thermodynamics (physical) Flashcards
What does Hess’s law state?
the enthalpy change for a reaction s independent of the route taken
Define standard enthalpy of formation?
the enthalpy change when 1 mole of compound is formed from its constituent elements in standard conditions with all products and reactants in their standard states
what is the standard enthalpy of an element
zero
define enthalpy change of combustion
when one mole of a substance is completely burnt in (excess) oxygen
define enthalpy change of atomisation
enthalpy change when 1 mole of gaseous atoms is formed from a compound in its standard state in standard conditions
define first ionisation energy
energy when one mole of electrons is removed from one mole of gaseous atoms to form 1 mole of gaseous 1+ ions
define 2nd ionisation energy
enthalpy change when one mole of electrons is removed from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
define first electron affinity
enthalpy change when 1 mole of gaseous atoms gains 1 mole of electrons to form one mole of gaseous 1- ions
define second electron affinity
enthalpy change when 1 mole of gaseous 1- ions gains one mole of electrons to form 1 mole of gaseous 2- ions
define lattice enthalpy of dissociation
enthalpy change when 1 mole of solid ionic lattice is dissociated (broken into) its gaseous ions
define enthalpy of hydration
enthalpy change when 1 mole of gaseous ions become hydrated/dissolved in water to infinite dilution (water molecules totally surround the ion)
define enthalpy of solution
enthalpy change when 1 mole of solute dissolves completely in a solvent to infinite dilutiom
define mean bond dissociation enthalpy
enthalpy change when 1 mole of covalent bonds is broken with all species in the gaseous state
what is the born haber cycle
thermochemical cycle showing all the enthalpy changes involved in the formation f an ionic compound. starts with elements n their standard states
what factors effect the lattice enthalpy of an ionic compound
size of the ions, charge on the 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 greater electrostatic forces of attraction between the oppositely charged ions.
NOTE increasing the charge on an anion has a much smaller effect than increasing the charge on the cation since increasing anion size also as the effect of increasing ionic size
how can the born-haber cycle e used to see if compounds theoretically exist?
use known data to predict certain values of theoretical compounds and then see if these compounds would be thermodynamically stable.
this was used to predict the existence of the first noble gas containing compound
what actually 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)
why is the perfect ionic model 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 lattice are not regular and the crystal structure can differ
which kinds of bonds will be most ionic and why?
between large positive ions and small negative ions
e.g. CsF
define the terms spontaneous and feasible
if a reaction is spontaneous and feasible, 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
define entropy
randomness/disorder of a system.
higher value for entropy = more disordered
what is entropy measured in?
JK-1mol-1
what is the second law of thermodynamics
entropy (of an isolated system) always increases as it is more likely for molecules to be disordered than ordered
is a reaction with positive or negative entropy change more likely to be spontaneous?
positive - reactions always try to increase the amount of disorder
how would you calculate entropy change for a reaction?
entropy change = sum of products’ entropy - sum of reactants’ entropy
define gibbs free energy change using an equation
change in gibbs free energy = change in enthalpy change - temperature change in entropy
what does the value of gibbs free energy for a reaction show
if G<O, reaction is feasible
if G=0, reaction is just feasible
f G>O, reaction is not feasible
what is the significance of the temperature at which G=0
this is the temperature at which the reaction becomes feasible
what are the limitations of sing 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).
if the reaction is exothermic and entropy increases, what is the value of G and what does this mean
G always negative so reaction is always feasible - product favoured
if a reaction is endothermic and entropy decreases, what is the value of G and what does this mean
G is always positive so reaction is never feasible - reactant favoured
if the reaction is exothermic and enthalpy decreases, what is the value of G and what does this mean
temperature dependant
if the reaction is always endothermic and entropy increases, what is G and what does this mean
temperature dependent
why is entropy 0 at 0K
no disorder - molecules/atoms are not moving or vibrating and cant be arranged any other way. maximum possible state of order
what are 2 key things to look out for to decide if entropy increases/decreases/stays relatively 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
