Entropy , enthalpy and Gibb’s free energy

.    Entropy , enthalpy and Gibb's free energy

The Enthalpy

Enthalpy is described in thermodynamics as an evaluation of the heat content of a chemical or physical system. Enthalpy (H) is an estimation of the total energy of a system and regularly denotes and demonstrates in a simpler way energy transfer between the reacting systems. A positive change in enthalpy denotes an endothermic reaction for the reason that energy is absorbed. A negative change in enthalpy denotes an exothermic reaction for the fact that the system has lost some energy to the environment.

The Entropy

A thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.

The Free energy

The free energy of a reacting system is the difference between the internal energy of a system and the product of its entropy and absolute temperature.

Gibbs free energy

The difference between the enthalpy of a system and the product of its entropy plus absolute temperature is referred to as Gibbs free energy. It is a determination of the useful work obtained from a thermodynamic system at constant temperature and pressure.


Heat is defined as the energy transferred from one system to another by thermal interaction.

Law of conservation of energy:

The law of the conservation of energy states that energy can neither be destroyed nor created but can be altered from one form to another. The total energy of a reacting system and the surroundings remains constant or unchanged.

Change in Enthalpy is the expression that is used to explain the energy swap over that occurs with the surroundings at a constant pressure. It is denoted with the symbol ΔH.

Enthalpy is the total energy content of the reactants. It is denoted with the symbol, H.

ΔH = ΔH products – ΔH reactants

The units of enthalpy are kilojoules per mole (kJmol-1)

An exothermic enthalpy change is constantly represented with a negative value, due to the fact that energy is expelled to the surroundings.

ΔH = -xkJmol-1

An endothermic enthalpy change is constantly represented with a positive value, since the energy is absorbed by the system from the surroundings.

ΔH = + ykJmol-1.

Standard enthalpy changes: standard conditions

When we want to place the enthalpy changes of different types of reactions side by side each other for comparison, we must make use of standard conditions like known temperatures, pressures, amounts and concentrations of reactants or products.

The standard conditions are listed below:

• A pressure of 100 kilopascals (102kPa)

• A temperature of 298K (25oC )

• Reactants and products in physical states, typical for conditions above.

• A concentration of 1.0mol dm-3 for solutions.

The o sign is used to denote a standard condition.

Standard enthalpy change of reaction


The standard enthalpy change of reaction is the enthalpy change of that reaction when the amounts of reactants shown in the equation for the reaction, react under standard conditions to form the products in their standard states.

Standard enthalpy change of formation


The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions. This means that both compound and elements are in their standard states.

The Standard enthalpy change of combustion


The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound is completely reacted with oxygen under standard conditions.

The Energy Content of Fuels

Energy content is a significant property of both food and matter that is made use of during the process of heating. The energy that our body utilizes for day to day activities like sleep, walk, talk etc comes from the food that we eat, for instance the candy bar. The energy that is generated a fuel is burned is a crucial quantity, and we’d like to be capable of measuring the effectiveness of fuel.

The energy content is the amount of heat produced by the combustion of 1 gram of a substance and is measured in joules per gram (J/g). Heat is a form of energy or in reality a flow of energy flow and it is usually calculated in calories.1 cal = 4.186 J.

Every now and then it is tricky to accurately measure the amount of heat that is produced by a substance. The measurement is made easier by burning a particular quantity of the fuel to heat up water. The energy expelled by the fuel can then be estimated by calculating the heat absorbed by the water as calculated by the change in temperature of the water. Heat gained by water can be denoted by

Q = change T * m * c

whereT is the temperature change, m is the mass, and c is the specific heat capacity constant of water.


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