Objective: Enthalpy of Neutralization To measure, using a calorimeter, the energy changes accompanying neutralization reactions. Background: Every chemical change is accompanied by changes in energy, usually in the form of heat. Energy changes at constant pressure are called enthalpies. When energy changes at constant pressure are due to reactions, we can determine the enthalpy of reaction or the heat of reaction, ΔH. If the chemical reaction being studied is considered to be the system, and the reaction takes place in a solution within a calorimeter, then the solution and calorimeter are the surroundings. Thus, we can use ΔT of the solution and calorimeter to determine the amount of energy exchanged in the form of heat due to the reaction. q = m*c*ΔT (1) qp=ΔH (2) ΔHsys = ‐ ΔHsurr (3) While styrofoam cups are often used to insulate hot liquids, they are not a perfect insulator. Using styrofoam cups in this lab to construct a calorimeter can minimize the heat lost from the system to the surroundings, but the calorimeter will still absorb some energy from the contents. So the overall equation incorporates the reaction, the water, and the calorimeter: ΔHrxn = ‐ qp = ‐ (qwater + qcal) (4) Procedure: A. Determining the Heat Capacity of a Calorimeter 1. Construct a calorimeter by nesting 2 styrofoam cups together. Sit the nested cups inside of a 400 mL beaker to help keep your calorimeter from tipping over. Use a piece of cardboard with a hole as a lid for your calorimeter. 2. Slip a thermometer through a rubber stopper with a hole in it, so that the thermometer will sit in the solution within the calorimeter without touching the bottom of the cup. 3. Place exactly 50.0 mL of tap water in the calorimeter cup and replace the lid and thermometer. Ensure that the thermometer is properly place to not touch bottom, but still in the water. 4. Place exactly 50.0 mL of water in a clean, dry 250 mL beaker. Gradually heat the water on a hot plate until the water temperature is ~20 °C above the temperature of the water from step 3. DO NOT OVERHEAT, as this will lead to the loss of water, introducing error into your results. Enthalpy of Neutralization 5. Make sure at least 5 minutes have passed since step 3. Record the temperature of the cold water to the nearest 0.1 °C Continue to record every 15 seconds until you have 3 or 4 constant temperature points. 6. Remove the beaker from heat, and allow to stand for a minute or two. Record the temperature of the hot water to the nearest 0.1 °C, and as quickly as possible without splashing or spilling pour the hot water into the calorimeter. Replace the lid. 7. Carefully stir the water in the calorimeter with your thermometer. Record the temperature every 15 seconds to the nearest 0.1 °C. 8. Once at least 4 temperature points have shown a constant temperature, you can stop recording data. This may take about 3 minutes. Plot your data, as temperature vs. time. Sketch two horizontal lines, for the starting temperature and the final temperature. The difference will be the ΔT for the cold water and the calorimeter. Calculate the heat capacity of the calorimeter from your recorded data. B. Enthalpy of Neutralization for the HCl + NaOH Reaction 9. Dry the calorimeter and the thermometer with a towel. Carefully measure 50.0 mL of 1.0 M NaOH and add it to the calorimeter. Place the lid on the calorimeter, but leave the thermometer out. 10. Measure out exactly 50.0 mL of 1.0 M HCl into a dry beaker. Allow both the acid and base solutions to stand 3‐4 minutes. 11. Measure the temperature of the HCl, then rinse off the thermometer over a sink, and wipe dry. Measure the temperature of the NaOH solution. Make sure the temperatures of both solutions are within 0.5 °C of each other. Warm or cool the HCl as needed. (holding in your hands, or cooling with tap water) 12. Lift the calorimeter lid, and carefully add the 1.0 M HCl all at once, being careful not to splash or spill. 13. Record the temperature of the mixture once every 15 s, while carefully stirring with the thermometer. Construct a temperature‐time curve as in step 8. Calculate the heat of neutralization per mole of water formed. You may assume the final solution has the same density and specific heat of water. Enthalpy of Neutralization Enthalpy of neutralization Pre Lab Questions: 1. Define exothermic and endothermic reactions in terms of the sign on ΔH. Consider equations 1‐4. 2. Calculate the final temperature when 50 mL of water at 60 °C are added to 25 mL of water at 25 °C. 3. A piece of metal weighing 5.10 g at a temperature of 48.6 °C was placed in a calorimeter containing 20.00 mL of water at 22.1 °C, and the final equilibrium temperature was found to be 26.8 °C. Assuming no loss of heat to the calorimeter, what is the specific heat of the metal?