Physical Science Lesson 14 Chemical Reactions

Chemical Reactions Reactions Chemical Reactions - The Collision Theory and Factors Affecting the Rate of a Chemical Reaction Reaction

Objectives  At the end of the lesson, lesson, you should should be able to explain explain the collision collision theory and describe describe the factors affecting the rate of reaction such as concentration, temperature, and particle size. Chemical kinetics is kinetics  is the branch of chemistry that studies reaction rates and factors affecting rates. Chemical reactions happen when the reactants collide. But will all collisions result in a reaction or are there requirements for a reaction to occur after collision?

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Learn about it!  According to the the collision theory , the rate of a chemical reaction reaction is directly proportional to the number of collisions between the reactants. $owe%er, only those  which collide collide effecti%ely will will result in a chemical chemical reaction. reaction. &or an effecti%e effecti%e collision collision to happen, the following conditions should be met by the reactants: ". 'hey must be properly oriented when they collide. (. 'hey must be energetic enough to surpass the acti%ation energy or the energy  barrier. Proper Orientation Consider the reaction reaction which has an acti%ation acti%ation energy of .

 Abo%e are the structures structures of and )grey * C, red * +, and below below are three possible possible ways the molecules can collide with one another:  Among the collisions collisions )A, -, and and C, only - is expected expected to proceed in a reaction reaction because because it

is the one which is properly oriented. 'his is based on the fact that this collision is expected to form a bond between the C atom of and + atom of which is what is seen from the structure of )shown below.

-ecause the molecules are not properly oriented in Collisions A and C, they will not lead to the formation of the product.

Learn about it! Energy Higher than Activation Energy 

'he activation energy  or the energy barrier is the energy needed to be surpassed by  the reactants for a reaction to occur. hen the energy barrier is higher, the reaction is slower. /t should be noted that not all properly oriented collisions ha%e enough energy to result in the transformation of the reactants. 'he inetic energy of the reactants should be high enough to exceed the acti%ation energy. 'herefore, for to react with , the inetic energy of the reactants should be greater than .

Try it! Consider the reaction . 1efer to the structures of , , and below )blue * 2, red * +: 3ae a diagram showing the correct orientation of collision between the reactants to form the product.

Learn about it! actors Affecting the !ate of !eaction 'he three factors that can affect the rate of a chemical reaction are concentration of reactants, temperature, and particle size. Concentration  An increase in concentration increases the rate of reaction. 1eason: Concentration is the number of molecules present in a specified %olume. hen the concentration is high, more particles are present4 therefore, there is a higher fre5uency of effecti%e collisions. "emperature  An increase in temperature increases the rate of reaction. 1eason: 6nergy is applied to increase the temperature. 'he added energy pro%ides additional inetic energy to the reactant molecules. $igher inetic energies means greater fre5uency of effecti%e collisions. Particle si#e The smaller the particle, the faster the rate of reaction.

1eason: 7maller particles ha%e larger surface area. 'he greater the surface area, the more sites of collision there will be.

What do you think?  Another factor that increases reaction rate is the presence of a catalyst. A catalyst is a chemical species that participates in the reaction but does not get used up. $ow does a catalyst speed up the reaction8

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 According to the collision theory , the rate of reaction is directly proportional to the number of collisions between the reactants.  An effective collision is characterized by reactants colliding with proper orientation and enough energy to surpass the acti%ation energy. 'he activation energy  or energy barrier is the energy needed to be surpassed by the reactants so that they will be transformed into products. 'here are three factors that affect the rate of the reaction: " concentration, ( temperature, and 9 particle si#e.

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/ncreasing the concentration or the temperature of the reaction leads to an increase in reaction rate. +n the other hand, decreasing the particle size increases the reaction rate. Question 1

'he theory states that higher fre5uency of effecti%e collisions results in a faster rate of chemical reaction. Correct! collision

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 According to the collision theory, the rate of reaction is directly proportional to the fre5uency  of collisions.

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Question 2

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 hich of the following are the re5uirements for a reaction to occur8 Incorrect!

1similar geometry of colliding molecules 4greater energy than the acti%ation energy 

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6ffecti%e collision is needed for a reaction to occur. /t is achie%ed when the reactant molecules collide with proper orientation and with sufficient energy to surpass the acti%ation energy. 2ext 5uestion

2e5ual energies of reacting species 3properly oriented collisions Question 3

 hich of the following is another term for acti%ation energy8 Incorrect!

2internal energy  'he acti%ation energy is also called the energy barrier because it restricts the reactants from  becoming products. 2ext 5uestion

1energy barrier3free energy 4reaction enthalpy