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Q & A: deuterium isotope effects and binding

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Most recent answer: 11/23/2012
why hydrogen bönd in D2O is stronger then H2O?
- heena (age 12)
kohat pakistan
The reason is, like most chemistry, quantum mechanical. The D (deuterium) has the same charge but about twice the mass (m) as the H (hydrogen). Classically, you might think that the H or D just sits at the position of lowest electrical potential energy, like a little ball at the bottom of a cup. It wouldn't have any kinetic energy. So you'd end up with the same binding energy.

However, actual particles are represented by quantum waves, with a spread in position and velocity. Imagine that you have a given wave function representing the position of the D or H. Quantum rules say that wave form determines the spread in momentum, p. The kinetic energy, p2/2m, is larger for the H than for the D because of the mass difference. That means that the D is more tightly bound. In the full quantum solution, the wave function itself will be a little different for H and for D. The difference shows up as a more negative potential energy of the D, with the D kinetic energy actually also up a bit, but still with reduced total energy for the D.

Sorry if this sounds weird, but some day you'll learn some quantum mechanics and this part at least will make sense.

Mike W.

(published on 11/23/2012)

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