When you energize water, when does it melt or evapourate, and when does it break the bond between hydrogen and oxygen and they are separated? I know electrolysis causes decomposition and heating it causes it to boil or melt, but how does that differ as it in both ways becomes energized, does the amount of energy given to it affects anything? If not, what does?
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1$\begingroup$ How do you energize water? $\endgroup$– Kyle KanosCommented Sep 29, 2015 at 10:25
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$\begingroup$ I mean by giving it energy like heat energy or electrical energy $\endgroup$– user3407319Commented Sep 29, 2015 at 10:49
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$\begingroup$ Go read about latent heat of fusion, latent heat of vaporization, specific heat capacity, and electrolysis. There are plenty of general resources. A good general physics book will talk about these things. $\endgroup$– Bill NCommented Sep 29, 2015 at 15:37
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1$\begingroup$ I'm voting to close this question as off-topic because OP hasn't indicated any previous research of phase transitions of water. $\endgroup$– Bill NCommented Sep 29, 2015 at 15:38
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1 Answer
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When you heat water you increase the kinetic energy of the water molecules. As an order of magnitude estimate, the kinetic energy at a temperature $t$ is about $kT$, where $k$ is Boltzmann's constant.
So when the molecules in water (or ice or steam) interact with each other the energy available is roughly $kT$. At the temperature of boiling water this energy is about 0.03eV. By contrast the energy required to break water into $H$ and $OH$ is about 5ev (424.4 kJ/mol). So at 100ºC the thermal energy available is nowhere near enough to split the water molecule. Heating water is just going to evaporate it, not decompose it.
Electrolysis procedes by a completely different mechanism. It is a type of reaction called a redox reaction. This transfers an electron to or from a molecule with the result that the molecule splits up.
At the risk of oversimplifying, in electrolysis we add energy to individual water molecules one at a time and we don't need much energy to decompose a single molecule. When we heat water we add the energy to all the $10^{23}$ or so water molecules simultaneously so no single molecule gets enough energy to dissociate.
answered Sep 29, 2015 at 10:53
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$\begingroup$ Another question, when we burn hydrocarbons, we must break the bond between hydrogen and carbon to make it react with oxygen, since that requires too much energy why doesn't the fuel evaporate instead of breaking the atom and burning? $\endgroup$ Commented Sep 29, 2015 at 12:03
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$\begingroup$ You are quite correct that you need to put in energy to make a hydrocarbon molecule react with oxygen. This energy is generically known as activation energy. If you heat fuel and air together gently the fuel will evaporate and you get a fuel vapour/air mixture instead of combustion. If you generate a locally high temperature, e.g. using a spark plug, then the mixture will burn (explosively!). The temperature at which this happens is known as the auto-ignition temperature. $\endgroup$ Commented Sep 29, 2015 at 14:21