Energy Changes in Electrochemistry

UNIT FOUR: ENERGY CHANGES IN ELECTROCHEMISTRY

Key Concepts

  • Chemical Reactions: Processes that result in a chemical change and produce energy, e.g., combustion of fuels like carbon:
    • C(s) + O2(g) \rightarrow CO2(g)
    • CH4(g) + O2(g) \rightarrow CO2(g) + 2H2O(g)
  • Energy Change (ΔE): Involvement in chemical reactions, particularly in combustion.
  • System vs Surroundings:
    • System: The part being studied.
    • Surroundings: Everything else.
  • Internal Energy (E): Total energy (potential + kinetic) within a system.
    • Formula: E = E{potential} + E{kinetic}

Energy Exchange in the Universe

  • The total energy of the universe remains constant.
    • When a system loses energy, the surroundings gain it:
    • \text{Total Energy} = \text{System} + \text{Surroundings}
  • Forms of Energy Transfer: Heat (q) and Work (w):
    • \Delta E = q + w

Types of Chemical Reactions

Exothermic Reactions
  • Energy is released to the surroundings (Heat out).
  • Examples:
    • Dissolution of NaOH in water:
      NaOH(s) \rightarrow Na^+(aq) + OH^-(aq) + \text{Heat}
    • Dissolution of CaCl2 in water:
      CaCl2(s) + 2H2O(l) \rightarrow Ca(OH)_2(aq) + 2HCl(g) + \text{Heat}
  • Enthalpy Change (ΔH < 0): Heat of products < Heat of reactants.
Endothermic Reactions
  • Energy is absorbed from the surroundings (Heat in).
  • Examples:
    • Dissolution of KNO3 in water:
      KNO3(s) + H2O + \text{Heat} \rightarrow K^+(aq) + NO_3^-(aq)
  • Enthalpy Change (ΔH > 0): Heat of products > Heat of reactants.

Measuring Energy Changes

  • Calorimeter: Device to measure heat released or absorbed during reactions.
    • q \propto \Delta T (where q is heat, and ΔT is temperature change)
    • Heat Capacity (C): The heat required to change temperature by 1 K, with units Joule per Kelvin (J/K).
  • Specific Heat Capacity (c): Defined as:
    • c = \frac{Heat : Capacity}{mass} = \frac{q}{mass \times \Delta T}
    • Relation: q = c \times mass \times \Delta T

Chemical Changes Importance

  • Chemical reactions: Key events for life processes (photosynthesis, digestion).
  • Energy production: Essential for various biological processes.

Types of Conductivity

  • Electrical Conductivity: Ability to transmit electricity (e.g., in solutions of ionic compounds).
    • Types:
    1. Metallic Conductivity: Movement of electrons (conductors like cooper, silver).
    2. Electrolytic Conductivity: Movement of ions in electrolytic solutions.
      • Strong electrolytes (like NaCl) dissociate completely in water.

Electrochemical Cells

  • Definition: Devices that convert electrical energy to chemical energy and vice versa.
  • Electrodes: Can be active (e.g., Zinc) or inert (e.g., Platinum).
Types of Electrochemical Cells
  1. Galvanic Cells: Convert chemical energy to electrical energy (spontaneous redox).
    • Example: Daniell cell:
      • Anode (negative): Zn \rightarrow Zn^{2+} + 2e^- (oxidation)
      • Cathode (positive): Cu^{2+} + 2e^- \rightarrow Cu (reduction)
      • Overall reaction: Zn(s) + Cu^{2+} \rightarrow Zn^{2+} + Cu(s)
  2. Electrolytic Cells: Convert electrical energy to chemical energy (non-spontaneous).
    • Oxidation occurs at the anode (+), reduction occurs at the cathode (-).

Additional Concepts

  • Electrolysis: Techniquefor obtaining elements from their compounds using electrical energy.
  • Secondary Cells: Rechargeable batteries (e.g., lead-storage battery) with reversible reactions.
  • Non-electrolytes: Substances that do not conduct electricity (e.g., sugar, alcohol).

Summary of Key Processes & Terms

  • Galvanic Cell: Converts chemical to electrical energy, spontaneous reactions.
  • Electrolytic Cell: Converts electrical to chemical energy, non-spontaneous reactions.
  • Heat Capacity: Amount of heat to change temperature by 1 K.
  • Specific Heat Capacity: Heat capacity per mass unit.
  • Endothermic and Exothermic Reactions: Defined by energy absorbed or released to/from the surroundings.