Rates of Reaction

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Rates of Reaction

• Honors Chemistry
• Mrs. Coyle

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How fast do reactions occur?
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Reaction Rate

• The change in concentration of a reactant or a
  product per unit time.
• Rate At2-At1 Rate ?A
• t2-t1 ?t
• Brackets indicate concentration.
• Units mol / L / s
• or mol/(Ls)

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Exo- and Endothermic Reactions

• Exothermic reaction reaction gives off energy.
• reactants products
  energy
• Endothermic reaction reaction takes in energy.
• energy reactants
  products

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Bonds and Energy

• When bonds are broken energy is absorbed.
• (analogy stretching a rubber band)
• When bonds are formed energy is released.
• (analogy releasing a stretched rubber band)

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Average bond energies, kcal/mole Average bond energies, kcal/mole
C-H 98
O-H 110
C-C 80
C-O 78
H-H 103
C-N 65
OO 116
CO 187
CC 145
( as found in CO2) ( as found in CO2)
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Ex Exothermic

• H2 Cl2 ? 2 HCl 183 kJ/mol

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Ex Endothermic

• 2HgO 181.7 kJ ? 2Hg O2

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Net Energy Change

• DH Potential Energy of Products
• -Potential Energy of Reactants
• DH lt0 exothermic
• DH gt0 endothermic

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Collision Theory of Reactions
Molecules must collide in order to react.
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Collision Theory Contd

• Molecules have kinetic energy before the
  collision. The KE is changed to potential energy
  as the bonds are breaking and new bonds are
  forming.

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Collision Theory Contd

• Activation energy is the minimum amount
  (threshold) of kinetic energy the reactants must
  have in order to react.
• The particles form an activated complex
  (transition state) that is an unstable
  arrangement of atoms that lasts a few moments.
• The products are formed.

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Analogy
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• Potential Energy vs Time
• (Exothermic)

Heat of Reaction DH
Time
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Energy vs Time (Endothermic)
Activation Energy
Heat of Reaction DH
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Reaction Rates are affected by

• Nature of the Reactants
• Concentration
• Temperature
• Catalyst
• Surface Area (Particle Size)
• Mixing (Stirring)

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Effect of Nature of Reactants on Reaction Rate

• Ionic reactions usually occur faster than
  reactions involving transfer of electrons.

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Concentration Effect on Reaction Rates

The more concentrated, the higher the reaction
rate. Why?
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Rate Expression

• Rate k Ax By
• k is the specific rate constant.
• The exponents and k are determined from
  experimental data.

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Homogeneous and Heterogeneous Reactions

• Homogeneous reactants are in the same phase.
• Heterogeneous reactants are in different phases.

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Ex H2(g)I2(g)?2HI(g)

• Homogeneous
• Gas phase
• What effect would increasing the pressure have?

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Ex Zinc(s) Sulfuric Acid (aq)

• Heterogeneous.
• Increasing the surface area will increase the
  reaction rate.

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Temperature Effect on Reaction Rates

The higher the temperature, the faster the
rate. Why?
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Catalysts

• A substance that speeds up the reaction without
  being consumed.

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Catalyst Effect on Reaction Rates
A catalyst lowers the activation energy, thus the
reaction proceeds faster.

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Catalysts

• Homogeneous catalyst is in the same phase as the
  reactants.
• Heterogeneous catalyst is in a different phase
  than the reactants. Provide a surface for the
  reaction to occur.

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Inhibitors

• Substances that negate the effect of the catalyst.

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Particle Size (Surface Area) Effect on Reaction
Rates

• The smaller the particle size the faster the rate.

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Effect of Mixing on Reaction Rates