Copyright © by Holt, Rinehart and Winston. All rights reserved. Ch. 17 Reaction Kinetics Understanding chemical reactions that occur at different rates
Copyright © by Holt, Rinehart and Winston. All rights reserved. The area of chemistry that is concerned with reaction rates and reaction mechanisms is called chemical kinetics. The change in concentration of reactants per unit time as a reaction proceeds is called the reaction rate. Rates are measured in a unit of something per time interval (Ex: M/s)
Copyright © by Holt, Rinehart and Winston. All rights reserved. Properties used to measure reaction rates Rates are measured by the rate of formation of a product or disappearance of a reactant keep track of how the concentration of one or more reactants or products changes over time. Ex: observe change in color. Measure pressure change use gas laws to calculate the concentrations. Measure temperature change
Copyright © by Holt, Rinehart and Winston. All rights reserved. As the concentration of the reactants decreases over time, the product concentration increases General Equation for the Rate of Reaction 2Br − (aq) + H 2 O 2 (aq) + 2H 3 O + (aq) → Br 2 (aq) + 4H 2 O(l)
Copyright © by Holt, Rinehart and Winston. All rights reserved. Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) time [Br 2 ] Absorption
Copyright © by Holt, Rinehart and Winston. All rights reserved. A B rate = - [A][A] tt rate = [B][B] tt time Chapter 17 Section 1 Reaction Rate
Copyright © by Holt, Rinehart and Winston. All rights reserved. Collision Theory: In order for reactions to occur between substances, their particles must collide. Reactant molecules must collide with a favorable orientation and with enough energy to merge the valence electrons and disrupt the bonds of the molecules to form to the products. Number of collisions per unit time determines how fast a reaction can take place.
Copyright © by Holt, Rinehart and Winston. All rights reserved. Particle Collisions
Copyright © by Holt, Rinehart and Winston. All rights reserved. Possible Collision Orientations for the Reaction of H 2 and I 2
Copyright © by Holt, Rinehart and Winston. All rights reserved. 1. Surface area 2. Temperature 3. Concentration 4. Presence of a catalyst 4 Major Rate-Influencing Factors Chapter 17
Copyright © by Holt, Rinehart and Winston. All rights reserved. The reaction rate depend on concentrations of the reacting species. example: A substance that oxidizes in air (18% O 2 ) oxidizes more vigorously in pure oxygen. Generally, an increase in the concentration of one or more of the reactants will increase the reaction rate Concentration
Copyright © by Holt, Rinehart and Winston. All rights reserved. An increase in surface area increases the rate of reactions. Because the reaction rate depends on the area of contact of the reaction substances. Surface Area An increase in temperature increases the reaction rate since the average kinetic energy of the particles increases; greater number of effective collisions. Temperature
Copyright © by Holt, Rinehart and Winston. All rights reserved. A catalyst is a substance that will lower amount of energy required for a reaction to take place (activation energy) Presence of a Catalyst E a is the initial input of energy needed to overcome the repulsion forces between molecules as come close together
Copyright © by Holt, Rinehart and Winston. All rights reserved. A catalyst lowers the energy barrier and the reaction proceeds at a fast rate
Copyright © by Holt, Rinehart and Winston. All rights reserved. Rate law: an equation that relates reaction rate and concentrations of reactants for a reaction Rate Laws for Reactions Rate law equation: R = k[A] n [B] m K = specific rate constant [A], [B] = molar concentration of reactant n, m = respective powers called “orders” n + m = overall order of the reaction For the reaction: nA + mB → pC
Copyright © by Holt, Rinehart and Winston. All rights reserved. F 2 (g) + 2ClO 2 (g) 2FClO 2 (g) rate = k [F 2 ][ClO 2 ] Rate Laws Rate laws are always determined experimentally. Reaction order is always defined in terms of reactant (not product) concentrations. The order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation
Copyright © by Holt, Rinehart and Winston. All rights reserved. An order of one for a reactant means that the reaction rate is directly proportional to the concentration of that reactant. An order of two means that the reaction rate is directly proportional to the square of the reactant. An order of zero means that the rate does not depend on the concentration of the reactant, as long as some of the reactant is present.
Copyright © by Holt, Rinehart and Winston. All rights reserved. 2H 2 (g) + 2NO(g)N 2 (g) + 2H 2 O(g) The initial reaction rate is found to vary directly with the hydrogen concentration: the rate doubles when [H 2 ] is doubled, and the rate triples when [H 2 ] is tripled. R [H 2 ] The initial reaction rate is found to increase fourfold when the [NO] is doubled and ninefold when the [NO] is tripled. R [NO] 2