The activation energy - Sample Essay

The rate of reaction tells us how quickly a chemical reaction happens. The rate can be determined by the speed with which one of the reactants is used up or the speed that one of the products is made. Time is a critical factor in the rate of any reaction. There are a number of factors that affect the rate of a reaction. These are: . Surface area . Concentration . Temperature Effect of surface area Small pieces of solids, especially powder, react faster than large pieces. Surface area is measured of how much surface is exposed.

As we increase the surface area, we increase the rate of reaction. For example, a nail. Only the iron atoms on the surface can react. If you cut the nail up, more iron atoms are exposed and ready to react: Collision Theory All substances are made up of particles. The particles might be atoms, molecules or ions. Before we can get a chemical reaction two particles must collide effectively. This is when a collision results in the formation of product molecules. The reaction rate is a measure of how frequently effective collisions occur.

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The more collisions between particles in a given time, the faster the reaction. This explains why an increase in the concentration of reactants will tend to lead to an increase in the rate of reaction. Effect of concentration As we increase the concentration, the rate of reaction increases. There are more particles in the same volume of water and so they are more likely to collide. Therefore there is a greater chance of acid particles colliding and reacting with particles on the surface of the marble.

Not all collisions between particles result in reactions, if they were then all reactions would be practically instantaneous, but the more collisions, the higher the chance that some will be effective. Kinetic Energy And The Effect of Temperature Even when the orientation of the molecules is correct, few collisions between molecules actually lead to a chemical change. This is because in order to react, the molecules must collide with a certain minimum kinetic energy known as the activation energy. If two slow moving molecules collide they will just bounce apart.

To meet with sufficient energy for bonds to be broken and reformed, the molecules must be moving quickly. An increase in temperature will increase the kinetic energy of the molecules and will increase the rate of reaction, as a greater proportion of the molecules will have sufficient energy to react. If you raise the temperature by just 10oc you roughly double the rate of many reactions: There are a number of signs that a chemical reaction has taken place. These are: . Bubbles of gas produced . Temperature change

. Change of colour e. g precipitation . Light produced . Sound produced . Smell produced . Kinetic energy (movement) . A reactant dissolving . A change in pH A reaction is always fastest at the beginning because this is the time that the numbers of reacting particles is greatest, and so the number of collisions between the reacting particles is greater. When the particles have reacted they can no longer react with other reactants. As the reaction proceeds the reaction slows down as the concentration of reacting particles decreases.

During a reaction molecules collide. When they collide with the correct orientation they slow down, stop and then fly apart. This happens even if they don’t have the required activation energy to react. In a collision that is unsuccessful the molecules separate and remain unchanged, whereas if a collision is effective the activation energy barrier is crossed and the particles that separate are chemically different from the others. Activation Energy The activation energy of a reaction can be thought of as an ‘energy barrier’.

The reactant molecules must have sufficient energy to get over this barrier if a reaction is to occur, or there will just be a mixture of unreacted chemicals left. Kinetic stability happens when the activation energy of a reaction is so large that there are virtually no molecules in the reaction mixture with sufficient energy to overcome the energy barrier, and so the reaction effectively does not happen. The difference between the potential energy of the reactants and the potential energy of the products indicates the enthalpy change of reaction.

When the products have a lower potential energy than the reactants, the reaction is exothermic, as the kinetic energy of the system and thus its temperature has increased. When the products have a higher potential energy than the reactants, the reaction is endothermic, as a net input of energy is needed to form in the products. The average kinetic energy of the system drops and so does the temperature. This is an exothermic reaction it provides the energy itself to keep the reaction going. If it was the other way round it would be an endothermic reaction. Prediction

I predict that the higher the molarity of the acid, the faster the rate of reaction. The reasons that I made this prediction, are that there are more acid particles in the same volume of water, so there is a greater chance of the acid particles colliding and reacting with the particles on the surface of the marble chips. So the rate of reaction will increase. Plan of Action I am going to find out how different acid concentrations affect the rate of reaction by setting up the equipment as shown in the diagram below. I will then measure out five lots of 2. 5 grams of small marble chips.

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