Collision+Theory

Collisions are necessary for a reaction to occur. Not all collisions will lead to a chemical reaction since the colliding particles must have a certain amount of kinetic energy for them to react. toc //The minimum amount of energy required for a reaction to occur is known as the // **activation energy**. In other words, the activation energy provides the necessary amount of energy in order to break the bonds (in the reactants). Activation energy is denoted by **E** a .



Referring to the enthalpy diagram, you can see that the activation energy is represented by the distance between the top of the curve and the reactants.

//What is enthalpy change (ΔH) represented by??? //

Correct. ΔH is represented by the distance between the reactants and the products on the diagram.

The following three conditions are necessary in order for two reactant particles to react:


 * The two particles must collide with each other.
 * The particles must collide with enough kinetic energy which is greater than the activation energy of the reaction (E>E a )
 * The particles must collide in the correct orientation so that the reactive parts of the two particles come into contact with each other.

These three conditions are known as //__ collision theory __//. Therefore any factor that alters on of the three conditions stated above will have an effect on the rate of the reaction.

= Maxwell-Boltzmann distribution curves =

Maxwell-Boltzmann distribution curve is a graph showing the distribution of kinetic energy amongst particles of a fixed mass of gas.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The area under the graph is directly related to the number of particles present. ( thus for a fixed mass of gas, at any temperature, this area remains constant).

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">At a particular temperature, most of the particles have a Kinetic energy close to the average value. At the average value, the are under <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> the graph is equal on both sides (because the number of particles having a lower kinetic energy is similar to the number of particles having a higher kinetic energy).

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If the temperature increases the shape begins to flatten since the distribution of kinetic energy has widened and thus fewer particles have a kinetic energy close to the new average value. However, the number of particles with a higher kinetic energy increases. The area of the curve remains constant since the number of particles has not changed.

= Factors that affect the rate of a chemical reaction =

The concentration of the reactants or the pressure of a gaseous reaction
<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Increasing the concentration/pressure increases the quantity of particles present in a given volume. Thus the number of successful collisions increases (collisions are more frequent).The rate of the reaction increases.

Surface area (or the particle size of solid reactants)
<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Increasing the surface area increases the rate of a reaction. When a solid and a liquid react, their particles can’t mix and only the surface particles of the solid come into contact with the liquid particles. Hence breaking the solid into smaller pieces, increases the surface area so more liquid particles are available to react (more particles are in contact with each other)

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<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">As can be seen the big particle has a small surface area. If you cut the big piece into smaller ones, along the red lines, you get smaller particles, the red ones, and the surface area exposed has also increased.

Temperature of the reaction mixture
<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Increasing the temperature increases the rate of the reaction due to the following two reasons:

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">1. At a higher temperature the particles are moving faster so the collisions are more frequent (however this isn’t the main reason why temperature increases the rate of a reaction)

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">2. The number of particles with the required activation energy increases. Thus the ratio of successful collisions to unsuccessful collisions increases so overall there are more successful collisions.

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<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The diagram shows the Maxwell-Boltzmann curve for the reactant gas particles at low temperature and high temperature. It can be seen that at a low temperature, '**a**' particles have the necessary activation energy to react. However at the higher temperature '**b**' particles had the necessary activation energy to react, which is much greater than before. Therefore the number of particles which possess the necessary activation energy has increases so the number of successful collisions has also increased.

Adding a suitable catalyst
<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A catalyst is a substance that increases the rate of a chemical reaction without itself being chemically changed at the end of the reaction.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A catalyst provides an alternative pathway with a lower activation energy. Since more particles will possess this lower activation energy, the rate of the reaction increases.

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Usually the efficiency of a catalyst decreases with time as it becomes inactive due to impurities in the reaction mixture, side reactions or if its surface becomes coated and unavailable for activity. // <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">


 * By the end of this lesson you should be able to: **
 * ** Define the term activation energy. **
 * ** Describe the collision theory. **
 * ** Explain using the collision theory, the effect of temperature, catalyst, concentration and particle size on the rate of the reaction. **
 * ** Sketch and explain a Maxwell-Boltzmann distribution curve for a fixed amount of gas. **
 * ** Using a Maxwell-Boltzmann distribution curve explain the effects of temperature on the rate of a reaction. **
 * ** Using a Maxwell-Boltzmann distribution curve explain the effect of catalyst on the rate of a reaction. **


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