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