2
$\begingroup$

The specific heat of water is lower than that of $\ce{NH3}$, but why?

$\ce{H2O}$ forms more hydrogen bonds if I am right, besides there's more electronegativity for Oxygen.

But still, $\ce{NH3}$ has higher Specific heat. I found this link saying $\ce{NH3}$ has lower molar mass, but its just by one gram so it shouldn't be having much effect right?

$\endgroup$
3
  • 3
    $\begingroup$ chemistry.stackexchange.com/questions/26651/…. May refer to the link for all clarification. $\endgroup$ Jan 14 at 8:52
  • 4
    $\begingroup$ $\ce{H20}\ $ is unknown. You may mean $\ce{H2O}$. Be aware chemists may be quite sensitive for O vs 0 replacement in chemical formulas. $\endgroup$
    – Poutnik
    Jan 14 at 9:33
  • 1
    $\begingroup$ Consider the effect of lower molar mass and of counts of vibration degrees of freedom on the specific heat capacity. $\endgroup$
    – Poutnik
    Jan 14 at 9:40
3
$\begingroup$

The link in the question states that ammonia has a higher specific heat than water. This is true, but it is important to compare the specific heat values under identical (or very similar) conditions.

  1. The isobaric specific heat of ammonia (gas) is 2.1636 kJ/kgK at 1 atmosphere and temperature = 26.9 C. https://www.engineeringtoolbox.com/ammonia-heat-capacity-specific-temperature-pressure-Cp-Cv-d_2016.html At -33.6 C (boiling point of $NH_3$ at 1 atm), the specific heat of $NH_3$ gas is 2.2946 kJ/kgK, not much different from the gas at 26.9 C; however, for liquid $NH_3$ at 1 atm and -33.6 C, the specific heat is 4.4464 kJ/kgK, a large increase from the gas at that temperature.

  2. Liquid water has one of the highest specific heats among common substances, about 4182 J/(K kg) at 20 °C https://en.wikipedia.org/wiki/Specific_heat_capacity At 100 C, liquid water has a specific heat of 4.2157 kJ/kgK (implied pressure = 1 atm). Again, not much difference because of temperature. https://www.engineeringtoolbox.com/specific-heat-capacity-water-d_660.html Now we find water vapor data in a different place: https://www.engineeringtoolbox.com/water-vapor-d_979.html where gaseous $H_2O$ at 216 F (102.2 C) has a specific heat of 1.890 kJ/kgK, a large drop, even bigger than in the ammonia system.

So although it is often stated that water has one of the highest specific heats among common substances, it seems that either ammonia is not considered common for this issue, or yes, it's true that water has one of the highest specific heats...And that is true of the gas and the liquid states!

But then among all the data, which are truly interesting, there is one other thing that stands out: the heat of vaporization of water is 40.8 kJ/mol, whereas for ammonia, it is only 23.5 kJ/mol. So water can still be considered an extreme molecule.

$\endgroup$
2
  • 3
    $\begingroup$ I believe you are comparing the specific heat of gaseous ammonia to that of liquid water. Presumably the linked page was referring to the difference in specific heat when both are in the same state. Liquid ammonia has a higher specific heat than liquid H2O, and gaseous ammonia has a higher specific heat than gaseous H2O. The same trend is observed with specific heat of PH3 vs SH2. $\endgroup$
    – Andrew
    Jan 15 at 13:27
  • $\begingroup$ @Andrew: Thank you for your comment! You are quite correct, and I will seriously edit my answer because it is currently a non-answer. $\endgroup$ Jan 15 at 19:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.