Rates of Reaction Mrs. Coyle

How fast does aging occur?

How fast does the candle burn?

Reaction Rate The 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-H98 O-H110 C-C80 C-O78 H-H103 C-N65 O=O116 (2 x 58) C=O187* (2 x 93.5) C=C145 (2 x 72.5) (* as found in CO 2 )

Ex: Exothermic H 2 + Cl 2  2 HCl kJ/mol

Ex: Endothermic 2HgO kJ  2Hg + O 2

Net Energy Change  H= Potential Energy of Products -Potential Energy of Reactants  H <0 exothermic  H >0 endothermic

Collision Theory of Reactions

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

Potential Energy vs Time (Exothermic) 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?

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.