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))
