04 - thermodynamics

• standard enthalpy of formation?

  the enthalpy change that occurs when 1 mole of a compound is formed from it's elements under standard conditions, with all substances being in their standard states.
• what's an example of the formation of an ionic lattice?


  NaCl.
• ionic lattice?


  the giant structure of ionic compounds. Large numbers of positively charged metal ions and negatively charged non-metal ions held together through ionic bonds in a regular arrangement.
• what DOES the standard enthalpy of formation describe for NaCl?

  the overall change that occurs when NaCl is formed from the elements in their standard state.
• what DOESNT the standard enthalpy of formation describe for NaCl?

  the stages that the elements (in their standard states) go through to become gaseous ions to form the ionic lattice.
• what can be described as the 1st possible route to the formation of an ionic lattice?

  the standard enthalpy of formation.
• what does the 2nd route involve?

  enthalpy changes needed to produce the gaseous ions for the lattice.
• how can both routes be represented?

  through a Born-haber cycle.
• what is a Born-haber cycle drawn as?

  an energy level diagram.
• what is a Born-haber cycle a visual representative of?

  the 2 possible routes to the formation of an ionic lattice.
• what is the 1st stage for a Born-haber cycle?

  draw a horizontal line across the bottom with the formula of the ionic compound being formed and its state symbol.

  
• what is the 1st possible route?

  shown by an arrow arrow. This route shows the formation of NaCl from its elements (standard states). Represents the standard enthalpy change of formation. Arrow points downwards the products (representing exothermic changes with negative enthalpy value).

  
• for an ionic compound, how is the standard enthalpy of formation always shown?

  using an arrow pointed down towards the ionic compound in the direction of the enthalpy change and a 2nd shorter line on the left. On top we write chemical symbols and state symbols.

  
• what does the 2nd route show?

  how the atoms of the elements (in their standard states) are converted into the gaseous ions which then form the ionic lattice through the stages of atomisation, ionisation and electron affinity. The final stage is the formation of the ionic lattice from the gaseous ions.
• how is the Born-Haber drawn for the 2nd route?


  each of the changes are drawn as individual steps in the Born-Haber cycle with the direction of the change represented by the arrows and equations written on the horizontal lines to show the product formed in each change.
• what is the 1st stage of the 2nd route?

  the atoms of the metal and non-metal elements must be converted into gaseous atoms through the process of atomisation.
• the standard molar enthalpy of atomisation?

  the enthalpy change for the formation of 1 mole of gaseous atoms from 1 mole of the element in its standard state under standard conditions.