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AP Chemistry 6.8 Enthalpies of Formation Study Notes

AP Chemistry 6.8 Enthalpies of Formation Study Notes - New Syllabus Effective fall 2024

AP Chemistry 6.8 Enthalpies of Formation Study Notes.- New syllabus

AP Chemistry 6.8 Enthalpies of Formation Study Notes – AP Chemistry –  per latest AP Chemistry Syllabus.

LEARNING OBJECTIVE

Calculate the enthalpy change for a chemical or physical process based on the standard enthalpies of formation.

Key Concepts: 

  • Enthalpy of Reaction
  • Enthalpy & Bond Energies
  • Enthalpy of Formation
  • Hess’s Law

AP Chemistry-Concise Summary Notes- All Topics

AP Chemistry Exam Questions MCQs and FRQs

6.8.A.1 Standard Enthalpy of Reaction Calculation:

1. Standard Enthalpy of Formation (ΔHf°):

The Standard Enthalpy of Formation (ΔHf°) is the heat change upon formation when 1 mole of substance is produced from elements in their stable forms at 298 K and 1 atm pressure.

– Standard Conditions: 298 K (25°C), 1 atm pressure.
– Standard State: The standard state of the element under normal conditions.
– ΔHf° of elements in standard state = 0 (e.g., O₂(g), C(s, graphite)).

It is used to calculate enthalpy changes in chemical reactions.

2. Enthalpy Change of a Reaction (ΔH°reaction):

The enthalpy change of a reaction (ΔH°reaction) is the heat released or absorbed during a chemical reaction under constant pressure. It is the difference in enthalpy between the products and reactants.

ΔHreaction​=Hproducts​−Hreactants​

Significance:

– Endothermic Reactions:
When ΔH°reaction is positive, the reaction absorbs heat from the surroundings (e.g., photosynthesis).

– Exothermic Reactions:
If ΔH°reaction is negative, the reaction releases heat into the surroundings (e.g., combustion).

Briefly, ΔH°reaction indicates if a reaction is heat-releasing (exothermic) or heat-gaining (endothermic).

3. Using Enthalpy of Formation Tables:

i. Reading Standard Enthalpy of Formation (ΔHf°) Values:
– Standard enthalpy of formation values (ΔHf°) are given in tables for various compounds at 298 K and 1 atm.
– The values are the heat change when 1 mole of a substance is formed from its elements in their standard state.
– Elements in their standard state have ΔHf° = 0 (e.g., O₂(g), N₂(g), C(s, graphite)).

ii. Using ΔHf° in Reactions:
To calculate the enthalpy change (ΔH°reaction) for a reaction, use the following equation:

iii. Stoichiometry Application:
1. Balance the chemical equation for the reaction.
2. Find the ΔHf° values in the table for the reactant and product.
3. Use stoichiometry: Multiply the ΔHf° of each substance by its coefficient in the balanced equation.
4. Calculate the total for products and reactants, then subtract:

Example:

For the reaction:

C(s)+O2​(g)→CO2​(g)

  • ΔHf° (C(s)) = 0

  • ΔHf° (O₂(g)) = 0

  • ΔHf° (CO₂(g)) = -393.5 kJ/mol

\Delta H^\circ_{\text{reaction}} = [\text{ΔHf° (CO₂)}] – [\text{ΔHf° (C)} + \text{ΔHf° (O₂)}]

\Delta H^\circ_{\text{reaction}} = (-393.5) – (0 + 0) = -393.5 \, \text{kJ/mol}

This reaction releases 393.5 kJ of energy.

 
 
 

OLD Content

Standard State

  • Standard enthalpy (ΔH⁰): the enthalpy change at standard conditions
    • ⁰ = recorded in standard state
  • Standard State conditions:
    • For a gas: pressure is 1 atm
    • For a pure substance in a condensed state (liquid or solid) = pure liquid or solid
    • For a solution, concentration is 1 molar
    • Temp is at exactly 25 C
  • Remember: of a pure element is 0
  • Standard heats of formation: the amount of heat needed to form 1 mole of a compound from its elements in their standard states (ie. how it exists in nature → oxygen is O2)
    • Unit: kJ
    • Have to make one mole of product to meet the definition
    • SS of an element is the form in which the element exists under 1 atm and 25 C
      • Not 0 = not how the element exists in nature
  • Ex: Write the equation for the formation of methanol (CH3OH)
    1. Step 1: write the elements (in standard state)
      • Note: The elements that exist as diatomic molecules are hydrogen (H2), nitrogen (N2), fluorine (F2), oxygen (O2), iodine (I2), chlorine (Cl2) and bromine (Br2
    1. Step 2: Balance
  • Formula:  (on RFS)
    • Use chart → add up all of heats of formation of products – all the heats of formation of reactants
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