Chapter 11
Thermochemistry




Thermodynamics --


Work -- energy used to cause an object with mass to move
Heat -- energy used to cause the temperature of an object to increase
Kinetic Energy -- energy of motion
Potential Energy -- energy not in motion - occurs when there is attraction and repulsion between objects
Calorie (cal) -- amount of energy required to raise the temperature of 1 gram of water 1 degree Celsius
Force -- any kind of push or pull exerted on an object against the force of gravity
Energy -- the capacity to do work or to transfer heat


First Law of Thermodynamics

Internal Energy - the sum of all the KE and PE of all the components of the system - represented by "E"

Relating ∆E (change in energy) to heat and work
∆H depends on the change within the system, no on how the change occurs



Enthalpies of Reactions

∆H = Hfinal - Hinitial (enthalpy of reaction = ∆Hrxn)



Guidelines for using thermochemical equations and enthalpy diagrams:

  1. Enthalpy is an extensive property (depends on the quantity of sample and includes measurements of mass and volume)
    • ∆H is directly proportional to the amount of reactant consumed in the process
  2. The enthalpy change for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction
  3. The enthalpy change for a reaction depends on the state of reactants and products (whether they are a gas, liquid, or solid)

Calorimetry - measure the heat flow in or out of a system

Hess's Law

Enthalpies of Formation


  1. The change depends on the state in which a substance exists - gas, liquid, or solid
  2. Standard state = pure form of a substance at 1 atm and 25C or 298 K
  3. Standard Heat of Formation = ∆Hf (kJ/mol) = 1 mol of substance is formed
  4. The ∆Hf of any element is zero (0)

Heat of Reaction

∆Hrxn = Σn∆Hf(products) - Σm∆Hf(reactants)

where, n and m are coefficients from the balanced equation

  • Chemical Kinetics: branch of chemistry that deals with rates of reactions and mechanisms of chemical reactions.


  • Thermodynamics: study of the changes in energy in chemical reactions, the influence of temperature on those changes, and the other factors that allow/cause chemical reactions to take place.


  • Collision Theory: states that particles must collide in order for chemical change to take place. (these particles may be ions, atoms, or molecules)


  • There are four main factors that affect the rate of a chemical reaction:
    • 1. the nature of the reactants
      2. the temperature of the system
      3. concentration of the reactants (including the pressure of reactants in gaseous form)
      4. the use of a catalyst


    Heat Content and Enthalpy

    • Every system has energy stored in it
      • energy in the chemical bonds, random motion of the atoms and molecules, and potential energy (stored energy)
    • The total of all these forms of energy is called the heat content or enthalpy--represented by "H".
      • the exact amount of enthalpy cannot be calculated
      • enthalpy will remain constant as long as no energy leaves or enters the material
      • if energy leaves or enters the substance, its enthalpy will change by an amount equal to the amount of energy gained or lost
    • a change in enthalpy is represented by "delta H"

  • Since energy must be added to ice to change it to the liquid phase, then:

  • delta H = HH2O(l) - HH2O(s)
    • we can measure the amount of heat absorbed when the phase change occurs (endothermic process)
    Heat of Fusion
    • quantity of heat needed to change a unit mass of solid to liquid at a constant temperature
    • for ice at zero degrees celsius it takes 335 joules per gram to change to liquid (endothermic process)
    • for liquid at zero degrees celsius it takes -335 joules per gram to change to ice (exothermic process)
    • the enthalpy for chemical reactions is different on both sides of the equation:

    reactants vs. products

    • this change of enthalpy is called the heat of reaction--represented by delta H
    • a change in enthalpy is measured in a unit called kilojoules (kJ)

    Heat of Formation

    • When you have 1 mole of a compound being formed, the delta H is called the "heat of formation"--delta Hf the heat of formation is dependent on the temperature and pressure at which the reaction occurs (it also depends on the phase of the substance)
    • the standard heat of formation, is the heat of formation at 25 degrees celsius and the pressure is at 1 atm
    • 25 degrees celsius = 298 K
    • 1 atm = 101.3 kPa
    • the heat of formation of water is -286 kJ/mol ---> 1 mol of water is formed from H2 + O2 at 25 degrees celsius and 1 atm (286 kJ of heat is released)
    • the minus sign indicates that heat is being given off
              Exothermic Reactions          vs.          Endothermic Reactions
    
              *energy is a product                       *energy as a reactant
    *large neg. values give *small neg. or pos. values-- off a lot of energy during unstable (explosives) its formation--very stable

    Calculating heat of formation:
    1 mole of ethyl alcohol is -9.5 x 102 kJ or -950 kJ. How much heat is produced when 11.5 grams of ethyl alcohol is burned?
      1. Change grams to moles:
            C2H5OH    x    1 mol  =  0.25 mol
                          46.08 g
    
      2. Change moles to kJ:
           0.25 mol C2H5OH   x   -950 kJ  =  -237.09 kJ
                                  1 mol
    

    Hess's Law of Constant Heat Summation
    • Law of Conservation of Energy-->energy can be neither created nor destroyed, but can change from one form to another
    • Hess's Law of Constant Heat Summation states that when a reaction can be expressed as the algebraic sum of 2 or more other reactions, then the heat of the reaction is the algebraic sum of the heats of these other reactions
    Method 2 for calculating heat of formation:
    Heat of formation of the reaction = heat of formation of the products - heat of formation of the reactants
    • any single element has a value of zero
    • if you have more than one mole of substance (based on the balanced equation) you have to multiply the heats of formation by the coefficient of the compound involved
    Entropy
    • measure of disorder, randomness, or lack of organization in a system
      • the more disorder, the higher the entropy value
    • entropy is represented by the capital letter "S"
      • therefore, delta S = Sf - Si
    • where Sf = final entropy--after the change has occurred
    • and Si = initial entropy--before the change has occurred
    • both are positive values but the change can be either positive or negative
    • an increase in entropy is positive
    • a decrease in entropy is negative
    • when a substance changes form the entropy will change
      • when a substance is in a solid form the entropy value is low
        • when a solid changes to a liquid, the entropy increases (the liquid is more "disordered" than a solid)
        • when a liquid changes to a gas, the entropy again increases (the gas is more "disordered" than the liquid--gas molecules/particles tend to act independently of each other)
        • when elements form a compound, a more stable condition is achieved-->entropy is low
        • the decomposition of a compound into individual elements creates a more unorganized system-->the entropy increases
        • (temperature can also cause a change in entropy)