Doc Brown's A Level Chemistry Advanced Level Theoretical Physical Chemistry - AS A2 Level Revision Notes - Basic Thermodynamics
GCE Thermodynamics-thermochemistry sub-index links below
Part 2 ΔH Enthalpy Changes contd. - Lattice Enthalpy, Born Haber Cycle, Enthalpies of Solution, Enthalpies of Ion Hydration
2.1a-c What happens when an ionic compound dissolves in water and the energetics of why?
What energy changes are associated with an ionic solid dissolving in water? A particle model is used to illustrate the dissolving of an ionic solid in water and the subsequent hydration of the free ions. The enthalpy of solution, lattice enthalpy and the hydration enthalpies of gaseous ions are then introduced and are all connected using a thermochemical cycle to theoretically explore 'some' of the factors that affect how soluble a salt/oxide etc. is soluble, i.e. does it dissolve appreciably or is it insoluble.
Energetics index: GCSE Notes on the basics of chemical energy changes - important to study and know before tackling any of the three Advanced Level Chemistry pages Parts 1-3 here * Part 1a-b ΔH Enthalpy Changes 1.1 Advanced Introduction to enthalpy changes - reaction, formation, combustion : 1.2a & 1.2b(i)-(iii) Thermochemistry - Hess's Law and Enthalpy Calculations - reaction, combustion, formation etc. : 1.2b(iv) Bond Enthalpy Calculations : 1.3a-b Experimental methods for determining enthalpy changes and treatment of results : 1.4 Some enthalpy data patterns : 1.4a The combustion of linear alkanes and linear aliphatic alcohols : 1.4b Some patterns in Bond Enthalpies and Bond Length : 1.4c Enthalpies of Neutralisation : 1.4d Enthalpies of Hydrogenation of unsaturated hydrocarbons and evidence of aromatic ring structure in benzene : Extra Q page A set of practice enthalpy calculations with worked out answers ** Part 2 ΔH Enthalpies of ion hydration, solution, atomisation, lattice energy, electron affinity and the Born-Haber cycle : 2.1a-c What happens when a salt dissolves in water and why? : 2.1d-e Enthalpy cycles involving a salt dissolving : 2.2a-c The Born-Haber Cycle *** Part 3 ΔS Entropy and ΔG Free Energy Changes : 3.1a-g Introduction to Entropy : 3.2 Examples of entropy values and comments * 3.3a ΔS, Entropy and change of state : 3.3b ΔS, Entropy changes and the feasibility of a chemical change : 3.4a-d More on ΔG, Free energy changes, feasibility and applications : 3.5 Calculating Equilibrium Constants : 3.6 Kinetic stability versus thermodynamic feasibility * EMAIL query?comment : PLEASE note that delta H/S/G values vary slightly from source to source, so I apologise in advance for any inconsistencies that may arise as I've researched and developed each section.
2.1 What happens when an ionic compound dissolves in water and the energetics of why?
Section 2.1 discusses a dissolving enthalpy cycle - another application of Hess's Law - in which the process of dissolving an ionic compound such as a halide salt or a metal oxide, is broken down into theoretical stages to help understand the factors involved in deciding whether of not a substance will dissolve readily or not at all. For the moment, only enthalpy changes will be considered but eventually entropy changes must be discussed too!
2.1a The changes that occur in the dissolving process
Despite the strong ionic bonding forces in most salts or simple binary compounds like oxide or chloride crystals i.e. the strong electrostatic attraction between positive ions (cations) and negative ions (anions) many ionic compounds readily dissolve in water. Therefore, not surprisingly, a great deal of energy is required to separate the ions, but dissolving can still take place. So how can we explain this?
Water consists of highly polar molecules due to the great electronegativity difference between hydrogen and oxygen (O > H hence the polarity of the bond δ-O-Hδ+). When salts dissolve in water a process of solvation occurs in which the ions become solvated by association with the solvent molecules. If water is the solvent, the process is called hydration and is exothermic because it involves particles coming together via intermolecular forces or covalent bonds in the case of aqua-cation complex ions - so this is a compensating source of energy.
In some cases cations become hydrated via dative covalent bonds to form a complex ions e.g.
Li(H2O)4]+, Cu(H2O)4]2+, Mg(H2O)6]2+, Al(H2O)6]3+ etc. because in the case of water, the most electronegative part of the highly polar water molecule (>Oδ-) will be attracted to the positive ion and, since the oxygen atom has two lone pairs of electrons, it is also the source of the dative covalent bond by donation of one of these pairs of electrons into a vacant metal ion orbital.
In the case of anions, the positive ends of the water molecules (-Hδ+) will orientate themselves towards the negative anion and the water molecules become weakly associated with anion, but no covalent bonds are formed.
This solvation of the ions means the ions are effectively bigger particles which makes the distance between the positive and negative ion centres greater, and by the laws of electrostatics, the attractive forces is weakened and the hydration process is always exothermic.
PLEASE note that dissolving a solute in this situation cannot be simply regarded as a physical change. The ionic lattice is broken down, but NOT by melting, and chemical bonds are formed between the cation and water molecules to form hydrated ions or aqua-ions such as those listed above.
2.1b Diagram illustrating the dissolving-solvation-hydration process for sodium chloride crystals forming salt solution
In order to try to understand processes you can use a Hess's Law cycle to break the process down into theoretical steps, each of which can measured experimentally or theoretically calculated. The relative magnitude of each energy change can help understand why a substance will dissolve or be insoluble. However, this cycle only uses enthalpy values and excludes entropy changes - which I'll deal with later.
2.1c The connection between lattice enthalpy, enthalpies of ion hydration and enthalpy of solution
(i) The energy change for a substance dissolving in a solvent is called the enthalpy of solution.
Comments on the enthalpy of solution values
To derive an alternative route to form the solution employing Hess's Law you then can further consider:
(ii) The ionic crystal lattice is vapourised into its gaseous positive ion (cation) and gaseous negative ion (anion) constituents.
(iii) The gaseous ions then interact with water to give the hydrated ions in aqueous solution.
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