Chemistry -Entrance Exam ( Universidad de Navarra) - unit 4

Energy Transformations in Oilic Reactions

Thermodynamics Overview

• Thermodynamics is a branch of physical chemistry focused on the relationships between heat, work, temperature, and energy.

  • Essential for understanding how energy transfers and transforms in chemical processes.

  • Explains phenomena like phase changes and reaction spontaneity.

• Studying thermodynamics enhances the appreciation of energy conservation and chemical reaction directionality.

Key Laws of Thermodynamics

1. First Law of Thermodynamics

   • States that energy cannot be created or destroyed, only transformed.

2. Second Law of Thermodynamics

   • Introduces entropy, suggesting that natural processes lead to greater disorder over time.

3. Predictive Role

   • Thermodynamics helps predict whether a chemical reaction will occur spontaneously.

4. Thermodynamic Cycles

   • Example: Carnot cycle, critical for understanding the efficiency of heat engines and energy conversion.

5. Real-World Applications

   • Important for areas such as calorimetry, refrigeration, and engines, affecting varied industries.

Thermodynamic Systems and Variables

• A thermodynamic system is defined as a matter or radiation body separate from its surroundings.

  • Characterized by mass/volume, composition, energy, temperature, and pressure.

• Types of Systems based on energy and matter exchange:

  • Closed, open, and isolated systems.

• State Variables (thermodynamic variables):

  • Pressure, volume, temperature, internal energy, enthalpy, entropy.

Heat Energy Measurement and Chemical Reactions

Calorimetry

• Heat energy in chemical reactions is measured using calorimetry.

  • Change in internal energy results in heat and work.

• Endothermic vs Exothermic Reactions:

  • Endothermic: Energy absorbed (ΔH > 0).

  • Exothermic: Energy released.

Endothermic Reactions

1. Characteristics:

   • Absorb energy, requiring input energy.

   • Products have higher energy content than reactants.

2. Factors Affecting Reactions:

   • Increasing temperature or using a catalyst can help reach activation energy.

3. Le Châtelier’s Principle:

   • System at equilibrium adjusts to counteract changes (e.g., adding heat shifts equilibrium right).

Exothermic Reactions

1. Characteristics:

   • Release energy, increasing environmental temperature.

2. Applications:

   • Drive chemical processes like combustion.

3. Examples:

   • Dissolution of ionic compounds in water is often exothermic.

   • Common in industrial processes such as fuel combustion.

Enthalpy Concepts

Enthalpy of Formation

• Definition: Change in enthalpy when one mole of a compound forms from its elements under standard conditions.

1. Standard Enthalpy of Formation: Zero for elements in their stable form.

2. Calculation: Values crucial for determining heat during reactions, especially combustion.

3. Negative Values: Indicate energy release when forming compounds.

4. Significance: Helps evaluate energy changes in combustion reactions.

Bond Enthalpy

• Definition: Energy required to break one mole of a specific bond type.

1. Strength Correlation: Stronger bonds have higher bond enthalpies.

2. Energy Changes:

   • Breaking bonds: endothermic.

   • Forming bonds: exothermic.

3. Estimation Formula: ΔH = Σ(bond enthalpies of broken bonds) - Σ(bond enthalpies of formed bonds).

Hess's Law and Gibbs Free Energy

Hess's Law

1. Principle: Total enthalpy change is the same regardless of reaction pathway.

2. Calculation Utility: Allows enthalpy changes for complex reactions to be calculated.

3. State Function: Dependence only on initial and final states.

Gibbs Free Energy

• Definition: Thermodynamic potential measuring maximum reversible work from a closed system at constant temperature and pressure.

1. Formula: ΔG = ΔH - TΔS

   • ΔH: Enthalpy change, ΔS: Entropy change.

2. Spontaneity Indicator:

   • ΔG < 0: spontaneous processes.

   • ΔG = 0: equilibrium.

3. Impact of Temperature: Influences spontaneity based on changes in entropy/enthalpy.

4. Phase Stability: Helps in understanding conditions for different phases to coexist, relevant for phase diagrams.

Applications of Chemical Reactions

1. Combustion of Fuels: Releases energy for transport (e.g., petrol).

2. Digestion: Releases energy from food through chemical processes.

3. Battery Usage: Conversion of stored chemical energy to electricity.

4. Power Generation: Utilizes exothermic reactions for electricity.

5. Chemical Industry: Relies on energy-changing reactions for production and synthesis.