Rates of Reactions

Rates Speed or rate is measured by the amount of change that occurs in a given time interval

REACTION RATE The amount of time it takes to use up the reactants (decrease in reactant concentration) OR The amount of time it takes to make products (increase in product concentration)

I.A. Reaction Rates 2 NO2  2 NO + O2 Reaction Rate-change in concentration of reactants and products in a given time period Consider the following reaction: 2 NO2  2 NO + O2 time (sec) [NO2] 0 4.0 M 5.0 3.1 M 10.0 2.5 M 15.0 2.1 M 20.0 1.8 M Sketch a graph with t on the x-axis and M on the y-axis.

I.A. Reaction Rates 2 NO2  2 NO + O2 How much NO is present at the beginning of the reaction? time (sec) [NO2] 0 4.0 M 5.0 3.1 M 10.0 2.5 M 15.0 2.1 M 20.0 1.8 M [NO]

I.A. Reaction Rates 2 NO2  2 NO + O2 How much NO2 was used up in the first 5 seconds? How much NO was produced? time (sec) [NO2] 0 4.0 M 5.0 3.1 M 10.0 2.5 M 15.0 2.1 M 20.0 1.8 M [NO] 0 M Calculate the moles of NO produced for each value of t (always use the total moles of NO2 used up to that point).

Collision Theory Reaction rate depends on the collisions between reacting particles. Effective collision Reaction occurs Elastic collisions  No reaction occurs Effective collisions occur if the particles... collide with each other have the correct orientation have enough kinetic energy to break bonds

Unsuccessful Collisions Particle Orientation Required Orientation Unsuccessful Collisions Successful Collision

I.B. Collision Theory Elastic Collisions Effective Collisions

Activation Energy (Ea) Even though the boulder will be more stable at the bottom of the hill, it needs a push to get up and over the bump first. depends on reactants low Ea = fast rxn rate

Activation Energy (Ea) Minimum energy required for a reaction to occur Transition state-complex that is neither reactant nor product Activated Complex Activation Energy Energy of products ∆H - Difference between energy of reactants and products Energy of reactants

Draw an energy diagram for an exothermic reaction Draw an energy diagram for an exothermic reaction. Label the axes, reactants, products, activated complex, activation energy, and change in enthalpy.

Factors Affecting Reaction Rate 1. Nature of the Reactants 2. Concentration of Reactants 3. Surface Area of Reactants 4. Temperature 5. Catalysts

Factors Affecting Reaction Rate 1. Nature of the Reactants Certain bonds are easier to break than others Ca(s) + 2H2O(l)  H2(g) + Ca(OH)2(aq) 2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq) Na reacts with water faster than Ca because Na is more reactive.

2. Concentration high concentration = the higher the concentration → the more molecules there are → the more collisions take place creating product

3. Surface Area the more surface a molecule can touch → the faster the reaction Increase surface area by… using smaller particles dissolving in water

Temperature high temp = fast reaction rate the hotter the molecules → the more they move → the more they collide & change into product

Lower temp, more reactant particles have low amts of kinetic energy As temperature increases, more reactant particles have higher kinetic energy Only a small # of particles will have enough energy to form an activated complex.

Catalysts 5. Catalyst substance that increases reaction rate without being consumed in the reaction lowers the activation energy, more collisions will have enough energy to pass over the energy barrier reaction is faster because a greater fraction of collisions is effective at any given point

Catalysts in body Enzymes Breakdown sugars, proteins, etc

The decomposition of ozone to oxygen is catalyzed by Cl. Reaction Mechanisms Many reactions occur in several steps. 2 O3 (g)  3 O2 (g) The decomposition of ozone to oxygen is catalyzed by Cl.

Cl + O3  ClO + O2 Elementary step O3  O + O2 Intermediates Reaction Mechanisms Cl + O3  ClO + O2 Elementary step O3  O + O2 Intermediates ClO + O  Cl + O2 Add steps 2 O3  3 O2

Determine the original equation. Rate-determining step. Reaction Mechanisms 2 NO  N2O2 Fast N2O2 + H2  N2O + H2O Slow N2O + H2  N2 + H2O Fast Determine the original equation. Rate-determining step.

What is the overall reaction? How many elementary steps? Reaction Mechanisms HBr + NO2  HOBr + NO Slow HBr + HOBr  H2O + Br2 Fast What is the overall reaction? How many elementary steps? What are the intermediates? What is the rate-determining step?