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Chem Knowt: Chapter 9

Chem Knowt: Chapter 9

  • Potential vs Kinetic Energy
  • Heat Flow
  • Exothermic vs Endothermic
  • Heat Capacity
  • Calorimetry
  • Enthalpy
  • Work
  • Energetics of Ionic Bond Formation
  • Lattice & Born-Haber Cycle
  • Potential Energy
    • Due to position or composition
  • Kinetic Energy
    • Due to motion of the object
  • Heat (q) is the transfer of thermal energy between two bodies at different temperature
  • Heat flow (q) increases the thermal energy of one body and decreases the thermal energy of the other
  • A change that releases heat is called an exothermic process
  • A change that absorbs heat is an endothermic process
  • Heat Capacity
    • Heat Capacity (Cp): the quantity of heat needed to raise the temperature of some particular object by 1 degree C at constant pressure.
      • q=Cp𝚫T
    • Specific Heat (Cg): heat required to raise the temperature of 1 gram of a substance at 1 degree C at constant pressure
      • q=mcs𝚫T
    • Molar Heat Capacity (Cp): is the heat required to raise the temperature of 1 mole of a substance by 1 degree C at constant pressure
      • q=ncp𝚫T
    • Specific Heat of Water:
      • Water can absorb a lot of heat energy without a large increase in its temperature due to its high specific heat capacity
  • System: the part of the universe that is the focus of a thermodynamic study. (Can be open, closed, or isolated).
  • Surroundings: everything in the universe that is not part of the system
  • Universe = system + surroundings
  • First Law of Thermodynamics (The Law of Conservation of Energy)
    • During a chemical or physical change, energy can be neither created nor destroyed although its form can change
  • Work: is done when a force moves an object through a distance
  • Lattice Energy: the extra stability that accompanies the formation of the crustal lattice is measured as the lattice energy.
  • The Born-Haber Cycle is a hypothetical series of reactions that represents the formation of an ionic compound from its constituent elements
  • Ion Size:
    • The force of attraction between charged particles is inversely proportional to the distance between them
  • Ion Charge:
    • The force of attraction between oppositely charged particles is directly proportional to the product of the charges

Chapter 9: Thermochemistry

*Write Summaries for each chapter

Energy Basics

  • Potential Energy
    • Due to position or composition
    • Chemical energy in a form of potential energy
    • Potential Energy = mgh
      • m = mass, g = force of gravity, and h = vertical distance
  • Kinetic Energy
    • Due to motion of the object
    • Thermal energy is a form of kinetic energy
    • Kinetic energy = ½ mv62
      • m = mass, v = velocity
  • Heat Flow
    • Heat (q) is the transfer of thermal energy between two bodies at different temperature
    • Heat flow (q) increases the thermal energy of one body and decreases the thermal energy of the other
    • When two substances are placed in contact, thermal energy will always flow from the high temperature substance to the low temperature substance
      • Heat flow will continue until both substances are at the same temperature
    • Matter undergoing chemical & physical changes can release/absorb heat
    • A change that releases heat is called an exothermic process
      • Example: the combustion reaction that occurs in a campfire
    • A change that absorbs heat is an endothermic process
      • The reaction in a cold pack used to treat muscle strains
    • Practice #1: Heat flow diagrams
      • Example: Biodiesel burning below a can of water
  • Energy Units
    • Historically, energy was measured in units of calories (cal)
      • Amount of energy required to raise one gram of water by 1 degree C
      • The Calorie (capital C), or large calorie, commonly used in quantifying food energy content, is a kilocalorie
    • The SI unit of heat, work, and energy is the joule
      • Defined as the amount of energy used when a force of 1 newton moves an object 1 meter
      • Named after James Prescott Joule
      • 1 J = 1 kg m^2/s^2
      • 1 calorie = 4.184 joules
  • Heat Capacity
    • Heat Capacity (Cp): the quantity of heat needed to raise the temperature of some particular object by 1 degree C at constant pressure.
      • q=Cp𝚫T
    • Specific Heat (Cg): heat required to raise the temperature of 1 gram of a substance at 1 degree C at constant pressure
      • q=mcs𝚫T
    • Molar Heat Capacity (Cp): is the heat required to raise the temperature of 1 mole of a substance by 1 degree C at constant pressure
      • q=ncp𝚫T
    • Specific Heat of Water:
      • Water can absorb a lot of heat energy without a large increase in its temperature due to its high specific heat capacity
      • The large amount of water absorbing heat from the air keeps beaches cool in the summer
        • Without water, earth’s temperature would be about the same as the moon's temperature on the side that is facing the sun
      • Water is commonly used as a coolant because it can absorb a lot of heat and remove it from the important mechanical parts to keep them from overheating
        • Water can even prevent melting
        • It can also be used to transfer the heat to something else because it is a fluid

Calorimetry

  • Terms Describing Energy Transfer
    • System: the part of the universe that is the focus of a thermodynamic study. (Can be open, closed, or isolated).
    • Surroundings: everything in the universe that is not part of the system
    • Universe = system + surroundings
    • An isolated system exchanges neither energy nor matter with the surroundings
  • First Law of Thermodynamics (The Law of Conservation of Energy)
    • During a chemical or physical change, energy can be neither created nor destroyed although its form can change
  • Change in Internal Energy

Enthalpy and Change in Enthalpy

  • Internal Energy (U) & Enthalpy (H) are… state functions!
    • State functions only depend on the present state of the system, not how it arrived there
      • State functions are independent of pathway
  • Work (W) & Heat Flow (q) are… non-state functions!
    • Non-state functions dependent of pathway
    • Work: is done when a force moves an object through a distance
    • Heat Flow: is energy transferred between objects because of a difference in their temperatures.
  • Heat of Reaction
    • Or enthalpy of reaction, is the heat absorbed or released by a chemical reaction at constant pressure
  • Methods of Determining Heat of Reaction
    • From calorimetry experiments
    • Calculations using others’ experiments:
      • Using Hess’ Law
      • From Enthalpies of Formation
      • From Bond Energies

Method 1: Calorimetry

  • The measurement of the heat flow that occurs during a physical change or chemical process
  • A calorimeter is the device used to measure the flow of heat by a physical or chemical process
  • Bomb Calorimeter: a constant-volume device used to measure the energy of a combustion reaction

Method 2: Hess’s Law

  • The change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps
  • If a reaction is reversed, 𝚫H sign changes
  • If the coefficients of a reaction are multiplied by an integer, 𝚫H is multiplied by that same integer

Method 2b: Enthalpies of Formation

  • The standard enthalpy of formation (𝚫H f °) (standard heat of formations) is the enthalpy change of a formation reaction
  • A formation reaction is the process of forming exactly one mole of a substance is in its standard state from its component elements in their standard states
  • Practice: Write the standard enthalpy of formation reaction for nitric acid
    • _____ → HNO3
    • H2 + O2 + N2 → HNO3
    • ½ H2 + 3/2 O2 + ½ N2 —> HNO3
  • Formation of Oxygen Gas
    • O2 (g) → O2 (g)
  • 𝚫H f °Al (s) = 0
  • Calculating 𝚫Hrxn degree from 𝚫Hf °values
  • CH3OH(g) + _O2(g) → CO2(g) + _H2O(l) → 1CO2(g) + 2H2O(l)
    • Use table 6.5: -238.6 kj/mol + 0 kj/mol → -395.5 kj/mol + 2(-285.8 kj.mol)
    • 𝚫Hrxn = -726.5 KJ
    • *practice problem

*Standard State = 0, look at Table 6.5

Method 2c: Bond Energies

  • Calculating 𝚫H rxn from Bond Energies
    • The 𝚫H f °reaction can be estimated by comparing the cost of breaking old bonds to the income from making new bonds

Energetics of Ionic Bond Formation

  • The ionization energy of the metal is endothermic
  • The electron affinity of the nonmetal is exothermics
  • But the heat of formation of most ionic compounds is exothermic and generally large. Why?

Ionic Bonding and the Crystal Lattice

  • The extra energy that is released comes from the formation of a structure in which every cation is surrounded by anions, and vice versa
  • The crystal lattice is held together by electrostatic attraction of the cations for all the surrounding anions
  • The crystal lattice maximizes the attractions between cations and anions, leading to the most stable arrangement
  • Lattice Energy: the extra stability that accompanies the formation of the crustal lattice is measured as the lattice energy.
    • The lattice energy is the energy released when the solid crystal forms from separate ions in the gas state.

Determining Lattice Energy: The Born-Haber Cycle

  • The Born-Haber Cycle is a hypothetical series of reactions that represents the formation of an ionic compound from its constituent elements
  • The reactions are chosen so that the change in enthalpy of each reaction is known except for the last one

Born-Haber Cycle

Trends in Lattice Energy

  • Ion Size:
    • The force of attraction between charged particles is inversely proportional to the distance between them
    • Larger ions mean the center of positive charge (nucleus of the cation) is farther away from the negative charge (electrons of the anion)
      • Larger ion: weaker attraction
      • Weaker attraction = smaller lattice energy
  • Ion Charge:
    • The force of attraction between oppositely charged particles is directly proportional to the product of the charges
    • Larger charge means the ions are more strongly attracted
      • Larger charge = stronger attraction
      • Stronger attraction = larger lattice energy
    • Of the two factors, ion charge is generally more important