Rates of Reaction

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Rates of Reaction

• Mrs. Coyle

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How fast does aging occur?
http//www.ging-strategie.de/Anti20Aging20Strate
gie.JPG
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How fast does the candle burn?
http//images.usatoday.com/tech/_photos/2006/03/28
/aprilfig1.jpg
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Reaction Rate

• The speed with which products form from the
  reactants.

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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 (2 x 58)
CO 187 (2 x 93.5)
CC 145 (2 x 72.5)
( 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
http//staff.um.edu.mt/jgri1/teaching/che2372/note
s/10/10_19.gif
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Activation Energy

• Minimum amount 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.
  Then the products are formed.

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Analogy
http//www.800mainstreet.com/7/0007-004-reac_rate2
.htm
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• Potential Energy vs Time
• (Exothermic)

Heat of Reaction DH
Time
http//staff.um.edu.mt/jgri1/teaching/che2372/note
s/10/theory.html
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Energy vs Time (Endothermic)
Activation Energy
Heat of Reaction DH
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Reaction Rates are affected by

• Concentration
• Temperature
• Catalyst
• Surface Area (Particle Size)
• Mixing (Stirring)

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Concentration Effect on Reaction Rates

The more concentrated, the higher the reaction
rate. Why?
http//content.answers.com/main/content/wp/en/thum
b/f/ff/525px-Molecular-collisions.jpg
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Temperature Effect on Reaction Rates

The higher the temperature, the faster the
rate. Why?
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Catalyst Effect on Reaction Rates
A catalyst lowers the activation energy, thus the
reaction proceeds faster.

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Particle Size (Surface Area)

• The smaller the particle size the faster the rate.

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Inhibitors

• Substances that negate the effect of the catalyst.