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. chemical kinetics: area of chemistry that is concerned with reaction rates and reaction mechanisms. A reaction rate: change in concentration of reactants or products per unit time Example of possible unit : M/s
Copyright © by Holt, Rinehart and Winston. All rights reserved. Over time, [reactants] decrease as the [products] increase 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. 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. Properties used to measure reaction rates Used to keep track of how the concentration of one or more reactants or products changes over time. 1. Observe change in color (Might include the use of an indicator) 2. Measure pressure change & use gas laws to calculate the concentrations. 3. Measure temperature change time
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 with enough energy to merge the valence electrons and disrupt the bonds of the molecules to form products. Ultimately, the 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. Concentration 2. Surface Area 3. Temperature 4. Presence of a catalyst 4 Major Rate-Influencing Factors Chapter 17
Copyright © by Holt, Rinehart and Winston. All rights reserved. Generally, an increase in the concentration of one or more of the reactants will increase the reaction rate Increased concentration of reactant species means more particles more collisions example: A substance that oxidizes in air (18% O 2 ) oxidizes more vigorously in pure oxygen. Concentration
Copyright © by Holt, Rinehart and Winston. All rights reserved. An increase in surface area increases the rate of reactions. Greater area of contact for the reactant species more collisions Surface Area An increase in temperature increases the reaction rate since the average kinetic energy of the particles increases, particles move faster more 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 Presence of a Catalyst
Copyright © by Holt, Rinehart and Winston. All rights reserved. Activiation Energy (E a ): the initial input of energy needed to overcome the repulsion forces between molecules as come close together Presence of a Catalyst
Copyright © by Holt, Rinehart and Winston. All rights reserved. How does a catalyst work? A catalyst lowers the energy barrier and the reaction proceeds at a fast rate
Copyright © by Holt, Rinehart and Winston. All rights reserved. To order to break bonds, energy must be absorbed When bonds are formed, energy is released. Presence of a Catalyst
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] = concentration of each 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 coefficients in balanced equation
Copyright © by Holt, Rinehart and Winston. All rights reserved. How do changes in concentration affect rate?? Rate Order = 1 : rate is directly proportional to the concentration of that reactant. Ex: double reactant; double rate R.O. = 2: rate is directly proportional to the square of the reactant. Ex: double reactant; rate 4x faster Rate Order = zero: rate does not depend on concentration of 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 = k [H 2 ] The initial reaction rate is found to increase fourfold when the [NO] is doubled and ninefold when the [NO] is tripled. R = k[NO] 2