PURPOSE To use coffee-cup calorimetry to determine the enthalpy of neutralization of two acids with sodium hydroxide. INTRODUCTION Enthalpy of neutralization is the heat released or absorbed when an acid and a base react to form water in a neutralization reaction. In this experiment, the enthalpy of neutralization for two different acid-base reactions will be determined using a coffee-cup calorimeter , which is a simple device used to measure heat changes in chemical reactions at constant pressure. The first reaction involves a strong acid (HCl) and a strong base (NaOH) , while the second reaction involves a weak acid (HC₂H₃O₂) and a strong base (NaOH) . Since strong acids and strong bases completely dissociate in water, their neutralization reaction typically releases more heat compared to a weak acid that only partially ionizes. By measuring the temperature changes during these reactions, the heat released can be calculated and used to determine the enthalpy of neutralization. Additionally, the heat capacity of the calorimeter will be determined to account for heat absorption by the apparatus. Understanding these enthalpy changes provides insight into acid-base thermodynamics and the energy differences between strong and weak acids. SAFETY PRECAUTIONS Sodium hydroxide (NaOH) and hydrochloric acid (HCl) are corrosive and can cause skin burns or eye damage. If spilled, rinse immediately with plenty of water. Acetic acid (HC₂H₃O₂) is an irritant and should be handled carefully. Although this experiment involves small amounts of chemicals, acetic acid vapors can be irritating. Work in a Well-Ventilated Area. EQUIPMENT AND CHEMICALS NEEDED Coffee-cup calorimeter with lid Two temperature probes or thermometers 100 mL beakers (at least 2) 10 mL graduated cylinder Hot plate 1.0 M Sodium hydroxide (NaOH) solution 1.0 M Sodium hydroxide (NaOH) solution 1.0 M Hydrochloric acid (HCl) solution 1.0 M Acetic acid (HC₂H₃O₂) solution EXPERIMENTAL PROCEDURE Part A. Finding the Heat Capacity of the Calorimeter Add 5.0 mL of tap water to the calorimeter. Place a temperature probe through a hole in the calorimeter cover and place the cover on the calorimeter. Allow the reading to stabilize and record the temperature on your data sheet. Place 5.0 mL of tap water in a small beaker and heat the water on a hot plate until the temperature is 15-20 °C above room temperature. Do not heat to boiling. Remove the beaker from the hot plate and allow it the stand for a minute. Measure the temperature of this warmed water with a second temperature probe. Allow the reading to stabilize before recording the temperature of the warmed water. Remove the cover from the calorimeter, quickly pour the warmed water completely into the calorimeter, and quickly replace the cover with the inserted temperature probe. Gently swirl the calorimeter while observing the temperature. The temperature will rise and level off. Record the temperature when the temperature levels off. Part B. Enthalpy of Neutralization of NaOH-HCl Rinse and dry the calorimeter. Add 5.0 mL of 1.0 M NaOH to the calorimeter. Measure 5.0 mL of 1.0 M HCl into a dry beaker and allow it to stand by the calorimeter for 3-4 minutes. Place a second temperature probe into the HCl and measure the temperature. Place the temperature probe with the cover onto the calorimeter and measure the temperature of the NaOH. The temperature difference between the solutions should be less than 0.5 °C. If the difference is greater, adjust the HCl temperature by holding the beaker or using an ice-water bath. When the temperatures of the NaOH and HCl are within 0.5 °C of each other, record the temperature of the NaOH. Quickly add the 1.0 M HCl and quickly replace the cover with the temperature probe. Carefully and gently swirl the calorimeter and observe the temperature. Record the maximum temperature reached. It may take up to 3 minutes. Part C. Enthalpy of Neutralization of NaOH-HC 2 H 3 O 2 Rinse and dry the calorimeter. Follow the same procedure as in PART B, substituting 1.0 M HC 2 H 3 O 2 in place of the 1.0 M HCl. PRE-LAB QUESTIONS What is the significance of the statement “ q system + q surroundings = 0” in relation to the First Law of Thermodynamics? A student measured the enthalpy of neutralization in a Styrofoam “Coffee Cup” calorimeter by mixing 25.0 mL of 1.0 M NaOH solution with 25.0 mL of 1.0 M HCl solution. Write the balanced equation and identify the limiting reactant in this experiment. If the change in enthalpy of this exothermic reaction was determined to be found −1295 J, what is the enthalpy of neutralization in kJ/mole? There are three approaches to determining enthalpy changes in a process. One can do direct calorimetric measurement, one can use Hess's Law, or one can use the standard enthalpy of formation, ∆ H f 0 . Write the acid-base neutralization equation for the strong base NaOH ( aq ) with the strong acid HCl ( aq ). Estimate ∆ H neutralization using the following ∆ H f 0 values: Unnamed: 0 H+ (aq) OH− (aq) H2O (l) ∆Hf0 (kJ/mol): 0 −230.0 −285.83 DATA AND OBSERVATIONS Part A. Heat Capacity of the Calorimeter 0 1 2 3 Temperature of calorimeter (and cold water) before mixing Tcalorimeter (measured) NaN °C Temperature of warm water Twarm (measured) NaN °C Final temperature Tfinal (measured) NaN °C Temperature change of warm water ∆Twarm = Twarm – Tfinal NaN °C Temperature change of calorimeter ∆Tcalorimeter = Tfinal – Tcalorimeter NaN °C Heat lost by the warm water qwarm = ∆Twarm × 5.0 g × 4.18 J/g·°C NaN J Heat gained by the cold water qcold = ∆Tcalorimeter × 5.0 g × 4.18 J/g·°C NaN J Heat gained by the calorimeter qcalorimeter = qwarm – qcold NaN J Heat capacity of the calorimeter Ccalorimeter = qcalorimeter / ∆Tcalorimeter NaN J/°C Part B. Enthalpy of Neutralization of HCl-NaOH 0 1 2 3 Temperature of calorimeter (and NaOH) before mixing Tcalorimeter (measured) NaN °C Maximum temperature reached Tmax (measured) NaN °C Temperature change ∆T = Tmax – Tcalorimeter NaN °C Heat gained by the solution qsolution = ∆T × 10.0 g × 4.18 J/g·°C NaN J Heat gained by the calorimeter qcalorimeter = ∆T × Ccalorimeter NaN J Total heat released qtotal = qsolution + qcalorimeter NaN J 0 1 2 3 Balanced equation for the reaction: NaN NaN NaN Moles of HCl in 5.0 mL of 1.0 M HCl Calculation: NaN mol Moles of NaOH in 5.0 mL of 1.0 M NaOH Calculation: NaN mol Moles of water produced (nwater) Calculation: NaN mol Enthalpy of neutralization per mole of water ∆Hneutralization = qtotal/nwater/(1000 J/kJ) NaN kJ/mol Part C. Enthalpy of Neutralization of HC 2 H 3 O 2 -NaOH 0 1 2 3 Temperature of calorimeter (and NaOH) before mixing Tcalorimeter (measured) NaN °C Maximum temperature reached Tmax (measured) NaN °C Temperature change ∆T = Tmax – Tcalorimeter NaN °C Heat gained by the solution qsolution = ∆T × 10.0 g × 4.18 J/g·°C NaN J Heat gained by the calorimeter qcalorimeter = ∆T × Ccalorimeter NaN J Total heat released qtotal = qsolution + qcalorimeter NaN J 0 1 2 3 Balanced equation for the reaction: NaN NaN NaN Moles of HC2H3O2 in 5.0 mL of 1.0 M HC2H3O2 Calculation: NaN mol Moles of NaOH in 5.0 mL of 1.0 M NaOH Calculation: NaN mol Moles of water produced (nwater) Calculation: NaN mol Enthalpy of neutralization per mole of water ∆Hneutralization = qtotal/nwater/(1000 J/kJ) NaN kJ/mol POST-LAB QUESTIONS An unknown metal sample weighing 9.25 grams is heated to 97.8° C in a hot water bath to establish thermal equilibrium. The hot metal sample is quickly transferred to 50.0 grams of water at 24.5 °C contained in a Styrofoam “Coffee Cup” calorimeter. The thermal equilibrium temperature of the unknown metal and water mixture is 27.4°C. What is the specific heat capacity of this unknown metal? The temperature of a mixture of a solution prepared by dissolving 2.50 grams of NaOH in 75.0 mL of water (both initially at 25°C) increases to 25.5 °C. Is this process exothermic or endothermic? What is the enthalpy of solution of NaOH in kJ/mol? How does the enthalpy in Part B compare to the calculated enthalpy in pre-lab question #3? How does the enthalpy in Part B compare to Part C?