AP Chemistry Unit 6.1 Endothermic and Exothermic Processes

Endothermic and Exothermic Processes

  • Energy: the capacity to do work or to produce heat
    • J = SI unit for energy
      • kJ = 103 J
  • Work: force acting over a distance.
    • W =  – P Δ V
  • Potential Energy: due to position or composition (stored energy) → can be converted to work
    • Ex: water behind a dam, saturated fat (energy stored in bonds), attractive and repulsive forces
  • Kinetic Energy: energy due to motion of the object
    • M = mass of object in kg; v = velocity of object in m/sec
  • State function: value that depends on the state of the substance, not how that state was reached
    • Ex: internal energy, pressure, volume, density, enthalpy
        • Note: work and heat are not state functions

System and Surroundings

  • System: that on which we focus attention
    • The solute, solution reactants and products, reaction
  • Surroundings: everything else in the universe
    • Ourselves (ie hands), a thermometer, reaction vessel, the solvent are part of the surroundings
    • When we measure temp changes during a chemical reaction, we are measuring the surroundings
  • Universe = System + Surroundings

Exo and Endothermic

  • Heat exchange/temp change accompanies:
    • Healing or cooling a substance
    • Phase changes
    • Dissolving solutes
    • Chemical reactions
  • Exothermic: heat (free energy) is released by the system (to the surroundings)
  • Endothermic: Heat is absorbed by the system (from the surroundings)

Exothermic Processes

  • Cooling an object
  • Phase changes:
    • Freezing; condensation; deposition
  • Some chemical & dissolution reactions (feel hot) → temp of surroundings is increasing
  • Electron affinity:
  • Solute and Solvent Interactions: in exothermic reactions, the solute and solvent particles are more strongly attracted to each other than they are too themselves
  • In an exothermic process, some kind of bonds or attractive force is forming!
    • “Free to form”
    • Justify: The energy required to break bonds (positive) is less than the energy released when the bonds are formed (negative) → process is energetically favorable → ΔH⁰ = negative

Endothermic Processes

  • Heating an object
  • Phase changes:
    • Melting; vaporization; sublimation
  • Some chemical and dissolution reactions (feels cold) → temp of surroundings is decreasing
    • In dissolving the change in energy is equal to the sum of the energies required to separate solute particles from one another and solvent particles from one another minus the energy released when attractions between solute particles and solvent particles form
  • → are breaking the forces of attraction between sodium and its valence electron
  • Solute and Solvent Interactions: in endothermic reactions, the solute and solvent particles are more strongly attracted to themselves (stronger interactions) than they are too each other.
  • In an exothermic process, some kind of bond or attractive force is breaking
    • “Takes (energy) to break”
    • Justify: The energy required to break bonds is more than the energy released when the bonds are formed → ΔH⁰ = positive
      • If temp graph is shown: because temp decreased, energy flows into the process

Why is dissolving an ionic compound an ambiguous change (can be classified as both physical and chemical)

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