lecture 3 Flashcards
under what condition is no work done in
under constant pressure
integral of v0 over v0 leading to an answer of 0
what is tricky to measure in an isolated system
measuring change in volume
what do we use to measure temp change
bomb calorimetry
what conditions are met when we use an isolated container eith a fixed volume
work is 0 (bc integral of v0 - v0 gives 0 work)
change in internal energy = change in heat
change U = change q
change in internal energy is given by
change in energy added as heat
change in work done TO the system
so to measure the internal energy of a system in an isolated system we must
find change in temp
find change in volume
in an isolated system, what is difficult to measure
change in volume
what does a bomb calorimeter include
the sample
high pressure o2
how do we measure the temp change in bomb calorimetry
ignite the sample
measure the temp of the water before and after ignition
what is heat capacity
energy supplied as heat required to raise temp by a fixed amount
equation for heat capacity
C = dq/dT
equation for change in work done
w = Pext x change in volume
equation for internal energy change at constant volume
U = q
if U = q at constant volume,, how else can heat capacity be expressed
C = dU/dT
what does the heat capacity at a constant volume tell us
tells us the internal energy needed to change the temp by a certain amount
C = dU/dT C=dq/dT
heat capacity at constant volume equation and why we partially differentiate U
Cv = (dU/dT)v
we partially differentiate U as it may change with pressure
bc U = q w
and w = Pext vol
kinetic energy of 1 particle
3/2 kBT
kinetic energy of n moles of particles
3/2 nRT
what is kB
R / avogadros
equation for the molar heat capacity of a monatomic ideal gas at constant volume
3/2R
what reaction conditions are needed in order to measure change in internal energy
no volume change
no heat change
why do we find enthalpy change instead of change in internal energy
bc not many reaction undergo with constant volume and constant temp
its easier to measure enthalpy at standard reactions
H equation // enthalpy equation
H = U +pV
why is enthalpy easier to use than internal energy
in gas reactions, most reactions occur under constant prssure aka atmospheric pressure
which allows enthalpy change to equal change in q (heat supplied) 🔺H = 🔺q
under constant pressure
if change in enthalpy = change in heat energy,,,, what else does it equal
change in internal energy + ( pressure x change in volume)
at constant pressure
in a system where pressure is constant, how can we measure enthalpy
measure enthalpy change by measuring heat change
exo = – endo = +
constant vol leads to
ZERO work done
constant pressure leads to
finding enthalpy change by measuring heat change
(using a bomb calorimeter)
enthalpy equation
H = U + pV
molar enthalpy and molar internal energy in solids and liquids
practically identical
due to their small molar volume
change in molar enthalpy and internal energy in gases ..
.. can be very different
ideal gas enthalpy equations
H = U + pV
H = U + nRT
change H = change U + ngRT
ng = moles of gas
at constant temp: 3rd equation
heat capacity at constant pressure
change H = change q (heat)
Cp = dq/dT =
Cp = (dH/dT)p
change in H = Cp x (change in temp)
molar heat capacity
for a monatomic gas equation
Cp = 5/2R
heat cap at constant vol
Cv = (dU/dT)v
heat cap at constant pressure
Cp = (dH/dT)p
in solids and liquids why is enthalpy so similar to internal energy
bc H = U + pV
but their change in volume is tiny tiny
so it can be written as H = U
molar enthalpy and molar internal energy is very similar due to such a small difference in molar volume
in solids and liquids why is enthalpy so similar to internal energy
bc H = U + pV
but their change in volume is tiny tiny
so it can be written as H = U
molar enthalpy and molar internal energy is very similar due to such a small difference in molar volume
in gases,, is the difference between molar enthalpy and molar internal energy large
yes!!!
H doesn’t equal U
change in moles is positive if
moles increased
change in moles is negative if
moles decreased
when does U = q
when no work is done
when volume is constant
when volume doesnt change do to using a container with fixed volume
bomb calorimetry is used to find what. and what conditions occur when using bomb calorimetry
measures ‘q’
however volume is constant and so
U = q
so bomb calorimetry shows us the change in internal energy
