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Wize University Chemistry Textbook > Thermodynamics Part 2: (2nd Law-entropy, 3rd Law, Gibbs Free Energy, Spontaneity)

3rd Law of Thermodynamics

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3 rd Law of Thermodynamics

  • The Third Law states: All perfect crystals in equilibrium at 0K have the same entropy, defined as 0
  • Conceptually this makes sense given our understanding of order and disorder. The system described in the third law would be the most ordered system imaginable.

Summary of the Laws of Thermodynamics

Wize Concept
It is more important to understand each law and the related equations than to memorize what each law is!


0th Law: 2 bodies that are in thermal equilibrium with a 3rd body are in thermal equilibrium with each other as well! All 3 bodies would be at the same temperature.


1st Law: Energy is conserved and cannot be created and destroyed. Energy can only be transformed into different forms like heat.

ΔUsurr=−ΔUsys\Delta U_{surr}=-\Delta U_{sys}

ΔU=q+w...where w=−PΔV\Delta U=q+w...where\ w=-P\Delta V

2nd Law: Entropy (disorder) in the universe will spontaneously increase. Energy like to be more spread out!

ΔSorxn=[(∑nΔSproductso)−(∑nΔSreactan⁡tso)]\Delta S^orxn=\left[\left(\sum_{ }^{ }n\Delta S_{products}^o\right)-\left(\sum_{ }^{ }n\Delta S_{reac\tan ts}^o\right)\right]

ΔG=ΔH−TΔS\Delta G=\Delta H-T \Delta S

3rd Law: A perfect crystal has 0 entropy at a temperature of 0K (hypothetical temperature that is the absolute lowest temperature). Any temperature higher than this would have entropy and there is more entropy as temperature increases!
Extra Practice
The Haber Bosch Process is an industrial process for “fixing” nitrogen, shown below. This process produces around 500 million tons of ammonia every year.
ΔfG0(NH3)=−16.4 kJ/mol\Delta_fG^0(NH_3)=-16.4\ kJ/mol
3/2 H2(g)+1/2 N2(g)→NH3(g)\rm 3/2\ H_{2(g)}\quad+\quad 1/2\ N_{2(g)}\quad\overrightarrow{\hspace{1.5cm}}\quad NH_{3(g)}
Entropy
Calculate the standard molar entropy change for the combustion of methane (J/mol K).
CH4 (g) + 2 O2 (g) →\rightarrow CO2 (g) + 2 H2O (g)
Thermodynamics
Which of the following statements disagrees with the laws of thermodynamics?
Thermodynamics
Indicate the FALSE statement about the laws of thermodynamics:
Thermodynamics
The Solubility Constant for AgI(s) at 25oC is 8.3x10-17. What is ΔG(rxn) given that the current concentrations of ions are: [Ag+]=9.1x10-9 & [I-]=9.1x10-9
1)Write the variables we know/trying to solve for
2) Out of the different equations for deltaG, which one should we use?