Energetics-NOTES

• 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 processes?

  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 transferred 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 determined?

  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?

  An experimental method for finding 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 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, as it cannot be created or destroyed.

  
• How does Hess's Law relate to overall enthalpy change?

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

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

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

  The direction of the arrows indicates whether the 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 A and B are formed from the elements at C.
• 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 A and B burn to form the products at C.
• 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 reaction arrows always point towards the central product.

  
• 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.