Thermodynamics Flashcards
enthalpy of lattice formation
- enthalpy change when one mole of a solid ionic compound
- formed from it’s gaseous ions
A calculation of the enthalpy of lattice formation of silver iodide based on a perfect ionic model gives a smaller numerical value than the value calculated
Explain this difference
- AgI contains covalent character
- Bonds (holding the lattice together) are stronger
The enthalpy of lattice formation for caesium iodide in Table 1 is a value obtained by experiment. –1 The value obtained by calculation using the perfect ionic model is –582 kJ mol
Deduce what these values indicate about the bonding in caesium iodide.
(Almost/Mostly) purely/ perfectly ionic
State the meaning of the term periodicity.
Repeating pattern/trends (of physical or chemical properties/reactions)
State why there is a difference between theoretical and experimental values
covalent character
Explain why the hydration becomes less exothermic from Li+ to K+
- size of ion increases from Li+ to K+
- attraction between lone pair on O and + ion decreases
state meaning of enthalpy change
- heat energy change at constant pressure
The enthalpy of hydration of Ca2+(g) is –1650 kJ mol–1 Suggest why this value is less exothermic than that of Mg2+(g)
- Ca2+ (ion) bigger/lower charge to size ratio (than Mg2+)
- weaker attraction/bond to (Oδ- in) water
explain why standard entropy value for CO2 is greater than carbon
CO2 / gas is more disordered (than solid)
State the temperature at which the standard entropy of aluminium is 0 J K–1 mol–1
0 K
standard enthalpy of formation
- enthalpy change when one mole of a compound is formed from its elements
- under standard conditions
- reactants and products in standard states
standard enthalpy of combustion
- enthalpy change when one mole of a compound
- completely burned in oxygen
- under standard conditions
- reactants and products in standard states
standard enthalpy of atomisation
- enthalpy change when one mole gaseous atoms are formed
- from an element in its standard state
mean bond enthalpy
- standard enthalpy change
- when one mole of gaseous molecules each break a covalent bond
- to form 2 free radicals
- averaged over a range of compounds
first ionisation energy
- standard enthalpy change when one mole of electrons
- is removed from one mole of gaseous atoms
- to form one mole of gaseous ions each with single positive charge
second ionisation energy
- standard enthalpy change when one mole of electrons
- is removed from one mole of gaseous 1+ ions
- to form one mole of gaseous ions each with 2+ charge
first electron affinity
- standard enthalpy change when one mole of gaseous atoms
- converted into one mole of gaseous ions
- each with single negative charge
- under standard conditions
second electron affinity
- standard enthalpy change when one mole electrons
- added to one mole of gaseous ions
- with single negative charge
- to form one mole of ions with 2- charge
lattice formation
- enthalpy change when one mole of solid ionic compound
- is formed from its gaseous ions
lattice dissociation
- enthalpy change when one mole of solid ionic compound
- dissociates into its gaseous ions
enthalpy of hydration
- standard enthalpy change when one mole of gaseous ions
- is converted into one mole of aqueous ions
enthalpy of solution
- standard enthalpy change when one mole of solute
- dissolves in enough solvent to form solution with ions are enough apart not to interact with each other
what does the perfect ionic model predict
- ions act as point charges
- ions are perfect spheres which cannot be distorted
- ions show purely ionic bonding with no covalent character
if a ionic compound has more exothermic lattice formation what does this suggest
covalent character
positive entropy change means
more disorder
negative entropy change means
more ordered
when is the reaction feasible
when Gibbs is less than or equal to 0
calculating temperature that reaction becomes feasible
T = enthalpy change/entropy change
T = H/S
equation that links Gibbs to y=mx + c
Gibbs = -ST + H
y = mx + c
Gibbs = y-axis
temp = x-axis
gradient = -S
y-intercept = H
In terms of electrostatic forces, suggest why the electron affinity of fluorine has a negative value.
- attraction between nucleus and extra electron
- energy released when electron is gained
write equation for first electron affinity for chlorine
Cl (g) + e- —> Cl- (g)
Explain why the bond enthalpy of a Cl–Cl bond is greater than that of a Br–Br bond
- bonding pair closer to nucleus/Cl smaller atom
- so attraction is stronger
Suggest why the electron affinity of chlorine is an exothermic change.
- attraction between chlorine nucleus and extra electron
explain why there is a difference between the hydration enthalpies between magnesium and sodium ions
- Mg2+ smaller ion AND more highly charged
- So more strongly attracted to water
state meaning of mean bond enthalpy of O-H bonds
- standard enthalpy change when 1 mole of O-H molecules break to form covalent bond
- averaged over a range of compounds
explain why value of Kw increases as temperature increases
- [H2O] is constant
- only partially dissociates
Explain why kw value increases as temperature increases
- breaking bonds is endothermic
- equilibrium moves to right hand side
state why there is a difference between theoretical and experimental values
covalent character
explain why enthalpy of hydration becomes less exothermic from Li+ and K+
- ion size increases
- weaker attraction between metal ion and O- on water
