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Chemistry LibreTexts

5.1: Energy

  • Page ID
    213151
  • Learning Objectives
    • Define energy.
    • Define work in a chemical context.
    • Identify the three most common expressions of energy.
    • Define heat.
    • Identify the three most common units for measuring heat.
    • Apply conversion factors to change a heat measurement to a corresponding value in a different unit.

    EnergyE, is defined as the ability to do work.  Because the context in which "work" is studied varies between scientific disciplines, this definition must be highly generalized.  Physicists are primarily interested in kinetic energy, which is the energy that is associated with the motion of objects.  Biologists investigate metabolic pathways, in which nutrients are converted to energy that, in turn, drives cellular processes.  Chemists explore the transfer of energy during physical and chemical changes.

    Studying energy is challenging because energy is not a physical substance.  Instead, energy is observed in non-tangible forms, including the emission of light, the generation of sound, and the transfer of heat.  Light and sound are primarily qualitative phenomena, meaning that they are easily observable, but are challenging to measure.  In contrast, heat is an expression of energy that can be quantified.  Therefore, this chapter will focus on measuring the amount of heat that is transferred during physical and chemical changes.

    Heat is the energy that is associated with the motion of particles, which, in turn, is directly related to the temperature of a substance.  Rubbing one object against another creates resistance, or friction, which results in the generation of heat.  These frictional forces are also present at the molecular level.  As the temperature of a substance rises, the particles that are contained within that chemical move more rapidly and, therefore, come into contact with one another more frequently.  These interactions generate friction, which produces heat.  Heat transfer is directional, in that the associated energy can only move from an object with a higher temperature to a material that has a lower temperature, and so will cease when both substances exist at the same temperature.

    Units for Measuring Heat

    Because heat, which is abbreviated as q, is a measure of the amount of energy that is transferred between substances, the identity and amount of the substance that is being studied must be clearly stated.  Therefore, the calorie (cal) which is the original unit that was utilized to measure heat, is defined as the amount of heat that is required to raise the temperature of 1 gram of water by 1 degree Celsius.  The word "calorie" is derived from the Latin term for heat, "caloric."  This translation is also reflected in the term "calorimeter," which is the name of the apparatus that is used to measure the amount of heat that is involved in a chemical or physical change. 

    The values that are specified within the definition of a calorie, "1 gram" and "1 degree Celsius," were selected because chemists usually perform experiments with relatively small quantities of chemicals.  However, when scientists began exploring applications of heat transfer in a nutritional context, the resultant measurements had incredibly large numerical values, due to the scale of the systems being studied.  As a result, individuals who were not familiar with the definition of a calorie were unable to correctly interpret these numerical values and, therefore, could not make informed nutritional decisions.  In response, scientists defined a larger-scale unit, the Calorie (Cal) for measuring the heat that is released when food is processed by a living organism.  In order to distinguish a dietary Calorie from a chemical calorie, the dietary Calorie is written with a capitalized "C."  The relationship between a dietary Calorie and a chemical calorie is shown below.

    1 Calorie (Cal) = 1,000 calories (cal)

    Unfortunately, because the pronunciations of "calorie" and "Calorie" are identical, these units can only be distinguished from one another in written format.  As a result, heat measurements that are relayed through spoken communication are ambiguous, as the relative capitalization of the associated units cannot be easily indicated.  In order to eliminate the misunderstandings that result from having two phonetically-identical units, scientists selected a third unit, the joule (J) as the SI unit for measuring heat.  As joules are primarily used to measure the amount of heat that is transferred during a physical or chemical change, its value is most often related to a chemical calorie, rather than a dietary Calorie, as shown below.

    1 calorie (cal) = 4.184 joules (J)

    Both of these relationships are measured equalities and are, therefore, exact.  Finally, these equalities can be applied as conversion factors that can be utilized to change a heat measurement to a corresponding value in a different unit. 

    Exercise \(\PageIndex{1}\)

    Using conversion factors based on the equalities that are provided above, convert 3.42 Calories to joules.

    Answer
    In order to completely eliminate the given unit, "Calories," a conversion factor based on the equality "1 Calorie (Cal) = 1,000 calories (cal)" must be applied first.  However, the unit that results upon the cancelation of "Calories" is "cal," which is not the desired final unit.  Therefore, the second equality must also be applied as a conversion factor.  Applying the correct number of significant figures to the calculated quantity results in the final answer that is shown below.

    \( {3.42 \; \cancel{\rm{Cal}}} \times \dfrac{1,000 \; \bcancel{\rm{cal}}}{1 \; \cancel{\rm{Cal}}} \times \dfrac{4.184 \; \rm{J}}{1 \; \bcancel{\rm{cal}}} = {\text {14,309.28 J}} ≈ {\text {14,300 J}}\)

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