Enthalpy: what is it, formula, types and examples

What is Enthalpy?

Enthalpy is the amount of heat that a thermodynamic system releases or absorbs from the surrounding environment when it is at a constant pressure, thermodynamic system meaning any object.

In physics and chemistry, enthalpy is a thermodynamic quantity whose unit of measurement is Joules (J) and is represented by the letter H.

The formula to calculate the enthalpy is:

H = E + PV

Where:

• H is enthalpy, E is the energy of the thermodynamic system, P is the pressure of the thermodynamic system, V is volume.

In this formula, the product of the pressure multiplied by the volume (PV), is equal to the mechanical work that is applied to the system.

Therefore, the enthalpy is equal to the energy of a thermodynamic system plus the mechanical work applied to it.

However, the enthalpy of a system can only be measured at the moment when an energy variation occurs. The variation, represented by the sign Δ, gives rise to a new formula:

∆H = ∆E + P∆V

This means that the variation in enthalpy (∆H) is equal to the variation in energy (∆E) plus the mechanical work applied to the system (P∆V).

Enthalpy comes from the Greek enthálpō , which means adding or adding heat. The term was first coined by Dutch physicist Heike Kamerlingh Onnes, winner of the Nobel Prize for Physics in 1913.

Types of enthalpy

There are several types of enthalpy depending on the substances and processes involved. When the process involves the release of energy, it is an exothermic reaction, while the capture of energy means that it is an endothermic reaction.

Based on the above, enthalpies are classified into:

Formation enthalpy

It is the energy that is required to form a mole of a substance from the elements that compose it. Recall that the mole is the unit of measurement of substance equivalent to 6.023x10 ²³ atoms or molecules.

An example of formation enthalpy is the union of oxygen (O) and hydrogen (H) to form water (H ₂ O), whose energy or enthalpy variation (ΔH) is -285,820 KJ / mol.

Reaction enthalpy

It is the energy that releases a chemical reaction under constant pressure.

An example of reaction enthalpy is the formation of methane (CH4) from the union of carbon (C) and hydrogen (H):

C + 2H ₂ → CH ₄

See also Chemical reaction.

Enthalpy of solution

Refers to the amount of heat released or absorbed by a substance when dissolved in aqueous solution.

An example of enthalpy of solution is what happens when dissolving sulfuric acid (H ₂ SO ₄) in water (H ₂ O). The amount of energy released by the acid is so high that it is a solution that must be used with certain safety measures.

Neutralizing enthalpy

It is the energy that is captured or released when an acid and a base are mixed, neutralizing each other.

An example of neutralization enthalpy is when we mix acetic acid (CH₃COOH) with bicarbonate (NaHCO₃).

See also Acids and bases.

Combustion enthalpy

It is the energy released when one mole of organic substance reacts with oxygen in the air and releases carbon dioxide (CO _2).

An example of combustion enthalpy is that generated by propane gas (C ₃ H ₈), which releases energy that is used as a domestic fuel:

C ₃ H ₈ + 5 O ₂ → 3CO ₂ + 4H ₂ O

Releases 2,044 x 10 ³ KJ / mol

The enthalpy variation (ΔH) = -2.044x10 ^ 3 KJ / mol

See also Combustion.

Decomposition enthalpy

It is the amount of heat or energy that is released when one mole of substance is broken down into simpler elements.

An example of decomposition enthalpy is when hydrogen peroxide or hydrogen peroxide decomposes to form water and oxygen:

2H ₂ O ₂ → 2H ₂ O + O ₂

96.5KJ / mol are released

Enthalpy variation (ΔH) = 96.5KJ / mol

Enthalpy of dissolution

It refers to the amount of heat or energy that a substance captures or releases when more water is added to the solution.

An example of dissolution enthalpy is when we add powdered detergent to the water.

See also Chemical solution.

Phase change enthalpy

It refers to the energy exchange that occurs when an element changes state (solid, liquid or gaseous). In this sense we have:

• Enthalpy of fusion: the change of enthalpy in the transition from solid to liquid Enthalpy of sublimation: the change of enthalpy in the transition from solid to gas. Evaporation enthalpy: the passage from liquid to gas.

An example of a phase change enthalpy is what happens in the water cycle, since when going from the liquid to the gaseous or solid state (or any of its possible combinations) the water releases or absorbs energy. In this case, the energy change in the transition of the water from liquid to gas at 100 ° C is equal to 40.66 KJ / mol.

See also:

• Endothermic reaction.Exothermic reaction.

What is enthalpy for?

Enthalpy is used to accurately measure the variations in energy that occur in a system, either when taking or releasing energy into the environment.

Enthalpy is a complex concept of thermodynamics that is not usually used in everyday life, since we do not calculate the energy needed to heat water for tea, for example. However, it is possible to understand how it works with an everyday example.

When we boil water, its temperature gradually rises until it reaches the boiling point (100 ° C). In this case, we are talking about negative enthalpy, since the thermodynamic system had to take energy from the environment in order to increase its temperature.

On the other hand, when we let that same water cool down a little after being boiled, its temperature begins to drop progressively without the need for external intervention. In this case, it is a positive enthalpy, since energy is being released into the environment.

Enthalpy and entropy

Entropy is a physical quantity that measures the amount of energy in a system that is not available. By calculating this magnitude it is possible to know the degree of disorder or chaos in the structure of a system.

The relationship between enthalpy and entropy is given by the equilibrium of the system. At less enthalpy (energy exchange), the system tends to equilibrium; but at the same time the entropy increases, since there is a greater possibility of chaos in the system.

For its part, a minimum entropy implies a lower level of chaos and therefore, the energy exchange (enthalpy) will be greater.