Lab 12
Calorimetry Calorimetry Objectives To perform simple calorimetry experiments. To use calorimetry results to calculate the specific heat of an unknown metal. To determine the enthalpy of neutralization for a strong acid-strong base reaction Calorimetry Fall18 | 1 Introduction Any chemical or physical change involves a transfer of heat (energy), where heat may exit the system (exothermic) or be absorbed by the system (endothermic). The amount of heat that flows into or out of the system is determined with a technique called calorimetry (heat measurement). A calorimeter is a laboratory apparatus that is composed of an insulated container, a thermometer, a mass of water, and the system to be studied, and is used to measure the quantity and direction of heat flow accompanying the chemical or physical change. Heat is measured in the energy units, Joules (J), defined as 1 kg.m2/s2. Another common unit is the calorie (cal) which is defined as the heat required to raise the temperature of 1-g of water by 1C. Enthalpy (or heat) of reaction, H, is the quantitative expression used to express the heat change in chemical reactions that are at constant pressure. H values are negative for exothermic reactions and positive for endothermic reactions and are often expressed as J/mol or kJ/mol. The specific heat of any substance can be determined in a calorimeter. The specific heat is an intensive physical property of a substance (independent of sample size) and is the quantity of heat necessary to raise the temperature of 1-g substance by 1C. Specific heat of some common substances are listed in Table 1. Note that either C or K can be used for the change in temperature, since the difference in a degree is the same for both scales. Table 1: Specific Heat Values for Some Common Substances Substance Specific Heat (cal/g C) Specific Heat (J/g C) Substance Specific Heat (cal/g C) Specific Heat (J/g C) Lead 0.0382 0.160 Iron 0.107 0.450 Tin 0.0502 0.210 Table salt 0.207 0.864 Silver 0.0564 0.236 Aluminum 0.216 0.902 Copper 0.0944 0.395 Wood 0.406 1.700 Zinc 0.0932 0.390 Water 1.00 4.184 The specific heat of a substance can be calculated by: Or, rearranging for energy, (2) q is the symbol given for heat and ΔT is the temperature change of the substance, where ΔT = Tfinal - Tinitial Part A: Specific Heat of a Metal When two objects at different temperatures come into contact, heat flows from the hotter to the colder object until the two objects reach the same temperature (Tf). In part A, the specific heat of a metal that does not react with water is determined by (1) heating a measured mass of metal, M, to a known (higher) temperature; (2) which is then placed into a measured amount of water at a known (lower) temperature; and (3) the final equilibrium temperature of the system after the two are combined is measured. The heat lost by the hot metal (indicated by negative sign – exothermic) is equal to the heat gained by the cooler water (indicated by positive sign – endothermic) which is expressed by, (3) Substituting equation (2) into equation (3), (4) Rearranging for specific heat of metal gives These equations assume no heat loss to the calorimeter when the metal and water are combined. Part B: Enthalpy (Heat) of Neutralization of an Acid-Base Reaction The transfer of heat that results from an acid/base neutralization reaction carried out at constant pressure is called the enthalpy of neutralization, ΔHn. The reaction to be studied is: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) As with any chemical reaction, the extent of the reaction is dependent on the amount of limiting reactant present. Given the moles of limiting reactant used in this reaction and the measured heat of the reaction (q), ΔHn can be determined. Heat evolved for the reaction is determined by (1) assuming the density and the specific heat for the acid and base solutions are equal to that of water, and (2) measuring the change in temperature, ΔT , when the two are mixed: (6) The mass (grams) of solution equals the combined masses of the acid and base solutions. Hn will be reported in units of kilojoules per mole (kJ/mol) of water produced. Thus, the amount of water that can be produced given the amounts of each reactant must be calculated, and ΔHn is calculated as follows: Calorimetry Experimental Procedure Part A: Determination of the Specific Heat of a Metal Materials: Calorimeter equipped with aluminum cup, stirrer, thermometer and lid, 25-mL graduated cylinder, 200-mm test tube, unknown metal, hot plate, 400-mL beaker, second thermometer for boiling water bath. Prepare a boiling water bath in a 400-mL beaker as shown in Figure 1. Prepare the metal 1. Obtain a jar of “unknown metal” and record the identification code on the Report Sheet. Obtain about 10cm3 volume of the metal using a dry 25-mL graduated cylinder. Transfer the metal into a pre-weighed, dry 200-mm test tube, and record the mass of the metal and test tube on the Report Sheet. Determine the mass of the metal by subtraction. 2. Place the 200-mm test tube with the dry metal in the water bath, making sure that the water level is well above the metal in the test tube (Figure 1). Heat the water to boiling and maintain the temperature for 10 minutes so the temperature of the metal will reach thermal equilibrium with the boiling water. Prepare the calorimeter while the water is heating. Prepare the water for calorimeter and data collection sheet 3. Obtain a calorimeter equipped with stirrer, thermometer and a lid (Figure 2). Measure and record the mass of the calorimeter cup on the Report Sheet. Using the 25-mL measuring cylinder, add ~20 mL water to the calorimeter and weigh the calorimeter cup with the water. Determine the mass of water by subtraction and record on the Report Sheet. 4. Observe the temperature of water in the calorimeter until the temperature remains constant for at least 20 seconds. Record this initial temperature of water on the Report Sheet and Data Table 1 (indicated by -20 sec). Record the temperature of the metal and transfer the metal to water 5. After the metal/test tube have been immersed in the boiling water bath for at least 10 minutes, record the temperature of the boiling water (the initial temperature of the metal) on the Report Sheet. NOTE: Have a timer ready for the next step and start the timing as soon as the metal is added to the cool water. Use the time intervals found in Data Table 1 as the guideline for collecting your data. 6. Remove the test tube from the hot water bath using a mitten and quickly transfer only the metal into the calorimeter through the small hole. Do this quickly but carefully so no metal pieces spill out. Replace the blue lid, swirl the contents gently and start the timer. 7. Record the temperature readings on Data Table 1. 8. Plot the data – instructions are given in ‘Plot the data – Figure 3’ box on next page. 9. Identify Ti(water) (initial temperature of water in calorimeter before metal is added), Tmax (Maximum temperature of metal and water mixture) and T (temperature change of water) clearly on the graph and record these values on the Report Sheet. 10. Complete the calculations for specific heat of a metal on the Report Sheet. Complete Trial 2 - Repeats parts A.1 through A.10 for the same dry metal sample. Allow the calorimeter to cool to room temperature before starting the second trial. DISPOSAL AND CLEANUP Dry the metal using a paper towel and place in the appropriately labeled “drying” beaker – do not place wet metal back in original container and do not mix dissimilar metals. Rinse and dry all calorimeter components thoroughly using a paper towel, and return to original storage. Part B: Enthalpy (Heat) of Neutralization of an Acid-Base Reaction Materials: Calorimeter equipped with plastic cup, stirrer, thermometer and lid, second thermometer, two 25-mL graduated cylinder, 25.0-mL of 1.1 M HCl, 25.0-mLof 1.1 M HNO3, 50.0-mL of standardized 1.0 M NaOH 1. Measure 25.0-mL of 1.1 M HCl in a clean 25.0-mL graduated cylinder. Record the molarity of the acid solution, as indicated on the bottle, on the Report Sheet. 2. Using a second clean graduated cylinder, transfer 25.0-mL of standardized NaOH solution to the dry calorimeter setup and replace the lid (Figure 2). Record the precise molarity of the NaOH solution, as indicated on the bottle, on the Report Sheet. 3. Initial temperature of HCl and NaOH can be recorded on the Report Sheet as well as on the Data Table 2 (indicated by -20sec) by reading the temperature from the thermometer (since all solutions are at room temperature) 4. Carefully but quickly add the acid to the base, replace the lid on the calorimeter and stir gently. 5. Read and record the temperature every 5 seconds for the first 1 minute and then every 30 seconds for about 5 minutes on Data Table 2. 6. Plot the Data – instructions are given in ‘Plot the data – Figure 3’ box on next page. 7. Indicate Ti(acid and base) (average initial temperature of acid and base in calorimeter), Tmax (Maximum temperature of mixture) and T (temperature change of neutralization) clearly on the graph and record these values on the Report Sheet. Repeat parts B.1 through B.7, replacing 1.1 M HCl with 1.1 M HNO3. Allow the calorimeter to cool to room temperature before starting the trial with HNO3. On the Report Sheet, compare your Hn values for the two strong acid-strong base reactions. DISPOSAL AND CLEANUP Dispose of all solutions in the sink with plenty of running tap water. Rinse and dry all calorimeter components thoroughly using a paper towel, and return to original storage. Page intentionally left blank Calorimetry Pre-Laboratory Assignment Date _____________ Name ________________________________ Partner(s) _______________________ 1. What is the purpose of the insulated wall in a calorimeter? As a calorimeter is being used to measure the teat/temperature change from a reaction, the insulated walls of the calorimeter is used to make sure that there is