Question

How can you determine if a reaction is nonspontaneous?

Answer 1

A reaction is a chemical change in which substances combine with each other and yields new substances. Spontaneity of a reaction means the feasibility of a reaction with or without any external source.

Complete step by step answer:
There are two categories of reaction of which one is spontaneous and the other is non-spontaneous. Both reactions occur to form new products. The difference lies between the initiation and the feasibility of the process to bring the chemical change.
Spontaneous reactions are those reactions which occur without the influence of other sources. These reactions are not driven by external forces. These reactions are natural processes and do not require and input energy. The products formed are lower in energy than the reactants.
Non spontaneous reactions are those reactions which require energy from outside to and do not proceed without any external source. The energy of the products is higher than the energy of the reactants.
The spontaneous and non-spontaneous reactions are clearly explained with the help of thermodynamics of a reaction. Based on the thermodynamics there are certain parameters which indicate the spontaneity of a reaction. The parameters are known as enthalpy (heat change), entropy and Gibbs free energy.
The enthalpy of a reaction is defined as the heat of reaction. It is either absorbed or released during the chemical change. The entropy of a reaction is the degree of randomness of a reaction. The total entropy is the sum total of the system entropy and the entropy of surroundings. The Gibbs free energy is the overall energy required during the transformation. The minimum energy required by the starting materials to convert into products is termed as the activation energy of the reaction.
The relation between the three parameters is written as
$\Delta G = \Delta H - T\Delta S$ where $\Delta G$ is the free energy change, $\Delta H$ is the enthalpy change, $\Delta S$ is the entropy change and $T$ is the temperature.
For a non-spontaneous change the following set of conditions must be satisfied:
a. $\Delta G$ must be positive
b. $\Delta H > 0$ and $\Delta S < 0$.
c. $\Delta H > 0$ and $\Delta S > 0$ at low temperatures.
d. $\Delta H < 0$ and $\Delta S < 0$ at high temperatures.

Note:
A non-spontaneous reaction is also known as endergonic reactions and a spontaneous reaction is known as exergonic reaction. The spontaneous reaction occurs faster and the non-spontaneous reactions occur slower.