Lattice Enthalpy, Entropy and Free Energy Flashcards
what is meant by lattice enthalpy?
enthalpy change that accompanies the formation of one mole of a solid ionic lattice from its gaseous ions under standard conditions
what is meant by enthalpy change of atomisation?
enthalpy change that acoompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
what is first electron affinity?
enthalpy change that takes place when one mole of electrons is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
is the first electron affinity endothermic or exothermic? why?
exothermic because an electron is being attracted towards the nucleus, so adds easier
is the second electron affinity endothermic or exothermic? why?
endothermic because the second electron is being gained by a negative ion, so repels the electron away
- so more energy is put in to force negatively charged electron onto a negative ion
what is the lattice enthalpy for KCl and MgCl2?
KCl: K+(g) + Cl-(g) –> KCl(s)
MgCl2: Mg2+(g) + 2Cl-(g) –> MgCl2(s)
what is the enthalpy change of atomisation of sodium and chlorine?
Na(s) –> Na(g)
1/2Cl2(g) –> Cl(g)
is enthalpy change of atomisation exothermic or endothermic?
always endothermic
- bonds are broken to form gaseous atoms
what is the electron affinity for chlorine?
Cl(g) + e- –> Cl-(g)
what is the ionisation energy for sodium?
Na(g) –> Na+(g) + e-
what is the order for the born haber cycle?
formation, atomisation. atomisation. ionisation energy, electron affinity, lattice enthalpy
what is enthalpy change of solution?
enthalpy change that takes place when one mole of a solute dissolves in a solvent
what is enthalpy change of hydration?
enthalpy change that accompanies the dissolving of 1 mole of gaseous ions in water to form 1 mole of aqueous ions
what is the dissolving process?
- the solid ionic lattice breaks up
- water molecules are attracted to and surround the ions
what is the enthalpy change of sodium chloride in water?
NaCl(s) + aq –> Na+(aq) + Cl-(aq)
what is the enthalpy changes of hydration during the dissolving of NaCl?
Na+(g) + aq –> Na+(aq)
Cl-(g) + aq –> Cl-(aq)
what are the factors affecting lattice enthalpy?
- ionic charge
- ionic radius
why does ionic charge affect lattice enthalpy?
- as you go across a period, ionic charge increases and electrostatic attraction between ions increases
- so the lactic enthalpy is more negative
- melting point increases
why does ionic radius affect lattice enthalpy?
- as you go down a group, ionic radius increases the electrostatic attraction between ions decreases
- lattice enthalpy is less negative
- melting point decreases
how can we predict that a compound should dissolve?
- sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change will be exothermic
- compound should dissolve
what is entropy?
measure of the dipersal of energy in a system
- the entropy is greater the more disordered a system is
order the states from the smallest entropy to the greatest entropy?
- solids
- liquids
- gases
how do we predict entropy changes?
- if a system changes to be more random, energy is more spread out and entropy is positive
- if a system changes to be less random, energy is more concentrated and entropy is negative
what ways can we see that there will be a change in entropy?
- changes in state
- change in the number of gaseous molecules
how can we see a change in entropy in the number of gaseous molecules?
- more moles of gas = positive value of entropy
- same mole of gas = entropy value is close to 0
what is the feasibility of a process?
the feasibility of a process depends on the entropy change and temperature in the system and the enthalpy change of the system
what is feasibility used to describe?
whether a reaction is able to happen
what is the Gibbs equation?
^G = ^H - T^S
for a reaction to be feasible what must happen?
- decrease in free energy
- negative ^G
- if ^G is zero the system is at equilibrium
what is the equation to find the temperature that the reaction is feasible at?
T = ^H divided by ^S
what are the units for ^G in Gibbs equation?
kJmol-1
what are the units for ^H in Gibbs equation?
kJmol-1
what are the units for temperature in Gibbs equation?
kelvin
what do we have to do after we find the value of ^S?
divide by 1000
what is the equation for ^S?
products- reactants
how can we link the Gibbs equation to the equation of a straight line?
y = mx + c
^G = -^ST + ^H
what does each part of the equation of a straight line show?
y = ^G
m = gradient = -^S
x = T
c = y intercept, ^H
why may a negative value of the Gibbs equation may not truly be feasible?
- slow rate of reaction
- high activation energy
when asked to give a range of the temperature for which a reaction is feasible what would you write if the reaction was exo/endothermic?
endothermic = temp and above
exothermic = temp and below
what factors affect the enthalpy of hydration?
- ionic radius
- ionic charge
how does ionic radius affect the enthalpy of hydration?
- down a group the ionic radius increases, the electrostatic attraction between ions and water molecules decreases
- hydration enthalpies is less negative
how does ionic charge affect enthalpy change of hydration?
- as you go across a period, the ionic charge increases
- the electrostatic attraction between water molecules and ions increases
- hydration enthalpies is more negative
