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Calorimetry

# Calorimetry - Dr McCord Calorimetry means that all heat...

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Calorimetry Concepts The whole point of calorimetry is to “trap” the heat that is entering/exiting the system and get a quantitative measure of it (how many joules?). Water is an excellent absorber of heat. It has one of the highest specific heats ( C s ) there is at 4.184 J/g˚C. Unfortunately water must be in a container and we must therefore account for the heat entering/exiting the container (the hardware) as well as the water. You can treat the water and the container as one whole unit – the calorimeter. As a unit the whole calorimeter (water + hardware) will have a definite heat capacity, C cal which will have units of J/˚C or kJ/K. Note that there is only energy per degree temperature for units for a plain heat capacity. All heat capacities have that for units and all heat capacities are extensive properties – meaning the bigger the calorimeter, the bigger the heat capacity will be. Heat capacities of calorimeters must be measured. You calibrate your calorimeter before you use it. How? Put a know amount of heat ( q cal ) into it via a standard reaction of some sort or a precise electric heater (see Calibration later in this review). Then measure the temperature response, T . The heat capacity of the entire calorimeter will simply be C cal = q cal / T Now you can turn that equation around and use C cal and T to get q cal . The heat from the chemical reaction you are studying ( q rxn ) is equal to but opposite in sign from the value of q cal . That is q rxn = - q cal Bomb Calorimetry and U system adiabatic walls water T q cal bomb calorimeter Bomb calorimetry is conducted under constant volume (isochoric) conditions. This is shown explicitly by writing q v for the heat term. The subscript “v” means conducted at constant volume. Because the volume is held constant and V is forced to be zero, no expansion work can be done throughout the process ( w = 0). This means that all heat flow ( q v ) is now equal to the internal energy change, U . The calculations are as follows for the heat flow in/out of the calorimeter.

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