E10-Thermo1 - Experiment#10 Thermochemistry I Heat Capacity the Dulong\/Petit Law OBJECTIVES In successfully completing this lab you will determine the

E10-Thermo1 - Experiment#10 Thermochemistry I Heat Capacity...

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Author: Dr. David Kelley Last Revised on 11/12/13 by JCV 10 – 1 Experiment#10 – Thermochemistry I: Heat Capacity & the Dulong/Petit Law OBJECTIVES In successfully completing this lab you will: ! determine the heat capacities (i.e. specific heats) of two metals: copper and an unknown metal; and ! assess the molar mass of each metal using the Law of Dulong and Petit. INTRODUCTION Chemical thermodynamics deals with the energy change that accompanies chemical reactions or physical changes. Thermochemistry is the branch of thermodynamics concerned with energy transfer by heat. The energy gained or lost by a system through heat (on a per-mole basis) as a result of a chemical or physical change is referred to as the enthalpy of reaction, Δ H rxn . A reaction in which energy is lost by the reactants to the surroundings (thereby raising the temperature of the surroundings) is termed an exothermic reaction and has Δ H rxn < 0. In this instance, energy may be considered a “product” of the reaction. A reaction in which energy is absorbed by the reactants from the surroundings (thereby lowering the temperature of the surroundings) is called an endothermic reaction and has Δ H rxn > 0. In this instance, energy may be considered a “reactant” in the reaction. Specific Heat. Measurements of energy transferred by heat are performed using a calorimeter. A calorimeter is simply a device that thermally insulates the reacting system and its immediate environment from the outside world. Typically, reactions are carried out in water (or an aqueous solution) and the calorimeter can be as simple as a Styrofoam cup. The energy transferred through heat results in a change in the temperature of the water, which is measured using a thermometer. The energy lost or gained by an aqueous solution is given by: q = m C s Δ T (1) where q is the quantity of energy transferred through heat (in Joules), m is the mass of the solution (in grams), C s is the constant-pressure, specific heat capacity of the solution (in J/g°C), and Δ T is the temperature change (in °C). The specific heat ( C s ) of a substance is the amount of energy required to raise the temperature of one gram of the substance by one degree (Celsius or Kelvin). Water has many unusual properties, including an extraordinary high specific heat of 4.184 J/g°C (or 1.00 cal/g°C). In comparison, the values for most metals are in the range of 0.1 to 0.8 J/g°C. In this experiment you will use the known value for the specific heat of water to determine the specific heat of two metals: elemental copper and an unknown metal. Determining Precise Values for Δ T. There are at least two basic complications to the experimental procedure employed in this study. First, the components of the reaction and the surrounding aqueous solution change temperature in a container, but the container also changes temperature as the result of an energy transfer. In this
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Experiment#10 – Heat Capacity & Enthalpy 10 – 2
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