Collision Theory of Reaction Rates

THE COLLISION THEORY OF REACTION RATES At the end of this topic, students should be able to: explain the concepts associated with reaction rates , activation energy, collision theory, (simple treatment only), and catalysis. explain the factors that affect the rate of reaction surface are...

THE COLLISION THEORY OF REACTION RATES At the end of this topic, students should be able to: explain the concepts associated with reaction rates , activation energy, collision theory, (simple treatment only), and catalysis. explain the factors that affect the rate of reaction surface area, concentration, pressure, temperature and catalyst  Reactions involving collisions between two species It is pretty obvious that if you have a situation involving two species they can only react together if they come into contact with each other. They first have to collide, and then they may react. Why "may react"? It isn't enough for the two species to collide - they have to collide the right way around, and they have to collide with enough energy for bonds to break.  The orientation of collision Consider a simple reaction involving a collision between two molecules - ethene, CH2=CH2, and hydrogen chloride, HCl, for example. These react to give chloroethane. As a result of the collision between the two molecules, the double bond between the two carbons is converted into a single bond. A hydrogen atom gets attached to one of the carbons and a chlorine atom to the other. The reaction can only happen if the hydrogen end of the H-Cl bond approaches the carbon-carbon double bond. Any other collision between the two molecules doesn't work. The two simply bounce off each other. Of the collisions shown in the diagram, only collision 1 may possibly lead on to a reaction. The energy of the collision Activation Energy Even if the species are orientated properly, you still won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction. Activation energy is the minimum energy required before a reaction can occur. You can show this on an energy profile for the reaction. For a simple over-all exothermic reaction, the energy profile looks like this: If the particles collide with less energy than the activation energy, nothing important happens. They bounce apart. You can think of the activation energy as a barrier to the reaction. Only those collisions which have energies equal to or greater than the activation energy result in a reaction. Any chemical reaction results in the breaking of some bonds (needing energy) and the making of new ones (releasing energy). Obviously some bonds have to be broken before new ones can be made. Activation energy is involved in breaking some of the original bonds. Where collisions are relatively gentle, there isn't enough energy available to start the bond-breaking process, and so the particles don't react The Maxwell-Boltzmann Distribution Because of the key role of activation energy in deciding whether a collision will result in a reaction, it would obviously be useful to know what sort of proportion of the particles present have high enough energies to react when they collide. In any system, the particles present will have a very wide range of energies. For gases, this can be shown on a graph called the Maxwell-Boltzmann Distribution which is a plot of the number of particles having each particular energy.  The area under the curve is a measure of the total number of particles present. The Maxwell-Boltzmann Distribution and activation energy Remember that for a reaction to happen, particles must collide with energies equal to or greater than the activation energy for the reaction. We can mark the activation energy on the Maxwell- Boltzmann distribution:  Factors that affect the rate of reaction THE EFFECT OF SURFACE AREA ON REACTION RATES The more finely divided the solid is, the faster the reaction happens. A powdered solid will normally produce a faster reaction than if the same mass is present as a single lump. The powdered solid has a greater surface area than the single lump. Some examples Calcium carbonate and hydrochloric acid In the lab, powdered calcium carbonate reacts much faster with dilute hydrochloric acid than if the same mass was present as lumps of marble or limestone. The catalytic decomposition of hydrogen peroxide This is another familiar lab reaction. Solid manganese(IV) oxide is often used as the catalyst. Oxygen is given off much faster if the catalyst is present as a powder than as the same mass of granules THE EFFECT OF CONCENTRATION ON REACTION RATES For many reactions involving liquids or gases, increasing the concentration of the reactants increases the rate of reaction. Some examples Zinc and hydrochloric acid In the lab, zinc granules react fairly slowly with dilute hydrochloric acid, but much faster if the acid is concentrated. The manganese(IV) oxide catalytic decomposition of hydrogen peroxide Solid is often used as a catalyst in this reaction. Oxygen is given off much faster if the hydrogen peroxide is concentrated than if it is dilute.  The reaction between sodium thiosulphate solution and hydrochloric acid This is a reaction which is often used to explore the relationship between concentration and rate of reaction. When a dilute acid is added to sodium thiosulphate solution, a pale yellow precipitate of sulphur is formed. As the sodium thiosulphate solution is diluted more and more, the precipitate takes longer and longer to form. Collisions involving two particles The same argument applies whether the reaction involves collision between two different particles or two of the same particle. In order for any reaction to happen, those particles must first collide. This is true whether both particles are in solution, or whether one is in solution and the other a solid. If the concentration is higher, the chances of collision are greater.  THE EFFECT OF TEMPERATURE ON REACTION RATES Collisions only result in a reaction if the particles collide with enough energy to get the reaction started. This minimum energy required is called the activation energy for the reaction. Increasing the temperature increases reaction rates because of the large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reaction.  THE EFFECT OF CATALYSTS ON REACTION RATES What are catalysts? A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction. When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning. Collisions only result in a reaction if the particles collide with a certain minimum energy called the activation energy for the reaction. Only those particles represented by the area to the right of the activation energy will react when they collide. The great majority don't have enough energy, and will simply bounce apart. If there are very few particles with enough energy at any time, then the reaction will be slow. Catalysts and activation energy To increase the rate of a reaction you need to increase the number of successful collisions. One possible way of doing this is to provide an alternative way for the reaction to happen which has a lower activation energy. In other words, to move the activation energy on the graph like this:  Adding a catalyst has exactly this effect of shifting the activation energy. A catalyst provides an alternative route for the reaction. That alternative route has a lower activation energy. Showing this on an energy profile: "A catalyst provides an alternative route for the reaction with a lower activation energy."