How do you calculate how much energy is needed to break a water molecule into its constituent atoms?
note
This is not homework. I just want to know what is required to get hydrogen out of water.
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Sign up to join this communityHow do you calculate how much energy is needed to break a water molecule into its constituent atoms?
note
This is not homework. I just want to know what is required to get hydrogen out of water.
The answer is given by the standard enthalpy of formation, $\Delta H_\mathrm f^\circ$ for water:
$$\begin{alignat}{2} \ce{H2 + 1/2O2 &-> H2O(l)}\quad&\Delta H_\mathrm f^\circ&=-285.8\ \mathrm{kJ/mol}\\[6pt] \ce{H2 + 1/2O2 &-> H2O(g)}\quad&\Delta H_\mathrm f^\circ&=-241.8\ \mathrm{kJ/mol} \end{alignat}$$
The energy required for the following reaction, your point of interest: $\ce{H2O -> H2 + 1/2O2}$ is simply a positive value for either one of the above $\Delta H_\mathrm f^\circ$ values depending on the state of the reactant. It means that per mole water splitting requires at least $241.8\ \mathrm{kJ}$ of energy.
Any method(thermal,electrolysis etc) followed, this much of energy will be always required. When using electrolysis, it may be convenient to express it in $\mathrm{eV}$.
Regarding calculation, I have to mention that $\Delta H_\mathrm f^\circ$s are experimental values. To know how they are calculated, here is a nice example of $\ce{NaCl}$.