Laboratory 4Grade_____________Chem 163Heat and WorkDr. MillerName_______________Discussion:During a chemical reaction, the energy of the system normally will change.Usually the energy change of the systemappears in the form of a heat change in the surroundings.It is also possible for some of the energy change of thesystem to appear as work done on the surrounding.The total energy change of the system,Usys, is the sum of heatchange of the system, qsys, and work of the system, wsys:Usys= qsys+ wsysWork done by the system during a reaction is usually in the form of the expansion of a gas at constant opposingpressure.The amount of expansion work done can be calculated:wsys= -PVIf the reaction is carried out in solution and in a calorimeter, the heat liberated or absorbed by the system can becalculated by measuring the temperature change of the surroundings, the water, by the formula:- qsys= qsur= (m)water(specific heat)water(t)waterNote that the heat change of the system (qsys) will be the same value but opposite in sign.If the reaction is carried out at constant pressure, the heat change of the system is proportional to the molar enthalpyof reaction,H, also called the molar heat of reaction.To find the molar heat of reaction, the number of moles of oneof the products or one of the reactants (with a stoichiometry number of one) must be found.(Not the total moles ofproduct just the moles of one product.)H = qsys/mole of a product formed = qsys/mole of a reactant usedThe theoretical value for molar heat of reaction,Ho, can be calculated from thermodynamic data using the equation:()(tan)oooffHnHproductsmHreacts=−Procedure:1.
