Chapter 5 - Thermochemistry

  1. The Nature of Energy
    1. Force - push or pull of matter
    2. Work - movement against a force
      1. W = F x d
    3. Heat
      1. Another form of energy
      2. Transferred because of difference in temp
        2. Temperature is a measure of the intensity of the heat
    4. Energy - capacity to do work (w) or transfer heat (q).
      1. Kinetic energy
        1. energy of motion
        2. Ek = ½ mv2
      2. Potential energy
        1. position in a field
          1. gravitational
          2. electronic
    5. Energy Units
      1. Joule (J) = kg m2 / s2
      2. kJ = 1000 J => more common for measurement of energy
      3. calorie and Calorie
        1. 1 Cal = 1000 cal
        2. 1 cal = 4.184 J
    6. Define system and surroundings
      1. systems tend to minimize energy

Problems: 1-7 odd

  1. The First Law of Thermodynamics
    1. The total energy gained by a system is equal to the energy lost by the surroundings
      1. Restatement of the Law of Conservation of Energy.
    2. Internal Energy (E)
      1. sum of kinetic and potential energies.
      2. very difficult to measure
      3. much easier to measure E = Ef - Ei
        1. i - initial state (reactants)
        2. f - final state (products)
        3. - always (final - initial)
      4. 3 parts
        1. number
        2. unit
        3. sign
      5. Esystem = qsystem + wsystem
        1. Heat added to system is positive
        2. Work done to the system is positive
    3. State Function
      1. Path independent
        1. Depends only on the state, and not how you get there!
        2. Symbolized by Upper Case Symbols
        3. Non-State function - symbolized by lower case letters

Problems: 9-13 odd

  1. Heat and Enthalpy Changes
    1. Endothermic - heat absorbed by the system (from the surroundings)
    2. Exothermic - heat given off by the system (to the surroundings)
    3. Enthalpy (H) - measure H
      1. H = qp
      2. H = Hf - Hi
  2. Enthalpies of Reaction
    1. Hrxn - energy for reaction as written
    2. Thermochemical equations
    3. Note - Enthalpy is an extensive property!
    4. H for a reaction = -H for the reverse reaction.
    5. Values are state dependent
      1. rxn to liquid H2O rather than gaseous H2O

Problems: 15, 17, 21-25 odd

  1. Calorimetry
    1. Heat capacity - heat required for change in temp of object by 1C
    2. Specific heat - heat required for change in temp of 1g of substance by 1C
      1. Note that T in K = T in C
      2. Water has very high specific heat!
    3. q = c m T
    4. Coffee cup calorimeter
      1. heat flow is into isolated 'surroundings' - usually water
      2. measure surroundings info (know cwater)
      3. remember interested in qsystem [qsys + qsurr = 0]
    5. Bomb calorimeter
      1. Constant volume (qv = E)
      2. usually combustion reactions (rxn w/O2)
      3. Must determine heat capacity of calorimeter with standard

Problems: 29-37 odd

  1. Hess' Law
    1. Sum of H for individual steps = H for overall process
      1. Combustion of methane (water vapor); H = -802 kJ
      2. Water vapor to liquid (2 mol) -88 kJ
      3. overall methane with water (l) = -890 kJ
    2. Use to determine enthalpy of difficult-to-observe reactions.
      1. carbon to CO
    3. Based on Law of Conservation of Energy and definition of State Function

Problems: 39-43 odd

  1. Enthalpies of Formation
    1. Many categories of enthalpy information (fusion, vaporization...)
    2. Enthalpy of Formation (Hf)
      1. Formation of 1 mole of substance from elements in their standard (thermodynamically most-stable) state
      2. H - plimsol mark (degree sign) indicates standard conditions.
        1. 1 atm pressure and some specified (usually 298K) temp
    3. Keys to recognizing formation reaction
      1. Only elements in reactants
        1. also standard state
      2. ONLY 1 mole of product
        1. leads to fractional amounts of some reactants
        2. This is OK because def'n of formation reaction is for i mol of product
    4. Hrxn = nHf (products) - nHf (reactants)

Problems: 45-53 odd

  1. Foods and Fuel - Read pp 162 - 167

Problems: 55-59 odd

Learning Goals:

Give examples of different forms of energy.

List the important units in which energy is expressed, and convert from one unit to another.

Define the first Law of thermodynamics both verbally and by means of an equation.

Describe how the change in internal energy of a system is related to the exchange of heat and work between the system and it's surroundings.

Define the term state function, and describe it's importance in thermochemistry.

Define enthalpy, and relate the enthalpy change in a process occurring at constant pressure to the heat added or lost by the system during the process.

Sketch an energy diagram like that shown in Fig. 5.13, given the enthalpy changes in the process involved, and associate the sign of H with whether the process is exothermic or endothermic.

Calculate the quantity of heat involved in a reaction at constant pressure given the quantity of reactants and the enthalpy change for the reaction on a mole basis.

Define the terms heat capacity and specific heat.

Calculate any one of the following quantities given the other three: heat, quantity of material, temperature change, and specific heat.

Calculate the heat capacity of a calorimeter given the temperature change and quantity of material involved; also calculate the heat evolved or absorbed in a process from a knowledge of the heat capacity of the system and it's temperature change.

State Hess' Law and apply it to calculate the enthalpy change in a process, given the enthalpy changes in other processes that could be combined to yield the reaction of interest.

Define and illustrate what is meant by the term standard state, and identify the standard states for the elements carbon, hydrogen, and oxygen.

Define the term standard heat of formation, and identify the type of chemical reaction with which it is associated.

Calculate the enthalpy change in a reaction occurring at constant pressure, given the standard enthalpies for formation of each reactant and product.

Define the term fuel value; calculate the fuel value of a substance given it's heat of combustion or estimate the fuel value of a substance given it's composition.

List the major sources of energy on which humankind must depend, and discuss the likely availability of these for the forseeable future.

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