MASTER THERMOCHEMISTRY Part 4 of 4

MASTER THERMOCHEMISTRY Part 4 of 4

STANDARD ENTHALPIES OF FORMATION (ΔHfº)

^What to catch from the above slide:

• Do note that Standard Enthaly of Formation (ΔHfº) for a pure element in its reference form under standard state is "0kJ/mol"

• Examples of pure elements in their reference forms include Na(s), H2(g), H2O(l), Br2(l), and C (graphite) --> note that their number of atoms and phases matter!

• The standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions. https://chem.libretexts.org/Courses/Valley_City_State_University/Chem_121/Chapter_7%3A_Thermochemistry/7.4%3A_Standard_Enthalpy_of_Formation#:~:text=The%20standard%20enthalpy%20of%20formation,elements%20under%20the%20same%20conditions.

ΔHfº AND ΔHº rxn

^What to catch from the above slide:

• Pay attention to the formula written in blue which shows that the Standard Enthalpy of Reaction (ΔHº rxn) can be determined by calculating the difference between Standard Enthalpy of Formation of products and Standard Enthalpy of Formation of reactants (also refer to the simplified version of the same formula below)

1st FORMULA YOU SHOULD KNOW:

ΔHº reaction = Standard enthalpy of reaction

ΔHfº (products)= Standard enthalpy of formation of products

ΔHfº (reactants) = Standard enthalpy of formation of reactants

Σ = sum up (you will be adding up all the values inside this symbol "sigma")

^What to catch from the above slide:

• This is a sample question

• IMPORTANT TO note that each molecule has its DISTINCT ΔHfº values as listed in the blue chart (e.g. CO2(g) = -393.5 kJ/mol, H2O(l) = -285.8kJ/mol)

• Also, make sure to multiply the COEFFICIENTS (n) of a given molecule to the ΔHfº (e.g there are 4 moles of CO2(g) in the above sample question, which translates to 4 x -393.5kJ/mol).

• Note that O2(g) has a "0kJ/mol" --> this is because it is in its pure form. Again, all pure forms are assigned "0kJ/mol"

^What to catch from the above slide:

• This is another sample question, except this time it involves ions and polyatomic ions (they have their own chart as seen above)

AVERAGE BOND ENTHALPIES (BE) & ΔHº rxn

^What to catch from the above slides:

• It requires ENERGY to break bonds (endothermic), and energy is released when bonds are formed (exothermic)

• ΔHº rxn can also be estimated (roughly calculated) by determining the difference between bond enthalpies of broken bonds and bond enthalpies of formed bonds (formula shown in the slide as well as below)

2nd FORMULA YOU SHOULD KNOW:

ΔHº rxn = standard enthalpy of reaction

BE = bond enthalpies (provided by a chart)

PRACTICE PROBLEMS FOR BOND ENTHALPIES & ΔHº rxn

PRACTICE PROBLEMS ON ΔHfº & ΔHº rxn

ANSWER KEY

PRACTICE PROBLEMS ON BOND ENTHALPIES (BE) & ΔHº rxn

ANSWER KEY