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Thermal Energy and Phase ChangesThermal Energy and Phase Changes
Which line segment represents a point at Which line segment represents a point at which you would use the heat capacity of which you would use the heat capacity of the liquid to calculate heat exchange?the liquid to calculate heat exchange?
1 2 3 4 5
13%16%
7%
16%
48%1. A->B2. B->C3. D->E4. F->G5. H->J
Enthalpy Changes and Chemical Enthalpy Changes and Chemical ReactionsReactions
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 RulesHess’ 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’ LawHess’ Law
If you can add the reactions, you can add the H’s.
Hess’s LawHess’s Law
Hess’ Law- Lab This Hess’ Law- Lab This WeekWeek
Mg(s) + ½ O2(g) MgO(s) H = ???
Constant Pressure Calorimetry (“Coffee Constant Pressure Calorimetry (“Coffee Cup”)Cup”)
Constant Volume Calorimetry (“Bomb”)Constant Volume Calorimetry (“Bomb”)
N2H4 + 3 O2 2 NO2 + 2 H2O
Ereleased = Eabsorbed by water + Eabsorbed by calorimeter
Ewater =
Ecalorimeter =
Total E =
H = energy/moles = 0.500 g N2H4
600 g water
420 J/oC
Calculating Reaction EnthalpiesCalculating 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 FormationStandard 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
Hrxn H f
products
H f
reactants
Table 5-2, p. 195
Using Standard Enthalpies of Using Standard Enthalpies of FormationFormation
What is the Hrxn for the detonation of nitroglycerin?
How much energy is released when 10g is detonated?
2C3H5(NO3)3(l) 3N2(g)1
2O2(g) 6CO2(g) 5H2O(g)
Bond EnthalpyBond 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= -)
Hrxn broken formed
Bond EnthalpyBond Enthalpy
Formation of water
2H2 O2 2H2O
Hrxn broken formed
Chemical Reactions and Enthalpy Chemical Reactions and Enthalpy ChangeChange
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) H = -2045 kJ
What does this mean?
How much energy is released by a 468-g tank of propane?