enthalpy

• what are the key things to remember and understand?

  • energy in a chemical reaction is neither created nor destroyed - they are merely transformed/converted
  • energy within chemicals is stored in bonds
• what is delta H?

  • "enpalthy change"- enpalthy = the measurement of energy in a thermodynamic system.
  • delta H = energy of product - energy of reactants
• what are the types of energy reactions?

  endothermic or exothermic
• what is an endothermic reaction?

  • results in a net gain of energy (absorbs energy)
  • delta H is positive
  • delta H = E(product) - E(reactants) = +ve

  
• what is an exothermic reaction?

  • results in a net loss of energy (releases energy)
  • delta H is negative
  • delta H = E(product) - E(reactants) = -ve

  
• thermochemical equations vs regular chemical equations

  • thermochemical equation = same as chemical equation but have delta H attached to the end of the equation
  • thermochemical equation: can have fractional coefficients
  • chemical equation: only whole number coefficients
• what is the formula for thermochemical equations involving stoichiometry?

  E1/n1 = E2/n2 or E1/m1 = E2/m2

  • E1 = delta H
  • n1 = coefficient of fuel in equation
  • E2 = energy absorbed/released
  • n2 = mol of fuel
  • left handside info is from data booklet and equations
  • right handside info is from question
• how to balance thermochemical equations involving combustion equations?

  • balance carbons (coefficient of CO2 = number of carbons on left)
  • balance hydrogens (coefficient of H2O = number of hydrogens on left divided by 2)
  • balance oxygens (coefficient of O2 = number of oxygens on right divided by 2) *be careful with this step in case there is an organic molecule
• key things to remember about delta H:

  • delta H is directly proportionate to coefficients (1/2 -> 1/2 delta H)
  • if you flip the equation -> delta H sign flips too
• what is the thermochemical equation involving complete combustion?

  CnHy + O2 -> CO2 + H2O
• what is the thermochemical equation involving incomplete combustion?

  CnHy + O2 -> CO + H2O
• what is the thermochemical equation for very incomplete combustion?

  CnHy + O2 -> C + H2O (usually for MCQ)
• what is heat capacity?


  how much energy is required to heat 1 gram of a substance by 1 degree Kelvin/degrees celsius
• what is the energy formula?

  q = mcAT

  q - energy (J)

  m - mass (g)

  c - specific heat capacity

  AT - change in temp. (degress Kelvin/degrees celsius)
• Define the term "activation energy" and explain its role in chemical reactions. How is activation energy related to the rate of a reaction?

  Activation energy is the energy required to initiate a chemical reaction. It represents the energy barrier that reactants must overcome. The lower the activation energy, the faster the reaction rate, as less energy is needed for reactants to transform into products.
• what is Calorimetry?

  • Measures the delta H of a substance (change in temp. indicates energy release)
  • Basic components of a calorimeter:
  • Electric heater - delivers specific amount of elec. energy for calibration
  • Thermometer - measures change in temp (temp increased - exo., temp decreased - endo.)
  • Stirrer - ensures even distribution
  • two types of calorimeters:
  • bomb calorimeter - no glass bulb (crucible)
  • solution calorimeter - glass bulb (crack it open to react it with other reactant)
• what are the equations used in calorimetry?

  • Electrical calibration
  • calibration
  • Ee (electrical energy) = V x I x t (secs) [in joules]
  • CF = Ee/change in Te (change in temp. due to electrical energy)
  • Calculating delta H
  • Ec (chemical energy) = CF x A Tc (change in temp. due to chemical reaction)
  • Delta H = Ec / n or m
• when is q = mcAt used vs E1/m1 = E2/m2?

  • q=mcAt : heating water
  • E1/m1 = E2/m2: combustion of fuel

  
• is the calorimeter well insulated?

  • no - heat loss when heater is turned off