Topic 8

• Why do chemical reactions have enthalpy changes?

  Some bonds are broken and some are made
• What ate standard conditions

  Temperature- 298K (-273.15 °C)

  Pressure - 100kPa
• What is enthalpy change?

  Is the heat energy change in a reaction at constant pressure

  Units - kj mol-1
• Exothermic reactions

  Give out heat energy - temp often increase

  AH is negative
• Endothermic Reactions

  Absorb heat energy

  AH is positive

  Temp often falls
• Enthalpy level diagrams: exothermic


  Reactants release energy to surroundings so products have less enthalpy than the reactants

  
• Enthalpy level diagrams: endothermic


  
 Reactants take in the energy from surroundings so the product have more enthalpy than the reactants

  
• If something has less enthalpy what does that mean?

  It's more stab
• What do reaction profile diagrams show?


  Enthalpy changes during reactions Activation energy is the minimal energy needed to begin breaking reactant bonds and start a chemical recation
• Endothermic reaction profile diagram

  Enthalpy increases

  
• Exothermic reaction profile diagram

  Enthalpy decreases

  
• Why do we need to specify the conditions for enthalpy change?

  Because changes in enthalpy are affected by temperature and pressure - using standard conditions = everyone knows
• Standard enthalpy change of reaction ArHø


  Is the enthalpy change when the recation occurs in the molar quantities shown in the chemical equation, under standard conditions

  
• Standard enthalpy change of formation AfHø

  Is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

  
• Standard enthalpy change of combustion AcHø


  Is the enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

  
• Standard enthalpy change of neutralisation AneutHø

  Is the enthalpy change when an acid and an Alkali recat together under standard conditions to form 1 mole of water
• Standard enthalpy change of neutralisation AneutHø



  Is the enthalpy change when an acid and an Alkali recat together under standard conditions to form 1 mole of water