What is Spontaneous Reaction
A spontaneous reaction is a chemical reaction that proceeds without any supply of energy. A reaction that is spontaneous will proceed without outside intervention. The Gibbs free energy of the products is lower than that of the reactants, which implies stable products. Therefore, the change in the free energy of the reaction is negative [1-8].
In chemistry and biology, a spontaneous reaction occurs naturally. It favors the formation of products under the given reaction conditions. An exergonic reaction is spontaneous.
On the contrary, a spontaneous process is one where a reaction does not occur, but the system undergoes a physical change. Examples of the spontaneous process include a ball rolling down a slide and ice melting into water.
Spontaneous Reaction and Gibbs Free Energy
What Causes a Reaction to be Spontaneous
The spontaneity of a reaction occurring at constant temperature and pressure can be determined by Gibbs free energy, which is given by [2-6]
ΔG = ΔH – TΔS
Here, ΔH is the change in enthalpy, ΔS is the change in entropy, and T is the temperature. ΔG is expressed in the unit of kJ mol-1. The sign of ΔG depends on the signs of ΔH and ΔS. A reaction is spontaneous and favorable if there is a decrease in enthalpy and an increase in entropy of the system.
In general, the conditions for a spontaneous reaction to occur are as follows:
1. When ΔG is negative, the reaction is spontaneous and proceeds in the forward direction.
2. When ΔG is positive, the reaction is nonspontaneous in the forward direction but is spontaneous in the reverse direction.
3. When ΔG is zero, the reaction is in equilibrium. The concentrations of the reactants and products will remain the same.
Examples of Spontaneous Reaction
Here are some examples of reactions that occur spontaneously.
1. The combustion of a fuel like ethanol (C2H5OH) in oxygen gas (O2) in air. The products of the combustion reaction are carbon dioxide gas (CO2) and water (H2O) at temperatures above 100°C. The reaction will progress in the forward direction until the fuel runs out.
C2H5OH (l) + 3 O2 (g) → 2 CO2(g) + 3 H2O (g)
2. When a soda bottle is opened, carbonic acid present in the aerated soda decomposes to carbon dioxide (CO2) and water (H20).
H2CO3 (aq.) → CO2 (g) + H2O (l)
3. Wood consists of hydrocarbon (CxHy). The burning of wood in the presence of oxygen (O2) produces carbon dioxide and water.
CxHy + (x+y/4) O2 → x CO2 (g) + (y/2) H2O (l)
4. The reaction of baking soda (NaHCO3) with vinegar (CH3COOH) gives sodium acetate (CH3COONa), carbon dioxide, and water.
NaHCO3 (s) + CH3COOH (aq.) → CH3COONa (aq.) + CO2 (g) + H2O (l)
5. The rusting of iron (Fe) in the presence of oxygen, water, and sunlight gives iron (III) oxide (Fe2O3).
4 Fe (s) + 3 O2 (g) → 2 Fe2O3 (s)
Spontaneous and Non-spontaneous Reactions [7-8]
A non-spontaneous reaction requires energy to proceed. This type of reaction does not favor the formation of products and is usually accompanied by an increase in Gibbs free energy. However, the reaction may proceed spontaneously in a reversible direction. A non-spontaneous reaction is generally endergonic. If a chemical reaction does not take place, then the transformation is known as a non-spontaneous process.
Examples of Non-spontaneous Reaction
1. Electrolysis of water: Energy in the form of an electric current is supplied to water (H2O) to form hydrogen (H2) and oxygen (O2). This process is known as electrolysis and is a redox reaction. Other examples of non-spontaneous redox reactions are electroplating and electrolysis.
2 H2O (l) → 2 H2 (g) + O2 (g)
2. Formation of nitrogen monoxide: Nitrogen (N2) and oxygen (O2) in the atmosphere combine at normal to high temperatures to form nitrogen monoxide or nitric oxide (NO).
N2 (g) + O2 (g) → 2 NO (g)
3. Photosynthesis: A photochemical reaction in which plants take in energy from the sun to convert carbon dioxide (CO2) and water (H2O) into sugar (C6H12O6) and oxygen (O2).
6 CO2 (g) + 6 H2O (l) + heat → C6H12O6 (s) + 6 O2 (g)
4. Ozone: Conversion of oxygen (O2) to ozone (O3) is a non-spontaneous process at all temperatures. It is driven by ultraviolet light from the sun.
3 O2 (g) + hν → 2 O3 (g)
Spontaneous vs. Non-spontaneous Reactions
The following table gives the difference between spontaneous and non-spontaneous reactions.
|Spontaneous Reaction||Non-spontaneous Reaction|
|Definition||A chemical reaction that takes place without any external energy||A chemical reaction that requires external energy to occur|
|Enthalpy and entropy change||Favors increasing entropy and decreasing enthalpy||Do not favor increasing entropy and decreasing enthalpy at normal conditions|
|Gibbs free energy||Negative||Positive|
|Example||Combustion of wood and fuel||Electrolysis of water|
How to Tell if a Reaction is Spontaneous or Non-spontaneous
The spontaneity of a reaction depends on the change in enthalpy (ΔH) and entropy (ΔS) as well as the temperature (T). The following table predicts whether a reaction will be spontaneous or not.
|ΔH < 0||ΔH > 0|
|ΔS > 0||Spontaneous at all T||Spontaneous at high T|
|ΔS < 0||Spontaneous at low T||Non-spontaneous at all T|