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Unformatted text preview: 303 HEAT OF FORMATION OF AMMONIUM NITRATE OBJECTIVES FOR THE EXPERIMENT The student will be able to do the following: 1. Calculate the change in enthalpy (heat of reaction) using the Law of Hess. 2. Find heats of reaction using a coffee-cup (open) calorimeter. 3. Calculate the heat capacity of a coffee-cup calorimeter. 4. Calculate the amount of heat involved in a specific temperature change of water. BACKGROUND The heat of formation of ammonium nitrate is the heat liberated when one mole of solid ammonium nitrate forms at constant pressure from its component elements in their standard states. N 2 ( g ) + 2H 2 ( g ) + 3 2 O 2 ( g ) NH 4 NO 3 ( s ) D H = D H f NH 4 NO 3 ( s ) Even though this reaction does not take place, we still would like to know the value so it could be used in calculations of D Hs for reactions that do take place. For example, we could use the heat of formations for ammonium nitrate, dinitrogen monoxide, and water to calculate the D H for the following reaction. NH 4 NO 3 ( s ) N 2 O( g ) + 2H 2 O( l ) D H rxn = D H f N 2 O( g ) + 2 D H f H 2 O( l ) - D H f NH 4 NO 3 ( s ) Because the reaction for the heat of formation of ammonium nitrate does not take place, we cannot determine it directly in the laboratory. Like many heats of reaction, we can determine this value indirectly using the Law of Hess. We will determine the heats of reaction for four reactions whose equations can be summed to yield the equation for the heat of formation of ammonium nitrate. The sum of the heats of reaction for each equation will yield the heat of formation of ammonium nitrate. N 2 ( g ) + 3 2 H 2 ( g ) NH 3 ( aq ) D H f NH 3 ( aq ) = - 81.17 kJ/mole H 2 ( g ) + N 2 ( g ) + 3 2 O 2 ( g ) HNO 3 ( aq ) D H f HNO 3 ( aq ) = - 206.0 kJ/mole NH 3 ( aq ) + HNO 3 ( aq ) NH 4 NO 3 ( aq ) D H neut NH 4 NO 3 ( aq ) NH 4 NO 3 ( s ) -D H solution _______________________________________________________ N 2 ( g ) + 2H 2 ( g ) + 3 2 O 2 ( g ) NH 4 NO 3 ( s ) D H f NH 4 NO 3 ( s ) = D H f NH 3 ( aq ) + D H f HNO 3 ( aq ) + D H neut + (-D H solution ) The heat of formation of aqueous ammonia is...
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