Thermodynamics

Thermodynamics: Bond Energy and Energy of reaction

Enthalpy Change, ΔH

​Basic rules of Enthalpy

When bonds are formed, energy is released

When bonds are broken, energy is absorbed

Enthalpy Change

​Replace with sub-header

ΔH= H_products - H_reactants

Math Response

2H₂ (g) + O₂ (g) --> 2H₂O (g)

Based on the information given in the table below, what is the ΔH⁰ for the above reaction

Type answer here

Correct Answer: -500

Deg^°

Rad

Enthalpy of Formation

Enthalpy of formation

•(also known as heat of formation) is the change in energy that takes
place when one mole of a compound is formed from its component
pure elements under standard state conditions (always calculated at a
temperature of 25o C)
•C + ½ O2 + 2 H3🡪 CH3OH
•ΔHo

f for a pure element, even diatomics, is 0

•If ΔHo

f is negative energy is released, product is more stable than

components. Exothermic
•If ΔHo

f is positive energy is absorbed, and the compound is less

stable than components. Endothermic

2 CH3OH + 3 O2🡪 2CO2 + 4 H2O

Compound

ΔHo

f

CH3OH

-201

O2 0

CO2 -394

H2O

-242

Enthalpy of Combustion

•Combustion is always Exothermic
•The amount of energy released when ONE mole of a hydrocarbon
combusts

How much heat is released when 5.00g of CH3OH
is combusted with excess oxygen

The End of Thermodynamics

Relationships involving ΔHrxn

•1. If a chemical equation is multiplied by some factor then ΔHrxn is
also multiplied by the same factor

•2. If a chemical rxn is reversed, then ΔHrxn changes signs

2H2O 🡪 2 O2 + H2 Hrxn = 481 J/mol

2 O2 + H2🡪2H2O Hrxn = -481 J/mol

Hess’s Law

•3. If a chemical equation can be expressed as the sum of a series of
steps, then for the overall equation is the sum of the heats of
reactions for each step

• A + 2B 🡪 C ΔH1
• C 🡪 2D ΔH2
• A + 2B 🡪 2D ΔH3 = ΔH1 + ΔH2

4 NH3 + 5 O2🡪 4 NO + 6H2O

•Equation A: N2 + O2 🡪 2NO ΔH = 180.6 kj/mol
•Equation B: N2 +3H2 🡪 2NH3 ΔH = -91.8 kj/mol
•Equation C: 2 H2 + O2 🡪 2 H2O ΔH = 180.6 kj/mol

Enthalpy of solution

•When Ionic substances dissolve in water, heat is emitted or absorbed.
•The bond between the cation and anion are broken which requires energy
•Energy is released when ions form new attractions to the dipoles of the water
molecules
•The amount of energy required to break the bond is equal to the lattice
energy. This step always has a positive ΔH
•The watr molecules must spread out to make room for the ions, this
requires energy to weaken but not break the intermolecular forces. This
step always has a positive ΔH
•Free-floating ions attracted to dipoles of water molecules. Energy is
released in the process. This step always has a negative ΔH

•The energy values from steps 2&3 combined are often called
hydration energy and is always negative.
•Hydration energy is a Coulombic energy and thus increases as the ions
either increase in charge or decrease in size

Heat of Fusion

•The energy put into a solid to melt it.
•The energy needed to overcome the forces holding the solid together
•Is also the heat given off by a substance when it freezes

Heat of Vaporization

•The energy required to turn a substance from liquid to a gas
•The energy given off when a gas condenses to become a liquid