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?

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