2.1 Chemistry - Thermochemistry

• Write the equation for the reaction which represents the enthalpy change of combustion of propanol.


  C3H7OH + 4^1/2 -> 3CO2 +4H2O
• How do we calculate enthalpy change?

  ΔH = H products - H reactants
• What is standard enthalpy of formation?

  The enthalpy change when one mole of a substance is formed from its elements in their standard state
• What happens to bonds during a 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
• 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 enthalpy change when energy is taken in?

  Positive

  
• What is the enthalpy change when energy is released?

  Negative

  
• What type of reaction is indicated by a positive overall enthalpy change?

  Endothermic
• What type of reaction is indicated by a negative overall enthalpy change?

  Exothermic
• Under what conditions are enthalpies measured?

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

  Enthalpy change when one mole of substance is produced from elements

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

  Enthalpy change when one mole of substance is burned in oxygen

  
• 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?

  Overall enthalpy change is independant of the route taken
• What method is used to determine enthalpy changes that cannot be found directly?

  Triangular cycle method with an intermediate product
• How are arrows used in the triangular cycle method?

  They indicate whether values should be added or subtracted
• How can Hess's Law be applied to calculate enthalpy changes?

  Using given enthalpies of formation or combustion
• What do the arrows point towards in the triangular diagram for combustion?

  They point towards the central product (H​2​O and CO​2​)
• What is average bond enthalpy defined as?

  Energy required to break one mole of a bond in gaseous state

  
• How can bond enthalpy data be used in reactions?

  To find overall enthalpy change for a reaction

  
• Why is the overall enthalpy change using bond enthalpies only an approximation?

  Average bond enthalpies vary across different compounds
• What is calorimetry and how is it used to find enthalpy change?

  • Experimental method for measuring temperature change
  • Conducted in a sealed, insulated container
  • Temperature noted at regular intervals
  • Reduces energy loss to surroundings
• What is the relationship between temperature change and energy change in calorimetry?

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

  q = mc∆T

  
• What does specific heat capacity measure?

  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 are the limitations of calorimetry in measuring enthalpy changes?

  • Energy loss to surroundings
  • Inaccuracies in temperature measurements
  • Assumes no heat loss to the environment