Enthalpy of Reaction and Hess’s Law Essay

I. PurposeThe purpose of this lab is to verify Hesss integrity through the three replys of NaOH and HCl, NH4Cl and NaOH, and NH3 and HCl. The snapper of the enthalpies of the first two reactions should equal the enthalpy of the third reaction.II. BackgroundHesss Law is used to determine the enthalpy of a reaction from adding two or more preceding reactions. To determine the enthalpies of certain chemical substance reactions, the change in must be measured. This is best done using a calorimeter to prevent disturb passing play to the surroundings. To calculate the heat change in solution, the q=mct equating must be used. A positive value for q means the solution gains heat, a oppose value means the solution loses heat and is exothermic. The reactions used in the lab are exothermic acid-base neutralizations. By calculating q, heat, the enthalpy of reaction sack be found by knowing the Molarity of the reactants. The specific heat of the calorimeter can be found by the equation qca l= t x heat capacity. Since a calorimeter is used the heat released from the reaction will be absorbed in the solution, while some heat is transferred to the calorimeter. So the q of the reaction is given by this equation qrxn= (qsol+qcal).III. Summary of ProcedurePart 1A calorimeter is to be arranged using Styrofoam cups and a hole on top of a cover to abridge the temperature while also preventing heat loss. 50mls of distilled is to be added to the calorimeter. 75mls must then be heated to 70 degrees Celsius and 50mls of that water is to be added to the calorimeter. The calorimeter has to be covered and then the temperature interpreted every 20 seconds for 3 minutes.Part 250mls of 2.0M HCl must be put into the calorimeter. A 50mls solution of 2.0M NaOH should then be added to the HCl solution and stirred. The temperature needs to be recorded every 20 seconds for 3 minutes. For the second reaction the same march has to be repeated using 2.0M NH4Cl and 2.0M NaOH. The third reactio n requires the same process using 2.0M solutions of NH3 and HCl.IV. Observations* Styrofoam cup becoming hot after mixture* thermometer scraping the Styrofoam cup sides* cover doesnt completely cover the cup* small touchstone of time between pouring the chemicals and covering and mixingVI. Results and QuestionsCalculations please see attached graphs and work.Post Lab Questions1. What is meant by calorimetry?Calorimetry is the scientific measuring of heat released during chemical and physical changes. It ensures that minimal heat is lost so the heat of reaction can be found accurately.2. How does graphical analysis improve the accuracy of the information?Once the points are plotted on a graph the line of best fit can be drawn and extrapolated toward the y-axis. Since the first data plots can be inconsistent the best fit line may ignore the first points making the data more accurate by disregarding the away data.3. What is the meaning of the negative sign in front of the equation f or heat of reaction?The negative sign in front of the brackets indicate that we are in fact solving for the reverse reaction. So the sign must have a negative to indicate the reaction that is the forward, exothermic, reaction.4. Do the lab results support Hesss Law?The lab does support Hesss Law. The percent shift is relatively small for this lab and algebraically it is proven that the sum of the enthalpies of the first two reactions is abutting the measured value of the third reaction.5. How could the procedure be modified to achieve greater accuracy?To improve the accuracy of the lab a formal capacitance could be used instead of average Styrofoam cups. An airtight seal between the cover and thermometer would also be more accurate by preventing heat loss to the surroundings.6. Find a table reference that lists standard heat of formation for the species included in your net ionic equations. Use them to calculate delta H for the reaction of the net ionic equations. Do these values support Hesss Law?See attached calculations.VII. ConclusionThis lab successfully verified Hesss Law by adding enthalpies of reactions to equal the enthalpy of a third reaction. The mix of NaOH with HCl, and NH4 with NaOH proved to give off a heat close to that of NH3 mixed with HCl. The percent error proved to be 3.72%. The sources of error could be the heat lost while the calorimeter was not covered and maybe not enough mixing throughout the while of the three minutes. The addition of a mixing unit and an airtight seal on the capacitors would provide an improved version of this lab.