Energy Changes Due to Acid-Base Reactions - Lab Report Example

Lecturer’s and Number Submitted Experiment on Energy Changes Associated With Acid-Base Reactions In thisexperiment, a calorimeter was used to measure the changes in temperature which normally occurs during an acid-base chemical reaction. The Results were obtained and tabulated in a table, which were later used in the calculations of the heat energy. The heat energy, which evolved as a result of the reaction, was calculated and the reaction enthalpies compared for the four acid-base reaction. Introduction It is a common phenomenon that whenever there is a chemical reaction in a system at constant pressure while, at the same time, there is no energy gained or lost to the surrounding; it is always evident that the system would either decrease or increase. In the first instance, energy will be lost in the form of heat if the reaction is exothermic when the system gets into thermal contact with the surrounding. On the other hand, energy will be gained in the form of heat if the reaction is endothermic when the system’s temperature decreases. Enthalpy, denoted by H, is the change in heat energy of a system at a constant pressure. The SI unit of energy, which is joule (J), is similar to that of enthalpy (Hughes). Enthalpy is known to be a safe function, and can be denoted as shown below; Heat absorbed = increase in enthalpy = H = Hfinal - Hinitial -----------(i) Adiabatic conditions arise when a system is configured in such a manner that there is no heat gained or lost by the system. Therefore, in adiabatic processes the heat absorbed is always equal to enthalpy change, which is zero. Enthalpy can be changed by changing the temperature of water or even by a chemical reaction under adiabatic conditions as shown below; Hsystem = + Hdue to reaction = 0 ----------------(ii) Hdue to reaction = - -------------------(iii) Thus, enthalpy change can be calculated as follows; = Specific heat capacity x mass x T -----------(iv) Where; T = Tf - Ti For exothermic reactions, T is always positive and negative for endothermic reactions. If the measurement of change in temperature of any chemical reaction is taken under adiabatic condition then change in enthalpy due to the reaction, can be ascertained from the enthalpy change producing the change in temperature (Hughes). Enthalpy is an extensive property that in many occasions depends on the amount of reaction that occurs. Therefore, in order to obtain an intensive property in the comparison of various reactions, the enthalpy change is always related to the amount of one of the products or reactants. From this explanation, the enthalpy reaction may be given as shown below; H = ----------(v) The SI unit for Hrxn is given as J/mol, even though reaction enthalpies are normally said to be in kJ/mol. Species A may be taken as the limiting reagent since some of it needs to be reacted. In addition, reaction enthalpies are known to be calculated from values that are tabularized at standard molar enthalpies of formation, from tabulated combustion enthalpies, and from calorimetry data (Hughes). The experimental calorimetry data is the method that is chosen for this lab experiment using the various principles discussed above. The Calorimeter A calorimeter is a device that can be used in measuring the enthalpy change in a system during a reaction, but at the same time taking caution of the increase in temperature (for exothermic reaction), or decrease in temperature (for endothermic reaction) (Hughes). There are several types of calorimeter, but for this experiment the calorimeter used is the Styrofoam cups and plastic lids. Moreover, the foam between the surrounding and the liquid helps as the thermal obstacle in preventing heat loss from the reaction to the surroundings. The reactions will take place in a cup of aqueous solution, and the temperature monitored with the thermometer. There are two assumptions that ought to be made about the system and the calorimeter. First, the Styrofoam cups and plastic lids are considered ideal calorimeter, which functions adiabatically at a constant temperature. Technically, all the heat that would be evolved from the reaction will be entirely used to increase the temperature of the reaction solution, and none is lost. Second, an assumption that the final solution is dilute and its specific heat capacity and density are similar to that of water (4.184J/gC and 1gm/L, respectively) (Hughes). The change in enthalpy associated with the change in temperature will then be calculated using equation (iv) above. From this solution, the change in enthalpy due to the reaction is then obtained by using equation (iii). The solution is then converted to kJ and then the amount of limiting reagent is determined. The solutions are then used to determine AHrxn using equation (v). Chemistry of the reactions The reaction enthalpy will be determined for the two acid-base reactions shown below: i) HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) ii) HNO3(aq) + KOH(aq) KNO3(aq) + H2O(l)NO HCl and HNO3 are strong acids while NaOH and KOH are strong bases. Requirements Styrofoam cups and plastic lids Thermometer Stir bars and stir plates Metal stands, clamps, and all normal laboratory glassware Strong acid (hydrochloric acid, HCl) at three aqueous concentrations: 1.0M, 3.0M, and 6.0M Strong base (sodium hydroxide, NaOH) at three aqueous concentrations: 1.0M, 3.0M, and 6.0M A second strong acid (nitric acid, HNO3) dissolved in water at 1.0M A second strong base (potassium hydroxide KOH) dissolved in water at 1.0M Procedure 1. Styrofoam cups and the plastic lids were arranged together with the thermometer, the stir bars and stir plates in position ready for the experiment. 2. 50ml of HCl was added into the Styrofoam cup calorimeter and then measured, and the initial temperature recorded as Tinitial. 3. 50ml of NaOH was then poured rapidly into the Styrofoam cup, and the plastic lids replaced. 4. The reaction solution, in the cup, was then stirred using the stir bar. 5. The temperature of the reaction mixture was observed while continuously stirring, and the highest reading of the thermometer recorded as the final temperature, Tfinal. 6. After finishing the measurement, the solution was poured out and the cup rinses plus the stir bar and the thermometer. 7. The same procedure was repeated for HNO3 and KOH and the results tabulated as shown below. 8. 50ml of cold water was added in to the Styrofoam cup, and the initial temperature recorded. 9. 50ml of hot water was then added into the Styrofoam cup after noting it initial temperature, and the final temperature of the stirred solution recorded. Results 50ml HCL(aq) + 50 ml NaOH(aq) 1 mol 3 mol 6 mol Initial Temperature, Ti () 24 24 29 Final Temperature, Tf () 29 42 61 T () 5 18 32 50ml HNO3(aq) + 50ml KOH(aq) 1 mol Initial Temperature, Ti () 24 Final Temperature, Tf () 31 T () 7 50ml H2O Hot Cold Hot Cold Initial Temperature, Ti () 35 27 34 28 Final Temperature, Tf () 31 32 T () 4 4 Calculations Heat (Q) for both hot and cold water is given by; Qsolution = (4.184J/g)(mass in g)(T) But; density of the mixture = 1g/ml Therefore; mass in g = density x volume = 1 x 100 = 100g Thus; Qsolution = 4.184 x 100 x 4 = 1673.6 J Heat (Q) gained by the calorimeter is given by; Qcalorimeter = QHot water – Qcold water But; QHot water = 4.184 x 50 x 3 = 627.6 J Qcold water = 4.184 x 50 x 4 = 836.8 J Therefore; Qcalorimeter = 627.6 – 836.8 = 209.2 J Calorimeter constant (Ccalorimeter) is then given by; Ccalorimeter = = = 52.3 J/ To find the enthalpy change from 50ml HCL(aq) + 50 ml NaOH(aq) reaction; -Hdue to rxn = +Qrxn + Qcalorimeter But; Qrxn = 4.184 x 50 x 5 = 1046 J Qcalorimeter = 52.3 x 5 = 261.5 J Therefore, -Hdue to rxn for will be; = 1046 + 261.5 = 1307.5 J Thus, H = = 1307.5 J/mol into kJ/mol = 1.3075 kJ/mol -Hdue to rxn for 3 moles will be; Qrxn = 4.184 x 50 x 18 = 3765.6 J Qcalorimeter = 52.3 x 18 = 941.4 J Hdue to rxn = 3765.6 + 941.4 = 4707 J Thus, H = = 1569 J/mol into kJ/mol = 1.5969kJ/mol -Hdue to rxn for 6 moles will be; Qrxn = 4.184 x 50 x 32 = 6694.4 J Qcalorimeter = 52.3 x 32 = 1673.6 J Hdue to rxn = 6694.4 + 1673.6 = 8368 J Thus, H = = 1394.7 J/mol into kJ/mol = 1.3947 kJ/mol To find the enthalpy change from 50ml HNO3(aq) + 50ml KOH(aq) reaction; -Hdue to rxn for 1 mole will be; Qrxn = 4.184 x 50 x 7 = 1464.4 J Qcalorimeter = 52.3 x 7 = 366.1 J Hdue to rxn = 1464.4 + 366.1 = 1830.5 J Thus, H = = 1830.5 J/mol into kJ/mol = 1.8305 kJ/mol Discussion Changes in energy are normally as a result of chemical reactions. When energy, in the form of heat, is released into the surrounding, the reaction is referred to as exothermic. On the other hand, if the energy is absorbed from the surrounding the reaction is referred to as endothermic. Thermochemistry is a branch of chemistry that mainly focuses on the measurement of the amount of heat absorbed or evolved (Hughes). The enthalpy of neutralization is the heat evolved in an acid-base reaction as depicted in this experiment. HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) + Q In the above equation, Q is expressed in kJ/mol of water. Neutralization reaction is always exothermic, and the value of Q is always negative. Neutralisation is generally the reaction between an acid and a base leading to the formation of sat and water. During such reactions, the metallic ions and anions normally don’t change. However, the most essential factor is the formation of non-ionized molecules water from hydrogen ions and hydroxide ions. H+(aq) + OH-(aq) H2O(l) The energy change in every reaction that was carried out ought to be the same. This so because in all the reactions only 1 mole of water was produced, and the metallic ions and the –ve ions in the acid are somewhat spectator ions, therefore, do not take part in the reaction so much as to cause an energy change. It is evident that changing the concentrations of 50ml HCl(aq) + 50 ml NaOH(aq) reaction affected Hdue to rxn. The change in concentrations of both HCl and NaOH from 1mole to 3 moles and from 3 moles to 6 moles increased the change in temperature and the heat evolved, as well. However, the heat of neutralisation ought to remain constant despite the change in concentration. This is because, when the concentration of HCl and NaOH is increased there is more number of HCl and NaOH particles in the 50ml, so more heat energy is needed to carry out the reaction completely. In fact, when the concentration was doubled from 3 moles to 6 moles, the heat energy was almost doubled from 4707 J to 8368 J, respectively. Therefore, when the energy needed increases the heat evolved also increases with the increase in temperature change. The heat of neutralisation needs not to change since it is the heat produced per mole, however, in the experiment it did not turn out to be same as it changed. This may be due to some errors form the experiment. Conclusion The experiment was successful, and the aim of the experiment was met. It was also noted that concentration of the reactants is directly proportional to the change in temperature and the heat evolved, at constant volume, but it does not affect heat of neutralisation. The success of the experiment was dependable on the accuracy made in the lab while carrying out the experiment. Works Cited Hughes, Ashley A. Enthalpy of Neutralization. 7 May 2008. 9 May 2013 . Read More