Thermochemistry Flashcards
what is an intensive property?
a property that does not depend on the amount of substance
eg. temperature
Kelvin temperature
K = ˚C + 273
what is temperature?
a measure of the average kinetic energy of particles in a sample
same Ek = same temp
change in the energy of a system results in
equal and opposite change in its surroundings
heat define
q
transfer of energy due to temperature difference
an EXTENSIVE property - depends on the amount
heat transfer between objects of different temperatures
thermal equilibrium
heat capacity define
C
quantity of heat required to raise temperature by 1˚C
depends on composition AND mass
define specific heat capacity
quantity of heat required to raise temp of 1g of pure substance by 1˚C
Jg-1˚C-1
for the same heat transfer, the object w/ LOWER heat capacity gets hotter
heat capacity formula
C = mc
heat capacity = mass x specific heat capacity
q = mcΔT
heat = mass x specific heat cap x change in temp
remember it by the units of specific heat cap
as C=mc, q=CΔT
q= heat capacity x change in temp
find specific heat cap from heating graph
sloped section
c = heat added over range / temp range
J ˚C-1
what three changes requires energy?
disrupting inter particle forces (solid–liquid–gas)
breaking chemical bonds
separation of charge
what two changes releases energy?
neutralising charge = lattice energy
Na+ + Cl- –> NaCl
dispersing charge over a bigger volume
hydrated cation Na(OH2)6 + is LOWER ENERGY than Na+
separating and bringing together particles that are attracted to one another energy
separating = ends
bringing together = exo
what does a Maxwell-Boltzmann distribution show?
the distribution of energies for particles in a sample at a given temperature
fraction probability per Jmol-1 vs energy
looks like a bell curve skewed to the right
0,0 but the tail never reaches 0 at the right
total energy of a sample of gas vs individual molecules
total energy of sample constant at a given temp
energy of individual molecules is constantly changing due to 10^9 collisions per sec
what does the peak of the Maxwell-Boltzmann curve show?
the fraction of particles having the most probable energy
on the left side - the line of average energy is higher than most probable energy
what does the area under a Maxwell-Boltzmann curve show?
related to the fraction of atoms/molecules having energies in the specified range
how does the Maxwell-Boltzmann curve look different for reactions of various temperatures?
higher temp, flatter curve (broader distribution)
same starting point (0,0)
same area under curve = same number of particles
what does a calorimeter measure?
the heat transferred by a reaction
q(reaction) =
= q(system)
= - q(surroundings)
= -m(soln) x c(sln) x T(sln)
so if ΔT is -ve, q is +ve, endothermic
define enthalpy
a characteristic of the reaction at standard temp/pressure
calculate by measuring heat transfer at constant pressure for a specific amount of reaction
∆rH = q / n
unit kJmol-1
does NOT depend on pathway!
define ∆H, and formula associated with it
observed enthalpy change
∆H / ∆rH = n(reaction)
Calculate ΔrH˚ from standard enthalpy of formation
= Σ(n x ΔfH˚(products)) - Σ(n x ΔfH˚(reactants))
Define bond enthalpy
Energy required for dissociation into the component atoms in the GAS phase
A-B(g) —> A(g) + B(g)
Bond breaking is always ENDO, +ve
standard state C
solid
standard state N
N2 gas
standard state calcium carbonate
CaCO3 (s)
define standard enthalpy change
enthalpy change when all reactants/products are in STANDARD STATE
delta H˚
fractions may be used for combustion
define standard enthalpy of formation
delta fH˚
enthalpy change for the reaction in which ONE MOL of substance in the standard state is formed from elements in their standard state
coefficient of product is 1, fractions used for others
calculate standard enthalpy from standard enthalpy of formation
sum of fH˚(products) - sum of fH˚(reactants)
define bond enthalpy
(D)
the energy required for dissociation into the component atoms in the GAS phase
A-B(g) –> A(g) + B(g)
requires energy to break bonds, ENDO, +Ve
find enthalpy of reaction from bond enthalpies
rH(reaction) = sum of D(broken) - sum of D(formed)
define energy of atomisation for metals
also energy of sublimation
= energy change on conversion of one mol of the metal in its standard state to its atoms in the gas phase
delta subH = delta aH
M(s) –> M(g)
endo
enthalpies of atomisation for non-metals molecular
conversion of the elemental form in its standard state to atoms.
eg. delta aH(X2): X2 –> 2X(g)
define ionisation enthalpy
GAS PHASE CATION formation
E(g) –> E+(g) + e-(g)
delta iH
electron affinity enthalpy
delta EAH
gas phase ANION formation for any atom
enthalpy change for ATTACHING AN e-
lattice formation enthalpy and trends in enthalpy
GAS phase ions to lattice, solid
M+(g) + X-(g) –> MX(s)
strongly exothermic
generally less exothermic for ionic solids having larger ions
e- transfer between atoms overall energy
ENDOthermic
Born-Haber cycle
METAL + NONMETAL metallic/covalent bond breaking (AiH, AeaH) GAS PHASE ATOM e- loss or gain (AaH) GAS PHASE IONS form lattice (AfH) IONIC SOLID
Metal+nonmetal straight to ionic = direct path, lattice enthalpy
ΔsolnH
enthalpy change on dissolving 1 mol of substance in sufficient solvent so that particles are well separated
two steps of dissolving ionic solid and the enthalpies involved
- separate ions in lattice
ALH, lattice dissociation enthalpy, endo - forming complexes
eg. M+(g) –(H2O)–> M+(aq)
AhydH = hydration enthalpy, exothermic (dispersing charge over larger vol)
