Spontaneity Of Chemical Reactions

Entropy is the measure of amount of randomness or disorder in a system. Entropy is an extensive property. It depends on the amount of matter a sample contains. Entropy is expressed by symbol S. It is measured in JK⁻¹ or kgm⁻²s²K⁻¹. Entropy is a state function. It depends on the state of the system rather than the path taken.

Lets take an example of melting of ice to understand entropy. We know that water molecules in ice are in ordered state. When ice is melted into liquid water, molecules become disordered. Or we can say, entropy of water molecules increases.

Gibbs free energy is the available energy in a system that can be used to perform some work.Gibbs energy is used to measure the maximum amount of work done in a thermodynamic system at constant temperature and pressure. It is used to determine the spontaneity of a chemical reaction. For example, if the Gibbs energy of reactants is greater than products then reaction will proceed spontaneously. If Gibbs energy of products is greater than reactants then reaction will be non-spontaneous.

Gibbs energy is a state function. It is defined in terms of three other state functions. These state functions are temperature, entropy, and enthalpy. These state functions are the driving factors behind chemical reactions. Gibbs energy being a state function does not depend on the path. It depends on the state of the system.

Gibbs energy is equal to sum of enthalpy minus product of temperature and entropy. So change in Gibbs energy is equal to sum of change in enthalpy and product of temperature and change in entropy.Because temperature is kept constant that is why we are not calculating change in temperature.

Before discussing the spontaneity of chemical reaction, lets first understand the terms change in enthalpy and change in entropy.As we know enthalpy is heat-content of the system. If change in enthalpy is positive then reaction is endothermic. This means heat should be added to the system for the reaction to take place.If change in enthalpy is negative, then reaction is exothermic. This means that heat is released by the reaction.

As we know entropy is the measure of disorder in a system. If change in entropy is positive or greater than zero, this means that system has moved from ordered state to disordered state.If change in entropy is negative or less than zero, this indicates that system has moved from disordered state to ordered state.

If Gibbs free energy is positive, the reaction is non-spontaneous. If it is negative, the reaction is spontaneous.Lets now discuss how entropy, enthalpy and Gibbs free energy are related in determining the spontaneity of a chemical reaction. If change in enthalpy is negative or less than zero and change in entropy is positive or greater than zero, then value of Gibbs energy will be negative or less than zero.Negative value of Gibbs free energy indicates that the reaction will be spontaneous.

For example, when wood burns, it releases heat into the environment. The reaction is exothermic. Change in enthalpy is less than zero in this case.Also system is moving from ordered state that is wood to disordered state which is ashes. Randomness in system is increased.Change in entropy is positive in this case. So the value of Gibbs energy will be less than zero. The burning of wood is a spontaneous reaction.

If the change in Gibbs free energy is positive or greater than zero, the reaction will be non-spontaneous. When reaction is endothermic and entropy is decreasing or less than zero, change in Gibbs free energy will be positive or greater than zero. The reaction will be non-spontaneous because we have to provide heat to the system to move it from less ordered state to more ordered state.

What if we have a particular reaction in which change in enthalpy is positive and change in entropy is also positive?.In simple words, the reaction is endothermic and is moving from ordered state to disordered state.Will the Gibbs energy be negative or positive.? Or we can say will the reaction be spontaneous or non-spontaneous?.In such cases, temperature will determine the spontaneity of the reaction.

If change in enthalpy and change in entropy are both positive, value of Gibbs energy will be negative at higher temperature. It will be positive at lower temperature. We can say, in such case, reaction will be spontaneous at higher temperature. It will be non-spontaneous at lower temperature.For Example, when ice melts into liquid water, change in enthalpy is positive. Reaction is endothermic. Entropy is also positive in this case. This is because ice in ordered state is converting to liquid water which is in less ordered state.

For Example, when ice melts into liquid water, change in enthalpy is positive. Reaction is endothermic. Entropy is also positive in this case. This is because ice in ordered state is converting to liquid water which is in less ordered state.At lower temperature, ice will not melt. The reaction will be non-spontaneous. At higher temperature, ice will melt. The reaction will be spontaneous.

What if change in enthalpy and change in entropy both are negative? Will the reaction be spontaneous or non-spontaneous?Temperature will determine spontaneity in this case. If change in enthalpy and change in entropy are both negative, value of Gibbs energy will be positive at higher temperature. It will be negative at lower temperature. We can say, in such case, reaction will be spontaneous at lower temperature. It will be non-spontaneous at higher temperature.