Topic 6 & 16: Kinetics...Topic 6 & 16: Kinetics Rates of chemical reactions Collision theory Monday, May 14, 2012

Topic 6 & 16: Kinetics

Rates of chemical reactions

Collision theory

Monday, May 14, 2012

recap!

last class, we introduced kinetics

we discussed how to measure rates of reaction, how to write rate expressions, and looked at some data

let’s practice...


try it!


0

0.7500

1.5000

2.2500

3.0000

0 750 1500 2250 3000

Concentration of N2O5 vs. Time

Time (s)

CONCENTRATION OF N2O5 (M)


kinetic theory

The movement of particles whose average energy is directly proportional to the temperature in kelvin (k).


kinetic energy

the energy of a particle depends on both the mass & velocity.

in a particular sample of gas, all the particles may not be moving at exactly the same velocity, and will have different energy

a distribution of this is shown below:


maxwell-boltzmann distribution


collision theory

Molecules can only react if they collide with each other.

These collisions must occur with sufficient energy (greater than activation energy)

Molecules must in the appropriate orientation.


activation energy

In other words, there is a minimum amount of energy required for reaction: the activation energy, Ea.

Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.


what affects rates?

The Nature of the Reactants Chemical compounds vary considerably in their

chemical reactivities.Concentration of ReactantsTemperatureCatalystsParticle sizePressuresurface area


but why?

let’s take temperature first...

At higher temperature, molecules have more energy.

therefore, more molecules will have enough energy to react


Effect of Temperature


effect of concentration

Recall - concentration is mol/volume (c = n/v)

increasing pressure of a gas has the same effect as increasing concentration:

more particles in a particular space means more chances of colliding


effect of surface area

what dissolves faster in your tea: a lump of sugar or a spoonful of fine sugar?

more surface area will give more opportunities for the reaction to take place


effect of catalysts

a catalyst is a substance that speeds up a reaction without actually being used up itself

catalysts provide an alternate pathway for the reaction with lower energy


Effect of Catalyst


reaction coordinate diagrams

It is helpful to visualize energy changes throughout a process on a reaction coordinate diagram like this one for the rearrangement of methyl isonitrile.


activated complex

It shows the energy of the reactants and products (and, therefore, ΔE).

The high point on the diagram is the transition state.

• The species present at the transition state is called the activated complex.• The energy gap between the reactants and the activated

complex is the activation energy barrier.Monday, May 14, 2012

try it!


Documents

Topic 6 & 16: Kinetics...Topic 6 & 16: Kinetics Rates of chemical reactions Collision theory Monday, May 14, 2012