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Heating Curves Qualitative- phase changes Quantitative- Heat of fusion/vaporization Measuring  H Constant Pressure Calorimetry (“coffee cup”) Constant Volume Calorimetry (bomb)  H and Reactions Hess’ Law (this week’s lab) Standard Heats of Formation

Thermal Energy and Phase Changes

Which line segment represents a point at which you would use the heat capacity of the liquid to calculate heat exchange? 1.A->B 2.B->C 3.D->E 4.F->G 5.H->J

Enthalpy Changes and Chemical Reactions   H = energy needed to break bonds – energy released forming bonds  Calorimetry is used to measure enthalpy changes  Constant Pressure gives  H  Constant Volume gives  E

Hess’ Law- The Rules  Enthalpy is a state function.  If a reaction can be written as the sum of two or more reactions, the  H for the net reaction is the sum of the  H's for the individual steps.  If a reaction is reversed, the sign of  H is reversed.  If the coefficients of the reactants/products are multiplied by a number, then the  H must be multiplied by that number, as well.

Hess’ Law  If you can add the reactions, you can add the  H’s.

Hess’s Law

Hess’ Law- Lab This Week Mg(s) + ½ O 2 (g)  MgO(s)  H = ???

Constant Pressure Calorimetry (“Coffee Cup”)

Constant Volume Calorimetry (“Bomb”) N 2 H O 2  2 NO H 2 O E released = E absorbed by water + E absorbed by calorimeter E water = E calorimeter = Total E =  H = energy/moles = g N 2 H g water 420 J/ o C

Calculating Reaction Enthalpies  Use values that have been determined from experiment  Two types of data that can be used  Standard enthalpies of formation  Bond enthalpies

Standard Enthalpy of Formation  Standard conditions: pure form, 1 bar pressure, usually at 298K (25°C)  Heat of formation for an element in its most stable form is zero  Standard heat of formation is given in Joules per mole  Results in fractional coefficients on occasion

Table 5-2, p. 195

Using Standard Enthalpies of Formation  What is the  H rxn for the detonation of nitroglycerin?  How much energy is released when 10g is detonated?

Bond Enthalpy  Remember that bond energy is the amount of energy required to break a bond in a gas phase molecule  Can only use bond enthalpies for reactions in which everything is in the gas phase  When calculating  H using bond enthalpies, assume all bonds are broken in the reactants (  H= +) and formed in the products (  H= -)

Bond Enthalpy  Formation of water

Chemical Reactions and Enthalpy Change C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O(g)  H = kJ What does this mean? How much energy is released by a 468-g tank of propane?