Lab 12 - Measuring Enthalpy Changes

However they must involve an increase in entropy when

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Unformatted text preview: ot involved. Therefore, spontaneous endothermic processes are relatively uncommon. However, they must involve an increase in entropy. When working with ΔG, ΔH, ΔT, ΔS, and in fact all chemical terms involving a Δ (change in) the quantity is calculated as final − initial. Thus, if a beaker containing water was originally at 22.5°C and a chemical process caused its temperature to fall to 17.3°C, the change in temperature would be (2) ΔT = Tfin al − Tin itial = 17.3°C − 22.5°C = −5.2°C It is important to remember final − initial in order to get signs right. This can sometimes be confusing. It is important to distinguish between s ys te m and s urroundings , the two parts of the thermodynamic universe, when dealing with reaction energetics. The first line of this document stated that spontaneous processes lower the free energy of the system. The s ys te m is the part of the universe being studied. The first law of thermodynamics states that energy is neither created nor destroyed. The energy released by a system must be converted to some other form or transferred to some other place. Often, the energy is transferred to the s urroundings , the part of the universe that interacts with the system being studied. In such studies, it is often practical to isolate a rather small "universe" of system and surroundings in which nearly all the energy is retained. In Parts A - C of this lab, we will assume that the thermodynamic universe is the test tube or beaker in which the experiment is being conducted. Of course, some heat is transferred outside the test tube (you will be able to feel beakers heat up or cool down) but most is retained. The energy released or absorbed by the systems being studied (chemicals dissolving or reacting) will be transferred to or from the water in which the processes occur. Thus, water is a major component of the surroundings. Returning to equation 2 and sign changes. In that example, the temperature of the wate r (the surroundings) decreased and heat was released. This...
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This document was uploaded on 01/22/2014.

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