Unit 1: Section 6 - Thermodynamics CDS * Flashcards
enthalpy definitions lattice enthalpy and born-haber cycles enthalpies of solution entropy free energy change
what is enthalpy change?
the heat energy transferred in a reaction at constant pressure
what are standard conditions?
298 K
100 kPa
what is the enthalpy change for exothermic and endothermic reactions?
exothermic - negative ΔH, heat is given out
endothermic - positive ΔH, heat is taken in
what is the enthalpy change of formation?
the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
what is bond dissociation enthalpy?
the enthalpy change when all the bonds of the same type in 1 mole of gaseous molecule are broken
what is the enthalpy change of atomisation?
the enthalpy change when 1 mole of monatomic gaseous atoms is formed from an element/compound in its standard state
what is the first ionisation energy?
the enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms
what is the second ionisation energy?
the enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions
what is the first electron affinity?
the enthalpy change when 1 mole of gaseous 1- ions is formed from 1 mole of gaseous atoms
what is the second electron affinity?
the enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions
what is the enthalpy change of hydration?
the enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions
what is the enthalpy change of solution?
the enthalpy change when 1 mole of an ionic compound dissolves into its aqueous ions
what is the lattice enthalpy of formation?
the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions
what is lattice enthalpy of dissociation?
the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions
what does Hess’s law state?
the total enthalpy change of a reaction is always the same, no matter which route is taken
what is a born-haber cycle?
start with the enthalpy of formation (elements in standard states –> ionic lattice)
then use enthalpy of atomisation and ionisation ( turn into gas, and turn diatomic molecules into monatomic e.g. Cl2 –> 2Cl then remove electrons)
use electron affinity (electron from ionisation added to other atom)
then lattice enthalpy of formation (turns gaseous ions into ionic lattice)
how to work out theoretical lattice enthalpy?
doing some calculations on the purely ionic model of a lattice.
what is the purely ionic model of a lattice?
it assumes that all ions are spherical, and have their charge evenly distributed around them
what is experimental lattice enthalpy?
finding lattice enthalpy from experiments, the value is often different to theoretical because most ionic compounds have some covalent character
what is covalent character?
the positive and negative ions in a lattice aren’t usually exactly spherical.
positive ions polarise neighbouring negative ions to different extents, the more polarisation, the more covalent the bonding
what does it mean if a ionic compound has covalent character?
the bonding in the lattice is stronger than expected, because covalent bonds are stronger than ionic.
experimental enthalpy will be a lot higher than theoretical
what happens when a solid ionic lattice dissolves?
bonds between ions break to give free ions - endothermic
bonds between ions and water are made - exothermic
enthalpy change of solution is the overall effect on the enthalpy of these
why can water molecules bond to ions?
oxygen is more electronegative than hydrogen, so it draws electrons towards itself, creating a dipole.
positive hydrogen can form bonds with negative ions
negative oxygen form bonds with positive ions
when do substances dissolve?
substances generally only dissolve if the energy released is roughly the same, or greater than the energy taken in.
so soluble substances tend to have exothermic enthalpies in solution
how can you use Hess’s law to work out enthalpy of solution?
solution - converts lattice to aqueous ions
hydration - converts gaseous ions to aqueous ions
lattice dissociation - converts gaseous ions into the lattice
a triangle can be constructed
ΔSOL = ΔHYD(+ve ion) + ΔHYD(-ve ion) - ΔLEFORM
how to tell if a substance is insoluble in water?
if the enthalpy of solution is positive and very big, so very endothermic, the substance is insoluble
what is entropy? (S)
a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles
the more disordered the particles, the higher the entropy
what factors affect entropy?
physical state affects entropy
more particles means more entropy
why does physical state affect entropy?
solid particles vibrate about a fixed point - not much disorder - low entropy
gas particles move fast wherever they want - most disorder - highest entropy
why do more particles mean more entropy?
the more particles, the more ways their energy can be arranged
what makes a reaction feasible?
substances tend to disorder because it makes them more energetically stable, so they move to increase their entropy, this means an endothermic reaction can happen without energy
how to calculate entropy change?
entropy of products - entropy of reactants
what is the standard entropy of a substance?
the entropy of 1 mole of that substance under standard conditions
what does it mean if total entropy of a reaction increases?
it is feasible for the reaction to occur, but it is not guaranteed
what is free energy change?
a measure used to predict whether a reaction is feasible
what is ΔG when a reaction is feasible?
when ΔG ≤ 0
then a reaction is feasible
what is the equation for Gibbs free energy change?
ΔG = ΔH - TΔS ΔH = enthalpy change (kJ/mol) ΔS = entropy change (J/mol/K)/1000 --> (kJ/mol/K) T = temperature in kelvins must all have same units so ΔS / 1000
when is a reaction always feasible?
when the reaction is exothermic and has a positive entropy change
when is a reaction never feasible?
when the reaction is endothermic and has a negative entropy change
how to calculate the temperature that a reaction becomes feasible?
when ΔG is 0 a reaction becomes feasible
so when
T = ΔH/ΔS
the reaction becomes feasible
what do the direction of the arrows show in the Born Haber cycle?
the direction of energy, if its pointing up its endothermic
down means exothermic
what happens if there is a 2- ion in a born Haber cycle?
the 1st electron affinity will go below the equation of the gaseous ions then back up in the second electron affinity
1st is exothermic
2nd is endothermic - repulsion between electron and negative ion means 2nd Ea requires energy to add the electron
how to know which ionic bond is stronger?
the bigger the ions, the weaker the ionic bonds
the higher the charge, the stronger the bond
charge takes priority over size
what does the graph of entropy by temperature look like?
as temperature increases, entropy increases
when a change of state happens, entropy increases dramatically (vertical line), this change is bigger going from liquid to gas then solid to liquid
when does a change of state become feasible?
ΔG = 0
this is when a substance melts or boils
How are diatomic molecules treated differently in born-Haber cycles?
The atomisation energy is needed twice e.g. to turn Br2 into 2 Br
They will also need 2x ionisation energy or 2x electron affinity
