energetics

• Define an endothermic reaction

  a reaction in which the heat is taken in as the reactants change to products; causing the temperature to drop
• Define an exothermic reaction

  a reaction in which heat is given out as reactants change to products; causing the temperature to rise
• Define enthalpy change


  a measure of heat energy absorbed or released when a chemical or physical change occurs at constant pressure
• what are the standard conditions for standard enthalpy?

  - 298K

  - 100kPa

  - 1 mol/dm^3
• what is standard enthalpy of formation?

  the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• what is standard enthalpy of combustion?

  the enthalpy change when 1 mole of a substance is completely burned in excess oxygen under standard conditions.
• what is mean bond enthalpy?


  The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds
• how is enthalpy change measured?


  1. calculate heat lost or gained - q=mcΔt

  2. calculate no. of moles of ONE of the reactants that caused the enthalpy change

  3. calculate ΔHr using actual heat change and number or moles
• explain why the value of the standard enthalpy of formation of water is the same as standard enthalpy of combustion of hydrogen


  it is the same reaction
• write an equation to represent the standard enthalpy of formation of calcium oxide CaO(s)

  Ca (s) + 1/2O2 (g) ---> CaO(s)
• write an equation to represent the standard enthalpy of formation of ethanol C2H5OH (l)

  2C (s) + 3H2 (g) + 1/2 O2 (g) ---> C2H5OH
• write an equation to represent the standard enthalpy of combustion of ethene C2H4 (g)


  C2H4 (g) + 3O2 (g) ---> 2CO2(g) + 2H2O(l)
• define Hess' law


  The enthalpy change of a reaction is independent of the route taken
• what are the three stages of the thermochemical cycle in energies of formation

  1. reactant

  2. standard states

  3. product
• what is calorimetry?


  the precise measurement of the heat flow into or out of a system for chemical and physical processes
• what are the three stages of the thermochemical cycle in energies of combustion

  1. reactants

  2. combustion products

  3. products