PMT flash card chapter 9

• What does "system" mean in a chemical reaction?

  The atoms and bonds involved in the chemical reaction
• What is the law of conservation?

  The amount of energy in an isolated system remains the same; energy cannot be destroyed or created, only transferred.
• What energy change is associated with breaking bonds?

  Energy is taken in to break bonds, resulting in an endothermic reaction.
• What energy change is associated with making bonds?

  Energy is released to make bonds, resulting in an exothermic reaction.
• What is an endothermic reaction?

  A reaction with an overall positive enthalpy change (+ΔH), where the enthalpy of products is greater than that of reactants.
• What is an exothermic reaction?

  A reaction with an overall negative enthalpy change (-ΔH), where the enthalpy of products is less than that of reactants.
• What are the characteristics of enthalpy change diagrams for endothermic and exothermic reactions?

  • Endothermic reaction:
  • Energy is absorbed
  • Diagram slopes upwards
  • Exothermic reaction:
  • Energy is released
  • Diagram slopes downwards

  
• What does activation energy mean?

  The minimum energy required for a reaction to take place.
• Which way does the arrow for activation energy point on an enthalpy profile diagram?

  The arrow always points upwards.
• What are the standard conditions in chemistry?

  100 kPa and 298 K.
• What does "in standard state" mean?

  The state an element or compound exists in at standard conditions (100 kPa, 298 K).
• Define enthalpy change of formation.

  The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions.
• Give an example of an equation that represents standard enthalpy of formation.

  H2 (g) + ½ O2 (g) → H2O (l)
• Define enthalpy change of combustion.

  The energy change that takes place when 1 mole of a substance is completely combusted.
• Give an example of an equation that represents standard enthalpy of combustion.

  C (s) + O2 (g) → CO2 (g)
• Define enthalpy change of neutralisation.

  The energy change that takes place when 1 mole of water is formed from a neutralisation reaction.
• What does enthalpy change of reaction mean?

  The energy change associated with a given reaction.
• How can you calculate enthalpy change from experimental data?

  Use the equation $Q =$$mc\Delta T$, where $m$ is the mass of the substance being heated, $c$ is the specific heat capacity, and $\Delta T$ is the change in temperature.
• What are the advantages of using a bomb calorimeter?

  • Minimises heat loss
  • Pure oxygen used to ensure complete combustion
• Why might experimental methods for enthalpy determination not be accurate?

  Heat is lost to the surroundings, conditions may not be standard, and reactions may not go to completion.
• What does average bond enthalpy mean?


  The mean energy required to break 1 mole of bonds in gaseous molecules.
• Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion or formation?

  Bond enthalpies are a mean for the same bond across different molecules, whereas standard enthalpy of combustion and formation apply just to that molecule, making them more accurate.

  
• How to calculate enthalpy change of reaction using average bond enthalpies?


  $\Delta H =$$\Sigma (\text{bond enthalpies of reactants}) - \Sigma (\text{bond enthalpies of products})$