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?

  It depends on how much energy is transferred when bonds are broken and formed.

  
• 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.

  
• Under what conditions is enthalpy measured?

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


  It is 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​)?

  It is the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.

  
• What do mean bond enthalpies represent?

  They represent the energy required to break different covalent bonds.

  
• Why do bond enthalpy values differ from data book values?

  They are not exact and vary in each situation, while data book values are averaged values.
• What is calorimetry?

  Calorimetry is an experimental method for finding enthalpy change by measuring temperature change over time.
• How is temperature change used in calorimetry?

  The measured change in temperature is proportional to the energy change.
• What is the formula for calculating energy change in calorimetry?

  q = mc∆T

  
• What is specific heat capacity?

  It is 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 and inaccuracies in measuring temperatures.
• What does Hess's Law state about energy in a reaction system?

  Energy must be conserved, as it cannot be created or destroyed.

  
• How is the overall enthalpy change for a reaction determined according to Hess's Law?

  It is the same regardless of the route taken.

  
• What is the triangular cycle method used for in Hess's Law?

  It is used to determine the enthalpy changes for reactions that cannot be found directly using an experimental method.

  
• How do the arrows in the triangular cycle method indicate calculations?

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

  It can calculate an enthalpy change using given enthalpies of formation by setting up a triangular diagram.

  
• What is the significance of the arrows pointing out from the central product in the 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 by setting up a triangular diagram.

  
• What is the significance of the arrows pointing towards the central product in the triangular diagram for enthalpies of combustion?

  It indicates that both A and B burn to form the products at C.
• What do bond enthalpy data represent?

  They 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?

  They can be used to find the overall enthalpy change for a reaction.

  
• What is the significance of the arrows in the triangular diagram for bond enthalpies?

  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.