5.4 Thermodynamics

• What does thermodynamics literally mean?

  Heat and movement
• What does thermodynamics study?

  Heat, energy, temperature, and matter properties
• What are energy cycles used for?

  Representing enthalpy changes for ionic compounds

  
• What is enthalpy change (ΔH)?

  Heat energy transferred during a reaction

  
• What are the key enthalpy change definitions to know?

  • Enthalpy of formation
  • Ionisation enthalpy
  • Enthalpy of atomisation
  • Bond enthalpy
  • Electron affinity

  
• What is the enthalpy of formation (ΔH)?

  Enthalpy change when 1 mole of compound forms

  
• What are the standard conditions for enthalpy of formation?

  298 K and 100 kPa
• Can ΔH of formation be endothermic or exothermic?

  Yes, it can be both
• Write the equation for the enthalpy of formation of sodium chloride.

  Na (s) + ½Cl (g) → NaCl (s) ΔH = -411 kJ mol

  
• What is the enthalpy of formation of elements?

  Zero
• What is ionisation enthalpy (ΔH)?

  Energy required to remove an electron from an atom
• Is ionisation enthalpy endothermic or exothermic?

  Always endothermic
• Write the equation for the first ionisation energy of sodium.

  Na (g) → Na+ (g) + e ΔH = +500 kJ mol
• What is the standard enthalpy change of atomisation (ΔH)?

  Enthalpy change when 1 mole of gaseous atoms forms
• Is the enthalpy change of atomisation endothermic or exothermic?

  Always endothermic

  
• Write the equation for the standard enthalpy change of atomisation of sodium.

  Na (s) → Na (g) ΔH = +108 kJ mol
• What is bond enthalpy?

  Energy required to break one mole of a specific bond
• What is bond dissociation energy (E)?

  Energy required to break a specific covalent bond
• How is bond enthalpy usually treated?

  As an endothermic energy change
• Write the equation for the bond enthalpy of chlorine.

  Cl (g) → 2Cl (g) E(Cl-Cl) = +242 kJ mol

  
• How does atomisation enthalpy differ from bond enthalpy?

  Atomisation produces 1 mole of atoms
• What does lattice enthalpy (ΔH) represent?

  Enthalpy change when 1 mole of ionic compound forms

  
• Is lattice enthalpy exothermic or endothermic?

  Exothermic
• Why is the lattice enthalpy value large and negative?

  Large energy release when forming ionic solid
• What does a large negative lattice enthalpy indicate?

  Ionic compound is more stable than gaseous ions
• What causes the stability of ionic compounds?

  Strong electrostatic forces between oppositely charged ions
• How is lattice energy determined?

  Using multiple experimental values and energy cycles
• Write the equation for the lattice energy of sodium chloride.

  Na (g) + Cl (g) → NaCl (s) ΔH = -776 kJ mol

  
• What is a Born-Haber cycle?

  • Application of Hess's Law for ionic compounds
  • Used to calculate lattice enthalpy
  • Shows steps to convert atoms to gaseous ions and ionic lattice

  
• What is the basic principle of drawing a Born-Haber cycle?

  Energy increases going up the diagram

  
• What does the Born-Haber cycle show?

  All steps to form ionic lattice from elements

  
• How do you start drawing a Born-Haber cycle for sodium chloride?

  Draw elements with state symbols about a third up

  
• What is the enthalpy of atomisation of sodium in the Born-Haber cycle?

  Na (s) → Na (g) ΔH = +108 kJ mol

  
• What is the enthalpy of atomisation of chlorine in the Born-Haber cycle?

  ½Cl (g) → Cl (g) ΔH = +121 kJ mol

  
• What is the first ionisation energy for sodium in the Born-Haber cycle?

  Na (g) → Na+ (g) + e ΔH = +500 kJ mol

  
• What is the electron affinity for chlorine in the Born-Haber cycle?

  Cl (g) + e → Cl- (g) ΔH = -364 kJ mol

  
• Where is the enthalpy of formation of sodium chloride placed in the Born-Haber cycle?

  At the bottom of the diagram

  
• What does the final change in a Born-Haber cycle represent?

  Lattice enthalpy, usually shown as formation

  
• What is the process to calculate lattice energy using Born-Haber cycles?

  1. Construct the Born-Haber cycle
  2. Apply Hess's law
  3. Rearrange the equation as needed
  4. Substitute known values

  
• Write the equation to calculate lattice energy (ΔH) using Hess's law.

  ΔH = ΔH + ΔH + IE + EA + ΔH