topic 6 & 16: kinetics...topic 6 & 16: kinetics rates of chemical reactions collision theory...
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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...
Monday, May 14, 2012
try it!
Monday, May 14, 2012
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)
Monday, May 14, 2012
kinetic theory
The movement of particles whose average energy is directly proportional to the temperature in kelvin (k).
Monday, May 14, 2012
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:
Monday, May 14, 2012
maxwell-boltzmann distribution
Monday, May 14, 2012
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.
Monday, May 14, 2012
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.
Monday, May 14, 2012
what affects rates?
The Nature of the Reactants Chemical compounds vary considerably in their
chemical reactivities.Concentration of ReactantsTemperatureCatalystsParticle sizePressuresurface area
Monday, May 14, 2012
but why?
let’s take temperature first...
At higher temperature, molecules have more energy.
therefore, more molecules will have enough energy to react
Monday, May 14, 2012
Effect of Temperature
Monday, May 14, 2012
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
Monday, May 14, 2012
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
Monday, May 14, 2012
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
Monday, May 14, 2012
Effect of Catalyst
Monday, May 14, 2012
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.
Monday, May 14, 2012
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!
Monday, May 14, 2012
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