(a)(i) What is meant by the rate of a chemical reaction? (ii) Explain in terms of the vision theory, the effect of temperature increase on reaction rate.
(b) When hydrogen peroxide is exposed to air, it decomposes (i) Write an equation for the reaction. (ii) Outline an experiment to illustrate that effect of a named catalyst on the rate of decomposition. (iii) Sketch an energy profile diagram to show the effect of the catalyst on the reaction rate, given that the reaction is exothermic. (c)(i) Explain why enthalpy data alone cannot be used to predict whether a reaction can occur spontaneously or not.
Explanation
(a)(i) How fast a reaction can take place. (ii) Collision theory states that the reactant molecules must collide with one another before forming the product. The number of collisions is directly proportional to the rate of reaction. This value of rate of reaction is however limited by the number of molecules attaining the activation energy and having appropriate orientation of the molecules. Increase in temperature only favours endothermic process and low temperature will favour the exothermic process. Low temperature however slows the rate of reaction and therefore catalyst is applied.
(ii) Thermodynamically, the decomposition of hydrogen peroxide is spontaneous. When hydrogen peroxide is put in a conical flask, nothing seem to happen. However. the addition of hydrogen peroxide to a little amount of manganese(IV) oxide (MnO\(_2\)) generates effervescence and a gas is vigorously given off. The reaction of MnO\(_2\) on the decomposition of hydrogen peroxide is catalystic. (iii) Energy profile.
(c)(i) Certain endothermic processes can be exothermic at a certain condition of temperature.
