Ch400Ch8LN2 - Finding Enthalpy Changes for Chemical...

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Finding Enthalpy Changes for Chemical Reactions and Physical Changes Enthalpy Changes for Physical Changes of State There are 6 changes of state that a chemical may undergo: Vaporization Condensation Sublimation Melting or Fusing Freezing Deposition There is a Δ H associated with all of these phase changes. Δ H vap is the enthalpy change associated with the vaporization process or the heat of vaporization; Δ H f is the heat of fusion; and Δ H sub is the heat of sublimation. As condensation if the reverse of vaporization, we don’t have a special term for the heat of condensation, it’s just – Δ H vap Also, when we sublime something, it goes from the solid state to the gas state directly. But as Δ H is independent of path (it’s a State Function), we could first melt the solid and then vaporize it to the gas. The Δ H value for the 2 paths would be the same, so long as the temperature was held constant. So for constant T, Δ H sub = Δ H vap + Δ H f 3 of the above processes are exothermic and 3 are endothermic. Which is which? Enthalpy Changes for Chemical Rxns • Chemical rxns may be exothermic or endothermic. • The following rxn is exothermic: • But what if we write the reverse of the above equation? • The reverse rxn is endothermic, with the same Δ H but an opposite sign. Calorimetry and Enthalpy Changes One common experimental method to find the Δ H for a rxn is to conduct the rxn inside a calorimeter. Calorimeters may be constant pressure or constant volume.
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Whatever type of calorimeter is used, the temperature change of the system, or the surrounding water reservoir, is measured. This gives us Δ T, where Δ T = T f - T i But how can we get from Δ T to Δ H? Because of something called the heat capacity of a chemical (or a mixture). Heat capacity is a measure of a substance’s (or a mixture’s) ability to store heat.
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Ch400Ch8LN2 - Finding Enthalpy Changes for Chemical...

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