energetics

• enthalpy change is the amount of heat energy taken in or given out during a change in a system provided the pressure is constant
• in an exothermic change energy is transferred from the system (chemicals) to the surroundings

  products have less energy than the reactants

  
• in an endothermic change, energy is transferred from the surroundings to the system (chemicals)

  requires input of heat energy

  products have more energy than reactants

  
• in an exothermic reaction the ∆H is negative
• in an endothermic reaction the ∆H is positive
• enthalpy change of formation is enthalpy change when one mole of compound is formed from its elements under standard conditions (298K and 100Kpa) with all reactants and product in their standard states
• enthalpy change of combustion is enthalpy change when one mole of substance is combusted completely in oxygen under standard conditions (298K and 100KPa) with all reactants and products being in their standard states
• Incomplete combustion will lead to soot, carbon monoxide and water

  it will be less exothermic than complete combustion
• standard conditions for enthalpy changes are

  100KPa pressure

  298K (room temperature or 25 degrees Celsius)

  solutions at 1mol dm-3
• energy change (j) = mass of solution (g) x heat capacity (j g-1 k-1) x temperature change (k)

  Q=mc∆T
• calculating enthalpy change of reaction ∆H from experimental data

  1. use q=mc∆t to calculate energy change
  2. work out moles of reactant
  3. use limiting reactant moles for ∆H = q/mol
  4. add a sign and unit (divide by 1000 to go from J to kJ)
• enthalpy of combustion can be calculated using calorimetry

  generally the fuel is burnt and flame is used to heat up water in metal cup
• errors with calorimetry

  energy loss from calorimeter

  incomplete combustion of fuel

  incomplete transfer of energy

  evaporation of fuel after weighing

  heat capacity of calorimeter not included

  measurements not carried out under standard conditions, H2O is gas not liquid
• calorimetric method

  1. wash equipment with solutions to be used
  2. dry cup and place in beaker for insulation/support
  3. measure desired solution volume with volumetric pipette and transfer to insulated cup
  4. clamp thermometer in place with bulb in solution
  5. measure initial temperature and every minute
  6. at minute 4 transfer reagent to cup and stir
  7. keep recording temperature
• calorimetric notes

  if reaction is too slow, exact temperature rise is difficult to obtain as cooling occurs simultaneously with reaction

  counteract by extrapolating readings to fourth minute
• errors in calorimetry

  1. energy transfer from surroundings (loss)
  2. approximate specific heat capacity and density of solution, we assume its same as water
  3. neglect heat capacity/absorbed by calorimeter
  4. reaction/dissolving mat be incomplete or slow
• Hess's law states total enthalpy change for a reaction is independent of the route by which the chemical change takes place
• hess's law can determine enthalpy changes from enthalpy of formation

  ∆H reaction = Σ ∆fH products - Σ ∆fH reactants
• Hess's law can determine enthalpy changes from enthalpy of combustion

  ∆H reaction = Σ ∆cH reactants - Σ ∆cH products
• mean bond energy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
• mean bond energy values are positive because energy is required to break a bond
• in an exothermic reaction, the sum of the mean bond energies in the reactant molecules will be less than the sum of bonds in the product molecules
• ∆H = Σ bond energies broken - Σ bond energies made

  where all substances are gases
• when comparing enthalpies of combustion for successive members of a homologous series such as alkanes and alcohols there is a constant rise in the size of enthalpies of combustion as the number of carbon atoms increases
• hess cycle for enthalpy change of combustion