1.8 Thermodynamics

• What is enthalpy change?

  The heat energy transferred in a reaction at constant pressure
• What are standard conditions for enthalpy change?

  298K and 100kPa
• What is enthalpy if the sign is negative?

  Exothermic
• What is enthalpy if the sign is positive?

  Endothermic
• What is the standard enthalpy change of formation?

  Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in its standard states
• What is the standard enthalpy change of combustion?

  the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
• What type of process is bond breaking?

  Endothermic process
• What type of process is bond making?

  Exothermic process
• What is first ionisation enthalpy?

  Enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ions
• What is second ionisation enthalpy?

  Enthalpy change when each ion in one mole of gaseous 1+ ions loses one electron to form one mole of gaseous 2+ ions.
• What is the first electron affinity?

  Enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions
• What is the second electron affinity?

  Enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions
• What is bond dissociation?

  Enthalpy change when one mole of covalent bonds is broken t in the gaseous state
• What is enthalpy of atomisation of an element?

  Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state
• What is enthalpy of atomisation of a compound?

  Enthalpy change when gaseous atoms are produced from one mole of a compound in its standard state
• What is lattice enthalpy of formation?

  Enthalpy change when one mole of a solid ionic compound is formed from its consistent ions in the gas phase

  *always -ve as forming bonds
• What is lattice enthalpy of dissociation?

  Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase
• What is hydration enthalpy?

  Enthalpy change when one mole of gaseous ions become hydrated
• What is enthalpy of solution?

  Enthalpy change when one mole of an ionic solid dissolve in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other
• What are the three main stages of born-haber cycles?

  1. Formation of free gaseous atoms from the elements in their standard states (enthalpy of atomisation)

  2. Addition or removal of electrons to form ions (enthalpy of ionisation energy or electron affinity)

  3. Attraction of the ions to form the ionic compound (enthalpy of lattice formation)
• What does lattice formation enthalpy?

  - charge of ion (higher charge = higher lattice enthalpy of formation)

  - sized of ion (smaller ion = higher lattice enthalpy of formation)
• What does a difference between theoretical value and experimental value indicate?

  Difference indicates compounds are not perfectly ionic, which assumes both ions are perfect spheres with no polarisation whereas in reality, the positive ion will attract the outer electrons of the negative io depending on size and charge of both. This causes the charge clouds to be distorted and is known as covalent character
• What makes an ion hydrated?

  Ions in an ionic lattice are strongly attracted to water, the cations to the oxygen and the anions to the hydrogen. This distorts the charge cloud f the ions and reduces the forces holding them together. This makes the ions in solution move from the lattice and become surrounded by water molecules
• What are the two processes in dissolving an ionic compound?

  • lattice enthalpy of dissociation
  • hydration of ions
• When given bond dissociation instead of atomisation what should you do?

  Divide value by 2 e.g Cl2(g) -> 2Cl(g)
• When given lattice dissociation what should you do?

  Flip the around or flip the sign as endothermic
• What is entropy?


  The degree of disorder in a system
• What factors affect disorder?

  • Temperature
  • Dissolving
  • Reactions that produce a gas
• How can you calculate entropy change?

  Products-reactants
• Give the equation for Gibbs Free Energy.

  ΔG = ΔH - TΔS
• When is a reaction feasible?

  When ΔG is less than or equal to zero
• How is ΔG calculated in a graph?


  Y = mx + c

  ΔG = (-ΔSxT)+ΔH

  
  

  ΔG=y

  ΔS=m

  T=x

  ΔH=c
• What affect will a negative enthalpy and positive entropy have on ΔG?

  ΔG always -ve and therefore feasible at all temperatures
• What affect will a negative enthalpy and negative entropy have on ΔG?

  Lower T: -ve Lower T: feasible

  Higher T: +ve Higher T: not feasible
• What affect will a positive enthalpy and positive entropy have on ΔG?

  Lower T: +ve Lower T: not feasible

  Higher T: -ve Higher T: feasible
• What affect will +ve enthalpy and -ve entropy have on ΔG?

  ΔG always +ve therefore never feasible at any temperature