* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 14 Enthalpy of neutralization
Survey
Document related concepts
Temperature wikipedia , lookup
Stability constants of complexes wikipedia , lookup
Transition state theory wikipedia , lookup
Work (thermodynamics) wikipedia , lookup
Equilibrium chemistry wikipedia , lookup
Chemical equilibrium wikipedia , lookup
Acid dissociation constant wikipedia , lookup
Heat transfer wikipedia , lookup
Calorimetry wikipedia , lookup
Acid–base reaction wikipedia , lookup
Thermoregulation wikipedia , lookup
Thermal conduction wikipedia , lookup
Electrolysis of water wikipedia , lookup
Transcript
Chemistry 151 Last Updated Dec, 2013 Lab 14: Determining the Enthalpy Change of an Acid-Base Reaction Introduction During the previous lab, calorimetry was used to determine the heat released from a combustion reaction. In this lab a similar experiment will be performed to find the enthalpy change (H) that occurs during a neutralization reaction between sodium hydroxide and one of three acids (HA) that will be assigned to you. HnA + nNaOH NanA + nH2O The calorimeter will be a “coffee cup” calorimeter, using two Styrofoam cups (traditionally, two cups are used to ensure the reaction is well insulated, reducing the amount of heat lost to the surroundings). As before, by measuring the amount of heat absorbed by the water, we can determine how much was released by the system, which in this case is the neutralization reaction shown above. This time, however, you will first estimate how much heat is absorbed by the calorimeter (the calorimeter constant) by mixing room temperature water with some that’s been heated. As they mix the hot water will lose heat, most of which will be absorbed by the cooler water, but some will be absorbed by the calorimeter (the better the insulator, the less heat will be absorbed). We can determine the heat lost and gained by the two using the calorimetry equation (see Lab 12 of your lecture notes for an explanation of each term). q = mcT The calorimeter constant, Ccal, is a measure of the heat absorbed by the calorimeter (qcal) by the T of the room temperature water. The heat absorbed by the calorimeter is assumed to be the difference between the amount lost by the heat water and the amount gained by the room temperature water. Ccal = |qcal | ΔTrt qcal = |qhot| - qrt This will give a value in units of J/C. In other words, the calorimeter is assumed to absorb this same amount of heat for every degree the water inside increases. When performing the neutralization reaction, the heat absorbed by the calorimeter (q cal) can be calculated by multiplying the calorimeter constant by the temperature change during the reaction. qcal = Ccal · Trxn The heat released by the reaction (qrxn) will be the heat absorbed by the solution (qsoln) plus that absorbed by the calorimeter. The sign will be negative to reflect an exothermic reaction. qrxn = -(qsoln + qcal) Remember that the enthalpy change of a reaction is expressed with respect to the stoichiometry of the equation. From the equation above (as well as the first Pre-lab question), we can see that for all acids studied in this lab, the stoichiometry of the reaction allows us to find ΔH by dividing qrxn by the moles of acid reacted. ΔHrxn = qrxn moles acid When determining qsoln, the mass and specific heat capacity of your solution will depend on the salt produced by the reaction. Use the following table to assist you with your calculations (the acid and base concentrations used in this lab all result in salt solutions that should be approximately 1.0 M) Solution Sodium chloride (1.0M) Sodium sulfate (1.0M) Sodium phosphate (1.0 M) Water Specific heat capacity (J/g·C) 3.89 3.76 3.80 4.184 Density (g/mL) 1.04 1.12 1.11 1.00 Procedure Part A: Determining the Calorimeter Constant 1. To make your calorimeter, simply obtain two Styrofoam coffee cups, place one inside the other, a stick a thermometer through a lid. 2. Using a graduated cylinder, measure 50.0 mL of deionized water and transfer it to the calorimeter. 3. Place the calorimeter on a ring stand and, using a clamp, suspend a thermometer in the water. Make sure the thermometer isn’t touching the sides or bottom of the cup. 4. Using a graduated cylinder, measure a second 50.0 mL sample of water and transfer it to a 100-250mL beaker. 5. Use a hot place to heat the beaker of water to approximately 15 C above room temperature. 6. Record the initial temperatures of two water samples. Immediately after, pour the hot water into the calorimeter. 7. Measure the maximum temperature reached by the mixture. 8. Repeat steps 1-7 for a second determination. Part B: Determining the Enthalpy Change of a Neutralization Reaction 1. Using a graduated cylinder, measure 50.0 mL of 2.0 M NaOH and transfer it to the calorimeter. Return it to the ring stand and reinsert the thermometer, as described in Part A. Note the initial temperature of the solution in the calorimeter. 2. Rinse the graduated cylinder and measure 50.0 mL of the acid assigned to you (0.67 M H3PO4, 1.0 M H2SO4 or 2.0 M HCl). 3. Add the acid to the calorimeter and measure the maximum temperature obtained by the solution 4. Repeat steps 1-3 for a second determination Waste Disposal All solution waste can be disposed of by pouring down the drain with running water. The coffee cups should be rinsed and returned to the reagent area. Name: _____________________________ Section: ________ Data Part A: Determining the Calorimeter Constant Trial 1 Trial 2 Volume of room temperature water, mL ________ ________ Mass of room temperature water, g ________ ________ Volume of hot water, mL ________ ________ Mass of hot water, g ________ ________ Initial temperature of room temperature water, C ________ ________ Initial temperature of hot water, C ________ ________ Final (max.) temperature of mixture, C ________ ________ ΔT of room temperature water, C ________ ________ qrt, J ________ ________ ΔT of hot water, C ________ ________ qhot, J ________ ________ In the space below, show your work for calculating q rt for trial 1 In the space below, show your work for calculating qhot for trial 1 Heat absorbed by calorimeter, J ________ ________ Ccal, J/C ________ ________ In the space below, show your work for calculating Ccal for trial 1. Average Ccal ________ Part B: Determining the Enthalpy Change of a Neutralization Reaction Acid ________________ Concentration ________ Base ________________ Concentration ________ Salt produced by reaction ________________ Trial 1 Trial 2 Volume of base, mL ________ ________ Volume of acid, mL ________ ________ Total volume of solution, mL ________ ________ Total mass of solution, g (assuming 1.0 M salt conc.) ________ ________ Initial temperature of solution, ºC ________ ________ Final (max.) temperature of solution, C ________ ________ ΔT, C ________ ________ Heat absorbed by solution, J ________ ________ ________ ________ In the space below, show your work for calculating q soln of trial 1 Heat absorbed by the calorimeter, J In the space below, show your work for calculating qcal of trial 1 qrxn, kJ ________ ________ Moles of acid reacted ________ ________ ΔHrxn, kJ ________ ________ Average ΔH, kJ In the space below, show your work for calculating q rxn and ΔHrxn from trial 1 ________ Name: _____________________________ Section: ________ Post-lab Questions 1. Write a balanced thermochemical equation (with phases) for the reaction you performed in this lab. 2. How would each of the following been affected (higher, lower, or no effect) if a beaker had been used as a calorimeter. Explain your answers. a) calorimeter constant b) ΔHrxn 3. How would the following affect your calculated value of q soln in Part B (incorrectly high, low, or no effect)? Explain your answers. a) The acid solution had been significantly warmer than the base. b) The thermometer in the calorimeter had an air bubble, making it read 2C too high. 4. If you used this same method to find the ΔHrxn of the reaction between Ca(OH)2 and your assigned acid, what modifications , if any, would you need to make to your calculations? Name: _____________________________ Section: ________ Pre-lab Questions 1. Write an equation for the reaction between NaOH hydroxide and each of the following a) Hydrochloric acid b) Sulfuric acid c) Phosphoric acid 2. In a calorimeter, 40.0g water at room temperature (20.4 C) was mixed with 60.0 g of water that was initially 48.9 C. The resulting mixture reached a maximum temperature of 37.3 C. Calculate each of the following. a) The heat released by the hot water. b) The heat absorbed by the room temperature water. c) The heat absorbed by the calorimeter. d) The calorimeter constant 3. 25.0 mL of a 0.50 M solution of acid HA was combined with 25.0 mL of a 0.50 solution of base MOH in a calorimeter with a calorimeter constant of 13.5 J/C. The initial temperature of the solution was 23.3 C and the maximum temperature was 34.7 C. The resulting solution had a specific heat capacity of 3.92 J/g·C and a density of 1.04 g/mL. Calculate each of the following a) The mass of the resulting solution. b) The heat absorbed by the solution. c) The heat absorbed by the calorimeter d) The heat released by the reaction e) The enthalpy change of the reaction