Calorimetry and Hesss Law Essay Example for Free

Calorimetry and Hesss Law EssayElemental atomic number 12 is angiotensin converting enzyme of the principal components of fl bes apply to illuminate nighttime activities, or to aid in signaling ones location to aircraft and ships. Your instructor may ignite a strip of atomic number 12 yarn to butt on the combustion of atomic number 12 in air. It will be evident that a great deal of alight cypher is released from this reception. A direct method for measuring the wake up produced by this chemical reply would be difficult, so we shall resort to an indirect method in this experiment as discussed below. Some chemical reactions (including the one above) are associated with the evolution of thermal energy and are called exothermic reactions. When there is absorption of energy in a chemical reaction, the process is called endothermic.The magnitude of the energy change is determined by the purposeicular reaction as well as the amount of product(s) formed. The thermal energy tr ansferred in a balanced chemical reaction carried bulge at constant pressure is called the total heat of reaction (or heat of reaction) and isgiven the symbol Hrxn. Hrxn is very much expressed in units of kJ/mole where mole refers to the amount of a reactant or a product mixed in the reaction. In general, the reactant or product must be specified. In this experiment, you will measure the hydrogen changes of several exothermic reactions utilizing a simple calorimeter. This calorimeter consists of an insulated vessel (a Styrofoam cup), a thermometer, and a lid (which is loose assignment to allow the pressure to roost constant. The energy given off by any reaction carried out in the calorimeter is absorbed by both the calorimeter and the solvent (water). This causes an increase in the temperature of the calorimeter and solvent that nookie be measured by a thermometer. The heat that is absorbed by the calorimeter and solvent is calculated from the equation qcal = C T (1)where C is the heat capacity of the calorimeter and solvent, and T is the change in temperature of the water (the solvent) in the calorimeter. awaken capacity is defined as the amount of energy required to raise the temperature of an object by 1 C. In this experiment, the vessel and the amount of solvent remain constant, so C is a constant. Enthalpy is an elongated quantity, so the amount of heat generated by the reaction is given by the expression qrxn = n H (2)where n is the number of moles of a specific reactant or product and H is the enthalpy change of the reaction in kJ/mol. Since the energy of the universe is conserved, the total energy change of the system (the reaction) and surroundings (calorimeter and solvent) is equal to zero. These relationships can be combined as shown in equation (3).qsystem + qsurroundings = qreaction + qcalorimeter = nH + CT = 0 (3)This equation can be rearranged to determine either C or H as shown in equations (4) and (5). C = nH/T (4)H = CT/n (5)For exothermic reactions, H 0 and T 0.The main experimental problem in any calorimetric measurement is containing an high-fidelity honour of T. The initial temperature, Ti, of the reactants can be determined directly victimisation a thermometer. However, it is difficult to defy a precise value for the final temperature, Tf (the instantaneous temperature when the reactants are mixed together and react), because (1) reactions do non occur instantaneously, and (2) calorimeters are non perfectly insulating, but actually allow some heat energy to slowly enter or turn tail from the calorimeter over time. This occurs both during the reaction and after its completion. If an exothermic reaction occurs in a hypothetical calorimeter that is perfectly insulated, all of the heat produced by the reaction will remain in the calorimeter, resulting in a constant final temperature. This would yield the same T whether or not the reaction is instantaneous.Now consider a hypothetical exothermic re action that occurs instantaneously, but in a realistic calorimeter that is not perfectly insulated. In this fountain, the temperature of the calorimeter would diminish over time due to the gradual escape of heat energy to the surroundings. The final temperature to be used in determining T in this case is actually the level best temperature reached immediately after reaction occurs, since this temperature change is due exclusively to the heat produced in the reaction, and no escaping of heat to the surroundings has occurred yet. For real calorimeter experiments, reactions neither occur instantaneously nor are calorimeters perfectly insulated. Thus, during an exothermic reaction the temperature of the calorimeter increases initially, but never has a chance to reach the correct maximum final temperature since heat is escaping to the surroundings even while the reaction is proceeding toward completion.A correction for this heat exchange is do by an extrapolation process using the tem perature vs. time curve (see Figure 1). First, a plot of the temperature readings as a function of time for the reaction is generated. By extrapolating only the linear portion of the curve (e.g., the points including and after the maximum temperature) back to zero time (the time when the reactants were mixed in the calorimeter), Tf is obtained. The Tf value determined in this flair will be the temperature that the calorimeter and the solvent would have reached, had the reaction occurred instantaneously and with no heat exchange to the room. This value should be used for the reckoning of change in temperature, T. Consult with your TA for specific instructions for extrapolation using Microsoft Excel.A. Determination of the Enthalpy of Combustion of Mg Using Hesss Law The calorimeter will be used to determine the enthalpy of combustion of magnesium by application of Hesss law. Consider the following reactions(a) H2(g) + O2 (g) piss (l) Ha = 285.84 kJ/mole(b) Mg(s) + 2 H+ (aq) Mg 2+ (aq) + H2 (g) Hb(c) Mg2+ (aq) + H2O (l) MgO (s) + 2 H+ (aq) HcBy adding equations (a), (b), and (c) we obtain(d) Mg (s) + O2 (g) MgO (s) Hrxn = Ha + Hb + Hcwhich represents the combustion of Mg(s).Reaction (a) represents the formation of liquid water from its constituent elements. The enthalpy change for this reaction, symbolized Ha above, is the standard heat of formation of liquid water (or Hf (H2O)) and is a cognise quantity. Hb and Hc will be determined experimentally by measuring the temperature rise when known batch of magnesium metal and magnesium oxide, respectively, are added to hydrochloric acid. Reaction (c) as written is an endothermic reaction. Since it is easier to perform the deform (exothermic) reaction, the data you collect will be of opposite sign to that needed for the Hesss law calculation for reaction (d). When data from your analysis is correctly combined with that for the known reaction (a), the enthalpy of combustion of magnesium metal can be obtaine d.PROCEDURENote Handle the Styrofoam cups gently. They will be used by other lab sectionsA. Determination of the Enthalpy of Combustion of MagnesiumReaction of Magnesium Metal and Hydrochloric loony toons1. Using the graduated cylinder, add 50.0 mL of 1.0 M HCl to the empty calorimeter. Wait for a few proceeding to allow the set-up to reach thermalequilibrium. 2. While waiting, determine the mass of a sample of magnesium ribbon (about 0.15 g) on the analytical balance, and then wrap it with a piece of copper wire. The copper will not react in the solution its purpose is to prevent the magnesium from floating to the surface during the reaction. Do not wrap the magnesium too tightly or it will not react quickly replete with the HCl solution. Do not wrap the magnesium too loosely since it may escape the copper batting cage and float. 3. Using LoggerPro, start a run of 500 seconds with the temperature probe in the 1.0 M HCl in the calorimeter (with lid). 4. The magnesium/copper bundl e is added to the HCl solution. Replace the lid with the thermometer in place, and begin swirling to mix. Be sure to support the temperature probe. watch swirling and collecting data and platter about 300 seconds or until the temperature starts decreasing. This will provide the linear part of the curve, and are the most important points for the extrapolation procedure. 5. When data collection is completed, rinse the calorimeter and thermometer with distilled water and dry as wholly as possible. Place the piece of copper in the container labeled copper waste. B. Reaction of Magnesium Oxide and Hydrochloric acid1. Place 50.0 mL of 1.0 M HCl into a clean graduated cylinder. 2. On a top-loading balance, transfer almost 0.7 to 0.8 g of MgO to a clean unhurriedness boat (no need to record this mass). Next, determine the mass of the MgO and the weighing boat on the analytical balance and record the data. Transfer the MgO to the dry calorimeter. 3. On the analytical balance, record the mass of the empty weighing boat after the transfer and calculate the mass of MgO actually transferred to the calorimeter. 4. memorialise the initial temperature (Ti) of the 1.0 M HCl solution in the graduated cylinder. 5. Note the time (time = zero) and add the 50.0 mL of 1.0 M HCl to the calorimeter containing the MgO. 7-8 points after the temperature maximum.In this reaction all the MgO should react since HCl is used in excess. However, if the hale MgO is allowed to sit on the bottom or sides of the cup it will not dissolve and hence it will not react. bring about sure the solution is mixed constantly but gently. (NOTE Before discarding this solution, check to see that all of the MgO has reacted. If solid MgO remains, the results from this portion of the experiment are not accurate. If any solid is present, this portion of the experiment must be repeated.)6. When data collection is completed, rinse the calorimeter and thermometer with distilled water and dry as completely as p ossible.