Enthalpy 2 Flashcards
Lattice enthalpy
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
the standard enthalpy change of atomisation
The enthalpy change that takes place for the formation of one mole of gaseous atoms from its elements, in its standard state under standard conditions.
The first ionisation energy
The enthalpy change required to remove one electron from each atom in a mole of gaseous atoms to form a mole of gaseous 1+ ions
The first electron affinity
The enthalpy change that takes place when one electron is added to each atom in a mole of gaseous atoms to form a mole of gaseous 1- ions.
The factors which affect hydration enthalpies
The smaller the ionic size, the greater the charge density and attraction for water molecules, and the more exothermic the hydration enthalpy. The larger the ionic charge, the greater the charge density and attraction for water molecules, and the more exothermic the hydration enthalpy.
Factors which affect lattice enthalpy enthalpy
As ionic radius increases, the attraction between ions decreases, the lattice energy is less negative. As ionic charge increases, the attraction between ions increases and lattice energy becomes more negative.
What is lattice enthalpy used to measure
The strength of ionic bonding in a giant ionic lattice.
The standard enthalpy change of solution
The enthalpy change that takes place when one mole of a solute dissolves in a solution
How do you calculate enthalpy change of solution
q=mc (triangle)T, to calculate the enthalpy change then divide by a 1000 to get KJ, you then divide it by moles and times it by -1 to find it per mole. M refers to the mass changing temperature.
The enthalpy change of hydration
The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
born haber cycle
gaseous ions
———————
(first electron affinity) (ionisation energy)
Gaseous atoms Gaseous ions
—————————- —————————-
(Atomisation)
(enthalpy change of formation)
Ionic lattice
-------------------
Enthalpy change of hydration cycle
Gaseous Ions
———————–
(enthalpy change of hydration) (Lattice enthalpy) Aqueous ions
-----------------------
(enthalpy change of solution)
Ionic lattice
----------------------
What is needed for the compound to dissolve
The sum of the hydration enthalpies must be larger then the magnitude of the lattice enthalpy
What is entropy
A measure of the dispersal of energy within a system. Systems that are more chaotic have a bigger entropy.
Units of entropy
JK-1mol-1
Entropy change
If a system changes to become less random, energy becomes more concentrated and the entropy change will be negative. You can predict whether entropy increases or decreases by comparing the physical states on wither side of the equation.
When does entropy increase
When there is a change in state that gives a more random arrangement of particles. When a substance changes from solid to liquid to gas its entropy increases. Melting and boiling increase the randomness of particles, energy is spread out more and delta S is positive.
How to predict the entropy change of a reaction
If there are more gas molecules in the products then the reactants then the entropy change is positive. The production of gas increases the disorder of particle, energy is spread out more and delta S is positive.
What is standard entropy
The entropy of one mole of a substance under standard conditions
standard entropy equation
S products - S reactants
Free energy equation
G= deltaH-T deltaS
How can a reaction be feasible with free energy
Free energy must be negative for a reaction to be feasible. The products must have a lower overall energy then the reactants and there must be a decrease in free energy.
Limitations of free energy
Not all reactions take place at the temperature calculated because the activation energy would result in a very slow rate. As free energy does not take into account kinetics or rate of reaction.
