thermochemistry

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thermochemistry
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2012-10-01 23:22:16
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  1. thermochemistry 
    the study of the energy changes that  accompany physical or chemical changes in matter 
  2. thermal energy 
    kinetic energy that results from the motion of molecules (results in physical change)
  3. physical change
    change in the arrangements of molecules (no chemical bonds are broken)
  4. chemical change
    change in electronic sturcture (chemical bonds change and atoms rearrange and form new substances)
  5. nuclear change
    change in the nucleus (formation of new atom)
  6. chemical system
    a set of reactants and products under study 
  7. surrondings 
    all matter around the system that is capable of absorbing or releasing thermal energy
  8. exothermic
    heat is released out of system and into surroundings
  9. endothermic
    heat is absorbed into system
  10. Heat
    the amount of energy transferred between substances

    change of thermal energy

    • factors :
    • temperature
    • mass
    • specific heat capacity
  11. temperature
    average kinetic energy of the particles in a sample of matter
  12. open system
    a system in which matter and energy can move in and out
  13. isolated system 
    and ideal system in which neither matter nor energy can move in and out 
  14. closed system 
    only energy can move in and out NOT MATTER 
  15. calorimetry
    the technological process of measuring energy changes in a chemical system (experimental technique)

    • - the surroundings point of view
    • -most of the energy change associated with the cheniccal reaction given/ taken from the environment as heat

    • when a closed system is used to contain a chemical reaction
    • the temperautre change of a set amount of water is used to measure the chemical energy difference
  16. the quantity of heat transferred (q) depends on 3 factors
    • -mass (m)
    • -Temperature change (delta T)
    • -type of substance (c) --> specific heat capacity
  17. formula for heat
    q=mc(delta T)
  18. specific heat capacity (c)
    • quantity of heat reaquired to raise the temperature of a unit mass of a substance 1degree C or 1K
    • vary from substance to substance
  19. molar enthalpy (delta Hx)
    the enthalpy change associated with a physical chemical or nuclear change involving one mole of a substance
  20. negative sign
    enthalpy changes for exothermic reactions
  21. positive sign
    enthallpy changes for endothermic reactions
  22. the amoud of energy involved in change (delta H per KJ) depends on
    the quantity of matter undergoing that change
  23. to calculate delta H you need molar
    • enthalpy value then use formula 
    • deltaH=nDeltaHx
  24. when investigating energy changes analysis is based on
    law of conservation of energy therefore total energy of change of chemical system=total energy change of the surroundings

    DeltaHsystem=+ lqsurroundingsl
  25. calorimetry of physical changes there are three assumptions
    • 1. no heat is transferred between calorimeter and environment 
    • 2. any heat absorbed/ released by calorimeter materials (ex container) is negligible 
    • 3. a dilute aqueous solution is assumed to habe a density and specific heat capacity equal to pure water (1g/m and 4.18 J/goC)
  26. an increase in the temperature of the surroundings implies
    a decrease in the enthalpy of the chemical system (change is exothermic)
  27. a decrease in the temperature of the surroundings implies
    an increase in the enthalpy of the chemical system (change is endothermic)
  28. 4 ways to communicate energy changes
    • -including energy value as a term
    • -writing a chemical equation and stating enthalpy change
    • -stating molar enthalpy of specific reaction
    • -drawing chemical potential energy diagram
  29. method #1 energy value terms
    • endothermic - certain quantity of energy is required to be supplied to reactants 
    • H20+285.8KJ--> H2+1/2 O2


    • exothermic - energy is listed with products 
    • Mg +1/2 O2 --> MgO + 601.6KJ
  30. Method #2 thermochemical equations with enthalpy values
    • 1. write balanced equation
    • 2. calculate deltaH (Hproducts -Hreactants) (H2 and O2 etc count as 0)
  31. method #3 molar enthalpies of reaction
  32. molar enthalpy of reaction
    (Hx) the energy change associated with the reaction of one mole of a substance 
  33. standard molar enthalpy of reaction
    • H0the energy change associated with the reaction of one mole of a substance at 100Lpa and specified temperature (usually 25oC)
  34. method #4 -- diagrams
    • exothermic -- high to low
    • endothermic -- low to high
  35. Hess's law
    if a set of reaction occurs in different steps but the final products and initial reactants are the same the overall enthalpy change is the same
  36. the value of delta H for any reaction that can be written in steps
    • equals the sum of the values of delta H for each of the individual steps 
    • Htarget=H1H2....
  37. 2 rules to hess's law
    • 1. if a chemical equation is reversed the sign of H changes
    • 2. if the coefficients of a chemical equation are altered by multiplying / dividing by a constant factor then the H is altered in the same way
  38. Hheat of formation
    Heat of formation- the amound of energy required/ released when a compound is formed from its standard elements 
  39. kinetic
    • -moving electrons
    • -vibrations of atoms connected by chemical bonds
    • -rotation and translation of molecules 
  40. potential
    • -nuclear potential energy of protons and neutrons in nuclei
    • -electronic potential energy of atoms connected by chemical bonds
  41. enthalpy change
    the difference in enthalpies of reactants and products during a change (energy absorbed from or released to the surroundings)
  42. energy is neither
    created nor destroyed only changes form
  43. in a closed system the total energy is 
    constant

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