8: Born Haber Cycles and Enthalpies of Solution Flashcards
Enthalpy Definitions Born Haber Cycles Enthalpy of Solution Perfect Ionic Model
Define enthalpy change.
The heat energy transferred in a reaction at constant pressure.
Why do exothermic reactions have a negative enthalpy change?
Heat energy is released to the environment.
Why do endothermic reactions have a positive enthalpy change?
Heat energy is absorbed into the reaction.
What is lattice enthalpy?
The energy required to break a bond between two ions. They essentially measure ionic bond strength.
How do you measure lattice enthalpy?
Using Born-Haber cycles. They cannot be measured directly.
Define enthalpy change of formation. Give an example using Ca and Cl2
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states.
Ca(s) + Cl2 (g) -> CaCl2(s)
Define bond dissociation enthalpy. Give an example using Cl2.
The enthalpy change when all of the bonds of the same type in 1 mole of gaseous molecules are broken.
Cl2(g) -> 2Cl(g)
Define enthalpy of atomisation of an element. Give an example using Cl2.
The enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard states.
0.5Cl2(g)->Cl(g)
Define enthalpy change of atomisation of a compound. Give an example using NaCl.
The enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms.
NaCl(s) - Na(g) + Cl(g)
Define first ionisation energy.
The enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms.
Define second ionisation energy.
The enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions.
Define first electron affinity.
The enthalpy change when 1 mole of gaseous 1- ions is made from 1 mole of gaseous atoms.
Define second electron affinity.
The enthalpy change when 1 mole of gaseous 2- ions is made from 1 mole of gaseous 1- ions.
Define the enthalpy change of hydration. Use the example of Na+
The enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.
Na+ (g) -> Na+(aq)
Define enthalpy change of solution
The enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.
State Hess’s Law.
The total enthalpy change of a reaction is always the same, no matter which route is taken
What is the difference between a theoretical and experimental lattice enthalpy?
How is experimental lattice enthalpy calculated?
The experimental lattice enthalpy assumes ionic compounds have some covalent character and is calculated using Born Haber cycles whereas the theoretical lattice enthalpy assumes that all ionic compounds are purely ionic.
What does the perfect ionic model assume?
It assumes that all ions are perfectly spherical and have an even distribution of charge.
Why is the perfect ionic model incorrect?
Most ionic compounds have some covalent character. This means that electron and charge is unevenly distributed.
Why can water form bonds with ions?
Water is a polar molecule.
Enthalpy of solution is a result of which two effect?
Bond making and bond breaking.
What 3 definitions of enthalpies are required to calculate enthalpy of solution?
Lattice dissociation enthalpy
Enthalpy of hydration of the positive ion
Enthalpy of hydration of the negative ion
State why there is a difference between theoretical and experimental values of enthalpy of lattice formation.
Theoretical model assumes there is no covalent character.
Explain why enthalpy of hydration becomes less exothermic from Li+ to K+.
Size of ion decreases
Therefore, electrostatic attraction between metal ion and slightly negative oxygen in water decreases.
