Topic B Collision Theory
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Transcript of Topic B Collision Theory
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TOPIC BCOLLISION THEORY
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• Chemical reactions occur at different rates. • rusting is a very slow reaction – many years• an explosion – fractions of a second
• The rate of a reaction can be determined by monitoring:• the change (decrease) in concentration of reactants over time• the change (increase) in concentration of products over time.
• Beer’s law can be used to determine the change in concentration of a colored solution over time.
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• The basis for the study of the rate of chemical reactions (chemical kinetics) is collision theory.
• Collision theory tells that a reaction will only take place (i.e., will only be successful) if three conditions are met:• 1. The reactants collide • 2. The collision occurs with a certain minimum energy• (known as the activation energy or Ea)• 3. The collision has the correct molecular orientation.
• This means that the reactants must collide in a certain physical, three-dimensional orientation for a reaction to take place.
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• No Reaction If:• reactants do not collide• collide with energies lower than the activation energy• collide without the correct molecular orientation
• These collisions do not lead to a chemical reaction
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• All chemical reactions take place via a series of elementary steps.
• An elementary step is a reaction that forms products in a single step, with only one transition state and no intermediates.
• An energy profile can be used to show the progress of a reaction from reactants, through a transition state and to products.
• Reactants with energies closer to the transition state at the beginning have lower activation energies, and faster rates of reaction.
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• Elementary steps have three molecuralities:
• Unimolecular:• – a single species reacts to form products (a rearrangement
occurs, activating a reactant molecule)• -- called first order reaction. • -- Rearrangements are caused by collisions between reactant
species and solvent or ‘background’ molecules• O3 --> O2 + O
• Bimolecular:• – two species collide and react to form products. • -- called second order reactions• NO + O3 NO2 + O2
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• Trimolecular (or termolecular):• - three species collide and react to form products. • -- called third order reactions• 2NO + O2 2NO2
• In an elementary reaction, with increasing molecularity, the chances of correctly orientated collisions goes down.
• Trimolecular reactions are relatively rare• they involve the need for all three species to be simultaneously in
the same area of space, and colliding with the correct energy and orientation – in terms of probability, this is relatively unlikely.
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• In all situations:• if the concentration of the reactants is increased:• there will a greater frequency of collisions• greater the chances of successful (correct energy, correct
orientation) collisions.
If we string together a series of elementary steps we get a more complicated reaction,
The more complex reaction is actually only a series of simple, elementary ones.
In the complex reaction on the next slide, there is an intermediate and more than one transition state.
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Potential Energy
Reaction Coordinate
Reactants
Products
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Potential Energy
Reaction Coordinate
Reactants
Products
Activation Energy Ea
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Potential Energy
Reaction Coordinate
Reactants
Products
Activated complex
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Potential Energy
Reaction Coordinate
Reactants
ProductsDE}
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Potential Energy
Reaction Coordinate
2BrNO
2NO + Br
Br---NO
Br---NO
2
Transition State
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Terms• Activation energy - the minimum energy needed to make
a reaction happen.• Activated Complex or Transition State - The arrangement
of atoms at the top of the energy barrier.
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ON
Br
ON
Br
ON
Br
ON
Br
O N Br ONBr ONBr
O NBr
O N BrONBr No Reaction