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Rates of ReactionMrs. Coyle

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How fast does aging occur?

http://www.ging-strategie.de/Anti%20Aging%20Strategie.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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Exothermic reaction: reaction gives off energy. reactants products + energy

Endothermic reaction: reaction takes in energy.

energy + reactants products

Exo- and Endothermic Reactions

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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

C-H 98

O-H 110C-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 CO 2)

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Ex: Exothermic

• H2 + Cl 2 2 HCl + 183 kJ/mol

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Ex: Endothermic

2HgO + 181.7 kJ 2Hg + O 2

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Net Energy Change

D H= Potential Energy of Products-Potential Energy of Reactants

D H <0 exothermic

D H >0 endothermic

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Collision Theory of Reactions

http://staff.um.edu.mt/jgri1/teaching/che2372/notes/10/10_19.gif

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Activation Energy

• Minimum amount of kinetic energy thereactants must have in order to react.

• The particles form an activatedcomplex (transition state) that is anunstable arrangement of atoms that lasts afew moments. Then the products areformed.

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Analogy

http://www.800mainstreet.com/7/0007-004-reac_rate2.htm

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Potential Energy vs Time(Exothermic)

http://staff.um.edu.mt/jgri1/teaching/che2372/notes/10/theory.html

Time

Heat of

ReactionD

H

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Energy vs Time (Endothermic)

Heat ofReaction D H

ActivationEnergy

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Concentration Effect on Reaction

RatesThe more concentrated, the higher the reaction rate.

Why?

http://content.answers.com/main/content/wp/en/thumb/f/ff/525px-Molecular-collisions.jpg

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Temperature Effect on ReactionRates

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 thereaction proceeds faster.

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Particle Size (Surface Area)

• The smaller the particle size the faster therate.

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Inhibitors

• Substances that negate the effect of thecatalyst.