Question

Maybe I'm wrong about this, but I thought I remembered from high school chemistry that all reactions are in equilibrium. Some equilibria are extremely far to the right or left, so they appear to react 100% or not at all, but even in those reactions there is a tiny amount on the unfavored side.

If that's not true, then the answer to this question will be short. But if it is true, then I have a question about the following reaction, and bodies of water like lakes and oceans.

2H2O (liquid)↽−−⇀2H2(g)+O2(g)

If I remember right, the energy change (ΔH or ΔG) in that reaction strongly favors the left side, so water molecules are not breaking apart into gas in large numbers. However, it's a gas on the right side, so I imagine that if a water molecule at the surface of the ocean breaks into gas, those products will escape, and the H2 and O2
gas molecules are no longer in physical contact and have no chance to react again.

That makes me think that gradually the ocean would turn into hydrogen and oxygen gas, even if the left side of that equation were favored. That doesn't seem to be happening, so what am I missing? I don't normally think there is hydrogen gas floating around in the air, but perhaps there actually is enough that some of it is colliding with O2 at the ocean surface and balancing the equilibrium.

Answer 1

All chemical reactions can theoretically reach equilibrium, but the equilibrium for the decomposition of water into hydrogen and oxygen strongly favors the liquid water side.

Step-by-step explanation:

You are correct that in principle all chemical reactions can reach a state of equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction. However, for the reaction 2H2O (ℓ) ⇌ 2H2(g) + O2(g), the equilibrium strongly favors the left side, meaning the formation of hydrogen and oxygen gases from liquid water is not favored under normal conditions.

The reaction for the formation of water from hydrogen and oxygen, 2 H2(g) + O2(g) → 2 H2O(ℓ), has a very large equilibrium constant, indicating that the reaction nearly goes to completion and the reverse reaction, the decomposition of water, is not appreciable under normal conditions.

Furthermore, although hydrogen and oxygen gases are produced when water decomposes, they generally do not escape the atmosphere to a significant degree because the rate of such decomposition is extremely low at room temperature and normal atmospheric pressure.

Additionally, there is a high activation energy barrier for the reverse reaction that prevents the reformation of water from these gases under standard conditions, without an external energy source like heat or a spark. Hence, while there might be tiny amounts of hydrogen and oxygen produced, they do not accumulate because the reaction rate is exceedingly slow, and the atmosphere does not provide the necessary conditions for their recombination into water.