Chem - topic 5/ 15

• Define Average Bond Enthalpy

  it is the energy required to break one mole of covalent bonds in the gaseous state under standard conditions
• Heat comes in many forms..

  Heat, Light, Sound, Electricity, and chemical energy
• Energy is always -- in a chemical reaction

  Conserved
• The heat content of a system is its --


  Enthalpy
• What is enthalpy

  Enthalpy is a measure of the amount of heat energy contained in a substance -- it is stored in the chemical bonds and intermolecular sources as potential energy.
• Heat is --

  Thermal energy caused by the movement of molecules
• Heat is expressed in --

  Joules
• What is temperature?

  measure of the avg kinetic energy of particles caused by the transfer of heat making molecules move faster, expressed in Kelvin or Celcius
• Exothermic reaction

  the release of energy (warm) the enthalpy of the products is less than the reactants. (NEGATIVE CHANGE IN ENTHALPY) heat is given out to the system
• Endothermic reactions

  The absorption of energy -- the enthalpy of the products is greater than the reactants (NEGATIVE change in enthalpy)
• What is Specific heat capacity

  the quantity of heat required to raise the temperature of a unit mass of a substance by one degree Celsius.
• Combustion and neutralization reactions are ---

  Exothermic thus negative enthalpy change
• Standard molar enthalpy of formation is

  the energy absorbed or released when 1 mole of a substance is formed from its constituent elements (in their normal state) at 25 degrees Celsius and 101kPa/1atm.
• Standard enthalpy of formation is ZERO for elements/ diatomic molecules because 


  they are already in their stable/standard state thus don't require energy to be formed (change of enthalpy of formation for an element is by definition 0).
• Sublimation is -- and deposition is

  sublimation is SOLID TO GAS deposition is GAS TO SOLID
• Standard molar enthalpy of combustion is

  the energy released when 1 mole of an organic compound combusts.
• IF calculating standard molar enthalpy of combusion from combustion values 

  REACTANTS MINUS PRODUCTS
• Standard molar enthalpy of combustion is →  the energy released when 1 mole of an organic compound combusts.
• If calculating standard molar enthalpy values from formation values → produces minus reactants.
• Assumptions made when calculating the standard molar enthalpy of combustion are →

  all the heat from the combustion reaction is transferred to the water, none of the heat is absorbed by the calorimeter, the combustion went to completion and the experiment was performed under standard conditions (25 degrees Celsius and 101kPa.
• if energy was added to the same mass and same initial temperature the hottest will be

  one with the lowest heat capacity
• Is bond breaking exo or endothermic

  Endothermic -- requires energy, bond making is exothermic and releases energy
• Average bond enthalpy can be inaccurate since ---

  it doesn't take into account the intermolecular forces/it is an average of all molecules.
• average bond enthalpies is calculated by

  bonds broken minus bonds formed
• Standard Enthalpy Change of Atomisation 

  enthalpy change when an element or compound is converted into 1 mole of free gaseous atoms under standard conditions
• If given atomization energy for a diatomic molecule → assume its the energy required to make one gaseous mole of free atoms, not two gaseous moles
• Enthalpy of atomisation and dissociation are different because --

  atomization enthalpy is energy needed for one mole to bond itself to itself (it is the energy for each atom not the two combined) while dissociation enthalpy is energy needed to split one mole of gasous bonds- dissociation is double atomization.
• Dissociation enthalpy is the same as 

  bond enthalpies
• dissociation enthalpy is --

  break one mole of a certain kind of gaseous bond. If there are multiple bonds in a structure, its the energy needed to break only one bond form the structure and keep the rest intact.
• Standard enthalpy change of hydration 


  enthalpy change when one mole of a gaseous ion is dissolved in large amount go water under standard conditions
• Standard Enthalpy Change of Solution

  Enthalpy change when 1 mole of a crystalline solid dissolves in such a large volume of solvent that addition of more solvent produces no further heat change under standard conditions
• Lattice enthalpy is

  enthalpy change when 1 mole of a crystalline solid in its solid form is broken into gaseous ion and it is always endothermic
• 1st ionization energy is always

  endothermic
• 1st electron affinity is always 


  exothermic
• what is Entropy

  a thermodynamic quantity related to the number of ways the energy of a system can be dispersed through the motions of its particles.
• All matter has the tendency to want 


  to be become more dispersed
• what is entropy affected by

  Temp - greater temperature means more particle movement so high disorder/entropy. Number of particles - more complex structures with more matter means higher likelyhood of disorder/entropy. atomic size and volume & pressure
• what states of matter have the highest entropy

  gases have the highest entropy, then liquid and then solid and aqueous has higher entropy than solid.
• Spontaneity

  very slow due to high activation energies
• Gibb's free energy must be -- to be spontaneous

  Negative