Topic 8 and 13: energetics I & II

• chemical potential energy is energy that lies between chemical bonds
• kinetic is how the bonds move
• potential is how strong particles are attracted
• when chemicals react bonds are broken and made which changes the chemical energy of atoms
• heat energy is a component of kinetic and potential energy that is responsible for the temperature of substances
• enthalpy is the thermal energy that is stored in a chemical
• unit for enthalpy is KJ/mol
• chemical system is the reactants and products
• surroundings is everything outside the chemical system
• exothermic is when heat energy is released to the surroundings and endothermic is when heat energy is taken in from the surroudnings
• exothermic enthalpy is negative while endothermic enthalpy is positive
• standard enthalpy change of reaction is the energy change associated with a given reaction under standard conditions and all reactants and products in there standard states
• standard enthalpy change of combustion is the enthalpy change that takes place when one mol of a substance reacts completely with oxygen under standard conditions and all reactants and products being in the standards states
• standard enthalpy change of formation of a compound is the enthalpy change that takes place when 1 mol of a compound is formed from its constituent element in there standard conditions and standard states
• enthalpy change of neutralisation is the enthalpy change when one mol of water is formed a neutralisation reaction in their standard conditions and standard states
• specific heat capacity is the amount of energy needed to raise the temperature of 1g of a substance by 1 degree
• heat transferred in J is mass in grams times specific heat times temperature change in K
• heat may be transferred to air, to the apparatus, conditions not standard and incomplete combustion would mean that not all reactants burned completely to make products
• enthalpy change of neutralisation add mass of both solutions when working out how much heat energy was transferred
• specific heat of water is 4.18
• average bond enthalpies is the mean energy needed for 1 mol of a given type of gaseous bonds to undergo homolytic fission
• average bond enthalpies equals to the sum of bond enthalpies of the reactants - the sum of bond enthalpies of the products
• bond enthalpies tells you how much energy is needed to break a given type of bond
• in endothermic reactions the energy required to break bonds is greater that the energy released from bond making
• in exothermic reactions the energy required to make bonds is greater that the energy required to break bonds
• standard enthalpy change of atomisation

  energy change when one mol of a gaseous atom is formed from an element in its standard states under standard conditions
• electron affinity

  energy change when each atom in 1 mol of atoms in the gaseous state gains an electron to form -1 gaseous ions
• standard lattice enthalpy

  energy change when one mol of an ionic solid is formed from its gaseous ions under standard conditions
• factors affecting magnitude of lattice energy

  charge on ions

  number of cation anion interactions

  sum of ionic radii
• lattice enthalpy of disassociation

  breaking ions of opposite charges apart
• lattice enthalpy of disassociation is the opposite of lattice enthalpy of formation
• to workout the theoretical lattice energy the assumptions are 


  ions are in contact with one another

  ions are perfectly spherical

  charge on each ion is equal
• the small the difference the closer to having a perfect ionic stricture

  the bigger the difference the more covalent character
• covalency in bonding

  caused by polarisation of anion by cation

  this distorts the electron density in the anion

  results in higher electron density near cation
• equation for charge density

  charge divided by radius squared
• standard enthalpy of solution

  enthalpy change when 1 mol of an ionic compound is dissolved in water to form infinity dilute solution

  this can be either endothermic or exothermic
• standard enthalpy of hydration

  the enthalpy change when 1 mol of gaseous ions is completely hydrated by water