Thermochem Flashcards
kinetic energy associated with
random motion of particles
thermal energy
potential energy related to the
arrangement of particles
chemical energy
an object, or collection of objects, being studied
system
everything outside the system that can exchange
energy and/or matter with the system
surrounding
heat is * and * property
energy transferred due to temperature difference
extensive property
temperature is * and * property
measure of average ke or intensity of heat?
intensive property
exothermic
endothermic
qsystem:
exo: q sys < 0
endo: q sys > 0
energy transferred as heat that is required to raise
the temperature of 1 g of a substance by 1 K
specific heat capacity
J/g-K
energy transferred as heat that is required to raise
the temperature of 1 mole of a substance by 1 K
molar heat capacity
J/mol-K
if two objects have the same mass, the object having the larger specific heat capacity will undergo the * temperature change for a given amount of energy transferred
if two objects have different masses, but the same specific heat capacity, the * object will undergo a smaller temperature change for a given amount of energy transferred
if two objects have different masses and different specific heat capacities, the temperature change must be calculated from the *
smaller
larger
conservation of energy
larger specific heat capacity = * temperature change
smaller
q = mc(delta T)
if heat of vap, sub or fus; q = ?
q = heat of (v,f,s) * mass
work associated with a change in volume (ΔV)
that occurs against a resisting external pressure (P)
formula and unit
pressure-volume work P - m^3 (1 Pa = 1 kg/m-s^2) delta V = m^3 J = kg-m^2/s^2
If work was done by
the system:
If work was done on
the system:
If work was done by
the system: w < 0 (expansion)
If work was done on
the system: w > 0 (compression)
First law of thermodynamics:
Energy can neither be created nor destroyed.
It can only be transformed into another form of
energy through the interaction of *
heat (q),
work (w), and internal energy (U).
First law of thermodynamics:
the energy change for a system (ΔU) is
the sum of the energy transferred as * and the energy transferred as * between the system and its surroundings
heat (q), work (w)
ΔU = q + w
The * in a chemical system is the sum
of the potential and kinetic energies inside the system,
that is, the energies of the atoms, molecules, or ions in
the system.
internal energy (U)
q>0 (+)
q<0 (-)
w>0 (+)
w<0 (-)
endothermic, U increases
exothermic, U decreases
work done on the system, U increases
work done by the system, U decreases
The total energy of the universe is *
formula
constant
ΔUuniverse = 0
ΔUuniverse = ΔUsystem + ΔUsurroundings
ΔUsystem = - ΔUsurroundings
thermodynamic property wherein changes in these
quantities depend only on the initial and final states
examples:
STATE FUNCTIONS
ΔT, ΔU, ΔV, ΔH, ΔP
quantities that depend on the pathway taken to
get from the initial condition to the final condition
examples:
PATH FUNCTIONS
q, w
measure of energy transferred as heat in constant pressure
formula:
enthalpy
ΔH = qp= ΔU + PΔV
SIGN CONVENTIONS
- ΔH =
+ΔH =
- ΔH = exothermic process
+ΔH = endothermic process
If a system is at constant volume:
ΔUsystem =
ΔUsystem = q(system) + w(system) = qv
If the system is NOT at constant volume:
ΔUsystem =
ΔUsystem = qp + w(system)
qv = qp + w Since w = -PΔV and qv = ΔU, then ΔU = qp – PΔV qp = ? ΔH = ?
qp = ΔU + PΔV ΔH = ΔU + PΔV
ØAt constant volume:
qv = ?
ØAt constant pressure:
qp = ?
ØAt constant volume:
qv = ΔU
ØAt constant pressure:
qp = ΔH
ΔH = qp= ΔU + PΔV
sign conventions
*ΔH = exothermic process
*ΔH = endothermic process
negative ΔH = exothermic process
positive ΔH = endothermic process
pure, unmixed reactants in their standard states* have
formed pure, unmixed products in their standard states
unit:
STANDARD REACTION ENTHALPY, ΔrH°
units: kJ/mol (or kJ/mol-rxn)
the most stable form of the substance in the
physical state that exists at a pressure of 1 bar
and at a specified temperature (usually 25 °C)
STANDARD STATE
enthalpy changes are * to each reaction
§ * equation + states are important!!
§ enthalpy change depends on the number of *
of reaction (number of times it is carried out)
§ for chemical reactions that are the reverse
of each other, ΔrH° values are numerically
the *, but *in sign
specific
balanced
moles
same, opposite
heat is shown as * of the chemical reaction
heat transferred is an * property
part
extensive
measurement of energy evolved or absorbed
as heat in a chemical or physical process
apparatus: *
à types of calorimetry:
§ constant pressure:
§ constant volume:
calorimetry
calorimeter
§ constant pressure
coffee cup calorimeter
§ constant volume
bomb calorimeter
measure the amount of energy transferred as heat under constant-pressure conditions
= enthalpy change, ΔH
formula
àcoffee cup calorimeter
qrxn + qsoln = 0
evaluate the energy released by the combustion
of fuels and the caloric value of food
constant volume = no P-V work done on/by the system
= energy is transferred as heat only = internal energy, ΔU
formula
bomb calorimeter
qrxn + qbomb + qwater = 0
qrxn + qbomb + qwater = 0 qrxn = unknown qwater = * qbomb = * Cbomb = * of the bomb in *
qwater = mwater Cwater ΔT qbomb = Cbomb ΔT
heat capacity in J/K
How to calculate for ΔrH°
- Using bond energies
- Using Hess’s Law
- Using heats of formation
Bond energy
During a chemical reaction, reactant bonds are broken
and product bonds are made.
Breaking bonds requires energy
Making bonds releases energy
(endothermic, BE > 0)
exothermic, BE < 0
Using bond energy theorem, how to compute for
ΔrH°?
ΔrH° = Σ𝐵𝐸 − Σ𝐵𝐸
Breaking bonds requires energy : endothermic, *
Making bonds releases energy : exothermic, *
If bonds are stronger in products than reactants,
ΔrH° is *
If bonds are stronger in reactants than products,
ΔrH° is *
BE > 0
BE < 0
negative
positive
if a reaction is the sum of two or more other
reactions, ΔrH° for the overall process is the
sum of the ΔrH° values of those reactions
Hess’s law
regardless of the multiple stages or steps of a
reaction, the total enthalpy change for the
reaction is the sum of all changes
a manifestation that enthalpy is a *
skl HAHAHAHA
state
function!
àenthalpy change for the formation of 1 mol
of a compound directly from its component
elements in their standard states
STANDARD MOLAR ENTHALPY OF FORMATION, 𝚫fH0
ΔfH0 for an element in its * state is zero
standard
ΔfH0 can often be used to compare
* of related compounds
stabilities
enthalpy change for a reaction, ΔrH0, under standard
conditions may be calculated from ΔfH0 values
equation:
the equation is a consequence of the
definition of ΔfH0 and Hess’s Law
ΔrH° = ΣnΔfH° − ΣnΔfH°
is the reaction product- or reactant-favored at equilibrium?
LOOK AT THE ΔrH0 VALUE!
product-favored = *
reactant-favored = *
product-favored = negative ΔrH0 (exothermic) reactant-favored = positive ΔrH0 (endothermic)
