Summary of heat/enthalpy changes for single-component systems A single-component (one-substance) system responds to an input/output of energy in the form of heat in one of two ways depending on how many phases are present. 1. If a single phase is present, e.g. liquid water, then the sample or system responds to the energy transfer by changing temperature according to: T C q ∆ = where C is the heat capacity in J/K which is assumed constant or independent of temperature. This equation also has two variant forms if we use the intensive quantities: specific heat, s, J/gºC, or the molar heat capacity, C , J/mol*K. T C n q T ms q ∆ = ∆ = where m is the mass in grams, n are the number of mols. 2. If two phases are present and in equilibrium at the appropriate phase transition temperature, then the system responds not by changing temperature, which indeed remains fixed, but by changing phase. For example, consider the phase transition of melting: We can write the “reaction” as follows:
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