February 05, 2014 Calorimetry the science of measuring heat generated or consumed during a chemical reaction or phase change What is going to happen when the burner is lit? February 05, 2014 Experimental Design Chemical changes involve potential energy, which cannot be measured directly Chemical reactions absorb or release energy with the surroundings, which cause a change in temperature of the surroundings (and this can be measured) In a calorimetry experiment, we set up the experiment so all of the energy from the reaction (the system) is exchanged with the surroundings ∆Esystem = ∆Esurroundings Chemical System Surroundings (water) Endothermic Change February 05, 2014 Chemical System Surroundings (water) Exothermic Change ∆Esystem = ∆Esurroundings February 05, 2014 There are three main calorimeter designs: 1) Flame calorimeter -a fuel is burned below a metal container filled with water 2) Bomb Calorimeter -a reaction takes place inside an enclosed vessel with a surrounding sleeve filled with water 3) Simple Calorimeter -a reaction takes place in a polystyrene cup filled with water Example #1 A student assembles a flame calorimeter by putting 350 grams of water into a 15.0 g aluminum can. A burner containing ethanol is lit, and as the ethanol is burning the temperature of the water increases by 6.30 ˚C and the mass of the ethanol burner decreases by 0.35 grams. Use this information to calculate the molar enthalpy of combustion of ethanol. February 05, 2014 The following apparatus can be used to determine the molar enthalpy of combustion of butan-2-ol. Initial mass of the burner: 223.50 g Final mass of the burner : 221.25 g water Mass of copper can + water: 230.45 g Mass of copper can : 45.60 g Final temperature of water: 67.0˚C Intial temperature of water : 41.3˚C Determine the molar enthalpy of combustion of butan-2-ol. February 05, 2014 February 05, 2014 Bomb Calorimeter In a bomb calorimeter a chemical reaction takes place in a strong reaction chamber that can withstand the intense pressure of heated gases The reaction chamber is surrounded by a jacket of water with a thermometer inserted The heat released from the chamber warms the water-filled jacket, allowing a scientist to determine the quantity of energy released by the reaction. February 05, 2014 To calculate Q for the calorimeter you must examine the type of information given about the calorimeter 1) If you are given the mass of water in the calorimeter: Q = mc∆t (This type of question assumes that the energy change of the material of the calorimeter is negligible compared to the energy change of the water. 2) If you are given the heat capacity (C) of the calorimeter Q = C ∆t (Heat capacity describes the amount of energy it takes to raise the temperature of the entire calorimeter by 1˚C. This value takes into consideration energy change of the water and all of the materials of the calorimeter) The units of heat capacity (C) are kJ/˚C February 05, 2014 Example A 1.00 g sample of propane was placed in a bomb calorimeter and ignited. The initial temperature of the calorimeter was 25.0˚C, and the final temperature was 27.3˚C. If the heat capacity of the calorimeter is 97.1 kJ/˚C, what is the molar enthalpy of combustion for propane? A 2.84 g sample of ethanol, C2H5OH, was burned in a bomb calorimeter filled with 2300 mL of water. If the temperature of the calorimeter rose from 25.0˚C to 33.7˚C what is the molar enthalpy of combustion of ethanol? .