Why can’t we just harvest the oxygen in H20 and breathe under water
In: Chemistry
When you say “we” do you mean with the aid of technology, or just our bodies?
Modern submarines do generate oxygen from water, allowing them to stay underwater for many days.
The human body, like all mammals and other land animals, have evolved to breathe air and don’t have gills to get oxygen from water.
“The electrolysis of water in standard conditions requires a theoretical minimum of 237 kJ of electrical energy input to dissociate each mole of water, which is the standard Gibbs free energy of formation of water. It also requires thermal energy to balance the change in entropy of the reaction.”
So, 237kJ minimum to get 32g of oxygen.
Apparently, humans use about 750g per day, so:
750g ÷ 32g × 237kJ = 5555kJ … which is about 1300 kcal. Presumably we use less getting it from the air.
It’s really energy intensive to split up water into oxygen and hydrogen, and it’s a completely different process from how things breathe. Even fish, they actually just suck up oxygen that’s dissolved in the water. But they don’t break up the water into hydrogen and oxygen.
That would require a very specific way of breathing; which would, paradoxically, leave you breathless. So you’d drown anyway!
We need oxygen so we can react it with sugar to make CO2 (notice that also has oxygen in it). I’m having trouble finding exact numbers, but from what I can see it takes more or less the same energy to break water into oxygen and hydrogen as you get from combining it with sugar to make CO2. The stability of these two molecules is why hydrogen gas burns just like sugar
The answer about submarines is the right and correct answer.
This question made me wonder just how much energy is needed to do this.
Google says that at rest you consume about 250ml of oxygen per minute. Using the quora link below, it takes about 2.6KWh of energy per minute to generate the oxygen your body needs at rest.
A fully charged Tesla Model 3 battery pack can electrolyze enough oxygen to keep you alive for a bit less than 30 minutes so a portable underwater electrolysis machine isn’t happening any time soon.
[https://www.quora.com/How-much-kWh-is-needed-to-split-one-liter-clear-water-into-hydrogen-oxygen](https://www.quora.com/How-much-kWh-is-needed-to-split-one-liter-clear-water-into-hydrogen-oxygen)
I’m going to first assume you mean splitting the water molecule:
Chemicals react one of two ways: by using up energy (endothermic), or releasing stored energy (exothermic).
Creating water from hydrogen and oxygen is an exothermic reaction – fusing oxygen and hydrogen **creates water** but also releases energy, in the form of heat.
In general, reversing an exothermic reaction is endothermic: you need to add heat BACK INTO the system for the reaction to go the other way.
What that means is to **split water** up into hydrogen and oxygen, you need energy, and a lot of it. It’s an amount of energy that would be impossible to reliably supply underwater, and would be nowhere near as efficient as filling a tank full of oxygen.
If you don’t mean splitting the water molecule, but you do mean extracting the dissolved oxygen in water (this is how fish breathe, they do not split molecules of h2o) the same answer as above pretty much still applies: the chemistry to do it efficiently would never outpace the reliability and efficiency of filling a tank with oxygen.
We need more oxygen than fish and water doesn’t have that much dissolved oxygen in it. You would need to reliably extract the dissolved oxygen from tens of liters of water PER MINUTE to stay alive down there. Or you can get a massive metal tank and fill it with oxygen that can be made comfortably on land.
Oxygen can be deadly below 20ft due to oxygen toxicity. Even if we were able to get over the hurdles the other comments have laid out, we would still need to mix the output with nitrogen or another inert gas to safely breathe below 20ft.
Source: SCUBA instructor
We can and do. That is how the people in a nuclear submarine breath.
Takes a large amount of energy, not a problem if you have a nuclear reactor.
Aside from the things others have said, our breathable atmosphere is made up of only about 20% oxygen, others is a mix of mostly hydrogen, carbon dioxide and water.
Even if you could separate oxygen from water, it would still not result in a pleasant breathable environment.
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