1.8 ThermoDynamics Flashcards
define enthalpy change of formation
- enthalpy change when one mole of a compound is formed from its elements under standard conditions, 298K and 100kPa, all reactants and products being in their standard states
define enthalpy of atomisation
- enthalpy change when one mole of gaseous atoms is formed from the element in its standard state
define bond dissociation enthalpy
- enthalpy change when one mole of a covalent bond is broken into two gaseous atoms.
define first ionisation enthalpy
- enthalpy change to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions with a +1 charge
define second ionisation enthalpy
- enthalpy change to remove one mole of electrons from one mole of gaseous 1+ ions to produce gaseous 2+ ions
define first electron affinity
- enthalpy change that occurs when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous ions with a 1- charge
define second electron affinity
- enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions
define enthalpy of lattice formation
- enthalpy change when one mole of an ionic crystal lattice is formed from its constituent ions in gaseous form
define enthalpy of lattice dissociation
- enthalpy change when one mole of an ionic crystal lattice is seperated into its constituent ions in gaseous form
define enthalpy of hydration
- enthalpy change when one mole of gaseous ions become aqueous ions
define enthalpy of solution
- enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well seperated and do not interact with one another
explain why first electron affinity is exothermic
- ion is more stable than the nucleus
- attraction between nucleus and electron
explain why second electron affinity is endothermic
- energy is required to overcome repulsive force between negative ion and electron
what do the strength of enthalpy of lattice formations depend on?
- the sizes of the ions
- the charges on the ion
how do the charge of the ions affect enthalpy of lattice formation?
- the greater the charge on the ion, the stronger the electrostatic attraction between the oppositely charged ions
- this results in a higher enthalpy of lattice formation
how do the sizes of the ions affect enthalpy of lattice formation?
- the smaller the ionic radii, the closer the ions can get to each other
- increasing strength of electrostatic attraction
- leading to a higher lattice enthalpy
what is the perfect ionic model?
- theoretical model that assumes ions are perfect spheres, purely ionic and the attractions are 100% electrostatic
when does the perfect ionic model fail?
- when compounds contain covalent character
- when compounds contain large ions with low charge density
what is meant by a spontaneous process?
- a process that will proceed on its own without any external influence
what is entropy?
- a measure of the disorder of randomness in a system
- the higher the entropy, the greater the disorder and the more ways the system can be arranged
why does the entropy increase when ice melts?
- water molecules are more disordered and have more freedom of movement than that in a solid form
what is the order of most entropy in terms of states
gas > liquid > solids
when can an increase in entropy occur?
- when there is a change of state from solid or liquid to gas
- when there is a significant increase in number of molecules between products and reactants
how can we calculate change in entropy and what are the units?
sum of the entropy of products - sum of the entropy of reactants
units are JK^-1mol^-1
do elements in their standard states have zero entropy?
- no
- only perfect crystals at absolute zero will have zero entropy
what is Gibbs free energy change?
- thermodynamic quantity used to predict weather a process or reaction will occur spontaneously at a constant temperature and pressure.
what is the formula for Gibbs Free Energy?
^G = ^H - T^S
^ = change
^H = change in enthalpy
^S = change in entropy
T = temperature K
what are units for Gibbs Free Energy?
^G = kJmol^-1
^H = kJmol^-1
^S = JK^1mol^-1
divide ^S by 1000
what are the conclusions of Gibbs Free Energy?
if ^G > 0, reaction is not feasible
if ^G < 0, reaction is feasible
if ^G = 0, reaction is at equilibrium
how can i calculate the temp when a reaction will become feasible?
- find T when ^G = 0
what is the effect on temperature on feasibility if ^H is exothermic
if ^S > 0, reaction is feasible at all temperatures, increasing temp makes it more feasible
if ^S < 0, reaction is feasible only at low temps, and increasing temp can make it non feasible
what is the effect on temperature on feasibility if reaction is endothermic?
if ^S > 0, reaction becomes feasible at higher temps
if ^S < 0, reaction is never feasible nomatter what the temp is
how can we plot a graph of Gibbs Free Energy?
y = mx + c
y = ^G
m = -^S
c = ^H
x intercept = T when reac is feasible
what is the equation for enthalpy of solution?
enthalpy of solution = enthalpy of lattice dissociation + the sum of enthalpies of hydration
why is enthalpy of dissociation enthalpy endothermic?
- ions are held together by strong electrostatic forces
- lattice dissociation involves breaking these ionic bonds to seperate the ions
- energy is required to break these bonds
why are enthalpy of hydration of ions exothermic?
- ions are surrounded by water molecules in the solution
- water molecules interact with ions via ion dipole interaction
- negative ions attracted to delta positive H ions on polar water molecules
- positive ions are attracted to delta negative oxygen on polar water molecules
