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Hess's Law

Hess's Law

There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally determined enthalpy changes. Some reactions are difficult, if not impossible, to investigate and make accurate measurements for experimentally. And even when a reaction is not hard to perform or measure, it is convenient to be able to determine the heat involved in a reaction without having to perform an experiment.

Reduction of Alkenes: Catalytic Hydrogenation

Reduction of Alkenes: Catalytic Hydrogenation

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the...

Nuclear Fusion

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process called fusion. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and ultimately produce one helium nucleus and two positrons.
A helium nucleus has a mass that is 0.7% less than that of four hydrogen nuclei; this lost mass is converted into energy during the fusion. This reaction produces about...

Hydrogen Bonds

Hydrogen Bonds

Hydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, and has a partial negative charge. The other is a hydrogen atom that has bonded with another electronegative atom and has a partial positive charge.
Hydrogen Bonds Control the World!
Because hydrogen has very weak electronegativity when it binds with a strongly electronegative atom, such as oxygen or nitrogen, electrons in the bond are unequally shared....

Free Energy Changes for Nonstandard States

Free Energy Changes for Nonstandard States

The free energy change for a process taking place with reactants and products present under nonstandard conditions (pressures other than 1 bar; concentrations other than 1 M) is related to the standard free energy change according to this equation:

where R is the gas constant (8.314 J/K·mol), T is the absolute temperature in kelvin, and Q is the reaction quotient. This equation may be used to predict the spontaneity of a process under any given set of conditions.
Reaction Quotient...

Limiting Reactant

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in reality, the reactants are not always present in the stoichiometric amounts indicated by the balanced equation.

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Related Experiment Video

Updated: Aug 29, 2025

Hydrogen Production and Utilization in a Membrane Reactor

Hydrogen Production and Utilization in a Membrane Reactor

Published on: March 10, 2023

Hydrogen production from the air.

Jining Guo¹, Yuecheng Zhang¹, Ali Zavabeti¹

¹Department of Chemical Engineering, The University of Melbourne, Parkville, Vic, 3010, Australia.

Nature Communications

|September 6, 2022

Summary

This summary is machine-generated.

Direct air electrolysis produces green hydrogen by capturing atmospheric water. This sustainable method overcomes freshwater scarcity, offering a scalable solution for low-carbon energy production.

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A Simple, Low-cost, and Robust System to Measure the Volume of Hydrogen Evolved by Chemical Reactions with Aqueous Solutions

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Published on: August 17, 2016

Area of Science:

Sustainable Energy
Electrochemistry
Environmental Science

Background:

Green hydrogen is crucial for a low-carbon economy.
Freshwater scarcity challenges traditional hydrogen production.
Geographic mismatch exists between renewable energy sources and water availability.

Purpose of the Study:

To demonstrate a novel method for direct hydrogen production from atmospheric water.
To overcome the limitations of freshwater dependency in green hydrogen generation.
To develop a sustainable and scalable hydrogen production technology.

Main Methods:

Utilizing a hygroscopic electrolyte for in situ water capture from the air.
Employing electrolysis powered by renewable energy (solar/wind).
Operating a direct air electrolysis (DAE) module prototype.

Main Results:

Achieved high current density up to 574 mA cm⁻².
Demonstrated stable performance over 12 days with ~95% Faradaic efficiency.
Successfully operated the DAE module in a low humidity environment (4% RH).

Conclusions:

Direct air electrolysis offers a sustainable solution for green hydrogen production.
This technology bypasses freshwater limitations, enabling production in arid regions.
The scalable DAE modules can supply hydrogen to remote and scattered areas.