Answer
Hint: A reaction is any process in which the starting materials usually called as the reactants are converted to the final products usually called the products. There are two types of reactions on the basis of their occurrence namely, (i) spontaneous reactions, and (ii) non spontaneous reactions. Both the reactions are determined by the physical properties of the thermodynamics.
Complete answer:
Thermodynamics is the branch of chemistry that deals with the flow of heat,
its study and the laws used to study and determine them.
A reaction is said to be spontaneous or feasible iff and only iff it has the tendency to take place in nature in a simple manner only. Like evaporation.
A reaction is said to be non spontaneous or non feasible if and only iff it has no tendency to take place in nature in a simple manner only. Like flow of liquids from bottom to top without any mechanical force.
For the determination of the spontaneity of the reaction, out of four
laws of thermodynamics two can be use which gives three factors which are as follows: -
In First law, the Enthalpy can be used. When the reaction releases the energy then it is said to be a spontaneous reaction.
In the second law, the Entropy and the Gibbs free energy tells us about the spontaneity of the reaction. The entropy should be positive for the spontaneous reaction. Whereas the gibbs free energy should be negative for the spontaneous reaction.
Note:
There could be
several factors that determine the spontaneity of the reaction, three of them are the enthalpy, the entropy and the Gibbs free energy. If a reaction satisfies any one condition then it is said to be a spontaneous reaction with conditions. But if all the conditions are satisfied then the reaction is spontaneous reaction without any pre required condition.
Chapter 18. Chemical Thermodynamics
Spontaneity: Free Energy and Temperature
Jessie A. Key
- To gain an understanding of the relationship between spontaneity, free energy, and temperature.
- To be able to calculate the temperature at which a process is at equilibrium under standard conditions.
In the Gibbs free energy change equation, the only part we as scientists can control is the temperature. We have seen how we can calculate the standard change in Gibbs free energy, ΔG°, but not all reactions we are interested in occur at exactly 298 K. The temperature plays an important role in determining the Gibbs free energy and spontaneity of a reaction.
If we examine the Gibbs free energy change equation, we can cluster the components to create two general terms, an enthalpy term, ΔH, and an entropy term, –TΔS. Depending on the sign and magnitude of each, the sum of these terms determines the sign of ΔG and therefore the spontaneity (Table 18.2 “Spontaneity and the Signs of Enthalpy and Entropy Terms”).
Table 18.2 Spontaneity and the Signs of Enthalpy and Entropy Terms| + | − | + | + | Nonspontaneous |
| − | + | − | − | Spontaneous |
| − | − | + | + or − |
|
| + | + | − | + or − |
|
Since all temperature values are positive in the Kelvin scale, the temperature affects the magnitude of the entropy term. As shown in Table 18.2 “Spontaneity and the Signs of Enthalpy and Entropy Terms,” the temperature can be the deciding factor in spontaneity when the enthalpy and entropy terms have opposite signs. If ΔH is negative, and –TΔS positive, the reaction will be spontaneous at low temperatures (decreasing the magnitude of the entropy term). If ΔH is positive, and –TΔS negative, the reaction will be spontaneous at high temperatures (increasing the magnitude of the entropy term).
Sometimes it can be helpful to determine the temperature when ΔG° = 0 and the process is at equilibrium. Knowing this value, we can adjust the temperature to drive the process to spontaneity or alternatively to prevent the process from occurring spontaneously. Remember that, at equilibrium:
We can rearrange and solve for the temperature T:
Using the appendix table of standard thermodynamic quantities, determine the temperature at which the following process is at equilibrium:
How does the value you calculated compare to the boiling point of chloroform given in the literature?
Solution
At equilibrium:
We must estimate ΔH° and S° from their enthalpies of formation and standard molar entropies, respectively.
Now we can use these values to solve for the temperature:
The literature boiling point of chloroform is 61.2°C. The value we have calculated is very close but slightly lower due to the assumption that ΔH° and S° do not change with temperature when we estimate the ΔH° and S° from their enthalpies of formation and standard molar entropies.
- The temperature can be the deciding factor in spontaneity when the enthalpy and entropy terms have opposite signs:
- If ΔH is negative, and –TΔS positive, the reaction will be spontaneous at low temperatures (decreasing the magnitude of the entropy term).
- If ΔH is positive, and –TΔS negative, the reaction will be spontaneous at high temperatures (increasing the magnitude of the entropy term).