Reaction rates and Collision Theory

Catalyst

- adding a catalyst to the reaction mixture will increase the rate of reaction. It does this by lowering the activation energy or creating an alternative pathway (mechanism) so that the activated complex (more later ) forms at a lower energy.

Particle Size

- if you decrease the size of the particles, you are increasing the surface area of the reactants, making more of the substances available to come in contact with each other, so reaction will go faster.

Concentration of reactants

- if you increase the concentration of the reactants you increase the chance that the particles will collide, so reaction will go faster.

Temperature

- if you increase the temperature of the reactants, you are increasing the kinetic energy of the particles. So, you are increasing the chance that when they collide that they will do so with enough energy to break existing chemical bonds, so reaction will go faster.

A chemical change

- new substances with new properties are formed Chemical bonds are broken and new chemical bonds are formed

Breakdown of Stratospheric Ozone

1. CFCs react with UV to break apart a Cl atom from the CFC 2. When chlorine is present (from CFCs), it can attach to an oxygen atom in an ozone molecule (O3 ) to form chlorine monoxide (ClO) and O2: 3. The chlorine monoxide molecule reacts with a free oxygen atom, which pulls the oxygen from the ClO to produce free chlorine again:

What are the necessary conditions for a reaction to occur?

1. Particles need to collide. 2. Must collide with enough energy to break existing chemical bonds 3. Must collide with the proper orientation so that new bonds form in the correct places. All 3 of these conditions must occur at the same time in order for a chemical reaction to occur (effective collisions).

And the cycle will continue. One Cl atom can catalyze the destruction of about _______ molecules per second!

And the cycle will continue. One Cl atom can catalyze the destruction of about 1 million ozone molecules per second!

Beginning in the 1920s, ___________ were used in refrigeration and air conditioning, as propellants in aerosol cans and as "blowing agents" to inject air into foam products like Styrofoam. ___________ were considered "safe" because they are ___________ But they had adverse effects in the upper atmosphere where they promote the breakdown of protective ozone, which became apparent after decades of use.

Beginning in the 1920s, chlorofluorocarbon compounds (CFCs) were used in refrigeration and air conditioning, as propellants in aerosol cans and as "blowing agents" to inject air into foam products like Styrofoam. CFCs were considered "safe" because they are non-toxic and non-flammable. But they had adverse effects in the upper atmosphere where they promote the breakdown of protective ozone, which became apparent after decades of use.

order the equations O3 + Cl → ClO + O2 CFCl3 + UV → Cl + CFCl2 ClO + O → Cl + O2

CFCl3 + UV → Cl + CFCl2 O3 + Cl → ClO + O2 ClO + O → Cl + O2

CFCs have lowered the average concentrations of ___________ in the stratosphere. Since ___________ CFCs are no longer manufactured

CFCs have lowered the average concentrations of ozone in the stratosphere. Since 1988 CFCs are no longer manufactured

Enzymes:

Catalysts inbiological systems Allow bodies to speed up complicated reactions that would be too slow to sustain life at normal body temperatures

During the course of the reaction the concentration of the reactants ____ and the concentration of the products ______.

During the course of the reaction the concentration of the reactants decreases and the concentration of the products increases.

∆Hrxn =

Ep - ER = ∆Hrxn

collision model

Idea that reactions occur during molecular collisions Reaction rate depends on reactant concentrations and temperature

If the value of ∆Hrxn is +, the reaction is _______. If the value is -, the reaction is _______.

If the value of ∆Hrxn is +, the reaction is endothermic. If the value is -, the reaction is exothermic.

Activation Energy (Ea)

Minimum energy required for a reaction to occur

Catalyst

Substance that speeds up a reaction without being consumed

Summary: 4 factors affect reaction rate (in addition to nature of reactants)

Temperature - if T increases, KE increases more collisions Concentration- if Conc increases, more chances of collision Particle size - smaller particle size, greater surface area, more collisions Catalysts - changes rate of rxn by lowering activation energy or creating a different activated complex with lower Ea or ensuring that reactants collide at correct orientation

WHAT must be met before reactants can be converted to products.

The EA

This is a graph of an endothermic reaction.

The products contain more energy than the reactants.

This is a graph of an exothermic reaction.

The reactants contain more chemical potential energy than the products, and this excess energy is released as heat during the reaction.

We represent reactions by graphing the progress of reaction against the energy of the components of the reaction. At the top of the "hill" you find the __________. This is unstable and very temporary (lasts ~_______) arrangement of all atoms in the reaction, in which bonds are _______. It falls apart to form the ______.

We represent reactions by graphing the progress of reaction against the energy of the components of the reaction. At the top of the "hill" you find the activated complex - a compound formed by reactants as an intermediate between the reactants and products. The activated complex is an unstable and very temporary (lasts ~10-11 seconds) arrangement of all atoms in the reaction, in which bonds are broken and reformed. It falls apart to form the products.

When CFCs are released into the troposphere they make their way to the ___________. The ultraviolet radiation present in the stratosphere has enough energy to break the bond connecting a ___________ to the CFC molecule, which can then break apart the ___________ molecules

When CFCs are released into the troposphere they make their way to the stratosphere. The ultraviolet radiation present in the stratosphere has enough energy to break the bond connecting a chlorine to the CFC molecule, which can then break apart the ozone molecules

Rate =

change in something over time

In both situations, the reactants and products contain chemical potential energy -

energy stored in the chemical bonds of the substances.

When you add a catalyst, it speeds up a reaction by

lowering the energy of the activated complex

Nature of the reactants -

some things just react faster than others.

The difference between the energy of the reactants (ER) and the energy of the activated complex is

the activation energy (EA).

Reaction rate is

the change in concentration of a reactant or product per unit of time as reaction proceeds = ∆concentration/time

The difference between the energy of the reactants and the energy of the products (EP) is

the overall change in energy (∆Hrxn)

In a chemical reaction, reactants become products. The rate at which this happens is

the rate of the reaction.