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Molar+Heat+of+Fusion+of+Ice - MBL1 Thermochemistry We...

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002.131 Chemistry Laboratory MBL1 page 47 MBL1: Thermochemistry We suggest you review Hess’s Law and Enthalpy Diagrams in your textbook. You will work with a partner but you should record the information in your own words. Have your T.A. initial your DATA before you leave the laboratory. OBJECTIVES: a) To determine the heat of fusion of ice using the temperature probe and MPLI. b) To determine the heat of the reaction of a strong acid with a strong base. INTRODUCTION The enthalpy, H , of a system is a state function. For a system open to the atmosphere ( i.e. kept at constant ambient pressure), the change in enthalpy, ∆ H , is equal to the energy supplied into the system H = energy supplied at constant pressure The Adiabatic Calorimeter Energy changes for processes ( e.g. chemical reactions or physical changes) may be measured in an adiabatic calorimeter. An adiabatic calorimeter is constructed so that there is no heat flow between the contents of the calorimeter and its surroundings. The calorimeter (cup and contents) is an isolated system. For an adiabatic process, q = 0, so that for any process taking place in the calorimeter: q process + q calorimeter = 0 (1) In this experiment, polystyrene coffee cups will be used to construct a "coffee cup calorimeter" which serves well as a cheap adiabatic calorimeter. The enthalpy of any process, ∆H process , can be found experimentally if you can measure the amount of energy required to restore the temperature of the system from the final value, T f , to the initial value, T i . If T f is less than T i , you must supply energy to the system; experimentally this is not difficult and can be accomplished by doing a measurable amount of mechanical or electrical work. If T f is greater than T i , you must remove energy from the system; this is possible, but is experimentally more difficult. Usually, rather than experimentally measuring the amount of energy removed, you calculate the amount of energy that needs to be removed using the concept of heat capacity. The heat capacity is the quantity of heat required to change the temperature of the system by one degree. The molar heat capacity, which applies only to pure substances, is defined as the energy required to raise the temperature of 1 mole of a substance ( e.g. water) by either 1 degree Celsius (°C) or 1 Kelvin (K). The known molar heat capacity of water at constant pressure is represented as C p (water) and has a value of 75.4 J K 1 mol 1 . Often the heat capacity is quoted in a different way; if it is the quantity of heat required to raise the temperature of one gram of substance by one degree. This specific heat capacity for liquid water is 4.184 J g 1 °C 1 . (Moore/Stanitski/Jurs, Chemistry the Molecular Science, page 219; Note : we use temperatures in K rather than °C in most thermodynamic calculations).
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