Calorimetry and Hess’s Law Essay

Elemental milligram is one of the principal components of fl atomic number 18s used to pass night cadence activities, or to aid in signaling ones location to aircraft and ships. Your instructor whitethorn ignite a strip of magnesium ribbon to demonstrate the flame of magnesium in air. It all toldow for be evident that a great deal of light postcode is released from this response. A direct order for measuring the warming produced by this reaction 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 aptitude 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 pitch is sterilised by the particular reaction as well as the aggregate of increase(s) formed. The thermal energy transferred in a braced chemical reaction carried out at continuous mash is called the enthalpy of reaction (or enkindle of reaction) and isgiven the symbol Hrxn. Hrxn is often expressed in units of kJ/ wall where mole refers to the amount of a reactant or a product involved in the reaction. In general, the reactant or product must be specified. In this experiment, you exit measure the enthalpy changes of several exothermic reactions utilizing a simple-minded calorimeter. This calorimeter consists of an insulated vessel (a Styrofoam cup), a thermometer, and a hat (which is loose fitting to allow the pressure to remain constant. The energy given off by any reaction carried out in the calorimeter is absorbed by both the calorimeter and the resolving power ( piddle). This causes an increase in the temperature of the calorimeter and solvent that can 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 force of the calorimeter and sol vent, and T is the change in temperature of the water (the solvent) in the calorimeter. Heat 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. total heat is an extensive 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 particularized reactant or product and H is the enthalpy change of the reaction in kJ/mol. Since the energy of the universe is conserved, the pith 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 m ain observational problem in any calorimetric measurement is obtaining anaccurate value of T. The initial temperature, Ti, of the reactants can be set(p) directly using a thermometer. However, it is difficult to obtain a precise value for the terminal temperature, Tf (the instantaneous temperature when the reactants are mixed together and react), because (1) reactions do non occur instantaneously, and (2) calorimeters are not abruptly insulating, but actually allow some heat energy to slowly enter or get around from the calorimeter over clip. This occurs both during the reaction and afterward its completion. If an exothermic reaction occurs in a hypothetical calorimeter that is absolutely 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 reaction that occurs instantaneously, but in a veri dicalistic calorimeter that is not perfectly insulated. In this case, 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 maximum 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 made by an extrapolation process using the temperature vs. time curve (see Figure 1). First, a plo t 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 manner will be the temperature that the calorimeter and the solvent would set out reached, had the reaction occurred instantaneously and with no heat exchange to the room. This value should be used for the calculation 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) H2O (l) Ha = 285.84 kJ/mole(b) Mg(s) + 2 H+ (aq) Mg2+ (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 known quantity. Hb and Hc will be determined experimentally by measuring the temperature rise when known masses 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 reverse (exothermic) reaction, the information you collect will be of opposite sign to that needed for the Hesss law calculation for reaction (d). When data from your analysis is powerful combined with that for the known reaction (a), the enthalpy of combustion of magnesium metal can be obtained. performanceNote 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 Acid1. Using the graduated cylinder, add 50.0 mL of 1.0 M HCl to the empty calorimeter. Wait for a few minutes to allow the set-up to reach thermalequilibrium. 2. man waiting, determine the mass of a sample of magnesium ribbon ( near 0.15 g) on the analytical balance, and then negligee it with a piece of bull wire. The copper will not react in the solution its objective 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 enough with the HCl solution. Do not wrap the magnesium too loosely since it may escape the copper cage and float. 3. Using LoggerPro, start a transport of 500 seconds with the temperature probe in the 1.0 M HCl in the calorimeter (with lid). 4. The magnesium/copper bundle is added to the HCl solution. Replace the lid wi th the thermometer in place, and begin swirling to mix. Be sure to support the temperature probe.Continue swirling and collecting data and record 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 completely 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 approximately 0.7 to 0.8 g of MgO to a clean advisement ride (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 t ransfer and calculate the mass of MgO actually transferred to the calorimeter. 4. Record 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 solid MgO is allowed to sit on the bottom or sides of the cup it will not dissolve and hence it will not react. Make sure the solution is mixed ever 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 possible.