O2(g) ? 2CO2(g) K 1.4
  x 102
• H2O (l) HNO2(aq) ? H3O(aq) NO2-(aq) K
  4.50 x 10-4

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LeChateliers Principle The effect of Changing
Conditions on Equilibia

• Le Châtelier's Principle When a stress is
  applied to a system at equilibrium, the
  equilibrium shifts to relieve the stress.
• The stress can be any
• change in concentration
• pressure
• Volume
• temperature that disturbs original equilibrium.

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LeChateliers Principle
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Le Chateliers Principle The Effect on Changing
Conditions on Equilibria
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Problem

• Methanol can be synthesized by combining carbon
  monoxide and hydrogen.CO(g) 2H2(g)   ?
   CH3OH(g) ?Hrxn -90.7 kJ
• What happens when
• The temperature is raised by 50oC?
• The pressure is raised?
• Methanol is added?
• Hydrogen is removed?

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Optional Homework

• Text 7.17, 7.18, 7.19, 7.20, 7.22, 7.23, 7.30,
  7.38, 7.40, 7.46, 7.48, 7.54, 7.56, 7.58, 7.62,
  7.64, 7.66, 7.68, 7.80
• Chapter 7 Homework online

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Required Homework

• Assignment 7