Chapter 7- Thermodynamics Flashcards
What is nature’s heat tax?
Natures cut on energy every time it’s used
It is in the form of heat
We cannot create more energy than what was there to begin with
How does the first and second thermodynamic laws associate with energy usage?
First- energy is conserved in chemical processes (cannot create more energy than what was initially there
Second- cannot break even with energy, energy is always lost to surrounding in processes
What is a spontaneous process?
A process that occurs without ongoing outside intervention
Dropping book
Ball on slope
How can a non spontaneous process be spontaneous?
By coupling it to another process that is spontaneous or by supplying energy from an external source
What does disorder change entropy?
Increasing disorder, increasing entropy in the more disorders states
Solid to liquid to gas
What is entropy?
What is it’s equation?
Determines the direction of chemical and physical change
Chemical system proceeds in a direction that increases the entropy of the universe
S=k ln(w)
K=gas constant over avogadros
W= # of energetically equivalent ways
What is microstate vs macrostate?
Micro- exact internal energy distribution among the particles at any one instant
Macro- result of a large number of different micro states
Pictures on pages 372-373
What is the variable W?
It is the number of energetically equivalent ways to arrange the components of a system
The number of possible micro states that can result in a given macrostate
What is the change in entropy for the universe in a spontaneous reaction?
ΔS > 0
Always positive
What phase changes does change in entropy (ΔS>0) increase for?
Solid to liquid
Solid to gas
Liquid to gas
An increase in moles of gas during reaction
The greater the temperature, the __________ the increase in entropy for a given amount of energy dispersed into the surroundings
Smaller
What is the sign and size of ΔSsys and ΔSsurr at low and high temperature in the equation:
ΔSuniv=ΔSsys + ΔSsurr
Δsys is always negative
ΔSurr= positive and large at low temp
Positive and small at high temp
What is the equation for ΔSsurr involving change in enthalpy and T?
ΔSsurr = -ΔHsys/T
What is the third law of thermodynamics?
The entropy of a perfect crystal at absolute zero (0K) is zero
What are allotropes?
Elements that exist in two or more forms
How does molar mass affect the entropy of gases?
The bigger the element, the greater the entropy
How is entropy affected by molecular complexity?
The more complex a compound, the greater the entropy
How do you calculate the standard entropy change for a reaction?
ΔSrxn = ΣnpS(products)-ΣnrS*(reactants)
N is the stoichemetric coefficients of the reactants and products
What is the Gibbs free energy equation and how can it be changed to incorporate ΔSuniv?
ΔG= ΔH -TΔS
ΔG= -TΔSuniv
What is spontaneity if ΔH is negative and ΔS is positive?
Spontaneous at all temps
Page 393
What is the spontaneity of ΔH is positive and ΔS is negative?
Non spontaneous at all temps
Page 394
What is the spontaneity of ΔH is negative and ΔS in negative?
(For high temp and low temp)
Spontaneous at low temp
Nonspontaneous at high temp
Page 394
What is spontaneity when ΔH is positive and ΔS is positive?
At high and low temps
Nonspontaneous at low temp
Spontaneous at high temp
Page 394
What are the three ways in calculating ΔG*rxn?
1. ΔG*rxn=ΔH*rxn-TΔS*rxn Page 397 2. ΔG*rxn=ΣnpΔG*f(products)-ΣnrΔG*f(reactants) *only works at temp which ΔG*f is tabulated (25C) Page 400 3. Use Hess’s laws and combine
ΔHrxn=-74.6 kJ
ΔSrxn= -80.8 J/K
ΔG*rxn= -50.5 kJ
Is the reaction endothermic or endothermic and how much thermal energy does it give off?
What is the maximum amount of energy available for useful work?
Exothermic and spontaneous and gives off 74.6 kJ of thermal energy
Max amount of energy= 50.5kJ
If G was positive it would represent the minimum amount of work
How do we calculate the free energy change of a reaction under non standard conditions?
ΔGrxn= ΔG*rxn + RTlnQ
What is the value of Q at standard condition for gas reactions (P=1atm)
Q=1 which results in ΔGrxn just equaling ΔG*rxn
What is the equation for ΔG*rxn at equilibrium with K?
ΔG*rxn= -RTlnK
What is the linear like equation for free energy change to equilibrium constant?
lnK=-ΔHrxn/R (1/T) + ΔSrxn/R
