Energetics

• Bond making releases energy
• Bond breaking requires energy
• Endothermic (enter)- if at the end of the reaction, energy has been taken in, the reaction is endothermic
• Exothermic (Exit)- if at the end of reaction, energy has been given out, then reaction is exothermic.
• What are some errors that could happen when doing an experiment

  • spillage
  • parallax error
• Example of endothermic:

  • thermal decomposition
• Examples of exothermic:

  • combustion
  • neutralisation
  • displacement
• What is thermal decompositions? The breakdown of a substance by heating in the presence of oxygen
• In a Reversible reaction, if one direction is exothermic the reverse direction is always endothermic
• The energy released is exactly the same as the energy taken in a reversible reactions
• Enthalpy is the total energy of some particular substance. It is given the symbol H
• A change in energy is measured at a constant pressure and it is called enthalpy change, the units are kilojoules per mole
• Enthalpy = triangle H
• The standard conditions for measuring enthalpy changes are:

  • Pressure of 100Kpa
  • a temperature of 298K
  • $Triangle H _o 298- delta H standard$
• The standard molar enthalpy of formation is the never change that takes place when 1 mole of a compound is formed from its constituent element in their standard state under standard conditions
• Example of standard molar enthalpy:

  Li(s) + 1/2 F2—————> LiF(s)
• The standard molar enthalpy of combustion is the energy change that takes place when 1 mol of a substance is completely combusted
• Example of standard molar enthalpy of combustion:

  H2 (g) + 1/2 O2 ————> H2O
• Exothermic - Diagram 


  This is the picture

  
• Endothermic - Diagram

  
• activation energy 


  Is the minu amount of energy need for a reaction to occur
• Hess law

  The enthralls change for a chemical reaction is independent of the route taken. we use Hess law as many Enthalpy change cannot be measured directly
• Equation for the Enthalpy of formation 


  $Products - reactants$
• Specific heat of a substance

  It is the amount of energy needed to increase the temperature of 1g substance by 1k same as 1c
• Measuring Enthalpy Change equation

  $Q=$$mc delta T$
• What Does Q stand for in the equation

  heat exchanged
• What does m stand for in the equation 


  Mass of substance being heated or cooled
• C in the equation

  Specific heat capacity
• Equation to measure mass

  mass= density x volume
• Bond enthalpy equation 


  $Reactants - products$
• Mean bond enthalpy 


  Enthalpy required to break a covalent bond averaged over a range of different molecules
• combustion equation 


  $Reactants - products$