Enthalpy

• Average bond enthalpy

  Energy required to break one mole of a particular bond (covalent) in a gaseous molecule
• forming bonds=exothermic

  energy is released
• breaking bonds= endothermic

  energy is required
• Enthalpy change from average bond enthalpies

  reactants - products
• bromine and chlorine do not exist in gaseous state under standard conditions as energy is needed to break individual dipole-dipole interactions
• limitations of average bond enthalpies

  actual vary on the chemical environment as the average is calculated across different chemical environments
• enthalpy change is the measure of heat energy in a chemical system
• enthalpy change is products - reactants and can be positive or negative
• energy transfer from system to the surrounding is an Exothermic change
• energy transfer from surroundings to the system is an endothermic change
• conservation of energy means that when a chemical reaction involving enthalpy change occurs heat energy is transferred between the system and surroundings
• Standard conditions

  100KPa (pressure)

  298K (temperature)

  1 moldm-3 (concentration)
• activation energy

  minimum amount of energy required for a reaction to take place (breaking of the bonds)
• exothermic

  -temperature of surroundings increases

  -chemical system loses energy and surroundings gain energy

  negative enthalpy change
• endothermic

  -temperature of surroundings decreases

  -chemical system gains energy and surroundings lose energy

  -positive enthalpy change
• standard enthalpy change of reaction

  enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with standard states
• standard enthalpy change of formation

  enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with standard states
• standard enthalpy change of combustion

  enthalpy change that takes place when one mole of a substance completely reacts with oxygen under standard conditions with all reactants and products in their standard states

  -complete combustion
• standard enthalpy change of neutralisation

  enthalpy change that accompanies the reaction of an acid and a base to form one mole of water under standard conditions with all reactants and products in their standard states
• specific heat capacity

  energy required to raise the temperature of one gram of a substance by 1K
• Lattice enthalpy

  Enthalpy change that accompanies the formation of one mole of an ionic solid from its gaseous ions under standard conditions
• standard enthalpy change of atomisation

  enthalpy change that occurs for the formation of one mole of gaseous atoms from the elements in its standard state under standard conditions

  endothermic
• first ionisation energy


  enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
• first ionisation energy is endothermic as energy is required to overcome the attraction between negative electron and positive nucleus
• first electron affinity

  enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
• first electron affinity is exothermic as the electron being added is attracted towards the nucleus
• second electron affinity

  endothermic as repulsion between two electron is needed to be overcome
• iodine= put as solid
• lattice enthalpy is the strength of ionic bonding in a giant ionic lattice
• second ionisation energy

  enthalpy change accompanying the removal of one electron from each ion in one mole of 1+ ions to form one mole of gaseous 2+ ions
• second electron affinity

  enthalpy change that takes place when one electron is added to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
• Enthalpy change is the amount of heat energy stored in a chemical system
• Why is the the enthalpy change of combustion less exothermic/ accurate?

  -heat loss to the surroundings

  -incomplete combustion

  -evaporation of alcohol from the wick

  -non-standard conditions
• Enthalpy change diagrams

  
• What are the limitations of average bond enthalpies?

  actual bond enthalpies can vary depending on the chemical environment of the bond
• avergae bond enthalpies are calculated from the actual bond enthalpies In different chemical environments
• Ionic compounds are much more stable than its gaseous ions due to the strong electrostatic forces of attraction between oppositely charged ions