enthalpy

History

The term 'enthalpy' was proposed by the Dutch physicist Heike Kamerlingh Onnes in 1909, but the idea of latent heat and heat content was developed earlier by Rudolf Clausius in the mid-19th century.

How it works

In an ordinary chemical reaction in air, pressure doesn't change, so the released heat equals the change in enthalpy. Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O releases 890 kJ/mol — that's the reaction enthalpy. In our bodies, the 'burning' of glucose with oxygen is also measured by enthalpy.

History

The term 'enthalpy' was proposed by the Dutch physicist Heike Kamerlingh Onnes in 1909, but the idea of latent heat and heat content was developed earlier by Rudolf Clausius in the mid-19th century.

How it works

In an ordinary chemical reaction in air, pressure doesn't change, so the released heat equals the change in enthalpy. Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O releases 890 kJ/mol — that's the reaction enthalpy. In our bodies, the 'burning' of glucose with oxygen is also measured by enthalpy.

History

The roots of the concept go back to the work of Hermann Helmholtz and Rudolf Clausius (1850s), who called it 'heat content'. In 1909, Heike Kamerlingh Onnes introduced the modern name from the Greek 'to heat'. Later, with the development of statistical physics, enthalpy received a microscopic foundation as the average energy of a system at fixed pressure and temperature.

How it works

From the first law of thermodynamics: dU = δQ - P dV. At constant pressure, dH = dU + P dV = δQ. Therefore, the heat effect of a reaction at P=const equals ΔH. The second law of thermodynamics, through the Gibbs free energy G = H - TS, determines the direction of spontaneous processes. In engineering, the enthalpy of steam or refrigerant allows calculation of cycle efficiencies.

Open problems & fun facts

Accurate quantum-chemical calculation of enthalpy of complex molecules at high temperatures.; Prediction of enthalpy of substances in extreme states (planetary interiors, stellar envelopes).; Relationship of enthalpy with microscopic fluctuations in nanoscale systems.

💡 The most powerful known chemical reaction — combustion of dicyanoacetylene in fluorine — releases about 10,000 kJ per gram, heating the flame to 5000 °C. The enthalpy of such a flame is comparable to the surface of a small star.