Making Water From Hydrogen and Oxygen


The chemical reaction between oxygen and hydrogen makes water.
The chemical reaction between oxygen and hydrogen makes water. (Anne Helmenstine)

Making water from hydrogen and oxygen is as simple as mixing hydrogen gas and oxygen gas and adding a spark or heat. The balanced equation for the chemical reaction is:

2 H2 + O2 → 2 H2O

This is a synthesis reaction that makes water from its elements. It’s also a combustion reaction. Burning hydrogen to make water produces a vivid red flame and a loud sound.

How Hydrogen and Oxygen Make Water

Simply mixing hydrogen and oxygen together won’t make water. Hydrogen and oxygen both exist as diatomic gases. A reaction between them requires energy to break the bonds between the atoms so they can form a new product. Once the bonds break, each hydrogen atom has a +1 positive charge, while each oxygen atom has a -2 negative charge. Two hydrogen atoms bound to one oxygen atom yield water, which is electrically neutral. An electrical spark works to initiate the reaction, as does heat. But, once the reaction starts, it’s highly exothermic and goes to completion.

Simple Demonstrations to Make Water

It’s easy to demonstrate making water from hydrogen and oxygen. The key is to keep the scale of the reaction small. Otherwise, too much heat is produced.

One method is to bubble hydrogen up through soapy water to form hydrogen soap bubbles. These bubbles float because they are lighter than air. Use a long-handled lighter or burning splint attached to a meter stick to ignite the bubbles. Get hydrogen either from a compressed gas container or via a chemical reaction.

Another method is filling a small balloon with hydrogen and touching the balloon with a burning splint attached to a meter stick. The hydrogen in the balloon reacts with oxygen in air. You can fill a balloon with both hydrogen and oxygen and ignite it, but only behind a safety shield and using hearing protection.

See the difference between using pure hydrogen and hydrogen with oxygen. Note the red color of the reaction (at 1:50).

Drinking Water and Fuel Cells

According to a 2006 UN Water Development Report, about one in five people don’t have access to clean drinking water. If water is so easy to make, why don’t we use it as a freshwater supply? There are two reasons. First, it’s dangerous to combine large amounts of hydrogen and oxygen. The Hindenburg accident is an example of the result. The other reason is that it’s not economically practical or environmentally sound. It takes more energy to produce hydrogen to make water than it does to get water from other sources.

However, the reaction between hydrogen and oxygen finds practical application in fuel cells. In a fuel cell, hydrogen (or another fuel) reacts with oxygen (or another oxidizer), producing electricity and heat. Fuel cells use catalysts to lower the activation energy of the reaction, so it’s easier to initiate. Nickel is a common catalyst, while water is the most common “waste” product. Hydrogen fuel cells are used for backup power generation, powering spacecraft and remote facilities, and in hydrogen cars.

Why Hydrogen and Oxygen Make Water Instead of Hydrogen Peroxide

Water isn’t the only common chemical made from hydrogen and oxygen. You might wonder why hydrogen and oxygen make water (H2O) instead of hydrogen peroxide (H2O2). The simplest explanation is that it is much more favorable for two hydrogen atoms to react with one oxygen atom than it is to add another oxygen into the mix. Even though oxygen gas is O2, the bond between the atom has to break for oxygen to form bonds with hydrogen to make water. Remember that while the usual oxidation state of oxygen is -2, it actually does display other states. Sometimes hydrogen and oxygen do form hydrogen peroxide, but the molecule is inherently unstable and eventually decomposes into water and oxygen.

Lavoisier Makes Water

Scientists knew oxygen and hydrogen made water long before they understood the molecular basis for chemical reactions. French chemist Antoine Lavoisier even named the element hydrogen for the reaction. Hydrogen’s name come from Greek words that mean “water-forming.” Lavoisier discovered the role played by oxygen in combustion, eventually using the reaction between hydrogen and oxygen to demonstrate conservation of mass for combustion reactions and disprove the phlogiston theory.

References

  • Grove, William Robert (1839). “On Voltaic Series and the Combination of Gases by Platinum”. Philosophical Magazine and Journal of Science. XIV (86–87): 127–130. doi:10.1080/14786443908649684
  • Hauch, Anne; Ebbesen, Sune Dalgaard; et al. (2008). “Highly efficient high temperature electrolysis”. Journal of Materials Chemistry. 18 (20): 2331. doi:10.1039/b718822f
  • Khurmi, R. S. (2014). Material Science. S. Chand & Company.
  • Schmidt-Rohr, K. (2015). “Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2“. J. Chem. Educ. 92: 2094–2099. doi:10.1021/acs.jchemed.5b00333

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