Energetics

• 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 are some uses of thermochemistry?

  • Measuring and comparing the energy values of fuels
  • Calculating the energy requirements for industrial processes
  • Working out the theoretical amount of energy released/taken in a reaction
  • Predicting if a reaction will take place or not

  
• What is an endothermic reaction?

  An endothermic reaction has an overall positive enthalpy change (+ΔH), where energy in breaking bonds is greater than energy out making bonds.
• What is an exothermic reaction?

  An exothermic reaction has an overall negative enthalpy change (-ΔH), where energy in breaking bonds is less than energy out making bonds.
• If a reversible reaction is endothermic one way, what type of reaction is the other way?


  If a reversible reaction is endothermic one way, the other way is exothermic.
• Give 2 examples of exothermic reactions.

  • Combustion of fuels
  • Neutralisation
• Give an example of an endothermic reaction.


  Thermal decomposition is an example of an endothermic reaction.
• Define enthalpy change and the symbol used to represent it.


  Enthalpy change is the energy change of a system at constant pressure, represented by ΔH.
• What are the standard conditions for thermochemical measurements?

  The standard conditions are 100 kPa (1 atm) pressure and 298 K (25°C) temperature.
• What does "in standard state" mean?

  "In standard state" refers to the state an element or compound exists in at standard conditions (100 kPa, 298 K).
• Draw an enthalpy change diagram for an endothermic reaction and one for an exothermic reaction.

  • Endothermic reaction: Energy increases as bonds are broken, resulting in a positive ΔH.
  • Exothermic reaction: Energy decreases as bonds are formed, resulting in a negative ΔH.

  
• Define standard enthalpy of formation.

  The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with reactants and products in their standard states.

  
• Give an example of an equation which represents standard enthalpy of formation.


  An example is H$_2$ (g) + ½ O$_2$ (g) → H$_2$O (l).
• Define standard enthalpy of combustion.

  The standard enthalpy of combustion is the enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions, with reactants and products in their standard states.

  
• Give an example of an equation which represents standard enthalpy of combustion.

  An example is C (s) + O$_2$ (g) → CO$_2$ (g).
• What is the difference between heat and temperature?

  Heat is the sum of all particles’ energy and is affected by the amount of substance; temperature is related to the mean kinetic energy of the particles and is independent of the number of particles present.

  
• How can you calculate enthalpy change from experimental data?

  You can use the equation Q = mcΔT, where m is the mass of the substance being heated, c is the specific heat capacity, and ΔT is the change in temperature.
• Draw a simple calorimeter.


  • A beaker
  • A thermometer
  • A lid
  • Insulation material
• How could this calorimeter be made more accurate?

  You could add draught screens, a lid on top of the beaker, and mineral wool around the beaker to insulate and reduce heat loss to the surroundings.

  
• What is a flame calorimeter and how does it differ from a simple calorimeter?

  A flame calorimeter reduces heat lost to the surroundings for more accurate results; it has a spiral chimney made of copper, an enclosed flame, and burns fuel in pure oxygen instead of air.

  
• How would you measure the enthalpy change for a reaction occurring in aqueous solution?

  You would use an expanded polystyrene cup as a calorimeter to reduce heat loss, measure the temperature change, and assume the heat capacity of the solution is 4.18 and its density is 1 g/cm$^3$.

  
• What can you use to make experimental determination of enthalpy change of reaction more accurate?

  You can use cooling curves to improve the accuracy of experimental determination of enthalpy change.
• What is Hess’s Law?

  Hess’s Law states that the enthalpy change for a reaction is the same regardless of the route taken.
• What is the enthalpy of an element?

  The enthalpy of all elements in their standard states is defined as 0.
• Define bond dissociation enthalpy.

  Bond dissociation enthalpy is the enthalpy change required to break a covalent bond, with all species in the gaseous state; it differs for the same bond type in different molecules.
• Define mean bond enthalpy.

  Mean bond enthalpy is the average value for the bond dissociation enthalpy of a given bond across different chemical environments.
• Why may experimental methods for enthalpy determination not be very accurate?

  Experimental methods may not be accurate due to heat loss to the surroundings, not being in standard conditions, or the reaction not going to completion.

  
• Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

  Using bond enthalpies is less accurate because they are a mean for the same bond across different molecules, while standard enthalpy of combustion and formation apply specifically to that molecule.