THERMOCHEMISTRY

• EXOTHERMIC REACTION

  it is one in which the heat is liberated from the system to the surrounding. temperature rises during this reaction
• ENDOTHERMIC REACTION

  it is one in which heat is absorbed from the surrounding. temperature decreases during an endothermic reaction.
• SIGNS ON ENDO AND EXO REACTIONS

  endo= +ve exo= -ve
• STANDARD ENTHALPY CHANGES

  standard enthalpy changes are enthalpy changes which have been measured under standard conditions
• WHAT ARE STANDARD CONDITIONS?

  1. a pressure of 1 atm or 101 kPa
  2. temp of 25 degree celcius
  3. conc of 1 mol per dm3 for aqueous solutions
• STANDARD ENTHALPY CHANGE OF FORMATION

  it is the enthalpy change that takes place when one mol of a compound in its standard state is formed from its constituent elements under standard state conditions

  sign = -ve except for NO formation
• STANDARD ENTHALPY OF COMBUSTION

  it is the enthalpy change that takes place when one mole of a substance is completely burnt in excess oxygen under standard conditions with all reactants and products being in their standard conditions.

  sign = -ve exothermic reaction
• STANDARD ENTHALPY CHANGE OF NEUTRALIZATION

  it is the heat liberated when an acid and a base are reacted together to form one mol of water under standard state conditions.

  sign= -ve exothermic reaction
• ENERGY DIAGRAMS FOR EXO AND ENDO REACTIONS

  graphs

  
• STANDARD ENTHALPY CHANGE OF REACTION

  it is the enthalpy change when molar amount of reactants as shown in the stoichiometry equation react together under standard state conditions.

  sign = -ve or +ve can be exo or endo
• WHY DOES ENERGY TRANSFERS OCCUR DURING CHEMICAL REACTIONS?

  because of formation and breaking of bonds
• BOND ENERGY

  it is the energy needed to break 1 mole of a particular covalent bond in 1 mole of gaseous molecule
• WHAT IS A BOND?

  it is a linkage between 2 atoms only
• THE STRONGER THE BOND , THE MORE TIGHTLY THE ATOMS ARE JOINED
• BOND ENERGIES ARE A MEASURE OF THE STRENGTH OF A COVALENT BOND
• BREAKING OF BONDS IS AN EXAMPLE OF AN ENDOTHERMIC PROCESS
• FORMATION OF A COVALENT BOND IS AN EXOTHERMIC PROCESS
• IN AN ENDOTHERMIC REACTION , ENERGY MUST BE PUT IN TO BREAK ANY CHEMICAL BOND.
• IN AN EXOTHERMIC REACTION , ENERGY IS RELEASED WHEN A BOND IS FORMED
• BOND ENERGIES ARE DIFFERENT IN DIFFERENT COMPOUNDS AS THEY ARE AFFECTED BY OTHER ATOMS
• IT TAKES MORE ENERGY TO BREAK THE FIRST SIMILAR BOND THAN THE SECOND.
• WHY ARE AVERAGE BOND ENERGIES USED?

  This is because the strength of a bond between 2 particular types of atoms is slightly different in different compounds.
• COVALENT BONDS FORMED BY SMALLER ATOMS ARE STRONGER THAN THOSE FORMED BY LARGER ATOMS
• POLAR BONDS ARE STRONGER
• BONDS BETWEEN TWO ELECTRONEGATIVE ELEMENTS TEND TO BE USUALLY WEAK.
• AS BONDS BECOME SHORTER, THEY BECOME STRONGER. WHY IS THIS?

  this is because electron density in the bond increases the attraction of the nuclei for these electrons , pulling the nuclei closer.#
• SPECIFIC HEAT CAPACITY

  q = vcΔT
• ENTHALPY CHANGES IN TERMS OF SPECIFIC HEAT CAPACITY

  ΔH = –mcΔT/n

  n= number of moles
• EQUATION TO CALCULATE ENTHALPY CHANGE USING BOND ENERGIES

  ENTHALPY CHANGE = BOND BROKEN - BOND FORMED
• ENTHALPY CHANGE OF REACTION = SUM OF ENTHALPIES OF COMBUSTION (reactants) - SUM OF ENTHALPIES OF COMBUSTION ( products)

  ENTHALPY CHANGE OF REACTION = SUM OF ENTHALPIES OF FORMATION (products) - SUM OF ENTHALPIES OF FORMATION (REACTANTS)

  combustion depend on reactants

  formation depends on products