Chapter 17 The Reaction Process

Reaction Mechanisms Step-by-step sequence of reactions by which the overall chemical change occurs. Intermediates Species that appear in some steps but not in the net equation. Homogeneous reaction Reactants and Products exist in a single phase.

Example Reaction I2 + H2  2HI

Collision Theory Set of assumptions regarding collisions and reactions. Particles must first collide with a favorable orientation and energy to merge the valence electrons and bonds.

Activation Energy Minimum energy required to transform the reaction into an activated complex. Exothermic Curve

Endothermic Curve

Energy Change ∆E forward = E products – E reactants ∆E reverse = E reactants - E products Ea = activation energy - E reactants Ea’= activation energy - E products

Reaction Rate The change in concentration of reactants per unit time as a reaction proceeds. Chemical Kinetics Area of chemistry that is concerned with reaction rates and the reaction mechanism.

Factors that affect Rate Nature of reactants Surface Area Temperature Concentration Catalysts Substance that changes the rate of a reaction without being consumed or part of the reaction. Catalysis- action of a catalyst.

Rate Laws Equation that relates the reaction rate and the concentrations of reactants. R = k[A]n[B]m R is the reaction rate k= specific rate constant A and B are concentrations of reactants. n and m is the order of the reactants

Order The power to which a reactant is raised. Value Meaning 0 – rate doesn’t depend upon the concentration of that reactant. 1 – rate is directly proportional to the concentration of that reactant. 2 – rate is directly proportional to the square of the concentration of that reactant. Overall Order Sum of the reactant orders.

Determining the order 2HI  H2 + I2 Experiment [HI] Rate 1 .015 .030 4.4x10-3 3 .060 1.76x10-2

Multiple Orders A + B  C Experiment [A] [B] Rate 1 1.2 2.4 8.0x10-8 2 3 3.6 7.2x10-7

Specific Rate Constant The proportionality constant relating the rate of the reaction to the concentration of the reactants. Found through experimental data. Affected by temperature and catalyst but unaffected by concentration of reactants.

Determining the K 2HI  H2 + I2 Experiment [HI] Rate 1 .015 1.1x10-3 2 .030 4.4x10-3 3 .045 9.9x10-3 Order of the reactant and the reaction is 2nd order.

Determining K, Higher order A + B  C Experiment [A] [B] Rate 1 1.2 2.4 8.0x10-8 2 4.0x10-8 3 3.6 7.2x10-7 Order of the reactant A is 2nd and the order of reactant B is 1st order and the reaction is 3rd order.