In a chemical reaction, either energy is released or absorbed. Based on these two factors, the chemical reactions are classified into two types:. In an endothermic reaction, the energy is absorbed; thus, energy change will be positive. In an exothermic reaction, the energy is released; thus, energy change will be negative. Catalyst — A chemical substance that alters either increases or decreases the rate of a chemical reaction is known as a catalyst.
This happens by altering the activation energy of a reaction. A catalyst lowers activation energy and increases the rate of a chemical reaction to a great extent. Apart from this, a catalyst does not bring any change in the reactants and the products. Based on the type of changes brought by a catalyst in a chemical reaction, a catalyst is classified into a positive catalyst and a negative catalyst.
A catalyst that increases the rate of reaction by decreasing or lowering the activation energy by changing the path of the reaction is known as a positive catalyst. A catalyst that decreases or retards the rate of a chemical reaction by increasing the activation energy by taking a longer alternative pathway is known as a negative catalyst or inhibitor.
In a nutshell, Activation Energy is the minimum amount of energy required by a chemical reaction to occur. For example, combustion does not take place at room temperature; at least a spark is needed for the combustion to occur. During a chemical reaction, firstly, reactants are excited to an intermediate form where the changes such as bond breaking and bond making occur. This energy level at which the intermediate product is formed is the activation energy. Lower the activation energy faster will be the conversion of intermediate to the products and vice versa.
This entire phenomenon is based on the collision theory, where an effective collision of molecules is needed for a reaction to occur.
Thus, we can infer that this Activation energy depends upon the temperature, surface area, the concentration of the reactants, and catalyst. It is classified as an exothermic and endothermic reaction based on whether the energy is released or absorbed during a chemical reaction. Inside our body, there are various metabolic reactions that need to be completed very fast to help us stay alive. For this, catalysts or enzymes are used.
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Activation Energy and the Arrhenius Equation The minimum energy necessary to form a product during a collision between reactants is called the activation energy E a. Example Determination of E a The variation of the rate constant with temperature for the decomposition of HI g to H 2 g and I 2 g is given here. Share Thoughts.
Collision Theory. Summarizing Collision Theory. Share Thoughts Post Image. Cancel Reply. Add Math. Math Editor. Edit math using TeX:. Math preview:. Close Insert Math. Let's try a simple problem: A first order reaction has a rate constant of 1. What is the half life of the reaction? What is the rate constant? What percentage of N 2 O 5 will remain after one day? The Activation Energy E a - is the energy level that the reactant molecules must overcome before a reaction can occur.
In order to calculate the activation energy we need an equation that relates the rate constant of a reaction with the temperature energy of the system. This equation is called the Arrhenius Equation:. Where Z or A in modern times is a constant related to the geometry needed, k is the rate constant, R is the gas constant 8.
If we rearrange and take the natural log of this equation, we can then put it into a "straight-line" format:. When the lnk rate constant is plotted versus the inverse of the temperature kelvin , the slope is a straight line. At o C the rate constant was found to be 2.
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