Rates of Reaction
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Transcript of Rates of Reaction
Rates of Reaction Mrs. Coyle
How fast does aging occur?
http://www.ging-strategie.de/Anti%20Aging%20Strategie.JPG
How fast does the candle burn?
http://images.usatoday.com/tech/_photos/2006/03/28/aprilfig1.jpg
Reaction RateThe speed with which products form from the reactants.
Exothermic reaction: reaction gives off energy.
reactants products + energy
Endothermic reaction: reaction takes in energy.
energy + reactants products
Exo- and Endothermic Reactions
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)
Average bond energies, kcal/moleC-H 98
O-H 110
C-C 80
C-O 78
H-H 103
C-N 65
O=O 116 (2 x 58)C=O 187* (2 x 93.5)
C=C 145 (2 x 72.5)(* as found in CO2)
Ex: Exothermic
• H2 + Cl2 2 HCl + 183 kJ/mol
Ex: Endothermic
2HgO + 181.7 kJ 2Hg + O2
Net Energy Change
H= Potential Energy of Products-Potential Energy of Reactants
H <0 exothermic
H >0 endothermic
Collision Theory of Reactions
http://staff.um.edu.mt/jgri1/teaching/che2372/notes/10/10_19.gif
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.
Analogy
http://www.800mainstreet.com/7/0007-004-reac_rate2.htm
Potential Energy vs Time (Exothermic)
http://staff.um.edu.mt/jgri1/teaching/che2372/notes/10/theory.html
Time
Heat of Reaction H
Energy vs Time (Endothermic)
Heat of Reaction H
Activation Energy
Reaction Rates are affected by:
• Concentration• Temperature• Catalyst• Surface Area (Particle Size)• Mixing (Stirring)
Concentration Effect on Reaction Rates
The more concentrated, the higher the reaction rate.
Why?
http://content.answers.com/main/content/wp/en/thumb/f/ff/525px-Molecular-collisions.jpg
Temperature Effect on Reaction Rates
The higher the temperature, the faster the rate.
Why?
Catalyst Effect on Reaction RatesA catalyst lowers the activation energy, thus the reaction proceeds faster.
Particle Size (Surface Area)
• The smaller the particle size the faster the rate.
Inhibitors
• Substances that negate the effect of the catalyst.