The rate of reaction is the speed whereby the reactants are converting into products. It may occur in a very slow or very fast rate. Some factors such as the presence of a catalyst, concentration of reactant, the surface area of reactants and temperature will affect the rate of reaction. In this experiment, we manipulate temperature and consider the time taken for permanganate to decolourize is the responding variable to determine the activation energy of the reaction between oxalic acid and potassium permanganate.
Every molecule undergoes its own minimum amount of energy during a molecular collision. When molecules collide effectively, the kinetic energy of the molecules will lead to a chemical reaction by stretching, bending and breaking bonds. Based on the collision theory, it shows that reaction will only occur when the reacting molecules collide with sufficient energy and in the correct orientation. When the reacting molecules reach these 2 criteria, they will overcome the minimum energy barrier because the kinetic energy in the collision is greater than the minimum energy barrier. In this case, the reaction occurs under low energy required. However, if molecules are moving too slowly with less kinetic energy until they cannot collide with each other or in the wrong orientation, the reaction will not occur. This is because the molecules fail to react and bounce off each other.
The minimum amount of kinetic energy required for an effective collision to take place is called as activation energy. It is usually marked as Ea with unit kJ/mol. Energy is needed for all chemical reactions even exothermic reaction and endothermic reaction. The diagram below shows that same amount of activation energy is needed so that the reactants can collide with each other, bend and break bonds. Exothermic reaction is a chemical reaction whereby energy released from reactants to the surrounding. The temperature of the surrounding increases. Although exothermic reaction is releasing energy ,it also requires a small amount of energy to start the reaction. To summarize, for exothermic reaction, total energy released by forming bonds is greater than total energy required to break bonds. Endothermic reaction is a chemical reaction whereby energy released from surrounding to reactants themselves. The temperature of the surroundings decreases. To summarize, for endothermic reaction, the total energy required to break bonds is greater than total energy released by forming bonds.
We can find the activation energy of a chemical reaction by using the Arrhenius equation. The equation is k = Ae-Ea/RT where k is rate constant, A is frequency factor with unit L mol-1s-1, R is ideal gas constant, Ea is activation energy and T is a temperature with unit Kelvin. From the Arrhenius equation, we know that it is a formula to show that the temperature dependence of the rate of reaction. The Arrhenius equation can also be written as, ln k = ln A – (Ea/R)(1/T)
Potassium permanganate is a strong oxidizing agent and permanganate ion is a purple colour. From the experiment, the colour changes for the reaction is from purple to pink and to colourless. Permanganate is reduced by oxalate, C2O42- under the acidic condition. The reaction of oxalate is very slow at room temperature. The permanganate ions turn from MnO4- to Mn2+. This can be proved with the purple colour of potassium permanganate is decolourised. At the equivalence point, the purple solution will turn into pink colour. Oxalic acid is oxidised to carbon dioxide by potassium permanganate. Sulphuric acid is added because a strongly acidic condition is needed for reduction of permanganate.
At 35C which is the lowest temperature in this experiment, the time taken for the permanganate to decolourize is 77 seconds. At 60C which is the highest temperature in this experiment , the time taken for the permanganate to decolourize is 13 seconds. From the reaction above, the changes from permanganate ion, MnO4- to Manganese ion, Mn2+ involved bond breaking under oxalic acid. Energy is required through this process. The energy produced by colliding molecules is not sufficient. However, they must collide with sufficient energy to overcome the energy barrier which is activation energy. When the temperature increases, the number of collisions between reactant molecules increases, the energy of the collision increases. In this case, the collisions between reactant molecules may have sufficient energy to overcome the barrier. Therefore, the reaction rate decreases when temperature increasing. Stirring can also affect the reaction rate. Stirring ensure that reactants are kept in contact with proper speed and orientation.
There are some precaution steps that we should take throughout this experiment. Firstly, the time taken for stirring must be consistent so that the results will fair. Secondly, we must be very careful when incubating the test tube into the water bath as the water bath is very hot. Thirdly, potassium permanganate must be decolourize completely when the time taken for potassium permanganate is recorded. Forthly, the test tube should be placed in water bath at least 5 minutes to achieve thermal equilibrium.
Conclusion : Activation energy is the minimum amount of kinetic energy required for an effective collision to take place. From the experiment, the activation energy that obtained is Ea=76.175kJ/mol. By increasing the temperature, the collision number increases and the collision energy increases. Therefore, a higher chance for the reactants have sufficient energy to overcome activation energy.