Enthalpy and Entropy
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What is lattice enthalpy?
A measure of strength of ionic bonding in a giant ionic lattice
Define lattice enthalpy of formation
Enthalpy change that occurs during formation of one mole of an ionic compound from its gaseous ions under standard conditions
Is lattice enthalpy endothermic or exothermic?
Exothermic
Define standard enthalpy change of formation
Enthalpy change that occurs when one mole of a compound is formed from its elements under standard conditions with all reactants and products in standard states
Define standard enthalpy change of atomisation
One mole of gaseous atoms formed from elements in standard states under standard conditions
Is standard enthalpy change of atomisation endothermic or exothermic?
Endothermic
What is the enthalpy change of atomisation equal to when an element is a gas in a standard state?
Enthalpy of atomisation is related to bond enthalpy of bond being broken. So the value is equal to the bond enthalpy
Define first ionisation energy
Enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous +1 ions
Is first ionisation energy endothermic or exothermic? Why?
Endothermic, energy required to overcome attraction between electron and nucleus
What is electron affinity and is it endothermic or exothermic?
Energy to gain electrons, exothermic
Define standard enthalpy change of solution
Energy change that takes place when one mole of a solute dissolves in a solvent
Define standard enthalpy change of hydration
Enthalpy change that accompanies dissolving gaseous ion water to form one mole of gaseous ions
What are the two processes that take place when a solid dissolves?
Ionic lattice breaks up Water molecules are attracted to and surround ions
Give 3 key properties of ionic compounds
High melting and boiling points Soluble in polar solvents Conduct electricity when molten or in aqueous solution
Describe the effect of increased ionic size on lattice enthalpy
Ionic radius increases Attraction between ions decreases Lattice energy is less negative Melting point decreases
Describe the effect of increased ionic charge on lattice enthalpy
Ionic charge increases Attraction between ions increases Lattice energy becomes more negative Melting point increases
How do you estimate melting points?
Lattice enthalpy may need to consider other factors such as packing of ions in an ionic lattice
Describe the effect of increased ionic size on enthalpy of hydration
Ionic radius increases Attraction decreases between ion and water molecules Hydration energy is less negative
Describe the effect of increased ionic charge on enthalpy of hydration
Ionic charge increases Attraction with water molecules increases Hydration energy becomes more negative
At what point does a substance dissolve?
When the sum of the hydration enthalpies is greater than magnitude of lattice enthalpy then overall enthalpy change is exothermic and it dissolves
Define entropy
The dispersal of energy within the chemical making up a chemical system
What is the general rule regarding entropy with solids, liquids and gases?
Solids have the smallest entropies Gases have the greatest entropies
How does an increase in the number of gaseous molecules affect entropy?
Increases entropy as there is more disorder
Define standard entropy
The entropy of one molecule of a substance under standard conditions