Spontaneity: Process, Reactions, Entropy, Gibbs Equation

Spontaneity comes under the First Law of Thermodynamics which is based on the law of conservation of energy which explains that energy can be transformed from one form to another but cannot be created or destroyed.

• The spontaneity in thermodynamics defines the direction of heat flow that can be developed by establishing a relationship between the work done by the system or by the system.
• All of the processes of heat flow that happen naturally tend to proceed spontaneously only in one direction.
• Many natural phenomena are having one straight path of heat flow. They do not have any limitations on their heat flow paths.

Key Terms: Spontaneous reaction, Gibbs equation, Entropy, Enthalpy, Laws of Thermodynamics, Conservation of energy, Heat, Work, Energy.

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Spontaneous Reactions

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In spontaneity, a spontaneous process is an irreversible one that happens without external energy or agents. This process may take place slowly or quickly as it is not related to the kinetics of the reaction rate. 

• A spontaneous reaction is a kind of reaction that happens naturally in a given set of conditions without any intervention.
• A spontaneous reaction tends to be complete without any outside help.
• A reaction cannot be said to have occurred spontaneously just because the reaction is spontaneous.
• Salt dissolves in water, fruit ripening, and ice melting that occurs naturally and doesn’t require any kind of energy or help. 

Examples: 

1. Let’s imagine that you are walking through a natural waterfall. The water that is falling from tumbles is happening on its own that doesn’t require any kind of help from leaves or dams. 
2. Take out a chilled bottle of soda from your fridge and keep it next to your computer. After 1 or 2 hours, the soda will come at room temperature; that happens naturally without any kind of help or intervention. 
3. When iron nails rust because of constant exposure to moisture, however, this process does not happen overnight; it can take days or months. Again, this process clarifies that it does not need any outside intervention or help. 

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Spontaneous Reactions and Entropy

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Total entropy is one of the important parameters which defines the spontaneity of any process of a chemical equation.

• Entropy is defined as a measure of disorder or randomness of a system. The concept of entropy was introduced in the year 1850 by a German physicist named Rudolf Clausius.
• Entropy is a state function and depends on the state of the system but not the path that is followed.
• Most of the chemical reactions are categorized into closed systems and open systems; we can say that there is a change in enthalpy too along with the change in entropy. 

Spontaneous and Non-spontaneous Processes

Spontaneous processes can be exothermic or endothermic. We can say the same thing in another way, it is not necessary that spontaneity is related to the enthalpy change of a process, which is represented with delta (ΔH). 

• Change in enthalpy causes to increase or decrease the randomness of the chemical equation that also influences molecular motions.
• Change in entropy alone is not responsible for the spontaneity of such a process.
• Therefore, Gibbs energy change has been used to explain the process of spontaneity.

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Spontaneity: Gibbs Equation 

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Gibbs energy can be defined as a state function and an extensive property. Gibb’s equation shows the energy change at a constant temperature It can be expressed in mathematical terms as follows:- 

ΔG_sys = ΔH_sys – TΔS_sys

Where,

• ΔG_sys - G is denoted as the Gibbs energy change of the system
• ΔH_sys - H is denoted as an enthalpy change of the system
• ΔS_sys - S is denoted as the entropy change of the system
• T - T is denoted as the Temperature of the system 

The above-mentioned equation is known as Gibbs Equation

For conducting spontaneous processes, the total entropy change (ΔS_total) in this is always greater than zero. The mathematical expression of this equation is mentioned below:- 

ΔS_total = ΔS_sys + ΔS_surr 

where, 

• ΔS_total is referred to as the total entropy change for the process.
• ΔS_sys is denoted as the entropy change of the system.
• ΔS_surr is denoted as the entropy change of the surrounding.

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Spontaneous Process

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In spontaneity a change in temperature between the surroundings and the system is always zero i.e., DT = 0 in the case of a thermal equilibrium state is known as a spontaneous process.

• It happens due to the change in enthalpy.
• The amount of enthalpy lost in the system is the same amount that is gained by the surrounding.
• Therefore, scientists have created the equations for the same that denote the change in entropy:

Here, 

• ΔH_surr is denoted as the enthalpy change of the surrounding
• ΔH_sys is denoted as an enthalpy change of the system

As expressed earlier, ΔS_total> 0.

As we earlier said that the change in entropy is always more than zero in the case of a spontaneous process. So conclusion is

TΔS_sys – ΔH_sys > 0

ΔH_sys– TΔS_sys < 0

When we use Gibb’s equation, it can be stated that ‘ΔGsys< 0’.

By the already provided equations, the spontaneity can be predicted.

• During an exothermic reaction, the system has a negative enthalpy which in turn makes all the exothermic reactions spontaneous.
• During an endothermic reaction when the temperature or the entropy change is extremely high then the Gibbs free energy becomes negative. 

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Things to Remember

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• Spontaneity is a part of Physics class 11 unit 8, Thermodynamics 
• It carries a total of 4 to 6 marks and 12 periods. 
• ΔS = ΔS_system + ΔS_surroundings
• ΔS = Complete change in the entropy
• ΔS_system = Change in the entropy of the system 
• ΔS_surroundings = Change in the entropy of the surroundings
• ΔS>0, + entropy change, the reaction is spontaneous
• ΔS<0, - entropy change, the reaction is nonspontaneous.
• Gibbs free energy equation: ΔG = ΔH - TΔS

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