Physical 2 Section 1 - Unit 17: Thermodynamics
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Enthalpy change of formation
The energy transferred when 1 mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
Enthalpy of atomisation
The enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state
Bond dissociation enthalpy
The standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals)
First ionisation enthalpy
The enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge
Enthalpy of lattice formation
The standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form
Enthalpy of lattice dissociation
The enthalpy of lattice dissociation is the standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form
Enthalpy of hydration
Enthalpy change when one mole of gaseous ions become aqueous ions
Enthalpy of solution
The standard enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another
In terms of electrostatic forces, suggest why the first electron affinity of an element has a negative value (2 marks)
• Attraction between the nucleus and the added electron - So energy is released when the electron is gained
Explain why the theoretical enthalpy of lattice dissociation for a compound may be different from the experimental value that can be calculated using a Born–Haber cycle (2 marks)
• The experimental lattice enthalpy value takes into account the covalent interaction • The theoretical lattice enthalpy value assumes only ionic interaction/perfectly ionic
Suggest why hydration of the chloride ion is an exothermic process (2 marks)
• Water is polar / water has Hδ+ - Chloride ion attracts (the H in) water molecules
Equation for entropy
∆S˚ = ΣS˚products - ΣS˚reactants
Equation for Gibbs free cycle
• ∆G = ∆H - T∆S
Equation for enthalpy of solution
Enthalpy of solution = Enthalpy of lattice dissociation + enthalpy of hydration OR Enthalpy of solution = - Enthalpy of lattice formation + enthalpy of hydration
Explain, in terms of molecules, why the entropy is zero when the temperature is zero Kelvin (2 marks)
• Particles are stationary / not vibrating - So there is perfect order / no disorder
Explain why the evaporation of water is spontaneous even though this change is endothermic (4 marks)
• The molecules become more disordered when water changes from a liquid to a gas / evaporates • Therefore the entropy change is positive • TΔS>ΔH • ΔG<0
The freezing of water is an exothermic process. Give one reason why the temperature of a sample of water can stay at a constant value of 0 °C when it freezes (1 mark)
• The heat given out escapes
Using a Born–Haber cycle, a value of –905 kJ mol–1 was determined for the lattice enthalpy of silver chloride. A value for the lattice enthalpy of silver chloride using the ionic model was –833 kJ mol–1. Explain what you would be able to deduce from a comparison of these values (3 marks)
• The model used assumes the ions are spherical and in a lattice • The calculated value is smaller than the cycle value or stronger attraction • Indicating some covalent character or ions are polarised