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/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)

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.