Chapter 22 - Enthalpy and Entropy Flashcards
Define lattice enthalpy.
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
Define standard enthalpy change of atomisation.
The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions.
Define first electron affinity.
The enthalpy change when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
Define first ionisation energy.
The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Define standard enthalpy change of formation.
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions with all reactants and products in their standard states.
Is ΔatmH endothermic or exothermic?
It is ALWAYS endothermic because bonds are broken to form gaseous atoms.
Is first electron affinity endothermic or exothermic?
First is exothermic because the electron being added is attracted in towards the nucleus.
Is lattice enthalpy endothermic or exothermic?
It is ALWAYS exothermic because it involves the formation of ionic bonds.
What is formation of gaseous atoms and is it endothermic or exothermic?
It is changing elements from their standard states into gaseous atoms and it is endothermic as it involves bond breaking.
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What is formation of lattice and is it endothermic or exothermic?
It is changing gaseous ions into the solid ionic lattice and it is exothermic as ionic bonds are formed.
Is ionisation energy endothermic or exothermic?
It is ALWAYS endothermic because energy is required to overcome attraction between electron and nucleus.
Why are successive electron affinities endothermic and not exothermic?
The negative ion repels the additional electron, so energy is required to force the electron into it.
What is a Born-Haber cycle?
A way of calculating lattice enthalpy using other known energy changes.
Define standard enthalpy change of solution.
The enthalpy change when one mole of a solute dissolves in a solvent.
What processes take place when a solid ionic compound dissolves in water?
The ionic lattice breaks up
So water molecules are attracted to and surround the ions.
What are the energy changes involved in the process of a solid ionic lattice breaking up in water?
Ionic lattice is broken up, forming separate gaseous ions.
Separate gaseous ions interact with polar water molecules to form hydrated aqueous ions.
Called enthalpy of hydration.
Define enthalpy change of hydration.
The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.
Describe the effect of increasing ionic size on lattice enthalpy and melting point.
Ionic radius increases
Attraction between ions decreases
Lattice energy becomes less negative
Melting point decreases
Describe the effect of increasing ionic charge on lattice enthalpy and melting point.
Ionic charge increases
Attraction between ions increases
Lattice energy becomes more negative
Melting point increases
Describe the effect of increasing ionic size on hydration enthalpy.
Ionic radius increases
Attraction between ion and water molecules decreases
Hydration enthalpy becomes less negative
Describe the effect of increasing ionic charge on hydration enthalpy.
Ionic charge increases
Attraction between ion and water molecules increases
Hydration enthalpy becomes more negative
Why is hydration enthalpy an inaccurate method of predicting solubility?
If overall enthalpy change of solution is exothermic, then compounds should dissolve, but many compounds with endothermic enthalpies of solutions are also soluble, so hydration enthalpy is not accurate.
Define entropy.
A measure of the dispersal of energy and disorder within chemicals in a chemical system.
What are the units of entropy?
J K-1 mol-1
How does entropy relate to physical states?
Gases > Liquids > Solids
Define standard entropy
The entropy of one mole of a substance, under standard conditions.
100kPa and 298K
How is entropy calculated for a reaction?
ΔS = (sum of products) - (sum of reactants)
What is feasability?
Whether a reaction is able to happen and is energetically feasible.
What is the equation for free energy change?
ΔG = ΔH - TΔS
ΔG = free energy change ΔH = enthalpy change T = temperature (K) ΔS = entropy change
What is the requirement of free energy change so that the reaction is feasable?
ΔG has to be negative indicating a decrease in free energy.
