Thermochemistry

• Thermochemistry


  The study of the relationships between chemical changes and energy, with a focus on thermal energy

  
• Societies at all levels of development can't function without the energy released from chemical reactions
• Energy is very high in demand as we use it world running
• We use the combustion of petroleum products to run cars, trucks, cook food (gas cylinders) etc.
• The combustion of coal is used to produce electricity that lights up our homes, watch TV, cook on the stove etc.
• Though these combustion reactions are meeting our essential energy needs, the downside is that they are a huge factor on global climate change
• There are other ways of harnessing energy besides combustion. For example, we use batteries on our TV remotes, cell phones, cars etc.
• This chapter introduces many of the basic ideas necessary to explore the relationships between chemical changes and energy, with a focus on thermal energy
• Energy


  The capacity to supply heat or do work
• Work (w)


  The process of causing matter to move against an opposing force
• Types of energy


  • Potential energy
  • Kinetic energy
• Potential energy


  The energy an object has because of its relative position, composition, or condition
• Kinetic energy


  The energy that an object possesses because of its motion
• Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form after the change is completed
• Law of conservation of energy


  During a chemical or physical change, energy can be neither created nor destroyed, although it can be changed in form
• Thermal energy


  Kinetic energy associated with the random motion of atoms and molecules
• Temperature


  A quantitative measure of "hot" or "cold"
• When atoms and molecules in an object are moving or vibrating quickly


  They have a higher average kinetic energy (KE), and we say that the object is "hot"
• Increasing the amount of thermal energy in a sample of matter

  Will cause its temperature to increase
• When atoms and molecules are moving slowly

  They have lower average KE, and we say that the object is "cold"
• Decreasing the amount of thermal energy in a sample of matter

  Will cause its temperature to decrease
• Heat (q)


  The transfer of thermal energy between two bodies at different temperatures
• Heat flow


  Increases the thermal energy of one body and decreases the thermal energy of the other
• Heat flow between high and low temperature substances

  1. Substance H has higher temperature and thermal energy

  2. Substance L has lower temperature and thermal energy

  3. Heat flows spontaneously from H to L

  4. Temperature of H decreases, temperature of L increases

  5. Heat flow continues until both substances are at the same temperature
• Exothermic process


  A change that releases heat
• Endothermic process


  A reaction or change that absorbs heat
• Specific heat capacity (c)

  The quantity of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius (or 1 kelvin)
• Specific heats of common substances at 25°C and 1 bar

  • water (H2O(l)): 4.184 J/g°C
  • ethanol (C2H6O(l)): 2.376 J/g°C
  • ice (H2O(s)): 2.093 J/g°C
  • water vapor (H2O(g)): 1.864 J/g°C
  • air: 1.007 J/g°C
  • oxygen (O2(g)): 0.918 J/g°C
  • aluminum (Al(s)): 0.897 J/g°C
  • carbon dioxide (CO2(g)): 0.853 J/g°C
  • iron (Fe(s)): 0.449 J/g°C
  • copper (Cu(s)): 0.385 J/g°C
  • lead (Pb(s)): 0.130 J/g°C
  • gold (Au(s)): 0.129 J/g°C
• Internal energy (U)


  The total of all possible kinds of energy present in a substance
• Energy is stored in a substance when the kinetic energy of its atoms or molecules is raised
• First law of thermodynamics

  Internal energy of a system changes due to heat flow in or out of the system or work done on or by the system

  ΔU = q + w

  
• Enthalpy (H)


  The sum of a system's internal energy and the mathematical product of its pressure and volume

  H = U + PV
• State function


  A property depending only on the state of a system, and not the path taken to reach that state
• Both the internal energy (U) and the enthalpy (H) are state functions
• Enthalpy change (ΔH)


  The heat released or absorbed by a system under constant pressure during a chemical or physical process

  ΔH = ΔU + PΔV
• Under constant pressure conditions, ΔH = q</b>
• Thermochemical equation


  The enthalpy change of a reaction is shown as a ΔH value following the equation for the reaction
• The enthalpy change of a reaction depends on the physical state of the reactants and products
• Exothermic reaction


  A reaction where ΔH is negative, meaning heat is released to the surroundings
• Endothermic reaction


  A reaction where ΔH is positive, meaning heat is absorbed from the surroundings