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5.6: Quantifying Heat Transfers: Introduction

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  • The previous section of this chapter identified the states of matter that exist and the phase changes that occur for a substance across a range of temperatures.  The resultant information could be summarized verbally or visually, as shown below in Figure \(\PageIndex{1}\), which is replicated from Figure 5.5.1 in Section 5.5.

    Conceptual Problem - Molecular Iodine.png
    Figure \(\PageIndex{1}\):  A summary of the states of matter that exist and the phases changes that occur for molecular iodine between 350. Kelvin and 500. Kelvin.

    Recall that the primary objective of this chapter is to quantify the amount of heat that is transferred during physical and chemical changes.  As stated in Section 5.4, heat must be applied in order to overcome the attractive forces between a substance's constituent particles, in order to transform that substance from a solid to a liquid, and, subsequently, from a liquid to a gas.  Heat must also be transferred to a substance in order to raise its temperature.  As stated previously, phase changes are isothermal, or "constant temperature," processes, because a state of matter conversion can only occur at its associated phase change temperature.  As a result, the temperature of a substance is unable to change while that chemical is transforming from one state of matter to another.  Additionally, as shown above in Figure \(\PageIndex{1}\), if the temperature of a substance is allowed to change, its state of matter must remain constant.

    Because these transformations cannot occur simultaneously with one another, the processes of changing the temperature of a substance and altering its state of matter are mutually-exclusive, and the heat transfers that are associated with these physical changes must be quantified separately.  As a result, two distinctive equations must be developed and applied to measure the heat transfers that are associated with the temperature changes and phase changes of a substance.

    Both of the heat transfer calculations that will be discussed in the following sections of this chapter are moderately challenging to apply, as several variables are involved in each equation.  As a result, the corresponding algebraic processes, as well as the associated unit cancelations that must be achieved in each, can be somewhat complex.  Finally, the numerical quantities that are incorporated into these equations are often presented within the context of a word problem.  Therefore, as was the case with the molar equalities that were developed in Chapter 4, chemists have chosen specific indicator phrases to correspond with each of the heat transfer processes.  Recall that an indicator phrase is a phrase that has a deeper meaning or application.  These key words, which will be specified when the heat transfer equations are further discussed, are intended to identify which physical change is being considered, and, therefore, which equation must be applied to solve the problem at-hand.