Thermodynamics Flashcards
law of conservation of energy
energy can be converted from one form to another but energy cannot be created or destroyed
thermodynamics
the study of the energy of a system
- can predict spontaneous reactions
AU
internal energy
AH
enthalpy
AS
entropy
AG
Gibbs free energy
AX are?
state functions and are usually a change in energy
state function
initial and final states of a system (ignores the journey)
delta = products - reactants
path functions
- heat (q) and work (w) are the consequences of change
- not predicable from AU (their sum)
- depend on the way change occurs (path function)
- the whole journey
system
might be chemical reaction and chemicals involved
boundary
separates the system and surroundings
universe
system and surroundings
temerpature
in kelvins - the transfer from hotter to cooler bodies
heat
the energy that transfers from hotter to cooler objects
work
motion against an opposing force
SI units for energy, work and heat
1J = 1Kgm^2s^-2
1J is the amount of KE possessed by a 2kg object moving at speed of one meter per second
open system
gain and lose mass or energy across boundaries eg. human body
closed systems
can absorb or release energy but not mass across a boundary.
- mass is constant
- light bulb
isolated systems
cannot exchange matter or energy with the surroundings
- energy is constant (cannot be created or destroyed)
- stoppered vacuum flask
- adiabatic: no heat transfer to surroundings
adiabatic systems
heat or matter cannot enter or leave the system
internal energy
sum of nuclear, electronic, vibrational, rotational, translational and interactional energy of all the individual particles in a sample of matter
- =0 in an isolated system - energy (heat, light, sound) can change
- AU = q + w
- AU = q - (P deltaV)
enthalpy
heat absorbed or evolved by a chemical system and may be determined y temp change or physical change under constant pressure
entropy
measure of the number of ways energy is distributed throughout a chemical system. value is related to enthalpy at a particular temperature
Gibbs equation
G = H - TS
enthalpy - temperature x entropy
exothermic
releases energy
- enthalpy of reactants is higher than enthalpy of products
- delta H is less than 0
endothermic
absorbs energy
- reactants have lower enthalpy than products
- delta H is positive
enthalpy and its equations
measure of energy in a thermodynamic system
H = U + pV
enthalpy (J)
internal energy (J)
pressure (Pa)
volume (m3)
but we can’t measure enthalpy of a system so we measure the change
measure of change in enthalpy equation
AH = q (subscript) p
q is positive with heat supplied (melt ice)
q is negative with heat given out (explosion)
+w and -w
+w is work done on the system (energy enters) - work done by a wind turbine
-w is work done by the system (energy leaves) - work done by car engine
standard enthalpy
- standard values so reactions can be compared
- pressure 10^5 Pa (gases)
- concentration 1M (solutions)
- 25 degrees or 298K usually (not one of the standard states though)
standard enthalpy of formation
enthalpy change of formation of 1 mol of substance in its standard form from its constitutive elements in their standard states
standard enthalpy of reaction
sum of the enthalpy of formation of the products - the sum of enthalpy of formation of reactants
phase diagrams
describes the existence of various phases of matter (solid, liquid, gas) for a substance as a function of pressure and temperature
- depends on bonding and intermolecular forces
- accompanied by changes in heat (energy)
equation of heat during heating of phase
q = mCAT
equation of heat during phase transitions
q = mAH
constant pressure during changes
- ice (more ordered) moves to a liquid which is less ordered (endothermic)
- stream (less ordered) moves to liquid (more ordered) - exothermic
why does H2O have a negative slope between solid and liquid?
ice is less dense than water
what does a bomb calorimeter measure and its equations?
- measures AU
- constant V AU = q - P delta V so delta V = 0 so AU = q (subscript) v
q reaction = - Ccalorimeter AT
C is the calorimeter constant
how does a bomb calorimeter work?
combustion causes water to heat. the heat gained by the calorimeter, q calorimeter, = the heat liberated by combustion
q calorimeter (J) = (C calorimeter J/k)(delta T sample k)
bomb calorimeter in a closed system
heat is constant q system + q surroundings = 0 q reaction (J) = - q calorimeter (J)
system and surroundings of a bomb calorimeter
system: reaction in calorimeter
surroundings: calorimeter including water
what does a coffee cup calorimeter measure? and equation
delta H at constant pressure (gas loss) so delta H = q (subscript) p
qp = mCAT
C is the heat for 1g to be heated by 1 degree (4.18J/k for water)
Hess’ Law
makes use of state functions by sequential reactions
- try to organise to make one thing from one equation equal something from another and can then determine enthalpy of that reaction
what is spontaneity?
order to disorder
- entropy goes from order to disorder (does not relate to exo or endo)
- less moles to more moles (reactants to products)
- increase in temp = more disporder
- change is state: solid to liquid to gas is less orderly
- solid dissolved in light
- gas dissolved in liquid goes into the atmosphere
spontaneity and entropy
AU universe = AU system + AU surroundings
AU sustem = - AU surroundings
spontaneous change
reactions occur without a constant input of energy
- order to disorder; entropy increases
non-spontaneous reaction
require work to be done
facts about sponteneity
exo and endo tell us nothing. delta H is not a good predictor. has nothing to do with rate of the reaction
entropy
the state of order of a system
standard molar entropy
the entropy per mole of a pure substance while in its standard state (1atm gas, 1M solution, pure substance)
units are J/mol/k and values are usually tabulated at 298k
- higher entropy = more disorder
- higher entropy in ionic solids with weaker bonds
equation for standard molar entropy of reaction
standard entropy of reaction = standard entropy of products - standard entropy of reactants
take mole ratios into account!!
entropy of the surroundings compared to enthalpy of system
AS surroundings = -AH system/T
If exo, heat from system to surroundings and AH system is negative
heat creates more disorder so AS surroundings is positive
keep units for AS and AH consistent
Gibbs energy and spontaneity
standard entropy, enthalpy and Gibbs can be used to determine spontaneity at standard conditions. standard enthalpy of formation of Gibbs and enthalpy are zero at standard states at all temperatures.
standard delta G = standard delta H - T x delta standard S
standard delta G < 0 for spontaneous
G = 0 for equilibrium
G > 0 for non-spontaneous
second law of thermodynamics
for a spontaneous process the total energy of the universe increases (AS total > 0)
AS universe = AS system + AS surroundings > 0
if AS is negative, the reaction is non-spontaneous
AS = 0 is equilibrium
AS system = -AS surroundings
Boltzmann’s formula
S = k x ln x w
third law of thermodynamics
a point at which a perfect crystal can form
enthalpy = 0
k = 0
equilibrium constant
k = concentrations of products over concentration of reactants
when not at equilibrium, k becomes?
Q
if only A and B are present initially, Q = 0
Q< k reaction is forwards
Q > k reaction is reversed
standard Gibbs free energy and Gibbs free energy
delta G reaction = delta standard G reaction + RTlnQ
standard delta G reaction is for standard conditions
this is a correction for non-standard conditions
Gibbs free energy decreases to a minimum equilibrium and then back up (AGr = 0 when K = Q)
the reaction isotherm
k = e ^ (-AGr/RT)
if AGr = 0, k = 1
G > 0, k < 1 and reactants predominate
G < 0, k > 1 and products predominate
