Exp 09: Heat of Reaction

Transcription

1 Your job is to use a calorimeter to determine the heat of reaction for three different chemical reactions. Each of these reactions is an acid-base neutralization reaction. Before using your calorimeter to determine the heat coming out of chemical reaction, you need to calibrate the device. You need to determine C, the heat capacity of that calorimeter, so you can use it determine heat (q) based on observations of temperature change (ΔT).

2 Part A: Determine Calorimetry Constant for your Calorimeter HOT WATER COLD WATER Setup your calorimeter by nesting one Styrofoam cup in a second. Record it's weight. In a beaker, heat ml of water to C. This will be your "hot water." Place 50 ml of "cold water" in the calorimeter, and record the new mass. By difference, record the mass of your cold water. Allow the calorimeter to set covered for 5 minutes. Hold a thermometer in the center of the cold water (do not let it touch the sides or bottom) and record the initial temperature of the cold water (this is also the initial temperature of your calorimeter). Measure out 50 ml (with a precision of.1 ml) of hot water in a graduated cylinder. Place a thermometer in the center of the graduated cylinder (do not let it touch the sides or bottom) and record the temperature of the hot water. Pour the hot water into the calorimeter and stir the solution briefly and record the temperature. This is the final temperature of the hot water, cold water, and calorimeter. HOT WATER qhot COLD WATER qcold q q + qcold = -qhot q = - qhot - qcold C ΔT = - qhot - qcold

3 Part B: Determine Heat of Neutralization for Three Reactions You will place a measured amount of an acid solution into your Styrofoam cup calorimeter. After measuring the temperature of this solution, you will add an equal amount of base solution. All solutions will be at the same initial temperature and the same concentrations. By measuring the final temperature of the liquid in the cup, you will be able to calculate the amount of heat energy released in the reaction. Measure out exactly 50.0 ml of the 2.0 M acid which you wish to react, and place the acid in the calorimeter (cup). Measure and record the temperature of the acid. Place exactly 50.0 ml of the 2.0 M base which you wish to react In a graduated cylinder, and then pour the base into the add. Stir the acid and base together with the thermometer and record the highest temperature reached by the mixture. Be sure to hold the tip of the thermometer in the center of the solution (do not let it touch the walls) in order to obtain the most accurate temperature reading. Using the data obtained, calculate the heat of neutralization (ΔH neut) for the reaction as carried out. The total volume of solution in the cup is ml. However, since the liquid is "salty water" the density is slightly greater than pure water, about 1.03 g/ml. Thus the mass of liquid in the calorimeter will be 103 g rather than 100 g. Likewise, we will assume the specific heat of a salt solution is slightly less than that of pure water, 4.05 J/g C. Carry out three different neutralization reactions so as to react a "strong + strong", a "strong + weak", and a "weak + weak" combination of acid and base. Thus, you should obtain three different values of ΔH neut. Repeat each measurement until you obtain a reasonably consistent result in each case. Acids Bases Strong HNO3 (nitric acid) NaOH (sodium hydroxide) Weak CH3COOH (acetic acid) NH4OH (ammonium hydroxide)

4 Exp 08: Gas Stoichiometry CALCULATIONS (2) Calculate the heat produced in your acid-base reaction. The heat produced will be the heat you observed as a temperature change, less the heat that was absorbed by the cup. Be cautious about the signs. The enthalpy of reaction is negative for an exothermic reaction and the q observed in the liquid and cup will be positive (heat entering the solution and cup). -1 x ΔH = qobserved + qcup qobserved = masssolution x Cssolution x ΔTsolution qcup = Ccup x ΔTcup (3) Calculate the moles of reactants consumed in the reaction. (heat of reaction is a coefficient, it must be reported not for your experiment but per mole of reactant!) ml L mol (4) Using a balanced equation, calculate the heat of reaction from your experiment. RXN RXN qrxn SOLUTION SOLUTION qsoln q ΔH = [ ( total heat observed ) / ( total moles of substance ) ] x coefficient of substance in the balanced equation 1 NaOH + 1 HNO3 1 NaNO3 + 1 H2O ΔHRXN =? kcal

5 Questions?