1.4 Energetics

• In a reaction, what two processes occur concerning bonds?

  Bonds are broken and made
• What happens to energy when bonds are broken in a reaction?

  Energy is taken in from surroundings
• What happens to energy when bonds are formed in a reaction?

  Energy is given out
• What determines the overall energy change of a reaction?

  Energy transferred when bonds break/form

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

  Positive
• What is the sign of the enthalpy change when energy is released to the surroundings?

  Negative
• Under what standard conditions is enthalpy measured?

  100 kPa pressure and 298 K
• What is the definition of enthalpy of formation?

  Enthalpy change for producing one mole

  
• Under what conditions is the enthalpy of formation measured?

  Standard conditions
• What is the definition of enthalpy of combustion?

  Enthalpy change for burning one mole

  
• Under what conditions is the enthalpy of combustion measured?

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

  Data book values are averaged
• What experimental method is used to find bond enthalpy values?

  Calorimetry

  
• What is calorimetry?

  Experimental method for finding enthalpy change
• Why are experimentally-derived bond enthalpy values not exact?

  They vary in each situation
• What is measured in calorimetry to determine enthalpy change?

  Temperature change over time
• What data treatment is applied to calorimetry data to improve accuracy?

  Extrapolation
• What is the relationship between measured temperature change and energy change in calorimetry?

  Proportional
• What does 'q' represent in the equation $q =$$mc∆T$?


  Energy change
• What does 'm' represent in the equation $q =$$mc∆T$?


  Mass
• What does 'c' represent in the equation $q =$$mc∆T$?


  Specific heat capacity
• What does '∆T' represent in the equation $q =$$mc∆T$?


  Temperature change
• What property of a substance does its specific heat capacity relate to?

  Energy required to raise temperature
• What is the definition of specific heat capacity?

  Energy to raise 1g by 1K
• After calculating 'q', what calculation is performed to find enthalpy change per mole?

  $∆H =$$q / n$

  
• Why are ∆H values found using calorimetry never completely accurate?

  Energy lost from the system
• Through what two processes is energy easily lost from a calorimetry system?

  Conduction or convection
• What fundamental principle underlies Hess's Law?

  Energy is conserved
• According to Hess’s Law, what is consistent about the overall enthalpy change for a reaction?

  It's the same regardless of route
• What name is given to the principle that the overall enthalpy change for a reaction is the same, regardless of the route taken?

  Hess's Law
• For what type of reactions is Hess's Law particularly useful?

  Reactions difficult to measure directly
• What visual aid is used with Hess's Law to determine enthalpy changes?

  Triangular cycle method

  
• What aspect of the arrows in a Hess's Law cycle dictates whether values are added or subtracted?

  Direction
• When using enthalpies of formation in a Hess's Law cycle, where do the arrows point?

  Out from the central product
• In a Hess's Law cycle using enthalpies of formation, what is formed from the elements at the central point C?

  A and B
• When using enthalpies of combustion in a Hess's Law cycle, where do the arrows point?

  Towards the central product
• In a Hess's Law cycle using enthalpies of combustion, what is the central product C?

  H2O and CO2
• What does bond enthalpy data represent?

  Averaged energy to break one mole

  
• Under what conditions is bond enthalpy defined?

  Gaseous state, standard conditions
• How can bond enthalpies be used in conjunction with Hess’s Law?

  Find overall enthalpy change