HesssLawLabReport - Hess Law Lab By Malia Clark Lab...

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Hess’ Law Lab By: Malia Clark Lab Partners: Amanda Paavola, Joey Oda Date: 2/10/16 I. Exploration: A. Introduction : According to Hess’ law, if a reaction can be carried out in a series of steps, the sum of the enthalpy changes for each step should equal the enthalpy change for the total reaction. There is no single instrument that can directly measure heat. However, it is possible to calculate the gain or loss of heat when a chemical reaction occurs. This is done by utilizing a simple calorimeter. One may also use ‘ q ’ to calculate the standard change in enthalpy (DH°, measured in kJ/mol) for a reaction by determining the number of moles of reactant and using the balanced chemical equation.
B. Aim: The purpose of this experiment is to experimentally verify Hess’ Law utilizing the three chemical reactions shown below: 1. NaOH (s) ® NaOH (aq) DH 1 = ? 2. NaOH (aq) + HCl (aq) ® NaCl (aq) + H 2 O (l) DH 2 = ? 3. NaOH (s) + HCl (aq) ® NaCl (aq) + H 2 O (l) DH 3 = ? Your first task is to experimentally determine the DH for each of the 3 reactions shown above using coffee-cup calorimetry. Your next task is to verify the experimentally-determined value of DH 3 utilizing Hess’ law (along with DH 1 and DH 2 ) . Be sure to include a percent difference in your calculations and conclusion. C. Background: Enthalpy (H) is the heat energy converted from energy contained in chemical bonds. Enthalpy change ( Δ H) is the amount of heat evolved or absorbed in a reaction carried out at constant pressure. In this experiment, coffee-cup calorimetry was used to calculate the experimental Δ H values of the three experiments. Calorimetry is a method used to determine the enthalpy change associated with a chemical reaction carried out in an aqueous solution, and is given by the equation: q = -mc Δ T where: q = heat energy (enthalpy) released/absorbed by the reaction measured in Joules (J) m = mass of the solution that absorbs thermal energy, measured in grams (g) c = specific heat capacity of solution ( J g 1 K 1 ) Δ T = change in temperature given in degrees Celsius or Kelvin for the solution. In the experiment, a liquid-filled styrofoam cup was nestled in an empty styrofoam cup to serve as the calorimeter, and a thermometer probe was put through a small hole in a styrofoam lid to measure the temperatures of the liquids. The calculated enthalpy changes were then compared to the theoretical enthalpy change verified through Hess’ Law. Hess’ Law states that the value of Δ H for a reaction is the same whether it occurs directly or in a series of steps. Two important items regarding Hess’ Law state that: if a reaction is reversed, Δ H is the same value, but with the opposite sign, and the magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction. In the investigation, the reactions being tested are acid/base neutralization reactions incorporating HCl and NaOH. These reactions are exothermic and will form salt and water . In an exothermic reaction, heat is released, producing a negative Δ H value and creating a more stable product. By conducting the neutralization reactions for the three given chemical

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