Reaction Rate

Reaction Rate: It’s the change in the concentration of reactants per unit time as reaction proceeds. The area of chemistry that is concerned with reaction rates and reaction mechanisms is called chemical kinetics

Rate-Influencing Factors The rate of the reaction depends on the collision frequency of the reactions and the collision efficiency. 1- Nature of Reactants 2- Surface area 3- Temperature 4- Concentration 5- Presence of Catalyst

Nature of Reactants Substances vary greatly in their tendencies to react. Bonds are broken and other bonds are formed in reaction. The rate of reaction depends on the particular reactants and bonds involved.

Surface Area Heterogeneous reactions involve reactants in two different phases. These reactions can occur only when the two reactants are in contact. Thus, the surface area of a solid reactant is an important factors in determining rate. An increase in surface area increases the rate of heterogeneous reaction.

Temperature An increase in temperature increases the average kinetic energy of the particles in a substances, this can result in greater number of effective collisions when the substances is allowed to react with another substance. If the number of effective collisions increases, the reaction rate will increase. To be effective, the energy must be equal or greater than the activation energy. At higher temperatures, more particles possess enough to form the activated complex when collisions occur. Thus, a rise in temperature produces an increase in collision energy as well as in collision frequency.

Beginning near room temperature, the reaction rates of many common reactions roughly double with each 10K or (10 C) rise in temperature.

Concentration In homogeneous reaction system, reaction rates depends on the concentration of the reactants. If the number of effective collisions increases, the rate increases as well. In general, an increase in rate is expected if the concentration of one or more of the reactants is increased. The actual effect of concentration changes on reaction rate, however, must be determined experimentally.

Presence of Catalysts A catalyst is a substance that changes the rate of the chemical reaction without itself being permanently consumed. The action of catalyst is called catalysis. A catalyst provides an alternative energy pathway or reaction mechanism in which the potential energy barrier between the reactants and products is lowered.

A catalyst that is in the same phase as all the reactants and products in a reaction system is called a homogeneous catalyst. When its phase is different from that of reactants, it is called a heterogeneous catalyst.

Rate Laws for Reactions The rate is determined experimentally by keeping the concentration of one reactant and the temperature of the system constant. Then, the reaction rate is measured for various concentration of the reactant.

Example: 2H 2 + 2NO  N 2 + 2H 2 O 4 moles of the reactants produces 3 moles of the products. A series of experiments is conducted by using same concentration of NO but different concentrations of H2. Doubling the concentration of H2 doubles the rate, tripling the concentration, triples the rate. If “R” represents the rate. [H2] represents the concentration of H2, then R∞ [H2] ∞: is proportional to.

2H 2 + 2NO  N 2 + 2H 2 O Now, if the concentration of H2 is kept constant, and the concentration of NO is varied. The rate of the reaction increases 4 folds when the concentration is doubled, and increases 9 folds when the concentration is tripled. Thus, R∞ [NO 2 ] So, R ∞ [H2]x [NO 2 ] Thus, R= k [H2]x [NO 2 ]

An equation that relates reaction rate and concentration of reactants is called rate law for the reaction.

Rate Laws and Reaction Pathways Rate law depends on reaction mechanism: A+ B  2C So, R= k [A]x [B] If the reaction is reversed: 2C  A+B So, R= [C]2

If the chemical reaction proceed in a series of steps, the rate law is determined from the slowest step, because it has the lowest rate. The slowest- rate step is called Rate Determining Step.

Example: NO2+ CO  NO + CO2, this reaction occurs in 2 steps: Step 1: NO2 + NO2  NO3 + NO ( slow) Step 2: NO3 + CO  NO2 + CO2 ( fast) So, R = k [NO2]x [NO2]= k [NO2]2 Practice p: 544