Experiment 4 - EXPERIMENT 4 Enthalpy of Formation of an...

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EXPERIMENT 4 Enthalpy of Formation of an Ammonium Salt Objective: The heat of formation of solid (NH 4 ) 2 SO 4 will be determined by combining measurements of enthalpy changes with known heats of formation using Hess’s law. The measured enthalpy changes will be determined by constant pressure calorimetry. Introduction: The enthalpy of formation per mole of a compound is the enthalpy change ( H ) accompanying the formation of 1 mole of the compound from its elements in their standard states. Direct measurement of enthalpies of formation is often difficult, so indirect methods involving enthalpies of reaction are used frequently instead. In this experiment the heat of formation of solid ammonium sulfate [(NH 4 ) 2 SO 4 ] will be determined by combining measurements of the enthalpy change of the neutralization reaction between aqueous ammonia (NH 3 ) and sulfuric acid (H 2 SO 4 ), 2 NH 3 ( aq ) + H 2 SO 4 ( aq ) (NH 4 ) 2 SO 4 ( aq ) Δ H neut (1) and the enthalpy change for the dissolution of solid (NH 4 ) 2 SO 4 in water, H 2 O (NH 4 ) 2 SO 4 ( s ) → (NH 4 ) 2 SO 4 ( aq ) Δ H diss (2) with the known heats of formation of aqueous NH 3 [–81.2 kJ/mol for 1.5 M NH 3 ( aq )] and aqueous H 2 SO 4 . Hess’s law states that the change in a thermodynamic property that is a state function, such as enthalpy, depends only on the initial and final states, independent of the particular combination of steps taken between them. In other words, adding several consecutive reactions gives a net reaction for which the H is simply the sum of the H s of the component reactions. For example, the H of formation of aqueous NH 3 is equal to the sum of the H of formation of gaseous NH 3 and the H that accompanies dissolving gaseous NH 3 in water: 1/2 N 2 ( g ) + 3/2 H 2 ( g ) NH 3 ( g ) Δ H of formation of NH 3 ( g ) = Δ H 1 (3) H 2 O NH 3 ( g ) → NH 3 ( aq ) Δ H of dissolution of NH 3 ( g ) = Δ H 2 (4) ______________________________ __________________________________ 1/2 N 2 ( g ) + 3/2 H 2 ( g ) NH 3 ( aq ) Δ H of formation of NH 3 ( aq ) = Δ H 1 + Δ H 2 (5)
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2 Given that H 1 is –45.8 kJ/mol and H 2 is –35.4 kJ/mol, the H of formation of aqueous NH 3 is calculated to be –81.2 kJ/mol. A similar calculation can be set up to get the H of formation of aqueous H 2 SO 4 . The enthalpy changes for Reactions (1) and (2) will be measured in this experiment using a calorimeter. A calorimeter is a device used to measure the amount of heat transferred during a chemical reaction. A calorimeter is just an insulated container equipped with a stirrer, a thermometer, and a loose-fitting lid (to maintain the contents at atmospheric pressure). The reaction is carried out inside the container, and the heat evolved or absorbed is calculated from the measured temperature change. Under conditions of constant pressure, the heat is equivalent to the enthalpy change, q = Δ H (6) The temperature change ( Δ T
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Experiment 4 - EXPERIMENT 4 Enthalpy of Formation of an...

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