energetics

• What happens to bonds during a chemical reaction?


  Bonds are broken and then made.
• What is required for bonds to be broken in a reaction?

  Energy is taken in from the surroundings.
• What occurs when bonds are formed in a reaction?


  Energy is given out.
• How does the overall energy change of a reaction depend on bond energy transfers?


  It depends on how much energy is transferred in breaking and forming bonds.
• What is the sign of the enthalpy change when energy is taken in?


  The enthalpy change is positive.
• What is the sign of the enthalpy change when energy is released?


  The enthalpy change is negative.
• How can the overall enthalpy change (∆H) be calculated?


  By measuring the energy changes during bond breaking and forming.

  
• Under what conditions is enthalpy measured?


  Under standard conditions of 100 kPa pressure and 298 K temperature.
• What is the enthalpy of formation (∆fH​o​)?


  The enthalpy change when one mole of a substance is produced from its elements under standard conditions.
• What is the enthalpy of combustion (∆cH​o​)?


  The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
• Why do different covalent bonds require different amounts of energy to be broken?


  Because bond enthalpy values vary based on the type of bond.
• How are bond enthalpy values typically obtained?


  Using calorimetry methods.
• Why do bond enthalpy values calculated experimentally differ from data book values?


  Because they are not exact and vary in each situation.
• What is calorimetry used for?


  To find enthalpy change by measuring temperature change over time.
• What does the measured change in temperature in calorimetry indicate?


  It is proportional to the energy change.
• What is the formula for calculating energy change (q) in calorimetry?


  q = mc∆T
• What is specific heat capacity?


  The energy required to raise 1g of a substance by 1K without a change of state.
• Why are ∆H values found using calorimetry never completely accurate?


  Because energy is easily lost from the system through conduction or convection.
• What does Hess's Law state about energy in a reaction system?


  Energy must be conserved; it cannot be created or destroyed.
• How does Hess's Law relate to overall enthalpy change?


  The overall enthalpy change is the same regardless of the route taken.

  
• What method is used to determine enthalpy changes that cannot be found directly?


  The triangular cycle method is used with an intermediate product.
• How are the arrows in the triangular cycle method used in Hess's Law?


  The direction of the arrows indicates whether values should be added or subtracted.
• How can Hess's Law be used with enthalpies of formation?


  It can calculate an enthalpy change using given enthalpies of formation.
• What is the significance of the arrows pointing out from the central product in a triangular diagram for enthalpies of formation?


  It indicates that both reactants are formed from the elements at the central product.
• How can Hess's Law be used with enthalpies of combustion?


  It can calculate an enthalpy change using given enthalpies of combustion.
• What is the significance of the arrows pointing towards the central product in a triangular diagram for enthalpies of combustion?


  It indicates that both reactants burn to form the products at the central product.
• What does bond enthalpy data represent?


  The energy required to break one mole of the stated bond in a gaseous state, under standard conditions.
• How can bond enthalpy data be used alongside Hess's Law?


  To find the overall enthalpy change for a reaction.
• What is the significance of the arrows pointing towards the central product in bond enthalpy calculations?


  The arrows always point towards the central product in the reaction.
• How is the enthalpy change represented when the reaction goes in the opposite direction to the arrow?


  It is subtracted from the total enthalpy change.