Rate Of Reaction

The rate of reaction or sometimes called the speed of reaction refers to how slow or fast a reaction takes place. The following concepts are associated with reaction rate and that you will see often.

Reaction Rate

Reaction rate is defined as the change in concentration of a substance divided by the time taken for that change to take place it is measured in mol dm-3 s-1 .

Rate constant

Proportionality constant in the relationship between reaction rate and reactant concentration

Rate = k [A] [B]

k = rate constant

Order of Reaction

Reactions have an order with respect to each reactant. E.g.The rate equation has been determined experimentally.

Rate = k [NO] O3

The reaction is first order with respect to NO, which means that the rate depends on the concentration of NO raised to the first power [NO]1.

It is also first order with respect to O3and that is [O3]1 . The sum of the individual orders gives the overall reaction order.

Rate – Determining Step

This is the slowest step in a reaction mechanism and is the one that determines the overall rate.

Half Life

Half life of a reaction is the time required for a reactant to reach half of its original concentration.

Activation Energy

Activation Energy Ea, is the minimum collision energy required to activate the molecules into a state from which reactant bonds can change into product bonds.
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Collision Theory

For a chemical reaction to occur the reacting molecules must collide with each other.
This forms the basis of the collision theory of chemical kinetics.
Basically, this theory states that the rate of a reaction is proportional to the number of collisions occurring each second between reacting molecules.
Rate is prpportional to Number Of Collisions divided by Seconds

It is important that you make note of the following:
– Molecules will only react if they collide with each other.
– Reactions will occur if there is enough energy in the collision
– Increased concentration increases the likely hood of collision, which increases reaction rate.
– Increased temperature increases the average energy of collisions, which increases the reaction rate.
– NB. The molecules must collide in the correct orientation to produce a chemical change.
Screen shot 2014-08-01 at 7.16.30 PM This reaction cannot produce any net chemical change
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This collision that can lead to a net reaction is known as an effective collision.


NB. Molecules must possess enough energy when they collide to produce a chemical change.
When two slow moving molecules collide, their electron clouds cannot inter-penetrate much and they just bounce on each other, chemically unchanged.
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When fast moving molecules collide atoms approach each other much more closely and their electron clouds inter-penetrate. This can lead to bond breaking and bond making. The net change here is  A B + C = A + B C

A catalyst is something added to a reaction that increases its rate, but does not change in concentration. This means the same amount of catalyst remains after the reaction as before.
Catalyst increases reaction rate by lowering the activation barrier for the reaction without altering the products of the reaction.
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A vast variety of significant catalysts are provided by enzymes. They occur in living systems and allow reactions to occur at relatively low temperatures like 37 degrees Celsius) our body temperature) and at relatively low pressure.
They are organic compounds that catalyze the reactions involved in the vital processes within animals and plants.
Enzymes are proteins, long chains of amino acids linked together by peptide bonds. Enzymes are involved in almost every reaction in your body. For example they help you to digest food and protect you from dangerous waste products that form in your body.
Enzymes are often used in industrial processes to catalyze biochemical reactions.





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