FACTORS THAT AFFECT RATES OF REACTION Chapter 7.3

COLLISION MODEL Kinetic Molecular Theory: States that the rate of reaction is affected by the number of collisions of reactant molecules per unit time. In general, the kinetic energy of particles increases as you go from a solid to a liquid to a gas (i.e. the water molecules in water vapour move faster than those in liquid water than those in frozen water).

4 FACTORS There are four factors that affect the rate of reaction. TEMPERATURE CONCENTRATION SURFACE AREA CATALYSTS

TEMPERATURE Higher temperatures increase the rate of a chemical reaction because particles move faster at high temperatures and this creates more violent collisions more often. Harder (and faster) collisions result in a better chance of breaking bonds so new ones can form.

CONCENTRATION Higher concentrations increase the rate of a chemical reaction because there are more particles to collide with. Since there are more collisions, there is a better chance of a chemical reaction taking place.

SURFACE AREA A larger surface area increases the rate of a chemical reaction because there are more exposed particles in the compound. What do you think is easier to ignite– a pile of sugar or fine particles of sugar (“sugardust”)? Check out this video: http://www.youtube.com/watch?v=Jg7mLSG-Yws&feature=related

CATALYSTS A substance that increases the rate of a chemical reaction but does not get consumed by the reaction. Catalysts are therefore not considered a reactant nor a product. e.g. 2 H2O2 MnO2 2 H2O + O2 Works by reducing the energy needed to break bonds, providing easier paths for reactions. Enzymes are considered biological catalysts

Homework Q.1-4 p.264. Read p.270-271 +p.274.