Thermodynamics

• What is thermodynamics the study of in chemical reactions?

  Energy changes
• Enthalpy is the heat energy stored in a chemical system
• Enthalpy can be measured directly using experimental methods

  False
• What is the key definition of enthalpy change?

  Heat energy change
• The units of enthalpy change are kJ mol⁻¹
• Standard enthalpy change occurs under standard conditions
• What are the standard conditions for temperature and pressure?

  298 K, 100 kPa
• Hess's Law states that the enthalpy change for a chemical reaction is independent of the route
• Steps to calculate enthalpy change using Hess's Law

  1️⃣ Identify the reactants and products

  2️⃣ Draw an enthalpy cycle

  3️⃣ Apply Hess's Law: ΔH₁ = ΔH₂ + ΔH₃
• Ionic compounds form giant ionic lattices
• What holds the ionic lattice together?

  Ionic bonds
• Ionic bonds are the electrostatic attraction between oppositely charged ions
• What is the enthalpy of lattice dissociation?

  Energy to separate ions
• Enthalpies of lattice dissociation are always positive
• What is the enthalpy of lattice formation?

  Energy to form solid
• Enthalpies of lattice formation are always negative
• The values for lattice dissociation and formation are equal in magnitude but have opposite signs
• Match the enthalpy change with its definition:

  Standard enthalpy of formation ↔️ Formation of 1 mole of compound

  Standard enthalpy of atomisation ↔️ Formation of 1 mole of gaseous atoms

  Second ionisation energy ↔️ Formation of 2+ ions from 1+ ions

  First electron affinity ↔️ Formation of 1- ions from atoms
• The standard enthalpy of formation for ionic compounds is always exothermic
• Atomisation is always endothermic because energy is needed to break bonds
• Why is the second ionisation energy greater than the first ionisation energy?

  Stronger nuclear attraction
• The first electron affinity is always exothermic
• The second electron affinity is always endothermic because of repulsion between the negative ion and the negative electron
• Steps to use a Born-Haber cycle to calculate lattice enthalpy

  1️⃣ Start with elements in their standard states

  2️⃣ Combine relevant enthalpy changes

  3️⃣ Apply Hess's Law
• The enthalpy change for elements in their standard states is taken as zero
• What is the formula to calculate enthalpy change using a Born-Haber cycle?

  $\Delta H =$$\Sigma \Delta H_{other}$
• The perfect ionic model assumes ions are perfect spheres with only electrostatic forces of attraction
• Experimental lattice enthalpies differ from theoretical values because ionic compounds have some covalent character
• Experimental lattice enthalpies from Born-Haber cycles are always equal to theoretical values calculated using the perfect ionic model.

  False
• The distortion of ions in an ionic compound is called polarisation
• What type of positive ions are good at polarising negative ions?

  Small and highly charged
• The enthalpy change when gaseous ions form aqueous ions is called the enthalpy of hydration
• Steps in the dissolution of an ionic compound

  1️⃣ Ionic lattice breaks down into gaseous ions

  2️⃣ Gaseous ions are hydrated

  3️⃣ Enthalpy change of solution is calculated
• Match the term with its definition:

  Enthalpy of Solution ↔️ Change when 1 mole dissolves

  Enthalpy of Hydration ↔️ Change when 1 mole forms aqueous ions
• Lattice enthalpy values are a measure of the strength of attraction between oppositely charged ions
• What happens to the lattice enthalpy of formation as the size of the halide ion increases?

  Becomes less exothermic
• What happens to the hydration enthalpy of halide ions as their size increases?

  Becomes less exothermic
• As the charge on an ion increases, its hydration enthalpy becomes more exothermic
• The enthalpy of lattice dissociation is the energy required to break one mole of an ionic lattice into gaseous ions.
• Entropy is a measure of disorder