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DEFINITIONS
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SPECIFIC RATE CONSTANT
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| Rate
constant determined when the concentration of each reactant is unity i.e.
one mole/dm3 then K is called specific rate constant.
In other words: Specific rate constant at a given temperature may be defined as the rate of reaction when the molar concentration of each reactant is unity. |
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MOLECULARITY OF REACTION
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The total number
of molecules taking part in the rate determining step of a chemical reaction
is called "molecularity" of the reaction. A reaction is unimolecular if only one molecule is involved in its rate-determining step. A reaction is bimolecular if two molecules take part in its rate-determining step. For example: C2H5OH è C2H4 + H2O Is a unimolecular reaction because only one molecule is involved. REMEMBER: The molecularity of a reaction can not always be predicted by the chemical equation alone, that every reaction must follow the rules of chemical kinetics. |
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ENERGY OF ACTIVATION
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| For a collision
to be effective, the sum of energy of colliding molecules must equals or
exceeds some minimum value called the activation energy of the reaction.
It is the minimum energy needed to convert reactants into products. It is denoted by Ea |
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| The activation depends upon the nature of the chemical bonds undergoing rupture. Another definition of activation energy is that: | ||||
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"The minimum energy
which the reacting species must posses in order to be able to
form an activated complex or transition state before proceeding to the products". |
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| Reactions having higher activation energy are slow while those having high energy of activation proceed with slow speed. | ||||
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EXPLANATION
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| A reaction takes place when molecules of reacting substances collide together. But all the collision are not effective i.e. do not lead to the formation of product. It has been found that only those collision are effective in which the colliding molecule possess a minimum amount of energy known as threshold energy. Before collision the molecules of reactants in their normal state do possess their respective internal energy, but their average internal energy is less than threshold energy. Now the molecules must acquire the difference of energy, to have effective collision. The excess energy that the reactant molecules, having average energy less than the threshold energy,must acquire in order to react and change into products is called activation energy. The speed of a reaction depends upon the activation energy of molecules. | ||||
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Activation energy = Threshold energy – Average internal energy |
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ACTIVATION ENERGY AND
RATE OF REACTION
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| The reactions having low activation energies take place with high speed. The reactions having high activation energies take place with slow speed. | ||||
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ACTIVATION ENERGY AND
TEMPERATURE
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| Reactions having low activation energies take place at low temperature Reactions having high activation energies take place at high temperature | ||||
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ACTIVATION ENERGY AND
CATALYST
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| The presence of positive catalyst increases the rate of a reaction by decreasing the energy of activation. | ||||
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