Rates of Reactions Lesson 13

Rates of reactions (P260-264) Review of Particle Theory All Matter is made up of extremely tiny particles Each Pure substance has its own kind of particles, different from the particles of other pure substances Particles are always moving. Particles at a higher temperature are generally moving faster, on average than particles at a lower temperature. Particles attract each other

Collision Model (theory) Atoms and molecules are constantly bumping into each other. If they are moving too slowly or are stable, no reaction will occur. Molecules that are moving quickly enough may break bonds between atoms and the atoms may combine to form new molecules (new products). The exact energy required for a particular old bond to be broken and a new one to be formed is called the 'Activation Energy. This is the threshold energy at or beyond which, if the collisions occur, new bonds form.

Collision Model (theory) Two factors increase the rate of reaction Number of collisions Number effective/successful collisions

4 Factors that affect rates of reactions Temperature When the reactants are heated, they bounce and contact more vigorously with other reactant molecules. This increases the number of successful collision and the rate of reaction. This is the most important factor.

4 Factors that affect rates of reactions Concentration Concentration is defined as the number of molecules of reactants per unit volume. The more the concentration of reactant molecules, the higher the probability of collision due to their sheer number. Excess concentration may have no effect if one of the reactants is used up.

4 Factors that affect rates of reactions Surface Area By increasing surface area, the number of molecules exposed for collisions is increased. This allows more collisions between molecules to occur and increases the rate of reaction.

4 Factors that affect rates of reactions Catalyst Catalysts, are chemicals or substances that catalyze or promote a chemical reaction to occur and remaining unchanged in the end. They are like parts of an assembling mechanism that help making the final product but then detach themselves from it. They lower the activation energy.

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