Heat effects and calorimetry lab report

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-pre lab questions located at the end of attachemtneCHEM 403 Exp 9 Experiment 9: HEAT EFFECTS AND CALORIMETRY* Heat is a form of energy, sometimes called thermal energy, that can pass spontaneously from an object at a high temperature to an object at a lower temperature. If the two objects are in contact, they will, given sufficient time, both reach the same temperature. Heat flow can be measured in a device called a calorimeter. A calorimeter is simply a container with insulating walls, made so that essentially no heat is exchanged between the contents of the calorimeter and the surroundings. Within the calorimeter chemical reactions may occur or heat may pass from one part of the contents to another, but, ideally, no heat flows into or out of the calorimeter from or to the surroundings. Specific Heat When heat flows into a substance, the temperature of that substance will increase. The quantity of heat (q) required to cause a temperature change (ΔT) of any substance is proportional to the mass (m) of the substance and the temperature change, as shown in Equation 1. The proportionality constant is called the specific heat, c, of that substance. q = specific heat * mass of substance * temperature change = c∙m∙ΔT (1) The specific heat is the amount of heat required to raise the temperature of one gram of the substance by 1 °C. Amounts of heat are measured in either joules or calories. To raise the temperature of 1 g of water by 1 °C, 4.18 joules of heat must be added to the water. The specific heat of water is therefore 4.18 joules/g∙°C. Since 4.18 joules equals 1 calorie, the specific heat of water is 1 calorie/g∙°C. Heat flow into or out of a substance is often determined by the effect that that flow has on a known amount of water. Because water plays such an important role in these measurements, the calorie, which was the unit of heat most commonly used until recently, was defined to be equal to the specific heat of water. The specific heat of a metal can readily be measured in a calorimeter. A weighed amount of metal is heated to some known temperature and is then quickly poured into a calorimeter that contains a measured amount of water at a known temperature. Heat flows from the metal to the water, and the two equilibrate at some temperature between the initial temperatures of the metal and the water. Assuming no heat is lost from the calorimeter to the surroundings, and that a negligible amount of heat is absorbed by the calorimeter walls, the amount of heat that flows from the metal as it cools is equal to the amount of heat absorbed by the water. In thermodynamic terms, the heat flow for the metal is equal in magnitude but opposite in direction, and hence in sign, to that for the water. For the heat flow q, qwater = –qmetal *Adapted from Slowinski, E. J., Wolsey, W. C. Chemical Principles in the Laboratory 9th ed. (2) CHEM 403 Exp 9 If heat flow is expressed in terms of Equation 1 for both the water and the metal, the result is: qwater = cwater ∙ mwater ∙ ΔTwater = –[cmetal ∙ mmetal ∙ ΔTmetal] (3) In this experiment, you will measure the masses of water and metal and their initial and final temperatures. Note that ΔTmetal < 0 and ΔTwater > 0, since ΔT = Tfinal - Tinitial. Given the specific heat of water, you can find the positive specific heat of the metal by Equation 3. The specific heat of a metal is related in a simple way to its molar mass. Dulong and Petit discovered many years ago that about 25 joules were required to raise the temperature of one mole of many metals by 1 °C. This relation, shown in Equation 4, is known as the Law of Dulong and Petit: 25 MM ≈ (4) c where MM is the molar mass of the metal. Once the specific heat of the metal is known, the approximate molar mass can be calculated by Equation 4. The Law of Dulong and Petit was one of the few rules available to early chemists in their studies of molar masses. Heat of Reaction When a chemical reaction occurs in water, the situation is similar to that which is present when a hot metal sample is put into water. With such a reaction, there is an exchange of heat between the reaction mixture and the solvent, water. As with measuring specific heat, the heat flow for the reaction mixture is equal in magnitude but opposite in sign to that for the water. The heat flow associated with the reaction mixture is also equal to the enthalpy change, ΔH, for the reaction, so the equation obtained is: qreaction = ΔHreaction = –qwater (5) By measuring the mass of the water used as solvent, and by observing the temperature change that the water undergoes, you can find qwater by Equation 1 and ΔH by Equation 5. If the temperature of the water goes up, heat has been given off by the reaction mixture,