Lab Manual Experiment 3

Lab Manual Experiment 3 - EXPERIMENT 3 Thermochemistry...

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Experiment 3- Page 1 EXPERIMENT 3 Thermochemistry INTRODUCTION Heat exchange is one of the most common processes explored in the chemical laboratory. The examination of the relationships between heat energy and chemical changes is called thermochemistry . If heat energy is released by a reaction, the change is termed exothermic . If, on the other hand, heat energy is absorbed, the change is said to be endothermic. The magnitude of heat energy which flows as a reaction proceeds is measured using a device called a calorimeter and the measurement of this heat flow is called calorimetry . Specific heat, heat of fusion and heat of reaction are a few of the heat exchange processes you will be exploring in this experiment. The amount of heat energy ( q ) required to raise the temperature of a substance by Δ T depends both upon the mass ( m ) and the identity (chemical composition) of the substance. The average heat capacity of a system is the amount of heat energy required, per degree of temperature rise, between any two given temperature points. The heat capacity per unit mass ( m ) of a substance is the specific heat capacity or specific heat ( c ), a physical property of the substance: c = Tm q Δ or q = (m)(c)( Δ T) If moles ( n ) are used in place of mass ( m ), the molar heat capacity ( C P ) is obtained: C P = Tn q Δ or q = (n)(C P )( Δ T) Cooling is the reverse of heating and either process can be used to obtain the heat capacity. The specific heat of water is calculated to be 4.182 J/K g and its molar heat capacity is 75.36 J/K mol at 20°C. Table 1 indicates that the specific heat of an unknown metal might be used to help in identifying it. The molar heat capacity, of course, cannot be calculated without knowing the molar mass. Note that Δ T has the same value in the Celsius and Kelvin scale. As illustrated in Table 1, the molar heat capacities of the metallic elements are about equal, with an average approximate value of 26 J/K mol. This discovery is known as the Law of Dulong and Petit , after the men who made this discovery in the early 1800s. They did not understand why this was so, but they used the observation to estimate the atomic masses of metals. You will also apply this law to estimate the atomic mass of an unknown metal from its measured specific heat. The law applies to monatomic substances in which the individual atoms are allowed to vibrate around their equilibrium positions in all three directions. Such conditions are met by the metallic elements in the solid state. Chemists can now predict that the molar heat capacities (under these circumstances) should be close to three times the value of the ideal gas constant ( R = 8.314 J/K mol). This is, indeed, the value observed by Dulong and Petit. TABLE I. Thermal Properties of Selected Materials at 25°C * Possible unknowns for Part II Material Specific Heat Capacity (J/K g) Molar Heat Capacity (J/K mol) Material Specific Heat Capacity (J/K g) Molar Heat Capacity (J/K mol) Bi 0.1221 25.52 *Zn 0.3886 25.40 *Pb 0.1276 26.44 Co 0.4210 24.81 Au 0.1290 25.42 *Ni 0.4440 26.07 Pt 0.1326 25.86 Fe 0.4494 25.10 Hg( l ) 0.1395 27.98 *Ti 0.5223 25.02 Sb 0.2072 25.23 Ca 0.6315 25.31 I 2 ( s ) 0.2145 54.44 Si 0.7121 20.00 *Sn 0.2274 26.99 K 0.7565 29.58
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern