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In order for a phase change to happen enough heat or energy has to be applied to overcome the phase and then the bond. That is why when you use the second method you add the heat of fusion or vaporization to the bond enthalpy to get the enthalpy of the reaction.
Although the temperature of a sample remains the same, heat is being supplied in order to provide the energy needed for a phase change. This is why the heating curve for water is horizontal at times. In calculations, the enthalpy for the phase change must be added to the bond enthalpies in order to get the correct answer.
Eesha Sohail 1D wrote:On the heating curve, one axis is temperature, but can someone explain what exactly temperature means on a molecular level vs heat?
Heat deals with thermal energy, whereas temperature is the molecular kinetic energy (the molecules moving around in the ice/water/vapor).
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