In the previous experiment you explored the heat of solution of potassium hydroxide. Using Hess s law, you will now use those results in conjunction with the data collected in this experiment to determine the enthalpy of neutralization for the reaction between potassium hydroxide (KOH) and hydrochloric acid (HCl). 10.1 Prelaboratory Assignment Read this lab guide and Section 6.7 in your textbook. Then, complete the prelab assignment in Chem21. The prelab assignment is due at the beginning of lab on the day your lab meets. 10.2 Introduction Enthalpy is a state function, meaning the enthalpy for a process only depends on the initial and final states of the system. Hess s law states that when several reactions are added together, their enthalpies can be added together to give the enthalpy for the sum of the reactions. For example: A + B C H 1 C D + E H 2 A + B D + E H = H 1 + H 2 If you need to reverse a reaction, the enthalpy is negated: C A + B H = H 1 Finally, if you need to multiply a reaction by a coefficient, the enthalpy is also multiplied by the same coefficient: 3(A + B C) H = 3 H 1 In the previous experiment, you determined H soln, the enthalpy for the dissolution of KOH (Reaction 1, below). In this experiment, you will determine H neut, the enthalpy for the neutralization of solid KOH (Reaction 2, below). You will then combine these 1
values using Hess s law to calculate H for the neutralization of aqueous KOH with HCl (Reaction 3, below). Finally, you will perform Reaction 3 and compare your experimental result with the theoretical result you calculated. 10.3 Precautionary Measures KOH is a strong base and HCl is a strong acid. Both are extremely corrosive chemicals that will burn your skin and eat holes in your clothing. Wear your safety goggles at all times and wear gloves when handling solid KOH, KOH solutions, and HCl solutions. If small quantities of solid KOH, KOH solution or HCl solution are spilled on you, rinse the chemicals off for several minutes with a stream of cold water from the sink. Also inform your instructor right away. In the case of a small spill on a countertop, wipe up the chemicals with a paper towel, and rinse the area thoroughly with water. In the case of a larger spill, seek the assistance of your instructor. 10.4 Experimental In this experiment, you will be graded on your accuracy. Set up your lab notebook. You will be recording temperatures over time as you did in the previous experiment. This data should be recorded in a table. Insert a thermometer in a Styrofoam cup that has been placed on a magnetic stir plate. (Do not turn on the heater!) Also, put a magnetic stirbar in the cup. For each reaction you will start with a measured volume of hydrochloric acid solution in the Styrofoam cup. You will turn on the stirrer and establish a baseline temperature by recording the temperature of the solution (to the nearest tenth of a degree) every 15 seconds for approximately two minutes. You will then add a known mass of solid KOH or a known volume of KOH solution to the HCl solution and continue to record temperatures every 5 seconds in a table in your notebook until the temperature of the water remains the same for 1 1 2 minutes or decreases for 5 measurements in a row. The initial temperature (T i) is the last baseline temperature reading before the KOH is added. The final temperature (T f ) is the highest temperature, not the last temperature reading recorded. Reaction 1: KOH(s) KOH(aq) Reaction 1 was completed during the last experiment, so we only need the data from that experiment for this part. You should have already recorded that data in your lab notebook as part of your prelaboratory assignment. 2
Reaction 2: KOH(s) + HCl(aq) H 2 O(l) + KCl(aq) Use a graduated cylinder to obtain approximately 100 ml of 1.5 M HCl solution. Record the exact volume and concentration, and then pour it into the cup. Establish a baseline temperature as described above. In the meantime, weigh between about 3 g of KOH into a weigh boat, being certain to record the exact mass of KOH. (Remove the weigh boat from the balance pan when adding KOH to avoid spills.) As quickly as possible, start timing, and add the KOH to the HCl solution. Record the temperature as described above. After recording the temperature, use litmus paper to determine whether your final solution is acidic or basic. Enter the result in your notebook and use that observation to determine which is the limiting reagent, KOH or HCl. Use tweezers to remove the stir bar, and pour the solution into an 800 ml beaker so you can dispose of it properly later. DO NOT POUR THE SOLUTION DOWN THE DRAIN. Rinse and dry your cup, stirbar, thermometer and weigh boat. Then, repeat this process so you will have two values for H neut to average together. Reaction 3: KOH(aq) + HCl(aq) H 2 O(l) + KCl(aq) Use a graduated cylinder to obtain approximately 50 ml of 1.5 M HCl solution. Record the exact volume and concentration, and then pour it into the cup. Establish a baseline temperature as described above. In the meantime, rinse and dry the graduated cylinder, and obtain about 50 ml of the 1 M KOH solution, being certain to record the concentration and the exact volume. As quickly as possible, start timing, and add this solution to the HCl solution. Record the temperature as described above. After recording the temperature, use litmus paper to determine whether your final solution is acidic or basic. Enter the result in your lab notebook and use that observation to determine which is the limiting reagent, KOH or HCl. Use tweezers to remove the stir bar, and pour the solution into an 800 ml beaker so you can dispose of it properly later. DO NOT POUR THE SOLUTION DOWN THE DRAIN. Rinse and dry your cup, stirbar, thermometer and weigh boat. Then, repeat this process so you will have two values for H to average together. 10.5 Hazardous Waste and Clean-up 10.5.1 Hazardous Waste At the end of the lab, transfer all of your waste material to the provided receptacle in the hood. 3
10.5.2 Clean-up Dry your glassware (including your cup - don t throw it away!) and return it to your station. Then, wipe down your benchtop with a wet paper towel and dry it. Have your instructor check your station before your leave. BEFORE YOU LEAVE LAB: Tear out the carbon-copy pages of your notebook. Make sure your name, your partner s name and your section number are on each page. Staple these pages together and turn them in to your instructor. 10.5.3 Lab Assignment Enter all of your data, calculations and answers to questions in the Lab 10 Assignment in Chem21. You will need to refer to your notebook for this. The Assignment is due approximately 15 minutes before your next lab meeting. 10.6 Calculations The calculations are somewhat repetitive, so you may want to set up a table in your lab notebook to keep track of the results. Your table should look something like the table below. Reaction 2 Reaction 3 Trial 1 Trial 2 Trial 1 Trial 2 Volume of HCl Volume of KOH - - Total Volume Mass of water T q H2 O q rxn Mass of KOH - - Volume of KOH (L) - - Moles of KOH Volume of HCl (L) Moles of HCl Litmus Color Moles of Limiting Reagent H rxn Average H rxn For each trial of each reaction, calculate the total volume of the solution. 4
the mass of water. 1.00 g/ml.) (Assume this is the mass of the solution and the density is T (= highest temperature - initial temperature) q H2 O (= m H2 OC T where C = 4.184 J/g C) q rxn (= q H2 O) the moles of KOH used. (When solid KOH is used, convert mass to moles using MW = 56.11 g/mol. When aqueous KOH is used, use the molarity of the solution to convert volume to moles.) the moles of HCl. (Use volume and molarity.) which is the limiting reactant, HCl or KOH? (You should be able to tell by the number of moles of each that was used and your result should be confirmed by the litmus test.) H rxn (= q rxn /n where n is the number of moles of the limiting reagent.) Average the H rxn from each trial for each reaction. Use Hess s Law to calculate H for Reaction 3 using your values for H soln and H neut according to the formula you wrote in your prelab. Calculate the percent difference between the experimental H for Reaction 3 in your table and your calculated value above. Percent difference = calculated value experimental value experimental value 100% 5