Cp and ∆H Estimates Reference Page
Feeling the Heat? How to Get Your Best Estimate

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Intro to Feeling the Heat? How to Get Your Best Estimate

In many thermodynamic problems, accurate property data may not be available for the exact substance or temperature range needed. In these situations, engineers often rely on estimation methods to approximate heat capacities and enthalpy changes.

These methods can provide surprisingly useful results with very little information. This video introduces several commonly used estimation techniques, including Kopp’s rule, Trouton’s rule, and correlations for estimating heats of vaporization.

Feeling the Heat? How to Get Your Best Estimate

This video explains several useful methods for estimating heat capacities and enthalpy changes when detailed thermodynamic data are not available.

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Examples and Definitions

Definitions

Kopp’s Rule
A rule for estimating the heat capacity of a compound by summing the approximate contributions from the atoms present in the molecule.
Sensible Heat Changes
Temperature changes that occur when heat is added to or removed from a substance without a phase change.
Latent Heat
Heat absorbed or released by a substance during a phase change at constant temperature.
Trouton’s Rule
An empirical rule used to estimate the enthalpy of vaporization of many liquids from their normal boiling temperature and the enthalpy of fusion of many solids from their normal melting temperature.
Clausius-Clapeyron Equation
An equation relating vapor pressure to temperature and the enthalpy of vaporization:

\[ \ln p^* = -\frac{\Delta H_v}{RT} + B \]

Watson Correlation
A correlation used to estimate the enthalpy of vaporization at one temperature from its value at another temperature using the critical temperature. A related expression is the Shimansky equation.