Reaction Rate

1.    Kinetic theory (three points)

a.    all matter is composed of particles

b.    particles are in constant motion

c.    all collisions are perfectly elastic

2.    Reversible reaction (from chapter 14)

Most reactions we have learned so far have gone to completion or from reactants to products but as we learned in the last chapter the products can combine and reform the reactants.

Example H₂ + I₂ à 2HI and 2HI à H₂ + I₂ (reverse reaction) or H₂ + I₂ ß> 2HI

3.    Reaction Rate

-     is the disappearance of a reactant or the rate of appearance of product (mol/L/s)

-     in the above reaction the rate could be measured by the rate of disappearance of H₂ or I₂ or the rate of appearance of HI (note these rates will not be the same, why?)

4.    Factors affecting reaction rate

a. nature of reactants

        b. reactant concentration

        c. pressure (only for a gas)

        d. temperature

        e. catalyst

Think of the three points of the kinetic collision theory to explain how the above factors affect the rate.

Kinetic Collision Theory

a.    Particles must collide

b.    Must collide with sufficient energy

c.    Must collide with the correct orientation

5.    Nature of reactants

-determines the kind of reaction and reaction rate

-reactions with bond rearrangement or e^- transfer take longer than those without like an ionic reaction.

-as molecules collide they usually rebound and move away unchanged however some collisions do

-have enough energy to cause changes in electron clouds. When this occurs may form an activated

  complex

6.    Concentration

-     quantity of matter that exists in a unit volume

-     affects reaction rate because of more particles per volume the more collision and more likely to react.

7.    Pressure

-     increase in pressure of a gas, decrease in volume and therefore increase in concentration and an increase in reaction rate.

8.    Surface area

-     heterogeneous reaction – a chemical reaction that takes place at the interface between 2 phases

-     example Zn + H₂SO₄ increase the surface area more surface exposed, therefore increase suface area increase the concentration

9.    Temperature

-     increase in temperature, more KE more collisions and harder collisions, therefore more activated complexesand a greater chance of product and an increase in reaction rate

10.    Catalyst

-     a substance that increases a reaction rate without being permanently changes itself

-     catalysis-is the process of increasing the reaction rate by the presence of a catalyst

-     the catalyst does this by changing the reaction mechanism in such a way that the activation energy is lowered.

-     Types of catalysts a. heterogeneous b. homogeneous

-     Opposite of a catalyst is an inhibitor

11.    Reaction Pathway (Draw Diagrams)

-activation energy – the energy required to form the activated complex

12.    Reaction Mechanism

-     most reactions occur in a series of steps each, each normally involving the collision of only two particles (three or more unlikely because won’t have enough energy or position)

-     reaction mechanism – the series of reaction steps that must occur for a reaction to go to completion.

-     Example

A à B

B à C

C à final product

-     one step will be slower than the rest this will be the rate determining step

-     at a given temperature the rate of the reaction varies directly as the product of the concentrations of the reactants in the slowest step.

-     Rate = k [A] if in the above example Aà B was the slowest step