energetics 2 p1 Flashcards
what is Hess’ Law?
the enthalpy change for a reaction is independent of the route taken
standard enthalpy of atomisation
enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions
1/2I2(g) –> I(g)
first ionisation energy
enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
Li(g) –> Li+(g) + e-
second ionisation energy
enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
Mg+(g) –> Mg2+(g) + e-
first electron affinity
enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- ions
Cl(g) + e- –> Cl-(g)
second electron affinity
enthalpy change when one mole of gaseous 1- ions gains one mole of electrons to form one mole of gaseous 2- ions
O-(g) + e- –> O2-(g)
lattice enthalpy of formation
enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
Na+(g) + Cl-(g) –> NaCl(s)
enthalpy of hydration
enthalpy change when one mole of gaseous ions become hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)
Na+(g) –> Na+(aq)
enthalpy of solution
enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution
NaCl(s) –> Na+(aq) + Cl-(aq)
mean bond dissociation enthalpy
enthalpy change when one mole of (a certain type of) covalent bond is broken, with all species in the gaseous state
Br2(g) –> 2Br(g)
what is a Born-Haber cycle?
thermochemical cycle showing all the enthalpy changes involved in the formation of an ionic compound. start with elements in their standard states (enthalpy of 0)
Born Haber cycle - up arrows and down arrows
up - atomisation, ionisation
down - formation, affinity, lattice formation
what factors affect the lattice enthalpy of an ionic compound?
size of ions, charge on ions
how can you increase the lattice enthalpy of a compound? why does this increase it?
smaller ions, since the charge centres will be closer together
increased charge, since there will be a greater electrostatic force of attraction between the oppositely charged ions
increasing charge on the anion has a smaller affect since increasing anion charge also increases ionic size
how can Born-Haber cycles be used to see if compounds could theoretically exist?
use known data to predict certain values of theoretical compounds then see if these compounds would be thermodynamically stable
used to predict the first noble gas containing compound
what happens when a solid is dissolved in terms of interactions of the ions with water molecules?
break lattice –> gaseous ions; dissolve each gaseous ion in water. the aqueous ions are surrounded by water molecules (which have a permanent dipole due to polar O-H bond)
what is the perfect ionic model?
assumes that ions are perfectly spherical and that there is an even charge distribution (100% polar bonds)
act as point charges
why is the perfect ionic model often not accurate?
ions are not perfectly spherical
polarisation often occurs when small positive ions or large negative ions are involved, so the ionic bond gains covalent character
some lattices are not regular and the crystal structure can differ
what kind of bonds are most ionic?
large positive ions and small negative ions e.g. CsF
what does spontaneous and feasible mean in terms of a reaction?
it will take place of its own accord; does not take account of rate of reaction
is a reaction with a positive or negative enthalpy more likely to be spontaneous?
negative - exothermic
entropy
a measure of the dispersal of energy in a system which is greater when the system is more disordered
symbol = S
what is the most disordered state?
gas
unit of standard entropy
J/K/mol
how does temperature affect entropy?
greater the temperature, particles have more energy and move more
thus the arrangement of particles becomes more random
more random arrangement = higher entropy
when a solid ionic lattice is dissolved in solution what happens to entropy?
entropy increases because the ions are more disordered
how does change in number of gas molecules in a reaction affect entropy?
increase in number of gas molecules = increase in entropy
decrease in number of gas molecules = decrease in entropy
equation to calculate entropy change
ΔSreaction = ∑Sproducts - ∑Sreactants
Gibbs free energy equation
ΔG = ΔH - TΔS
what does a positive Gibbs value mean for feasibility?
positive = not feasible
=0 JUST feasible
negative = feasible
what is the second law of thermodynamics?
entropy (of an isolated system) always increases, as it is overwhelmingly ore likely for molecules to be disordered than ordered
what is the significance of temperature when G=0?
temperature in K at which the reaction becomes feasible
how would you calculate the temperature at which a reaction becomes feasible?
rearrange to T = ΔH/ΔS since G=0
what are the limitations of using G as an indicator of whether a reaction will occur?
Gibbs free energy only indicates if a reaction is feasible
it does not take into account the rate of reaction (the kinetics of the reaction)
many reactions that are feasible at a certain temperature have a rate of reaction that is so slow that effectively no reaction is occurring
another equation for Gibbs!
ΔG = -RTlnK
why is entropy zero at 0K?
no disorder - molecules/atoms are not moving or vibrating and cannot be arranged in any other way
maximum possible state of order
what are the two key things to look out for to decide is entropy changes or stays constant?
number of moles - more moles made –> increase in entropy
going from solid –> liquid/gas or liquid –> gas
how is it possible for the temperature of a substance undergoing an endothermic reaction to stay constant?
the heat that is given out escapes to the surroundings
