EXPERIMENT 3: HEAT, TEMPERATURE, AND CALORIMETRY Introduction Some physical chan

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EXPERIMENT 3: HEAT, TEMPERATURE, AND CALORIMETRY Introduction Some physical changes and many chemical changes are accompanied by the emission or absorption of heat q. A chemical reaction which emits heat is said to be an exothermic reaction and the quantity of emitted heat is given a negative sign. A chemical reaction which absorbs heat is termed endothermic, and the absorbed heat is given a positive sign. Although it is common language usage to speak of heat emitted or heat absorbed, heat is not a property of a system by itself. In thermodynamics heat is defined as the transfer of thermal energy. Thermal energy is the energy associated with random motions of atoms and molecules. Since heat is a form of energy transfer it is generally reported in the SI energy unit joules J Temperature is a property which provides a measure of thermal energy. Temperature is an intensive property, i.e., it is independent of the quantity of substance present. One manner in which temperature can be measured is with a thermometer, which relies upon the volume of an expanding liquid (as the temperature rises) to measure the temperature. In chemistry the temperature is generally measured either in the metric unit, degrees Celsius °C, or in the Systeme Internationale SI unit, kelvin K. Additional information about these units can be found in your lecture text, if needed. A lower case t is generally used to indicate the temperature is measured in °C whereas an upper case T normally is used to indicate the temperature is measured in K. The specific heat c of a substance is another intensive property. It is the amount of heat required to increase the temperature of one gram of a substance by 1 °C. Specific heat is a physical property which varies from one substance to another substance. Consequently, specific heat can be used to help in identifying a substance. At constant pressure the relationship between the heat required to raise the temperature of an object and its specific heat is given by Equation (3-1) where m is the mass of the object measured in units of grams. The units for the quantities used in the equation are listed in the brackets. (3-1) As is customary the Greek letter is used to indicate "change in", ie., in this particular case it is the final temperature tr minus the initial temperature t Heat changes can be measured with a device called a calorimeter. The device was named from an energy unit, the calorie, which is now rarely used in chemistry, but which continues to be used in other fields. A calorimeter is a highly heat insulated vessel in which temperature changes can be monitored without fear of losing heat to the environment. In this experiment a calorimeter will be constructed from two foam (insulation) cups. The purpose of this exercise is to determine the specific heat of a metal. A hot metal will be placed into cooler water in the calorimeter. As heat is transferred from the hot metal to the cooler water, the temperature of the water rises and is measured. Since heat is a form of energy 24-

Explanation / Answer

QW1.

m = 31.78 g of Ti

Ti = 93°C,

m = 60 g of water

Tw = 22°C

Tf = 26.5 °C

Cp water = 4.184 J/gC, Cp Ti ?

energy balance

-Elost = Egained

-Qtitanium = +Qwater

Qtitanium = mass Ti *Cp Ti * (Tf-Ti)

Qwater= mass water *Cp water * (Tf-T)

- mass Ti *Cp Ti * (Tf-Ti) = mass water *Cp water * (Tf-T)

substitute

-31.78 * Cp Ti * (26.5-93) = 26.5 * 4.184*(26.5-22)

Solve for Cp Ti

Cp Ti = -26.5 * 4.184*(26.5-22)/ (26.5-93) / 31.78

Cp Ti = 0.23608J/gC

Q2.

sig fig must be --> 3 since it is the least value on all data given

Cp Ti = 0.236 J/gC

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