Thermochemistry

• Combustion reactions involve putting a hydrocarbon fuel with oxygen to produce carbon dioxide and water
• Incomplete combustion occurs when there is not enough oxygen present
• Incomplete combustion of heptane produces carbon monoxide and water
• In exothermic reactions, the reactants have more energy than the products
• Delta H is the change in enthalpy and is negative for exothermic reactions
• Endothermic reactions absorb energy and have a positive Delta H
• Enthalpy is measured in kilojoules per mole
• Thermochemical equations only need to be written if specifically asked for
• If coefficients are doubled in a thermochemical equation, Delta H is also doubled
• If an equation is reversed, Delta H becomes negative
• When adding two equations, you add their Delta H values
• Equation starts with N2O4 and finishes with nitrous oxide and O3 (Ozone)
• Equation 1: n2o4 gas goes to nitrous oxide with Delta h = -11.1
• Equation 2: No changes needed
• Equation 3: O3 (Ozone) needs to be on the other side
• Add equations 1 and 2, then add the product to equation 3
• Halve the values to get one N2O and one O3
• Final equation: 213 kilojoules per mole
• Heating and combustion
• Heat of combustion and molar heat of combustion are in the data booklet
• Always use kilojoules per mole unless specified
• Universal gas law (PV = nRT)
• Units: Pressure in kilopascals, volume in liters, n in moles, R is 8.31
• Temperature in kelvins (Kelvins = degrees Celsius + 273)
• SLC conditions: 25 degrees Celsius or 298 Kelvin, 1 molar concentration, 100 kilopascals pressure
• Derivative equation: V = n * 24.8 at SLC conditions
• V = n * 24.8 can be used for quick calculations at SLC
• Vika may ask questions using this equation
• Convert all units to standard units for calculations
• Vika may provide values in different units to test understanding