ThermochemistryThermochemistry

Exothermic reactions release heat to the surroundings.

Fe2O3 + 2 Al 2 Fe + Al2O3 + 851.5 kJ

Potassium Permanganate Reaction Demo

Or ΔH = -851.5 kJ

Thermite Reaction Demo

ThermochemistryThermochemistry

Endothermic reactions absorb heat from the surroundings.

Ba(OH)2 + 2 NH4NO3 + 102.2 kJ

Ba(NO3)2 + 2 NH3 + 10 H2O

Barium Hydroxide Reaction Demo

Or ΔH = +102.2 kJ

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EnthalpyEnthalpy

Energy that is gained or lost by substances during a reaction.

Symbolized by the letter H.

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EnthalpyEnthalpy

Example:Given equal amounts (mass) of

both, which produces more energy: Methane (Natural Gas!) or Octane (Gasoline!)

EnthalpyEnthalpy

Combustion of Methane (Natural Gas!) CH4(g) + O2(g) CO2(g) + H2O(g)

ΔH = -890.4 kJ (Exothermic!)

22

EnthalpyEnthalpy

Taken from reaction ratio!

Your furnace turns on and burns 32.0 g of methane, how much heat is produced?

32.0 g CH4

16.0 g CH4

1 mol CH4

1 mol CH4

-890.4 kJ= -1780 kJ

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

ΔH = -890.4 kJ

EnthalpyEnthalpy

Combustion of Octane (Gasoline!) C8H18(l) + O2(g) CO2(g) + H2O(g)

ΔH = -5430 kJ (Exothermic!)

2 25 16 18

EnthalpyEnthalpy

Taken from reaction ratio!

Your car turns on and burns 32.0 g of octane, how much heat is produced?

32.0 g C8H18

114.0 g C8H18

1 mol C8H18

2 mol C8H18

-5430 kJ

= -762 kJ

2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(g)

ΔH = -5430 kJ

EnthalpyEnthalpy

Methane produces more energy when burned…as long as mass is constant.

Enthalpy of FormationEnthalpy of Formation

This is the amount of energy (enthalpy) that is involved in “creating” a compound.

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Enthalpy of FormationEnthalpy of Formation

The “ΔH” given in a reaction is calculated by using these values in this equation:

)(H - )(H H ReactantsProducts

Enthalpy of FormationEnthalpy of Formation

Example:CH4 (g) + O2 (g) CO2 (g) + H2O (l)

What is ∆H for this reaction?1st Step - Balance the Equation

2 2

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (l)

Enthalpy of FormationEnthalpy of Formation

2nd Step – Find Enthalpy Values Using Chart

-75 kJ/mol -393.5 kJ/mol

-286 kJ/mol

Any element that is “pure”, has an enthalpy of 0 kJ/mol

Enthalpy of FormationEnthalpy of Formation

3rd – Plug into ΔH equation and solve:

ΔH = [ (-286) + (-393.5)] – [(-74.8) + 0]

Coefficient from balanced equation!

ΔH = (Hproducts) – (Hreactants)

2

Enthalpy of FormationEnthalpy of Formation

Final Answer: -890.7 kJ/molNo significant digits needed since

all the numbers are found on tables!

Hess’s LawHess’s LawA series of reactions can be added

together to find their overall enthalpy.

N2 + O2 2 NO

2 NO + O2 2 NO2+

N2 + 2 O2 2 NO2

ΔH = +181 kJ

ΔH = -131 kJ

ΔH = +68 kJ

Hess’s LawHess’s Law

Example: C2H2 + H2 C2H4 ΔH = -174.4 kJ

C2H6 C2H4 + H2 ΔH = 137.0 kJ

What is the enthalpy for the net reaction:

C2H2 + 2 H2 C2H6

C2H2 + H2 C2H4

C2H6 C2H4 + H2+

C2H2 + 2 H2 C2H6

ΔH = -174.4 kJ

ΔH = 137.0 kJ

ΔH = - 311.4kJ

If you cannot simply add to find the net reaction, “flip” a reaction to make it

work!

C2H4 + H2 C2H6 ΔH = -137.0 kJ