Hess’s LawSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9CHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s LawGeneral Chemistry 101/102General Chemistry 101/102Laboratory ManualLaboratory ManualUniversity of North Carolina at WilmingtonUniversity of North Carolina at WilmingtonCHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s Law•PurposePurposeTo use Hess’s Law to determine the enthalpy To use Hess’s Law to determine the enthalpy change (change (H) for a chemical reaction that cannot H) for a chemical reaction that cannot be easily measured by normal experimental be easily measured by normal experimental methods.methods. •Safety and Waste ManagementSafety and Waste ManagementThe concentration of HCl used in this experiment The concentration of HCl used in this experiment will eat holes in your clothing and will cause burns will eat holes in your clothing and will cause burns to your skin. Handle with care.to your skin. Handle with care.The products of the reactions in this experiment The products of the reactions in this experiment can be poured down the sink and flushed with can be poured down the sink and flushed with water.water.CHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s Law•BackgroundBackgroundWhen a chemical reaction is carried out in When a chemical reaction is carried out in aqueous solution, the heat lost or gained by the aqueous solution, the heat lost or gained by the reaction is equal to the heat gained or lost by the reaction is equal to the heat gained or lost by the solution.solution.(1) qsolution = - qreactionThe minus sign is necessary since heat loss is an exothermic process (q is negative) while heat gain is an endothermic process (q is positive).CHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s Law•BackgroundBackgroundThe enthalpy change (The enthalpy change (H) for a chemical reaction H) for a chemical reaction taking place in aqueous solution can be calculated taking place in aqueous solution can be calculated from the following relationship :from the following relationship :specific heat capacity of solution (4.18 J/gºC)mass of solution (in grams)temperature change (in ºC)moles reactantHrxn =s x g x T-qsolnmoles reactant=CHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s Law•BackgroundBackgroundIn today’s experiment, you will determine the value In today’s experiment, you will determine the value of of H for this reactionH for this reaction(1) CaCO3(s) + H2O(l) Ca(OH)2(s) + CO2(g) Hrxn(1)By experimentally determining the value of By experimentally determining the value of H for H for each of these two reactionseach of these two reactions(2) CaCO3(s) + 2 HCl(aq) CaCl2(s) + H2O(l) + CO2(g) Hrxn(2)(3) Ca(OH)2(s) + 2 HCl(aq) CaCl2(s) + 2 H2O(l) Hrxn(3)According to Hess’s LawAccording to Hess’s LawHrxn(1) = Hrxn(2) + (-Hrxn(3))CHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s Law•ProcedureProcedureWeigh 0.90 g of CaCOWeigh 0.90 g of CaCO33 into a coffee into a coffee cup calorimeter. Add 5.0 mL of water cup calorimeter. Add 5.0 mL of water to the calorimeter and stir the mixture.to the calorimeter and stir the mixture.Place a temperature probe in the calorimeter Place a temperature probe in the calorimeter and start measuring the temperature. and start measuring the temperature. After 1 minute, pour 20.0 mL of After 1 minute, pour 20.0 mL of HClHCl(aq)(aq) into the calorimeter. Stir into the calorimeter. Stir the solution and continue measuringthe solution and continue measuringthe temperature until it finally begins the temperature until it finally begins to decrease.to decrease.Clean and dry the calorimeter and repeat this Clean and dry the calorimeter and repeat this procedure using 0.60 g of Ca(OH)procedure using 0.60 g of Ca(OH)22..CHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory ManualHess’s LawHess’s Law•ProcedureProcedureCHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory Manualpower connector and power connector and serial/USB connectorserial/USB connectorUSB connector on computerUSB connector on computertemperature probe connected to temperature probe connected to the Microlab unitthe Microlab unitCHM 101/102CHM 101/102Laboratory ManualLaboratory ManualLaboratory Manual112233graph areagraph areadata tabledata tablelive datalive