energetics

• What happens to bonds during a chemical reaction?

  Bonds are broken and then made
• What is required for bonds to be broken?

  Energy is taken in from surroundings
• What occurs when bonds are formed?

  Energy is given out to surroundings
• How does the overall energy change of a reaction depend on bond processes?

  It depends on energy transferred in breaking and forming bonds

  
• What is the sign of enthalpy change when energy is taken in?

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

  The enthalpy change is negative
• Under what conditions is enthalpy measured?

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

  Enthalpy change when one mole of substance is produced from elements

  
• What is the enthalpy of combustion (∆cH​o​)?

  Enthalpy change when one mole of substance is burned in oxygen

  
• What do mean bond enthalpies represent?

  Energy required to break different covalent bonds

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

  They are averaged values and vary in situations
• What is calorimetry?

  An experimental method for finding enthalpy change
• How does calorimetry measure enthalpy change?

  By measuring temperature change over time
• What does the measured change in temperature indicate in calorimetry?

  It is proportional to the energy change
• What is the formula for calculating energy change in calorimetry?

  q = mc∆T

  
• What is specific heat capacity?

  Energy required to raise 1g of substance by 1K
• Why are ∆H values found using calorimetry never completely accurate?

  Energy is lost through conduction or convection
• What does Hess's Law state about energy in a reaction system?

  Energy must be conserved; cannot be created or destroyed
• How does Hess's Law help in determining enthalpy changes?

  It allows calculation of enthalpy changes indirectly

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

  A method using an inbetween product to calculate enthalpy changes

  
• How are arrows used in the triangular cycle method?

  They indicate whether values should be added or subtracted
• How can Hess's Law be used with enthalpies of formation?

  To calculate enthalpy change using given enthalpies of formation

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

  They point out from the central product

  
• How can Hess's Law be used with enthalpies of combustion?

  To calculate enthalpy change using given enthalpies of combustion
• What is the significance of the arrows in the triangular diagram for enthalpies of combustion?

  They point towards the central product

  
• What do bond enthalpy data represent?

  Energy required to break one mole of a bond

  
• How can bond enthalpy data be used with Hess's Law?

  To find the overall enthalpy change for a reaction

  
• What direction do reaction arrows point in bond enthalpy diagrams?

  They always point towards the central product