Question

how much electricity we have to pass from the water to release hydrogen and oxygen

Answer 1

We can combine hydrogen and oxygen to make water. And we can even get energy out of the process that we might be able to use to light a home or drive a car. But, of course, there are some details that make it a bit challenging
First of all, if we simply mix a container of oxygen gas with a container of hydrogen gas, at room temperature and pressure, then nothing (or not much) will happen. The molecules of oxygen and hydrogen would be “happier” to join together to form water, but they need a little kick before that reaction will happen. Using an imperfect analogy, you can think of a hydrogen molecule as two magnets joined together, and the oxygen molecule as two differently shaped magnets joined together. A more tightly packed magnet can be formed by joining the hydrogen and oxygen magnets into a water magnet, but unless they bang into each other fast enough this won’t happen. Chemists say that an activation energy is needed for the reaction to occur.

We can supply this activation energy with a spark. But then, as you might guess, kaboom! If the magnets rearrange themselves into the more tightly packed water molecule, there is now energy left over that makes the water (vapor, actually) hot. Based on my calculations, if we combine a typical canister of hydrogen at 2,500 pounds per square inch pressure with a similar oxygen canister, the resulting water vapor will heat up by several thousand degrees Fahrenheit. This is why NASA uses liquid hydrogen and oxygen in the big, orange, external fuel tank that helps the space shuttle get into orbit.

So, we can mix oxygen and hydrogen gas to make hot water vapor, but that probably wasn’t the intent of this question. Can we mix them to make water without an explosion? Yes, if we use what is called a hydrogen fuel cell. A hydrogen fuel cell is kind of like a battery that needs a continuous input of hydrogen and oxygen gas to make electrical energy, with water as the only output. The Toyota Mirai, for example, is one of the first cars that you can buy that is powered by a hydrogen fuel cell. The fuel cell doesn’t go kaboom (hopefully!) because it is designed to extract electrical energy from the reaction that can be used to drive a circuit and only gives off about one-fifth as much heat. The fuel cell contains other components to do this that I don’t have the space to discuss.

First of all, if we simply mix a container of oxygen gas with a container of hydrogen gas, at room temperature and pressure, then nothing (or not much) will happen. The molecules of oxygen and hydrogen would be “happier” to join together to form water, but they need a little kick before that reaction will happen. Using an imperfect analogy, you can think of a hydrogen molecule as two magnets joined together, and the oxygen molecule as two differently shaped magnets joined together. A more tightly packed magnet can be formed by joining the hydrogen and oxygen magnets into a water magnet, but unless they bang into each other fast enough this won’t happen. Chemists say that an activation energy is needed for the reaction to occur.

We can supply this activation energy with a spark. But then, as you might guess, kaboom! If the magnets rearrange themselves into the more tightly packed water molecule, there is now energy left over that makes the water (vapor, actually) hot. Based on my calculations, if we combine a typical canister of hydrogen at 2,500 pounds per square inch pressure with a similar oxygen canister, the resulting water vapor will heat up by several thousand degrees Fahrenheit. This is why NASA uses liquid hydrogen and oxygen in the big, orange, external fuel tank that helps the space shuttle get into orbit.

So, we can mix oxygen and hydrogen gas to make hot water vapor, but that probably wasn’t the intent of this question. Can we mix them to make water without an explosion? Yes, if we use what is called a hydrogen fuel cell. A hydrogen fuel cell is kind of like a battery that needs a continuous input of hydrogen and oxygen gas to make electrical energy, with water as the only output. The Toyota Mirai, for example, is one of the first cars that you can buy that is powered by a hydrogen fuel cell. The fuel cell doesn’t go kaboom (hopefully!) because it is designed to extract electrical energy from the reaction that can be used to drive a circuit and only gives off about one-fifth as much heat. The fuel cell contains other components to do this that I don’t have the space to discuss.