How to Determine if a Chemical Reaction is Spontaneous
Spontaneity is part of the first law of thermodynamics. In this section, students can understand the fact associated with an isolated system's fixed energy level.
There is a direction of heat flow that can be elaborated by establishing a relation between the work done by the system or on the system. This is spontaneity in thermodynamics.
Spontaneous meaning in Chemistry is not that hard to understand. Many natural phenomena are having one straight path of heat flow. They do not have any limitations on their heat flow paths.
What Is a Spontaneous Reaction?
A spontaneous chemical reaction is an irreversible process where you can’t get the ingredients back without the external agents.
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Equation of Spontaneous Reaction
We can predict the spontaneity Chemistry of a reaction with the change in its total entropy. This is defined as the spontaneity of any process. Almost all types of chemical reactions come under this category.
Scientists also predicted that the change in enthalpy helps the increase or increase in the randomness of the chemical reactions. They also affect the molecular motions. This is not possible as entropy changes only possible due to spontaneity. Many other processes are also in the queue of participation.
Students can gain proper knowledge about Spontaneity Chemistry and spontaneous equations by studying Gibb’s energy.
Spontaneous Equation
Gibb’s equation can be the best option to understand spontaneous reactions in chemical composition. It is a state function. Also, Gibb’s equation is an extensive property. At constant temperature, Gibb’s equation shows the energy change.
It can be expressed as,
ΔGsys = ΔHsys – TΔSsys
Here,
Change in Gibbs energy of the system = ΔGsys
Change in enthalpy of the system = ΔHsys
Change in Entropy of the system = ΔSsys
Constant Temperature of the system = T
Also, if we conduct a spontaneous process, the total change in entropy is always greater than zero.
Mathematical expression for the above spontaneous reaction meaning expression is
ΔSsys + ΔSsurr = ΔStotal
Here,
ΔStotal= total change in entropy for the process
ΔSsurr = change in entropy of the surrounding
ΔSsys = change in entropy of the system
What Is Spontaneous Process?
When the system is in a thermal equilibrium state, the change in temperature between the surroundings and the system is always zero.
i.e. dT = 0
Do you know how this is happening? It is due to the change of enthalpy. As the amount of enthalpy is lost by the system, the same amount is gained by the surrounding.
So, scientists have put forward the equations that stand for the change in entropy for both the system and the surrounding.
\[\Delta S_{surr}=\frac{\Delta H_{surr}}{T}=- \frac{\Delta H_{sys}}{T}\]
\[\Delta S_{Total}=\Delta S_{sys}+\left ( -\frac{\Delta H_{sys}}{T} \right )\]
Here, ΔHsurr = enthalpy change of the surrounding
ΔHsys = enthalpy change of the system
As expressed earlier, ΔStotal> 0.
The change in entropy is always more than zero when it is a spontaneous process. So, we conclude that
TΔSsys – ΔHsys > 0
ΔHsys– TΔSsys < 0
When we use Gibb’s equation, it can be said that ‘ΔGsys< 0’.
Conclusion
In a spontaneous chemical reaction, if we notice any energy change in Gibb’s energy of the system as less than zero otherwise, it is not a spontaneous process.
It can be concluded that relation is also predicted for a spontaneous reaction.
When it is an exothermic reaction, the enthalpy of the system is negative. This is why it makes all exothermic reactions spontaneous.
When it is an endothermic reaction, Gibbs’s free energy turns into negative. It happens in certain conditions only, such as when the temperature rises or the change in entropy is very high.
FAQs on Spontaneity in Chemistry: Explained for Students
1. What does spontaneity mean in the context of chemistry?
In chemistry, spontaneity refers to the inherent tendency of a process or chemical reaction to occur on its own, without the need for a continuous input of external energy once it has started. A spontaneous process is one that moves a system towards a more stable, lower-energy state or a state of higher disorder. A common example is iron rusting when exposed to air and moisture.
2. What is the main difference between a spontaneous and a non-spontaneous process?
The key difference lies in the energy requirement. A spontaneous process proceeds naturally under a given set of conditions (e.g., ice melting at room temperature), while a non-spontaneous process requires a continuous supply of external energy to happen (e.g., charging a phone battery). Spontaneity is determined by the change in Gibbs Free Energy (ΔG); a negative ΔG indicates a spontaneous process, whereas a positive ΔG indicates a non-spontaneous one.
3. If a reaction is spontaneous, does that mean it will happen quickly?
No, this is a common misconception. Spontaneity is a thermodynamic concept that only tells us if a reaction is energetically favourable and can proceed without external intervention. It says nothing about the rate of the reaction, which is governed by chemical kinetics and factors like activation energy. For example, the conversion of diamond to graphite is a highly spontaneous process, but it is so slow that it takes millions of years to observe.
4. What is Gibbs Free Energy and how does it help predict spontaneity?
Gibbs Free Energy (G) is a thermodynamic potential that combines enthalpy (H) and entropy (S) to determine the maximum reversible work a system can perform at constant temperature and pressure. Its change (ΔG) is the single most reliable criterion for predicting spontaneity, as per the equation ΔG = ΔH - TΔS.
The criteria are as follows:
- If ΔG is negative, the process is spontaneous.
- If ΔG is positive, the process is non-spontaneous.
- If ΔG is zero, the system is at equilibrium.
5. How do enthalpy and entropy together determine if a reaction is spontaneous?
Both enthalpy (ΔH), the change in heat, and entropy (ΔS), the change in disorder, influence spontaneity. A reaction is most likely to be spontaneous if it is exothermic (releases heat, negative ΔH) and leads to an increase in disorder (positive ΔS). However, a reaction can still be spontaneous if one factor is unfavourable, provided the other is favourable enough to compensate, depending on the temperature (as seen in the Gibbs equation, ΔG = ΔH - TΔS).
6. Can an endothermic reaction (one that absorbs heat) ever be spontaneous? Provide an example.
Yes, an endothermic reaction (positive ΔH) can be spontaneous if the increase in entropy (positive ΔS) is large enough. If the TΔS term in the Gibbs equation becomes greater than the ΔH term, the overall ΔG will be negative. A perfect example is the dissolving of ammonium nitrate in water. The process absorbs heat from the surroundings, making the beaker feel cold (endothermic), but it happens spontaneously because the solid salt dissolving into ions greatly increases the system's disorder (entropy).
7. How is the concept of spontaneity applied in electrochemistry, for example, in batteries?
In electrochemistry, the functioning of a galvanic or voltaic cell (like a battery) is based on a spontaneous redox reaction. This spontaneous chemical reaction releases energy, which is harnessed as electrical energy. The cell potential (E°cell) is directly related to Gibbs Free Energy by the equation ΔG° = -nFE°cell. A positive cell potential signifies a negative ΔG°, confirming that the reaction is spontaneous and can be used to generate an electric current.
8. What are some everyday examples of spontaneous processes?
Many everyday occurrences are examples of spontaneous processes. These include:
- An ice cube melting in a warm room.
- Sugar or salt dissolving in water to form a solution.
- A hot cup of coffee cooling down to room temperature.
- The diffusion of a perfume's scent across a room.
- A ball rolling down a hill under the force of gravity.
