Does liquid iron have a constant specific heat capacity? Everywhere I've looked (save one sourceless online reference to $c_p = 611\ \mathrm{J\ kg^{–1}\ K^{–1}}$ here), I find no value listed for liquid iron. Is this because the heat capacity is temperature-dependent? Or is this related to a measurement problem? Or does an accepted constant value exist, but I'm just a poor researcher?
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2 Answers
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Okay, let's answer bit by bit:
Measurement of heat capacity of liquids: this isn't particularly hard to do, and in fact it's much easier to obtain $C_p$ experimentally than $C_V$… however, once you know one, the other can be determined from it and some other thermodynamic properties. Different types of calorimeters are used for liquids, but the best-known is probably the Callender and Barnes apparatus.
“Does an accepted value exist?” and “Is the specific heat of liquid iron temperature-dependent?” — Well, I found a good paper which includes measurements and comparison with previously published values: Thermophysical Properties of Liquid Iron, M. Beutl, G. Pottlacher and H. Jäger, Intl. J. Thermophys., 1994, 15, 1323.
which confirms that considering that heat capacity is roughly constant in the 1800–4000 K range. The authors thus extract the constant pressure specific heat capacity as 825 J/kg/K, apparently in reasonable agreement with previous work that they cite:
- “Or am I a poor researcher?” — Well, the above paper is the first result return by Google for a search for "specific heat" and "liquid iron", so I'll let you be the judge of that ;-)
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$\begingroup$ Thanks. In regard to the third bullet, I don't have access to the article since I'm not affiliated with any subscribing institution. But hey, I finagled my way into the answer, so that's worth something. $\endgroup$– ExcellllMay 23, 2012 at 21:23
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2$\begingroup$ But then, if F'x googles frequently for thermodynamic properties and Excellll doesn't, the results the two of you get up top may be different... $\endgroup$ May 24, 2012 at 16:59
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Heat capacities are usually dependent on temperature except for ideal gases. You will generally find the values tabulated at specific temperature and pressure values. For instances for which one is interested in heat capacity as a function of temperature, this is usually approximated through a regression equation. The NIST JANAF Themochemical Tables use the Shomate equation with 5 coefficients, of the form:
Cp° = A + B t + C t2 + D t3 + E / t2
where t is T (in K) / 1000, and A, B, C, D and E are regression coefficients.
If one looks at the Condensed phase thermochemistry data page for iron, one can find the relevant coefficients (A, B, C, D and E in the above equation) and thus calculate heat capacity of liquid iron in the 1809 - 3133 K range. Since all coefficients except A are close to zero, for liquid iron heat capacity is nearly constant in this range.
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