4. Dispose of the neutralized solution in the cup, by pouring it down the drain. Calculation Determine the enthalpy change for the process per mole of NaOH as in part III. The mass of water will be 30 g (the mass of solid NaOH in the final solution will be ignored). This enthalpy change is ∆ H(net) for the overall reaction. Indirect determination of heat of reaction for NaOH (s) + HCl (aq) → NaCl (aq) + H 2 O (l) Heat of solution of NaOH (s) 1. Using a graduated cylinder, measure 30 ml of distilled water, then pour the water into a small Styrofoam cup. Measure the temperature of the water to the nearest 0.1ºC 2. Weigh approximately 1.0 g solid NaOH to the nearest 0.01 g 3. Once the initial temperature of the water has been measured, pour the solid NaOH into the water in the cup. Stir gently with the thermometer, and observe the temperature. Record the final temperature when it stops changing. Ensure all the NaOH has dissolved. This is the final temperature. Do not discard this solution. It will be used in the next step. Calculation Determine the heat of solution of per mole of NaOH in a way similar to that used in part II Heat of neutralization of NaOH (aq) 1. Using a graduated cylinder, measure 30 ml of 1.0 M HCl, Measure the temperature of the HCl solution to the nearest 0.1ºC 2. Measure the temperature of the NaOH solution prepared above. Ensure the temperature of the HCl and NaOH solutions are within 1ºC. You may need to allow the NaOH solution to cool.
16 3. Once the initial temperature of the HCl and NaOH solutions are similar, record the temperature, then pour the HCl solution into the Styrofoam cup containing the NaOH solution. Stir gently with the thermometer, and observe the temperature. Record the temperature when is stops changing. This is the final temperature. 4. Dispose of the neutralized solution in the cup, by pouring it down the drain. Calculations Determination of the enthalpy of neutralization of the NaOH solution (per mole of NaOH) is similar to the calculation used in part III. Verification of Hess’ Law The heat of neutralization of solid NaOH with HCl (aq) ( ∆ H(net)) should be the same as the sum of the heat of dissolution of solid NaOH ( ∆ H(diss) plus the heat of neutralization of aqueous NaOH ( ∆ H(neut)) ∆ H(net) = ∆ H(diss) + ∆ H(neut) Pre-lab Exercise on Blackboard After reading through this experiment, log on to your Blackboard account and complete the pre-lab assignment for this experiment. The closing time for the assignment can be found on Blackboard. No extensions will be given to this deadline. The pre-lab assignment is worth 6 points. Post-lab questions 1. Estimate the specific heat capacity of iron. Use a suitable resource to find the actual heat capacity and compare. 2. Is the acid, or the base, the limiting reagent in part III. 3. Determine the amount of heat, in joules, absorbed by 300 g copper, if its temperature changes from 25.0ºC to 150.0ºC 4. Determine the final temperature when 10.0 g lead, at 100.0 0 C, is dropped into 50.0 g water, at 25.0 0 C.
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