What Is The Net Energy Released In A Reaction?

Definition of Net Energy

The net energy released or absorbed in a chemical reaction is the difference between the total energy of the products and the total energy of the reactants. This net energy change determines whether a reaction is exothermic (releases energy) or endothermic (absorbs energy).

Net energy should not be confused with activation energy. Activation energy is the initial energy input needed to get a reaction started. It does not affect the overall energetics of the reaction. Activation energy is like the push needed to start a ball rolling down a hill. Once the reaction starts, the net energy change depends only on the relative energies of starting materials and final products.

While activation energy relates to the initial energy barrier, net energy relates to the overall energetics of the reaction. Net energy, also called reaction energy or enthalpy change, indicates the total energy released or absorbed when the reaction goes to completion. This energy change determines the heat and work potential of a given chemical reaction.

Endothermic vs Exothermic

Chemical reactions can be classified as either endothermic or exothermic based on whether they absorb or release energy. The difference lies in what happens to the energy of the chemical bonds before and after the reaction.

In an endothermic reaction, energy must be added for the reaction to proceed. This is because the chemical bonds in the reactants have more energy than the products. As the reaction occurs, energy is absorbed and used to break bonds in the reactants. This makes endothermic reactions feel cold, as heat is removed from the surroundings.

In contrast, exothermic reactions release energy. Here, the bonds in the products contain less energy than the bonds in the reactants. As the bonds reform, this excess energy is released, usually as heat. Exothermic reactions warm up the surroundings as the energy flows out of the system.

One way to distinguish endothermic vs exothermic reactions is by looking at the enthalpy change (ΔH). For an endothermic reaction, ΔH is positive, meaning it requires more energy input. Exothermic reactions have a negative ΔH, indicating a release of energy.

Measuring Net Energy

The net energy released or absorbed in a chemical reaction can be measured experimentally using a calorimeter. This device allows you to directly measure the heat transferred between the reacting chemicals and the surroundings. By carefully measuring the temperature change in the calorimeter and accounting for heat losses, the enthalpy change of the reaction can be determined. This directly correlates to the net energy change.

Net energy can also be calculated based on the chemical bonds broken and formed during the reaction. Using bond dissociation energies, you can find the energy required to break all the bonds in the reactants. Similarly, you can calculate the energy released when new bonds are formed in the products. The net energy is simply the difference between the energy required to break bonds versus the energy released in forming new bonds. If more energy is released than required, the net energy change is negative and the reaction is exothermic. If more energy is required, the net change is positive and the reaction is endothermic.

Entropy Considerations

The change in entropy (disorder) of a chemical reaction also affects the net energy released or absorbed. Reactions that increase entropy tend to release energy, while reactions that decrease entropy tend to absorb energy.

For example, the combustion reaction of methane and oxygen to produce carbon dioxide and water vapor results in an increase in entropy. More moles of gaseous products are formed than the number of moles of gaseous reactants. This increase in entropy contributes to the large amount of energy released during combustion.

In contrast, the formation of diamond from graphite results in a decrease in entropy, as the orderly crystal structure of diamond is formed from the less ordered graphite structure. This decrease in entropy makes diamond formation an endothermic process that requires energy input.

By considering both enthalpy change and entropy change, the Gibbs free energy change can be calculated, which determines whether a reaction will release or absorb energy.

Gibbs Free Energy

Gibbs free energy is a thermodynamic potential that allows the prediction of the direction of a chemical reaction. It relates the net energy released in a reaction to the change in entropy that occurs.

Gibbs free energy, often represented by G, is defined as:

G = H – TS

gibbs free energy chemical reaction equation

Where H is the enthalpy, T is the temperature, and S is the entropy. For a reaction to proceed spontaneously, the Gibbs free energy must decrease. Therefore, reactions where the change in G (represented as ΔG) is negative are exergonic – they release energy. Reactions where ΔG is positive are endergonic, meaning they absorb energy.

Using the Gibbs free energy equation, we can predict whether a reaction will occur spontaneously based on the values of H, T, and S. Even if a reaction results in an increase in enthalpy (positive ΔH), if the increase in entropy is large enough, the reaction can still have a negative ΔG and proceed spontaneously.

Gibbs free energy provides a useful thermodynamic measure of the driving force of a chemical reaction that accounts for both enthalpy and entropy changes. It allows chemists to determine if a reaction will occur spontaneously given a set of conditions.

Reaction Quotient

The reaction quotient (Q) is an expression that describes the ratio of concentrations of reactants and products for a given chemical reaction at any point during that reaction. It indicates how far the reaction has proceeded towards equilibrium. Q is similar to the equilibrium constant (Keq) but applies to non-equilibrium conditions.

For a general reaction: aA + bB ⇌ cC + dD

The reaction quotient is defined as:

Q = [C]c[D]d / [A]a[B]b

Where [C], [D], [A], and [B] are the molar concentrations of C, D, A and B respectively. The stoichiometric coefficients a, b, c, and d are exponents.

If Q < Keq, the reaction will proceed towards products until equilibrium is established. If Q > Keq, the reaction shifts towards reactants. At equilibrium, Q = Keq. Therefore, the reaction quotient indicates whether a reaction will be spontaneous based on the relative concentrations of reactants and products at any given time.

Reactions in Nature

Photosynthesis and cellular respiration are two of the most important reactions that occur in nature. Photosynthesis is the process plants use to convert carbon dioxide and water into glucose and oxygen using energy from sunlight. This process stores energy from the sun in the glucose molecules. The overall reaction for photosynthesis is:

6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2

Cellular respiration is the process that releases energy from glucose to produce ATP for cells. Respiration is the opposite reaction of photosynthesis. The overall reaction for cellular respiration is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP

These two reactions work together to flow energy through ecosystems. Photosynthesis captures and stores energy from the sun, while cellular respiration releases that stored energy for use by living organisms. The cycling of matter and energy between photosynthesis and respiration drives the flows of energy that sustain food webs and ecosystems.

Industrial Applications

The concept of net energy is critical for many industrial processes and technologies. Understanding whether a chemical reaction releases or absorbs net energy allows us to harness exothermic reactions as energy sources and endothermic reactions for energy storage.

Some key examples of applying net energy principles in industry include:

Fuel Cells

Fuel cells produce electricity from exothermic oxidation-reduction reactions. The net energy released from reactions of fuels like hydrogen or natural gas is converted to electrical energy. Fuel cells are much more efficient than internal combustion engines at converting chemical potential energy to electrical energy.


Batteries store electrical energy through endothermic chemical reactions during charging. The reverse exothermic reactions release energy when discharging the battery. The net energy absorbed during charging can be quantified based on the battery’s storage capacity.


Explosives and propellants involve exothermic chemical reactions that release large amounts of net energy very rapidly. This generates high temperatures and pressures that can be harnessed for purposes ranging from mining to rocket propulsion. The explosive power correlates directly with the net energy released.

Understanding net energy is thus essential for optimizing these and many other industrial technologies that involve chemical reactions.

Nuclear Reactions

Nuclear reactions involve changes in the nucleus of atoms. The two main types of nuclear reactions are nuclear fission and nuclear fusion.

Nuclear fission is the splitting of a large atomic nucleus into smaller nuclei. This process releases energy because the total mass of the resulting nuclei is less than the original mass. Fission reactions are used in nuclear power plants to generate electricity.

Nuclear fusion is the joining of two lighter atomic nuclei to form a heavier nucleus. This process releases a very large amount of energy, even more than fission reactions. The reason is that fusion results in a loss of mass according to Einstein’s famous equation E=mc2. Fusion powers the Sun and other stars.

Both fission and fusion reactions demonstrate the conversion of mass into energy. Even a small amount of mass loss, when converted to energy, produces a tremendous amount of power according to the equation E=mc2. This makes nuclear reactions a unique and extremely concentrated source of energy.


The net energy released or absorbed in a chemical reaction provides crucial insights into the thermodynamic favorability and real-world applications of that reaction. This article covered key concepts related to net energy, including:

– The difference between exothermic reactions that release energy, and endothermic reactions that absorb energy.

– How net energy is mathematically calculated from changes in enthalpy, entropy, and Gibbs free energy.

– The various methods for experimentally measuring net energy, such as calorimetry.

– The applications of net energy calculations in predicting chemical equilibria and reaction direction.

– The role of net energy in industrial processes, nuclear reactions, and natural phenomena.

Understanding the net energy of reactions helps scientists and engineers optimize chemical processes for practical applications. Exothermic reactions can provide energy for power generation, heating, or cooking. Endothermic reactions can absorb heat for cooling or refrigeration. Net energy also reveals whether a reaction will occur spontaneously, and to what extent. By mastering the concepts in this article, readers can apply net energy principles to a wide range of contexts.

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