chem mod5.2 lattice enthalpy/entropy/gibbs free energy
Enthalpy change of formation: The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kPa), all reactants and products being in their standard states
Enthalpy of atomisation: The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
First Ionisation enthalpy The first ionisation enthalpy is 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
First Electron affinity The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge O (g) + e- O- (g) [eaH] = -141.1 kJ mol-1 ] The first electron affinity is exothermic for atoms that normally form negative ions because the ion is more stable than the atom and there is an attraction between the nucleus and the electron
Second Ionisation enthalpy The second ionisation enthalpy is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions.
second electron affinity The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.
Lattice Enthalpy LEH The Lattice Enthalpy is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form.
Enthalpy of Hydration hydH Enthalpy change when one mole of gaseous ions become aqueous ions .
Enthalpy of solution solH The enthalpy of solution is 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.
The strength of a enthalpy of lattice formation depends on the following factors 1. The sizes of the ions: The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them. 2. The charges on the ion: The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).
The lattice enthalpies become less negative down any group.
A reaction that is exothermic will result in products that are more thermodynamically stable than the reactants. This is a driving force behind many reactions and causes them to be spontaneous (occur without any external influence).
Entropy is a description of the number of ways atoms can share quanta of energy. If number of ways of arranging the energy (W) is high, then system is disordered and entropy (S) is high.
Substances with more ways of arranging their atoms and energy (more disordered) have a higher entropy.
Solids have lower entropies than liquids which are lower than gases. When a solid increases in Temperature its entropy increases as the particles vibrate more. There is a bigger jump in entropy with boiling than that with melting. Gases have large entropies as they are much more disordered
An increase in disorder and entropy will lead to a positive entropy change
In general, a significant increase in the entropy will occur if: -there is a change of state from solid or liquid to gas - there is a significant increase in number of molecules between products and reactants.
When an ionic substance dissolves the lattice must be broken up. The enthalpy of lattice dissociation is equal to the energy needed to break up the lattice (to gaseous ions). This step is endothermic. The size of the lattice enthalpy depends on the size and charge on the ion. The smaller the ion and the higher its charge the stronger the lattice
Hydration enthalpies are exothermic as energy is given out as water molecules bond to the metal ions. The negative ions are attracted to the δ+ hydrogens on the polar water molecules and the positive ions are attracted to the δ - oxygen on the polar water molecules
The higher the charge density the greater the hydration enthalpy (e.g. smaller ions or ions with larger charges) as the ions attract the water molecules more strongly. e.g. Fluoride ions have more negative hydration enthalpies than chloride ions Magnesium ions have a more negative hydration enthalpy than barium ions
We must consider entropy, however, to give us the full picture about solubility. When a solid dissolves into ions the entropy increases as there is more disorder as solid changes to solution and number of particles increases.
Enthalpy change of formation: The standard enthalpy change of formation of a compound is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kPa), all reactants and products being in their standard states
Enthalpy of atomisation: The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
First Ionisation enthalpy The first ionisation enthalpy is 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
First Electron affinity The first electron affinity is the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a –1 charge O (g) + e- O- (g) [eaH] = -141.1 kJ mol-1 ] The first electron affinity is exothermic for atoms that normally form negative ions because the ion is more stable than the atom and there is an attraction between the nucleus and the electron
Second Ionisation enthalpy The second ionisation enthalpy is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produces one mole of gaseous 2+ ions.
second electron affinity The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.
Lattice Enthalpy LEH The Lattice Enthalpy is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form.
Enthalpy of Hydration hydH Enthalpy change when one mole of gaseous ions become aqueous ions .
Enthalpy of solution solH The enthalpy of solution is 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.
The strength of a enthalpy of lattice formation depends on the following factors 1. The sizes of the ions: The larger the ions, the less negative the enthalpies of lattice formation (i.e. a weaker lattice). As the ions are larger the charges become further apart and so have a weaker attractive force between them. 2. The charges on the ion: The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).
The lattice enthalpies become less negative down any group.
A reaction that is exothermic will result in products that are more thermodynamically stable than the reactants. This is a driving force behind many reactions and causes them to be spontaneous (occur without any external influence).
Entropy is a description of the number of ways atoms can share quanta of energy. If number of ways of arranging the energy (W) is high, then system is disordered and entropy (S) is high.
Substances with more ways of arranging their atoms and energy (more disordered) have a higher entropy.
Solids have lower entropies than liquids which are lower than gases. When a solid increases in Temperature its entropy increases as the particles vibrate more. There is a bigger jump in entropy with boiling than that with melting. Gases have large entropies as they are much more disordered
An increase in disorder and entropy will lead to a positive entropy change
In general, a significant increase in the entropy will occur if: -there is a change of state from solid or liquid to gas - there is a significant increase in number of molecules between products and reactants.
When an ionic substance dissolves the lattice must be broken up. The enthalpy of lattice dissociation is equal to the energy needed to break up the lattice (to gaseous ions). This step is endothermic. The size of the lattice enthalpy depends on the size and charge on the ion. The smaller the ion and the higher its charge the stronger the lattice
Hydration enthalpies are exothermic as energy is given out as water molecules bond to the metal ions. The negative ions are attracted to the δ+ hydrogens on the polar water molecules and the positive ions are attracted to the δ - oxygen on the polar water molecules
The higher the charge density the greater the hydration enthalpy (e.g. smaller ions or ions with larger charges) as the ions attract the water molecules more strongly. e.g. Fluoride ions have more negative hydration enthalpies than chloride ions Magnesium ions have a more negative hydration enthalpy than barium ions
We must consider entropy, however, to give us the full picture about solubility. When a solid dissolves into ions the entropy increases as there is more disorder as solid changes to solution and number of particles increases.
