bonus points if you can explain why water has no caloric value, but hydrogen can power huge machinery.
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Mixing hydrogen and oxygen does not in itself produce water. Reacting hydrogen with oxygen does, that is, hydrogen makes water when burned with oxygen. This reaction naturally also produces a lot of heat and can be violent or even explosive. Water has no caloric value because it does not burn any further. All the energy has already been released by burning hydrogen.
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You can think of water as the “ash” left over from burning hydrogen. It’s gotten all the energy out of its system and it’s incredibly stable; it takes that much energy and more to rip it back apart.
Similarly, sodium is a fairly reactive alkali metal, and chlorine gas will tear your lungs up good. But you put salt (sodium chloride) on your food like it’s no big deal.
Water is stable for exactly the same reason that hydrogen and oxygen mixed together (which is *not* the same thing) aren’t.
In a water molecule, the bond between hydrogen and oxygen is very strong. That means it takes a lot of energy to break it. But since chemical processes are reversible, that also means it *releases* a lot of energy to *form* it. This isn’t quite an exact analogy, but you can think of it as being sort of like putting two magnets on a smooth table: they’ll slide together and collide with considerable force, and then it *takes* a lot of energy to pull them back apart.
That released energy is what makes hydrogen a good fuel. But in water, the bonds are already formed, so you’re not able to release extra energy by forming what’s already there.
Water has no caloric value for the same reason. Your body extracts energy from foods by reacting them with oxygen and capturing the energy released by the formation of strong carbon-oxygen and hydrogen-oxygen bonds. In water, those bonds are already formed, so there’s no energy to extract.
Coal and wood are pretty good fuel sources too, no? You can light them on fire and do things like power furnaces, steam engines, ovens, etc.
When the coal is burnt, it leaves behind some things. Ashes, carbon dioxide, and water vapor. Are any of *those* things good fuel sources? No, of course not, they’re already burnt. Spent. Used up.
Hydrogen and oxygen, in their pure forms, may be very reactive molecules, especially in the presence of each other. But after they combine into water, that’s it. They are no longer in their pure forms. They’re spent. Used up.
Water is effectively the “ash” of a reaction between oxygen and hydrogen.
Yeah – the basic answer is that the hydrogen and oxygen have already been used up when they’ve become water, so the combustion and energy release already happened and this is what’s left.
Remember that this is all about chemical energy, and chemistry is about electrons, not nuclei. The properties that hydrogen and oxygen have when we speak in general are the ones that come from their atomic or molecular forms; H2 or O2, if not just H and O. A water molecule, though, isn’t a mere combination of those two things, it’s a transformation of them into something new, a bonded set of electrons and protons and neutrons which is far, far more stable.
Hydrogen ions can be used to power engines. Hydrogen fuel cells split hydrogen into hydrogen ions and electrons, the electrons are then used for power and the hydrogen ions, electrons and oxygen are then combined to make water. https://youtu.be/gh95X3Qb6zo
Hydrogen can also be fused together in stars releasing nuclear fusion energy.
Hydrogen’s potential for high energy release is due it’s single electron in a shell that needs two. Oxygen’s facilitation of combustion is due to having two vacancies in its outer shell of electrons to make up the eight needed for stability.
Two hydrogens and an oxygen share their electrons in such a way that their requirements are fulfilled and no further energy can be released by combining with different atoms. They have formed a more stable molecule than the individual molecules of hydrogen and oxygen gas.
Do not confuse the properties of compound with those of the elements from which it is made, they are inevitably different else the compound would not form.
For the same reason that you can combine a deadly hundred-foot sinkhole with a treacherous 100-foot tower of stone to create flat ground that’s safe to walk on.
Even though the two have a lot of dangerous potential energy alone, together they are quite secure and relatively inert.
Hydrogen and oxygen are each molecules of high available energy, they “want” to react with other molecules to get to a lower energy level. Water is a very low energy molecule and thus the result of most chemical reactions involving hydrogen and oxygen. The low energy level is what makes it so stable. We drink water because it is useful in many chemical reactions that our body must perform to sustain itself.
Now water does not power any machinery, it is the mechanism that allows the machinery to channel power though. Like a hammer it only allows you to nail something, it does not do the work. Water is useful because it’s cheap and available, and when you boil water with a fire you can use the high pressure steam to perform work, the fire is the power source and the water is just the tool
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