Water has an unusually high specific heat capacity due to it's hydrogen bonds.
Why is it that the change in isotope causes a 10% difference in the heat capacity? How does the added neutron in the nucleus make any difference to the bonds?
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$\begingroup$ Googling 'heat capacity of heavy water' yields various scholarly reports dating back to the 1930s. $\endgroup$– Jon CusterNov 20, 2015 at 23:11
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$\begingroup$ True, but the ones I've looked at are extremely long with lots of equations I can't make sense of, so an understandable answer would be nicer if possible... Should I edit the question? $\endgroup$– TethysNov 20, 2015 at 23:19
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$\begingroup$ The Wikipedia page (3rd hit for me from Google) on 'Orders_of_magnitude_(specific_heat_capacity)' lists Water as 4186 J/kg/K and Heavy water as 3767 J/kg/K - so it makes about a 10% difference. $\endgroup$– Jon CusterNov 20, 2015 at 23:40
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9$\begingroup$ The added mass of a deuteron relative to a proton reduces the O-D stretch vibration frequency relative to O-H. This reduces the energy that can go in to that degree of freedom, reducing the heat capacity. $\endgroup$– Jon CusterNov 20, 2015 at 23:57
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5$\begingroup$ Specific heat capacity // H2O: 74.539 J mol-1 K-1 // D2O: 84.42 J mol-1 K-1 // www1.lsbu.ac.uk/water/water_properties.html // So "heavy water" takes more energy to heat than "normal" water. $\endgroup$– MaxWNov 21, 2015 at 1:14
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1 Answer
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I posted an answer here the other day which was completely wrong! I apologise and have left the incorrect text below for posterity to record how wrong people who should know better can be!
Looking on the NIST fluid properties database I see that the specific heat capacity (i.e. the heat capacity per unit mass) of D2O is just 1.5% higher than the specific heat capacity of H2O. So at 298 K this makes the molar heat capacity of D2O (84.963 J/K/mol) an astonishing 12.7% higher than H2O (75.38 J/K/mol). I am astounded and have no explanation.
Everything below this line is WRONG!
The molar heat capacity is almost exactly the same. i.e. it takes the same energy to heat the same number of molecules by one degree Celsius. The difference in the *specific* heat is just because of the difference in density. D2O molecules have a relative molar mass of about 20 compared with 18 for H2O. Since the bond lengths are similar, the density is higher by a factor 20/18 - about 11% higher. So a given mass of D2O has about 11% fewer molecules in it.
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$\begingroup$ According to this page www1.lsbu.ac.uk/water/water_properties.html the molar heat capacity of D2O is more than 10% higher than H2O. Are these values wrong then? $\endgroup$– diogomFeb 2, 2018 at 20:05
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1$\begingroup$ I will check with some independent sources and get back to you. The 'internet' seems to have specific heat capacities i.e. per unit mass that are within 1.5% of the light water value (e.g. hedhme.com/content_map/?link_id=22146) . That is the level of change I would have expected for the molar heat capacity. It seems suspicious to me that the molar heat capacity should change by almost the factor as the density. As it happends, this answer really matters to me (!) but I think I need more reliable sources than are openly available on line. $\endgroup$ Feb 3, 2018 at 21:45
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$\begingroup$ Hi. I was completely wrong. I apologise. I have changed the original post to reflect that. $\endgroup$ Feb 4, 2018 at 17:53