The Difference between Exothermic and Endothermic Chemical Reactions

Thermodynamics of chemical reactions has three basic laws which govern the behavior of chemical reactions. Classical thermodynamics Deals with chemical systems under equilibrium conditions. Two other areas which are related to thermodynamics. These are statistical thermodynamics and non-equilibrium thermodyamics.

An example of a system that non-equilibrium thermodynamics can be applied to is biological systems of our body.

Two basic rules of thermodynamics that are experimental facts and that there is no way of proving them are: The conservation of energy principle that describes the behavior of thermodynamic quantities under equilibrium states. This conservation of energy is called the first law of thermodynamics.

The other experimental fact that is found is that heat cannot flow from cold places to hotter places without the application of work in order to do this process.

The first law of thermodynamics states that the internal energy of a system under equilibrium is equal to the heat that is stored in that system plus the work done on that system. The mathematical formula to this law is:


Where E denotes the internal energy and Q is the heat that the system has and W denotes the work done on the system.

The second law of thermodynamics states that it is not possible for a cyclic process that heat flows from cold to hotter space without doing work in the process. Another formulation to this law is that it is not possible to convert all the heat that we put in a system to work in 100%.

The third law of thermodynamics states that it is not possible to lower the temperature of a given system to absolute zero in a finite number of steps. Another formulation to this law is that when the temperature of a given system is approaching zero, the change in entropy under isothermal conditions converges to zero.

For each of these thermodynamic laws there is a mathematical formula which can be found in any chemistry book. These mathematical formulas make relations between thermodynamic functions important for chemical reactions.

One of these functions is called the enthalpy. Enthalpy means the amount of energy that is stored in a molecule or a system.

In a chemical reaction we talk about difference in enthalpy or energy between the starting materials or reactants and the end products of the reaction. If the enthalpy or energy of the products is higher than that of the reactants, we call this reaction and endothermic reaction, because we need to put energy into it to drive it to completion.

On the other hand, if the enthalpy or energy of the reactants is higher than that of the products, we talk then of an exothermic reaction, because heat is released during the reaction.

There is an important principle of chemical reactions, which is called the Hammond postulate. This postulate states that exothermic reactions have low energy barrier while endothermic reactions have high energy barriers. In other words, exothermic reactions are characterized by early transition state, while endothermic reactions are characterized by late transition states.

If a reaction is exothermic it does not necessarily mean that it is spontaneous. Spontaneous reactions are governed by another thermodynamic function which is called the free energy of Gibbs which has the symbol G. A positive value of G means that the reaction is not spontaneous. A zero value of G means that the reaction is under equilibrium state and a negative value of G means the reaction is spontaneous.

Another thermodynamic function which has importance in chemical reactions is the entropy and has the symbol S. Entropy is a measure of disorder of the system. A reaction with positive entropy is not favored entropically because the order is increased in that reaction. If the reaction has a negative entropy value then this means that the reaction is entropically favored because the disorder is increased.

An example of a reaction which is entropically disfavored is the Diels-Alder reaction in which two fragments are combined together to form a cycloadduct which is one fragment. Thus increasing the order. An example of an entropically favored reaction is the dissociation of water to OH and H ions. In this case we start with one molecule and ends up with two fragments. Thus increasing the disoder and entropy.