BackThermochemistry: Enthalpy Changes and Hess's Law (General Chemistry, Chapter 7)
Study Guide - Smart Notes
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Thermochemistry and Enthalpy Changes
Introduction to Thermochemistry
Thermochemistry is the study of energy changes, particularly heat, that occur during chemical reactions. The most important thermodynamic quantity in chemical reactions is enthalpy (ΔH), which measures the heat exchanged at constant pressure.
Relationships Involving Enthalpy (ΔH)
Scaling Chemical Equations
When a chemical equation is multiplied by a factor, the enthalpy change (ΔH) is also multiplied by the same factor.
Key Point: If the reaction is scaled by n, then ΔHnew = n × ΔHoriginal.
Example:
Application: Formation of 1 mole of CaO(s): kJ; formation of 2 moles: kJ = kJ.
Reversing Chemical Equations
If a chemical equation is reversed, the sign of ΔH is also reversed.
Key Point:
Example:
Context: The heat released in the forward reaction equals the heat absorbed in the reverse reaction.
Hess's Law: Summing Enthalpy Changes
Hess's Law states that if a chemical equation can be expressed as the sum of a series of steps, the overall enthalpy change is the sum of the enthalpy changes for each step.
Key Point:
Example:
Numerical Example: If kJ and kJ, then kJ.
Applying Hess's Law: Sample Calculations
Manipulating Equations to Find ΔHrxn
To determine the enthalpy change for a target reaction, combine given reactions (and their ΔH values) by reversing and scaling as needed so that their sum yields the desired equation.
Example: Find for
Reaction | ΔH (kJ) |
|---|---|
Reverse and scale reactions as needed, then sum their ΔH values:
Standard States and Standard Enthalpy Changes
Definitions
Standard State:
Gas: Pure gas at 1 atm pressure
Liquid/Solid: Pure substance at 1 atm and 25°C
Solution: 1 M concentration
Standard Enthalpy Change (ΔH°): The enthalpy change for a process when all reactants and products are in their standard states.
Standard Enthalpy of Formation (ΔH°f): The enthalpy change when one mole of a compound forms from its elements in their standard states. For pure elements in their standard state, .
Calculating Standard Enthalpy Changes Using ΔH°f
General Formula
The standard enthalpy change of a reaction can be calculated using the enthalpies of formation:
Example: Combustion of methane:
values: kJ/mol, kJ/mol, kJ/mol
Calculation: kJ/mol$
Summary Table: Key Relationships Involving ΔH
Operation on Equation | Effect on ΔH |
|---|---|
Multiply equation by n | Multiply ΔH by n |
Reverse equation | Change sign of ΔH |
Add equations | Add ΔH values |
Example Problems
Sample Calculation Using Hess's Law
Given:
values: kJ/mol, kJ/mol, kJ/mol
Calculation:
Important Notes
Most compounds have a negative ΔHf, indicating exothermic formation.
Elements in their standard state have ΔHf = 0.
Conclusion
Understanding the relationships involving enthalpy changes, especially Hess's Law and the use of standard enthalpies of formation, is essential for predicting and calculating the energy changes in chemical reactions. These principles are foundational in thermochemistry and widely applicable in both laboratory and industrial chemistry.
