Chemistry Notes: Enthalpy, Entropy, Gibbs Free Energy, and Chemical Reactivity

Instructor Information

  • Name: Dr. Anny Leudjo Taka

  • E-mail: anny.leudjo@uregina.ca

  • Office: LB 246

  • Consultation Times: Tuesday and Thursday (2pm-4pm)

Research Interests

  • Interdisciplinary Research:

    • Nanotechnology

    • Material Science

    • Synthetic Chemistry

    • Analytical Chemistry

  • Aim:

    • Address pressing global challenges

    • Develop sustainable hybrid nanomaterials for water purification and energy saving devices

Territorial Acknowledgment

  • University of Regina and federate colleges located on Treaty 4 and Treaty 6 territories: homeland of nêhiyawak, Anihšināpēk, Dakota, Lakota, Nakoda, and Métis/Michif Nation.

Chemical Reactivity and Mechanisms (CHEM 140)

Key Concepts

  • Factors affecting chemical reactions:

    • Nature of Reactants

    • Concentration of Reactants

    • Pressure (for Gaseous Reactions)

    • Solvent Effects

    • Energy Considerations (Activation Energy)

    • Enthalpy Changes ($\Delta H$)

    • Entropy Changes ($\Delta S$)

    • Gibbs Free Energy ($\Delta G$)

Important Definitions

  1. Enthalpy ($\Delta H$):

    • Heat energy exchange between reaction and surroundings.

    • Positive $\Delta H$ indicates endothermic; negative indicates exothermic.

  2. Entropy ($\Delta S$):

    • Measure of disorder or freedom in a system.

    • $\Delta S$ increases in reactions with more product moles than reactants.

  3. Gibbs Free Energy ($\Delta G$):

    • Indicator of spontaneity.

    • Negative $\Delta G$ means spontaneous; positive means nonspontaneous.

Predicting Signs of Changes

  • For Enthalpy:

    • Can estimate based on bond energies: the amount of energy needed to break vs. form bonds.

    • Example: For the reaction $\text{CH}3\text{Cl} + \text{H}2\text{O} \rightarrow \text{CH}_3\text{OH} + \text{HCl}$.

      • Calculating $\Delta H$ involves:

      • Bonds Broken: Gains energy (positive BDE).

      • Bonds Formed: Releases energy (negative BDE).

Exothermic vs Endothermic

  • Energy Diagrams:

    • Exothermic Process:

      • Products lower in energy; $\Delta H < 0$; temperature of surroundings increases.

    • Endothermic Process:

      • Products higher in energy; $\Delta H > 0$; temperature of surroundings decreases.

Entropy Change in Processes

  • Total Entropy Change ($\Delta S_{tot}$):

    • $\Delta S{tot} = \Delta S{sys} + \Delta S_{surr}$.

    • Positive $\Delta S_{tot}$ indicates spontaneity.

Gibbs Free Energy Relations

  • Spontaneity and relations:

    • $\Delta G = \Delta H - T\Delta S$.

    • For spontaneity, consider the negative values of $\Delta G$ in relation to temperature and entropy.

Equilibrium Constants

  • The equilibrium constant ($K_{eq}$) indicates favorability:

    • $\Delta G = -RT \ln(K_{eq})$ where $R = 8.314 \, \text{J/mol} \cdot \text{K}$ .

    • $K{eq} > 1$ favors products; $K{eq} < 1$ favors reactants.

Summary Points for Exam Preparation

  • Key Terms:

    • $\Delta H$ = Enthalpy; $\Delta S$ = Entropy; $\Delta G$ = Gibbs Free Energy.

    • Exergonic and endergonic reactions in terms of $\Delta G$.

    • Spontaneity linked with both entropy increases and favorable enthalpy changes.

  • Practice Problems:

    • Predict signs and magnitudes of $\Delta H$, $\Delta S$, $\Delta G$ for various reactions.

    • Use bond energies and reaction coordinates to understand energy diagrams.

Remember to review tables of bond dissociation energies and practice problems related to $\Delta H$, $\Delta S$, and $\Delta G$ calculations!