Enthalpy of Neutralization Procedure/Experiment

Enthalpy of Neutralization Jonniel S. Vince Cruz#1 Physical Sciences Department, College of Science, Pamantasan ng Lungsod ng Maynila General Luna St., Intramuros, Manila 1jsvincecruz@plm.edu.ph Abstract— This experiment aimed determining the enthalpy of neutralization for the strong acid-strong base and weak acid-strong base reactions. Coffee-cup calorimetry was applied to equimolar concentrations of both hydrochloric acid and acetic acid with sodium hydroxide. From the calculated heat capacity of solution, it was found out that the reaction of strong acid-strong base reaction had a mean enthalpy of neutralization of - 74844.25 kJ/mol, yielding a percentage error of 28.18%. Weak acid neutralization yielded a value of –73.60 kJ/mol-- a percentage error of 12.43%. In the analysis it is assumed that a linear relationship exists in the extrapolation of the final temperature, necessitating stabilities in temperature determinations. Keywords— Thermodynamics, Calorimetry, Enthalpy, Heat Capacity, Neutralization Introduction The enthalpy of neutralization (ΔHneutralization) is defined as the change in enthalpy for when equivalent amounts of acid and base reacts to form each weaker conjugate acid-base forms. It is also an enthalpy of reaction describing the energy released for every 1 mole of water produced in the reaction. The reaction results to an absence of separate hydrogen and hydroxide ions in solution. [1-2]. The net ionic equation for a reaction that goes to completion is H+(aq)+OH-(aq)——>H2O(l) Heat measurements from reactions are commonly carried out using a coffee-cup calorimeter in an open-air environment, a thermodynamic process occurring at constant pressure (1 bar). [3] The heat evolved in the reaction is equal to the heat calculated using the equation dqp= mcpΔT and the heat absorbed by the calorimeter at constant pressure: -ΔHneutralization = ΔHreaction + ΔHcal This also translates to the equation ΔHneutralization=- [msolutioncsolution(Tf-Ti(base))+ Cp (Tf – Ti(cal))] where Ti(cal)=Ti(acid) and csolution = cwater = 4.184 J/mol-K. To evaluate the calorimeter constant (also known as its heat capacity) in J/oC, one adds a known mass of hot water to a known mass of cold water which is in the calorimeter. Heat (Q) is lost by the hot water and is absorbed by the cold water and the calorimeter. Thus the heat absorbed by the calorimeter is the heat lost by the hot water minus the heat gained by the cold water. The initial temperature can also be approximated by Ti(cal)= (Ti(acid) + Ti(base))/2 The determination is used assuming that strong acids and strong alkalis are fully ionized in solution, [4] and that the ions behave independently, unaffected with molecular interactions for ionic pairs. [5] When a reaction is carried out under standard conditions at the temperature of 298 K (25 degrees Celsius) and 1 atm of pressure and one mole of water is formed it is called the standard enthalpy of neutralization (ΔHneutralization o). Enthalpies of neutralization are always negative – a net amount of energy is released in the bond-forming reaction. The standard state values for most reactions of strong acids and bases range from -57 to -58 kJ/mol. Weak acids and bases aren't fully dissociated in solution; thus, reactions of bound acid-base substrates will not go to completion accounting for equilibrium effects. [4] The enthalpy change in the reaction thus depends on the molecular interactions, and equilibrium constants, as affected by the nature of the reactants. This factor increases the sensitivity to changes in temperature and pressure of the reacting vessel. The bond between the proton and its conjugate base requires energy to be broken, hence the lower measured value enthalpy change. [1], [6-7] In this experiment, the enthalpy of neutralization would be determined for strong acid-strong base and weak acid weak acid-weak base reactions. METHODOLOGY A. Determination of the Cp of the Calorimeter Two hundred milliliters of distilled water was heated to boiling in a 500 mL beaker using a hot plate. A calorimeter was set-up as similar as shown in figure 1. Two Styrofoam cups were place in a 250 mL beaker, with the cover as a 4”x 4”piece of cardboard with a punched hole at the center. The top of the Styrofoam cup was taken and added with 40g water. The cup was taken back to the calorimeter and inserted with thermometer. The initial temperature was recorded and a weighed 40 mL hot water was recorded for its temperature before immediately pouring into the calorimeter. Temperature rise of the cool water was recorded every minute until the temperature levels off to a constant value. The temperature (y-axis) versus time (x-axis) was plotted to extrapolate the instantaneous final temperature after mixing of hot and water. Linear regression analysis was used to determine the equation and y-intercept of the curve. The formula for the average heat capacity was calculated using the following equation: Cp= [-mhotcwater(Tf-Ti(hot)+mcoolcwater(Tf-Ti(cool))]/(Tf-Ti(cool)) B. Determination of Enthalpy of Neutralization (Strong Acid-strong Base A clean, dry calorimeter was prepared, dried and weighed using analytical balance. Using dilution, 100 mL of 0.5 M HCl was added to the calorimeter, noting the initial temperature by placing the thermometer with the cover. The solution was stirred slowly, and recorded for its temperature Ti (cal). Then, a 100 mL of 0.5 M NaOH was placed in a clean, dry Erlenmeyer flask. Also, the temperature of the base was recorded (Ti(base)). The base solution was transferred into the calorimeter with acid. The temperature was tracked every minute starting when one-half of the solution had been transferred. The solution was slowly stirred while recording its temperature at 30-second intervals. This procedure was continued for five to six minutes after the temperature reaches the maximum level. The calorimeter and its contents were weighed to find the salt solution produced in the reaction. Three trials were conducted and the enthalpy of neutralization was calculated. C. Determination of Enthalpy of Neutralization (Strong Acid-strong Base Similar procedure was done for weak acid-strong base reactions, using a prepared 100 mL 0.5 M acetic acid by dilution. For both reactions, the final temperature Tf was calculated by performing regression analysis by using time as x, and y as temperature, obtaining the y-intercept that would be generated from the curve, as shown below: RESULTS AND DISCUSSIONS A. Determination of the Cp of the Calorimeter In the experiment, boiling water was mixed with a room-temperature distilled water, and was tracked for its immediate temperature to determine the heat capacity of calorimeter. The following measurements were gathered: TABLE Heat Capacity (Cp) of the Calorimeter Trial 1 2 3 Mean s wt. of water/g 42.5826 40.5735 41.2108 41.455 1.0267 Ti (water) /K 298.15 299.15 301.15 299.48 1.5275 wt. of hot water /g 21.9753 32.606 28.4593 27.680 5.358 Ti ( hot water)/K 373.15 373.15 373.15 373.15 0 Tf /K 320.271 324.525 322.792 322.52 2.1391 Cp/ (J/K) 397.951 431.181 449.478 426.20 26.112 The specific heat of water used is 4.184 J/ g-K, and the specific heat of ice is 2.110 J/ g-K. The extrapolation graph was plotted as shown in Figure 3. From subsequent extrapolations and calculations using Cp equation a mean value of 426.2039 J/K was obtained using three trials. B. Determination of Enthalpy of Neutralization (Strong Acid-Strong Base For the enthalpy of neutralization using hydrochloric acid and sodium hydroxide, the following data was gathered: jkj TABLE II Enthalpy of Neutralization (strong acid-Strong base); ’; Trial 1 2 3 Mean s wt. of water/g 194.1477 200.143 197.873 197.38 3.0272 Ti (acid) /K 301.95 301.35 300.95 301.42 0.5033 TABLE II, cont’d. Trial 1 2 3 Mean s Ti (base)/K 302.65 301.95 301.95 302.18 0.4041 Ti (cal)/K 301.95 301.35 300.95 301.4 0.503 Tf /K 305.596 304.869 304.27 304.9 0.66 ΔrxnH (J/mol) -78930.76 -78785.45 -66816.53 -74844 6952 Enthalpy of Neutralization (strong acid-Strong base) Extrapolations of values are shown Figure 4 below. The accepted value for enthalpy of neutralization is -104.21 kJ/mol, whereas the mean experimental enthalpy of neutralization is - 74844.25 kJ/mol, yielding a percentage error of 28.18%. Some points of error could be accounted by erroneous methods during actual measurements. Concentrations of solutions are not analytical since a beaker instead of volumetric flask was used. A graduated cylinder was used for measuring volumes of relatively large quantities, where a pipette was used repeatedly, propagating the errors in calculations. Uncertainties in the glass wares even led to the problematic errors for the final calculations. Chaser solvents cannot be used since water is the analyte, which causes inaccuracies due to incomplete amount as expected in the experiment, leading to negative error; a false positive for an analysis. The pH meter was used just as an alternative, which could even lead to larger uncertainties for expired applicability of calibrations. A digital laboratory thermometer could have prevented errors in temperature readings. Also, it is assumed that the specific heat of the solution is the same as the specific heat of water, which is not true, and depends on the species present in the solution. The temperature is not exactly met at room temperature, and the actual atmospheric condition does not match the theoretical values; even causing serious errors which causes deviation at probable conditions. The temperature was not regulated, that is, no conditioning system sustains the devices inside the laboratory causing resistance error due to heat generated. C. Determination of Enthalpy of Neutralization (Weak Acid-Strong Base For the enthalpy of neutralization using acetic acid and sodium hydroxide, the following data was gathered: TABLE II Enthalpy of Neutralization (strong acid-weak base) Trial 1 2 3 Mean s wt. of water/g 199.3111 209.2778 201.3404 203.3097667 5.2671 Ti (acid) /K 300.75 301.45 302.05 301.416667 0.6506 Ti (base)/K 302.55 302.45 302.25 302.416667 0.1528 Ti (cal)/K 300.75 301.45 302.05 301.416667 0.6506 Tf /K 304.8264706 305.1136364 304.95 304.963369 0.14405 ΔrxnH (J/mol) -72715.93292 -77875.69764 -70209.87074 -73600.50043 3908.717217 Extrapolations of values are shown Figure 5 below. The accepted value for enthalpy of neutralization is -84.050 kJ/mol, whereas the mean experimental enthalpy of neutralization is – 73.60 kJ/mol, yielding a percentage error of 12.43%. The primary error in this experiment could be attributed to the temperature changes and resistance error provided by the devices in the weight and temperature measurement. Secondary error could be mainly due to the inaccuracy of instruments used especially the purity of the reagent being used. The grade of both acids and bases are unknown, even the assay was not included in the label of the container. The values for extrapolation are inconsistent for every trial, which signals the imprecision of results. Also, the specific heat capacity used was determined long time ago, which may change during the actual experimentation method. Thus, it was recommended to use analytical reagents, and regulate the temperature, and use high-grade glass wares intended to decrease the error in the experiment. The experiment led to the insight for the presence of deviations of most solutions from ideality of theoretical values depending on the environmental conditions, since the magnitude of these heat effects depends on the following parameters pressure, temperature, physical state of the reactants and the products, and amount of substance involved in the reaction. [8] The thermodynamic principles could then be applied to supply energy for industrial applications. With the increasing complexity of molecules, its interactions with each reactant became larger, causing more corrections and calibration techniques while adopting more principles from specie being studied. Conclusions The enthalpy of neutralization (ΔHneutralization) is the change in enthalpy that occurs when equivalent amounts of acid and base reacts to form each weaker conjugate acid-base forms. Determination of the heat evolved in a reaction is done using a calorimeter. The heat evolved in the reaction is equal to the heat calculated using the equation dqp= mcpΔT and the heat absorbed by the calorimeter at constant pressure. ∆H is negative, in neutralization, implicating an exothermic reaction. [9] In the analysis, coffee-cup calorimetry was applied to equimolar concentrations of both hydrochloric acid and acetic acid with sodium hydroxide. From the calculated heat capacity of solution, it was found out that the reaction of strong acid-strong base reaction had a mean enthalpy of neutralization of - 74844.25 kJ/mol, yielding a percentage error of 28.18%. Weak acid neutralization yielded a value of –73.60 kJ/mol—giving a percentage error of 12.43%. In the analysis it is assumed that a linear relationship exists in the extrapolation of the final temperature, necessitating stabilities in temperature determinations. Thus, it was recommended to perform calibration checks on each of the devices, and to use glass wares intended for every aspect of determination to decrease the error in the experiment. References “Enthalpy of Neutralization.” Wikipedia, the Free Encyclopedia, September 6, 2014. http://en.wikipedia.org/w/index.php?title=Enthalpy_of_neutralization&oldid=596918521. “Heat of Neutralization.” Chemwiki.ucdavis.edu. Accessed September 28, 2014. http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/Calorimetry/Virtual%3A_Calorimetry/Heat_of_Neutralization. “Enthalpy of Neutralization.” Accessed September 28, 2014. http://www.ccri.edu/chemistry/courses/chem_1100/wirkkala/labs/Enthalpy_of_Neutralization.pdf. “Exp: Enthalpy of Neutralization.” Accessed September 28, 2014. http://chemskills.com/?q=heat_of_neutralization. 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