2)The enthalpy of reaction(ΔHrxn) is an extensive property of a system that can beused to obtain the heat absorbed/evolved in a reaction at a given temperature andpressure.a.If at constant pressure, the qpcan be determined from the followingformula:i.ΔHrxn= qp.Introduction to Hess’s Law: 1)Hess’s law of heat summationstates that for a chemical equation that can bewritten as the sum of two or more steps, the enthalpy change for the overallequation is the sum of the enthalpy changes for the individual steps.2)An example calculation for the decomposition of 2 moles of CaCO3.a.Unknown equation: 2 CaCO3(s) barb2right2 CaO(s) + 2 CO2(g)ΔH= ? b.Known equations:i.Ca(s) + C(s) + 3/2 O2(g) barb2rightCaCO3(s)ΔHo1= –1208 kJii.C(s) + O2(g) barb2rightCO2(g)ΔHo2= – 393.5 kJiii.Ca(s) + ½ O2(g) barb2rightCaO(s)ΔHo3= – 635.0 kJc.Step 1: Flip Equation #1 (and the sign of ΔHo1)i.CaCO3(s) barb2rightCa(s) + C(s) + 3/2 O2(g)ΔHo1= –(–1208 kJ)d.Step 2: Cancel Like Terms and Add ΔHoi.CaCO3(s) barb2rightCa(s) + C(s) + 3/2 O2(g)ΔHo1= +1208 kJii.C(s) + O2(g) barb2rightCO2(g)ΔHo2= –393.5 kJiii.Ca(s) + ½ O2(g) barb2rightCaO(s)ΔHo3= –635.0 kJiv.CaCO3(s) barb2rightCaO(s) + CO2(g)ΔHorxn= +179.5 kJe.Step 3: Multiply by Entire Equation by 2 &ΔHorxni.2(CaCO3(s) barb2rightCaO(s) + CO2(g))ΔHorxn= 2(+179.5 kJ)f.Final Answer:i.2 CaCO3(s) barb2right2 CaO(s) + 2 CO2(g)ΔHorxn= +359 kJIntroduction to Specific Heat: 1)Specific heatis the quantity of heat required to raise the temperature of one gramof a substance, one degree Celsius (or Kelvin) at constant pressure.a.Specific heats are characteristic for that substance at 25oC and constantpressure.Experiment #8B Barnett
49b.Example specific heats of pure materials are listed below:Element/Compound Specific Heat (J/(g•oC))Aluminum 0.901 Iron 0.450 Lead 0.128 Copper 0.387 Water (l) 4.184 Procedure for the Specific Heat of an Unknown Metal: 1)Place approximately 400. mL of tap water into a 600 mL beaker and place onto ahotplate.a.Heat the water to the boiling point.2)While waiting for the water to boil, obtain an unknown metal cylinder anddetermine the mass on an ANALYTICAL balance.a.Record this mass in the data table attached.b.Record the unknown number/letter in data table.c.The unknown metals are aluminum, iron, lead, and copper.3)Carefully, place the metal cylinder into a dry test tube and clamp the tube intothe boiling such that the metal cylinder is below surface.a.The metal should NOT be in direct contact with the water.4)Obtain the mass of two clean and dried empty Styrofoam cups with a cap on anANALYTICAL balance.a.These two cups should be stacked on top of one another and the capshould have a hole in it to facilitate the use of your thermometer.b.This set of Styrofoam cups will function as your calorimeter.c.Record mass in data table.
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