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Third Order Reactions: ( 3) Are Quite Rare

A reaction is considered third order if its rate depends on the concentrations of three reactants. For a third order reaction to occur, a minimum of three molecules must collide simultaneously. There are three cases for third order reactions based on whether the concentrations of the three reactants are equal or different. Third order reactions are rare according to collision theory, as it is unlikely that three or more molecules will collide at the same time. An example of an actual third order reaction is the hydrolysis of sucrose in acid solution.

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151 views2 pages

Third Order Reactions: ( 3) Are Quite Rare

A reaction is considered third order if its rate depends on the concentrations of three reactants. For a third order reaction to occur, a minimum of three molecules must collide simultaneously. There are three cases for third order reactions based on whether the concentrations of the three reactants are equal or different. Third order reactions are rare according to collision theory, as it is unlikely that three or more molecules will collide at the same time. An example of an actual third order reaction is the hydrolysis of sucrose in acid solution.

Uploaded by

Lily Antonette Agustin
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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THIRD ORDER REACTIONS

A reaction is said to be of third order if the rate is determined by the variation of three
concentration terms. In other words, the minimum number of molecules necessary for
the reaction to take place is three.

There may be three different cases in third order reaction.

(i) All the three species have equal concentrations.

(ii) Two species have equal concentrations and one different.

(iii) All three species have unequal concentrations.

WHY THIRD ORDER REACTIONS ARE RARE

According to the collision theory, the reactions occurs due to the collision of the
reactant molecules.

In first and second order reactions the probability of collision is quite high as compared
to the third and higher order reactions (It is quite unlikely that three or more than three
molecules will collide at the same time).

Due to this very low probability of colliding of molecules, the higher order reactions
(>3) are quite rare.
EXAMPLE OF AN ACTUAL THIRD ORDER REACTION IN

The hydrolysis of sucrose in acid solution is third-order,

r = k[sucrose][H+][H2O];

however, the concentrations of both the catalyst H+ and the solvent H2O are normally
constant, so that the reaction is pseudo–first-order.

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