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An introduction to calorimetry, a technique used to measure heat of reaction. It explains how calorimeters work, the importance of calibration, and provides examples of calculations for determining energy released or absorbed during chemical reactions using calorimeters. The document also discusses the differences between well-insulated and poorly-insulated calorimeters, and includes a temperature vs time graph for both types.
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Lesson 1 Calorimetry A calorimeter is a device used to measure heat of reaction. The reactants are enclosed in a vessel surrounded by a known amount of water. This amount of water is enclosed in a well, insulated vessel where heat loss is minimised. The reactants are allowed to react and at the end of the reaction the temperature change of the water is measured. Calorimeters can be expensive or cheap and easy to construct such as the polystyrene cup calorimeter shown on the right. Measuring the temperature change of the known body of water should be enough to calculate the amount of energy produced or absorbed by the reaction according to the formula E = 4.18 J/Co/g X mass X ΔT, where ΔT is the change in temperature and mass is the mass of water. This, however, assumes that all the energy given out or absorbed by the reaction goes into or comes from the water. This is not the case as energy is used to heat the physical parts of the calorimeter and also escapes into the environment from poorly insulated containers, so not 100% of the energy involved can be measured. In order to accurately determine the energy lost or absorbed by the reaction we must first calibrate the calorimeter. This is done by delivering a known amount of energy, most often by passing a known electric current at a given voltage for a period of time measured in seconds. A known amount of energy can also be delivered by burning an accurately known mass of fuel whose molar heat of combustion is accurately known. The change in temperature of water is then measured and the calibration factor (Cf) determined. Cf = Energy / ΔT The formula used to determine the electrical energy delivered is given by E = VIt (Joules) Example 1 A calorimeter is calibrated by passing a current of 1.20 A at 6.00 volts through the heating coil for 90.0 seconds. The temperature of the water rose by 12.2 oC. a) Calculate the Cf of the calorimeter. Step 1 find the amount of energy delivered> => E = 6.00 X 1.20 X 90.0 = 648 J Step 2 Calculate Cf = 648 / 12.2 = 53.1J/Co b) Magnesium metal was placed into a calorimeter containing 100 mL of 1.00 M HCl solution at 25.0oC. Calculate the amount of energy released in kJ if the final temperature of the water reached 29.6oC. => E = Cf X ΔT => E = 0.0531 kJ/Co^ X (29.6 – 25.0) = 0.244 kJ
c) When calculating the Cf, in a) above, 100.0 mL of water was used. How would the Cf change if only 50.0 mL of water was used? => Since less water is used, with the same amount of energy, the temperature of the water would rise more with 50.0 mL of water than with 100.0 mL of water. Hence Cf would be lower with 50.0 mL of water than with 100.0 mL. d) After the Cf has being calculated, what volume of solution should be used in the calorimeter every time it is used to measure energy? => Exactly the same volume as used in determining the calibration factor. Eg 100.0 mL Working out the change in temperature (ΔT) is not difficult for a well insulated calorimeter. Simply subtract the starting temperature from the final temperature. A temperature vs time graph for a well insulated calorimeter is shown on the right. Working out ΔT for a poorly insulated calorimeter, however, is more involved. A temperature vs time graph for a poorly insulated calorimeter is shown on the right. Finding the final temperature is not that easy. To find the final temperature of the solution in the calorimeter we extrapolate backward the line of decreasing temperature, as shown on the right, in red. The point where the extrapolated line and the starting temperature intersect is the final temperature. Two types of calorimeters exist, a bomb calorimeter and a solution calorimeter. A bomb calorimeter is one that burns compounds to calculate the heat of combustion while a solution calorimeter is one that measure the heat of reaction of solutions. Well insulated calorimeter Poorly insulated calorimeter