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Enthalpy
EQ: How do you predict the sign of delta H?
A. Enthalpy (ΔH)
1. Enthalpy is the amount of heat in a system2. In order to determine the amount of heat
change in a reaction we use…
3. Reactions can be…
ΔHrxn = Hproducts - Hreactants
A. Enthalpy (ΔH)A) Endothermic1. Products have more energy than the reactants2. ΔH=+ (positive)3. heat into system4. Heat is being absorbed5. Surroundings feel cool6. Ex: 25 kJ + NH4NO3 NH4
+ + NO3-
Heat is going into the arrow (energy on the left)
A. Enthalpy (ΔH)A) Exothermic1. Reactants have more energy than products2. ΔH= - (negative)3. heat exits system4. Heat is being released5. Surroundings feel warm6. Ex: 4Fe + 3O2 2 Fe2O3 + 1625 kJ
Heat exits the arrow (energy on the right)
HEATHEAT HEATHEAT HEATHEAT HEATHEAT
Endo or Exo?
Endo or Exo
Endo or Exo?
B. Enthalpy of Combustion (ΔHcomb)
• 1. change in enthalpy when 1 mol of a substance is completely burned
B. Enthalpy of Combustion (ΔHcomb)• Ex: How much heat is evolved when 54.0 g C6H12O6
is burned?• C6H12O6 + 6O2 6 CO2 + 6H2O (ΔHcomb= -2808 kJ)
54.0 g C6H12O6
180.14 g C6H12O6
1 mol C6H12O6-2808 kJ
1 mol C6H12O6
= -842 kJUse coefficient in balanced equation
xx
C. Molar Enthalpy (heat released per mole)
1. Molar Enthalpy of vaporization: Heat required to evaporate one mole of a substance.
2. Molar Enthalpy of condensation -ΔHCond = + ΔHVAP
3. Molar Enthalpy of fusion: Heat required to melt one mole of a substance
4. Molar enthalpy of freezing (ΔHsolid) solidification
• -ΔHSolid = + ΔHFusion
C. Molar Enthalpy (heat released per mole)
5. If the ΔHfus is 6.01 kJ/mol, than the ΔHsolid =
-6.01 kJ/mol
C. Molar Enthalpy (heat released per mole)
6. Ex: Calculate the energy required to melt 250 g of ice. (ΔHfus = 6.01 kJ/mol)
250 g H2O
18.02 g H2O
1 mol H2O 6.01 kJ
1 mol H2O=83 kJ
C. Molar Enthalpy (heat released per mole)
x x
Calculate how much energy is absorbed when 7.20 moles of O2
reacted with an unlimited amount of chlorine. 2Cl2 + 7O2 + 130 kcal 2Cl2O7
7.20 moles of O2
7 moles O2
+130 kcal = 134 kcal
D. More Enthalpy Stoichiometry
ΔH = +130 kcal (+ because it’s endo)x
Determine the mass of iron used to produce 250 kJ of heat according
to the balanced reaction below. 4Fe + 3O2 2Fe2O3 + 1625 kJ
-250 kJ
D. More Enthalpy Stoichiometry
ΔH = -1625 kJ (- because it’s exo)
x-1625 kJ
4 mol Fe x1 mol Fe
55.85 g Fe= 34 g Fe
Produce = - energy