Chemistry 151 Last Updated Dec, 2013 Lab 14: Determining the Enthalpy Change of an Acid-Base Reaction

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1 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. H na + nnaoh Na na + nh 2O 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 = mc T The calorimeter constant, C cal, is a measure of the heat absorbed by the calorimeter (q cal) 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. C cal = q cal ΔT rt q cal = q hot - q rt 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. q cal = C cal T rxn The heat released by the reaction (q rxn) will be the heat absorbed by the solution (q soln) plus that absorbed by the calorimeter. The sign will be negative to reflect an exothermic reaction. q rxn = -(q soln + q cal) 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 q rxn by the moles of acid reacted. ΔH rxn = q rxn moles acid

2 When determining q soln, 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 Specific heat Density capacity (J/g C) (g/ml) Sodium chloride (1.0M) Sodium sulfate (1.0M) Sodium phosphate (1.0 M) Water 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 mL 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 H 3PO 4, 1.0 M H 2SO 4 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.

3 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 q rt, J In the space below, show your work for calculating q rt for trial 1 ΔT of hot water, C q hot, J In the space below, show your work for calculating q hot for trial 1

4 Heat absorbed by calorimeter, J C cal, J/ C In the space below, show your work for calculating C cal for trial 1. Average C cal

5 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 q cal of trial 1

6 q rxn, kj Moles of acid reacted ΔH rxn, kj Average ΔH, kj In the space below, show your work for calculating q rxn and ΔH rxn from trial 1

7 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) ΔH rxn 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 2 C too high. 4. If you used this same method to find the ΔH rxn of the reaction between Ca(OH) 2 and your assigned acid, what modifications, if any, would you need to make to your calculations?

8 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

9 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