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Reaction rate is affected by the following:
- Temperature: Increase in temperature results increase in reaction rate, decrease in temperature results
in decrease in reaction rate.
- Concentration: Increase in pressure increase in reaction rate.
- Nature of reactants: Chemical properties of substance to react quickly or slowly.
- Surface Area: The greater the surface area the greater the rate of reaction
Collision Theory:
Molecules act as a small hard sphere which bounce off eachother and transfer energy
among themselves during their collisions.
- Effects of concentration: If [reactants] increases more collisions are possible b/w mcolecules the number
of collision per second increases as a result reaction rate increases as well.
- Effects of Temperature: Increase in temperature increases the kinetic energy of the molecule as well
and the speed at which tehy are moving increases. Molecule move faster with more energy therefore reaction rate increases.
Activation Energy: minimum Potential energy required to change the reactants into the activated
complex.
Note: if the reactants don't have energy they can only climb pathway up the hill and don't react.
Activated Complex: An intermediate molecule which occurs when reactants are in the process of rearranging
to form product.
Note:
- As moplecules approach eachother KE is converted into PE and climb up the hill.
- A collision b/w two particles is effective if the collision results in reaction.
- Two species collde two requirement must be met befor a succesful reaction can occur. the molecule must possess:
- Sufficent KE: if insufficent KE is converted to PE. Insufficent PE will be gained.
- Correct alignement: if the reactants are not correctly alligned. the reaction needs more energy to be
completed.
Reaction mechanism: Actual sequence of steps which make up an overall reaction.
Elementary process: It is an individual step in a reaction mechanism.
Notes:
- The slowest step in reaction is called rate determining step.
- The overall reactin rate is unaffected by the addition of reactants intermediate which are already in the fast step because
they are not used rate determining step.
- To determine the overall reaction mechanism simply add up all the step in reaction equation.
HBr
+ O2 à HOOBr
HBr
+HOOBrà2HOBr
2HBr
+ 2HOBr à 2H2O + 2Br2
4HBr
+ O2 à2H2O + 2Br2
- HOBr and HOOBr are reaction intermediate
they can exist indefinitely on their own but react quickly in above mechanism.
- activated comples is found by adding
up all the atoms invovled int two reacting molecules.
catalyst reaction:
The catalyst is an active participtant which is generated in a later step of reaction mechanism.
enthalpy doesn't change by adding catalyst in reaction.
Homogeneous reaction: Reaction in which all reactats are in same phase.
Example:
two gases reacting.
Heterogeneous reaction: Reaction in which all reactants are present in diferent phase.
Example: solid and liquild reacting together.
Note:
All factors effect the reaction rate of heterogenous reaction but in homogeneous surface area doesn't
effect the reaction rate.
Fastest to slowest reaction:
AQUEOUS IONS > GASES OR LIQUID > SOLIDS
Catalyst: A chemical can be added to a reaction which provide an overall reaction with alternate reaction
mechanism having lower the activation energy.
Example: water.
Note: Catalyst increases the forward and reverse reaction rate.
Inhibitor: A chemical which reduces reaction rate by combining with one of the reactants or catalyst
to prevent the reaction from occuring.
Examples: poison, antibiotic
Potential energy (PE):
Energy existing as a result of the position of the object in space and sum of all attractive forces existing among
particles which make up the object.
Kinetic energy (KE):
Energy which a system posses because of movement with in system.
Entahlpy:
The total kinetic and potential energy which exist in a system when at a constant pressure.
Enthalpy = Hprod –
Hreact
Hprod =
combined enthalipies of all the products.
Hreact= combined enthalipies of all the reactants.
Endothermic:
If the reaction is endothermic
the products have more energy than reactant.
Hprod>Hreact
enthalpy>0
Example:
N2 + O2 + 164 Kj à 2N2O
Study Clue:
ENdothermic means heat ENter the system.
Exothermic:
If the reaction is exothermic the products have less energy than reactants.
Example:
H2 + Cl2 à 2HCl+ 184
Kj
Study Clue: Heat EXit the system in EXothermic
reation.
+ >>>>>>>> -
Kinetic Energy Distributions:
Reaction rate due to an increase in temperature is primarliy due to the increased numbers of molecules with sufficent
energy to react and not to the increased number of collisions.
Rule of Thumb: For slow reaction at 10 º C temperature increase DOUBLES the reaction rate.
Note: The distributions tells us that
a few particles have low energy and few have very high energy.
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