# Rates of Reaction Mrs. Coyle How fast does

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 Rate The speed with which products form from the reactants. Exo- and Endothermic Reactions

Exothermic reaction: reaction gives off energy. reactants products + energy Endothermic reaction: reaction takes in energy. energy

+ reactants products 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/mole C-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)

Heat of Reaction H Time http://staff.um.edu.mt/jgri1/teaching/che2372/notes/10/theory.html Energy vs Time (Endothermic)

Activation Energy Heat of Reaction H 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 Rates A 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.

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