5 Energetics

• what is the energy store in a compound called?

  its enthalpy (H)
• in exothermic reactions is delta H positive or negative?

  negative
• in endothermic reactions is delta H positive or negative?

  positive
• what is the standard enthalpy of combustion?

  the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states.
• is standard enthalpy of combustion exo of endo?

  always exo
• what is standard enthalpy of formation?

  the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states.
• is the standard enthalpy of formation exo or endo?

  usually exo
• what is the standard enthalpy of neutralisation?

  the enthalpy change when one mole of water is formed from its ions in dilute solutions.
• is the standard enthalpy of neutralisation exo or endo?

  always exo
• what is the standard enthalpy of reaction?

  the enthalpy change when the dumber of reactants shown in the equation react under standard conditions to give the products in their standard states.
• What is the law of conservation of energy?

  energy cannot be created or destroyed, only transferred from one store to another.
• what is hess's law?

  enthalpy change is in dependant of the path taken.
• breaking bonds ... energy

  requires
• forming bonds ... energy

  releases
• what is bond dissociation energy?

  the energy required to break one mole of gaseous bonds to form gaseous atoms.
• why do we use the mean value of bond dissociation energy?

  because as a molecule begins to break down the remaining bonds exist in slightly different environments and therefore have different bond enthalpies.
• what is mean bond enthalpy

  the average enthalpy change when one mole of bonds for the same type are broken into gaseous molecules under standard conditions.
• what equations are used to calculate the enthalpy change for a reaction by measuring the change of temperature?


  q = mc deltaT enthalpy change (per mole) = q/moles
• what do each letter stand for and what are their units in q = mc deltaT?

  q (J) = heat energy given out m (g) = mass of substance heatedc (J g^-1 K^-1) = SHCdeltaT (K) = temperature rise