Standard Thermodynamic Properties Table
Interactive reference for Enthalpy (ΔH), Entropy (S), and Gibbs Free Energy (ΔG) with built-in spontaneity calculator.
Oxygen
Hydrogen
Nitrogen
Carbon (Graphite)
Carbon (Diamond)
Iron
Water (Liquid)
Water (Gas)
Carbon Dioxide
Carbon Monoxide
Ammonia
Nitrogen Dioxide
Methane
Ethane
Propane
Butane
Acetylene
Methanol (Liquid)
Ethanol (Liquid)
Calcium Carbonate
Calcium Oxide
Sodium Chloride
Glucose
The Big Three: H, S, and G
Thermodynamics tells us whether a reaction can happen, not how fast it happens. To make these predictions, chemists rely on three fundamental quantities measured at standard conditions (25°C, 1 atm).
Enthalpy (ΔH)
The heat content. Exothermic reactions (release heat) have negative values and are often favorable.
Entropy (S)
The measure of disorder. The universe tends towards chaos (positive S). Gases have much higher entropy than solids.
Gibbs Energy (ΔG)
The ultimate judge of spontaneity. It combines Enthalpy and Entropy into a single value.
The Gibbs Equation
The relationship between the three variables is the most important equation in chemical thermodynamics:
Spontaneity Rules
If ΔG < 0, the reaction is spontaneous.
If ΔG > 0, the reaction is non-spontaneous (requires energy).
If ΔG = 0, the system is at equilibrium.
Frequently Asked Questions
What does ΔH°f mean?
ΔH°f (Standard Enthalpy of Formation) is the heat change when one mole of a substance is formed from its elements in their standard states at 1 atm and 25°C.
Why is ΔH°f for O2 equal to 0?
By definition, the standard enthalpy of formation for any element in its most stable form (standard state) is zero. Examples include O₂ gas, C graphite, and Fe solid.
What is Entropy (S°)?
Entropy is a measure of the disorder or randomness of a system. Gases having high disorder have high entropy values, while solids have low entropy values.
How do I calculate ΔG reaction?
You can calculate ΔG reaction using the formula: ΔG°rxn = ΣnΔG°f(products) - ΣmΔG°f(reactants). Alternatively, use the Gibbs equation: ΔG = ΔH - TΔS.
What does a negative ΔG mean?
A negative ΔG indicates that a reaction is spontaneous (thermodynamically favorable) under the given conditions. A positive ΔG means the reaction is non-spontaneous.
Why are the units for S different from ΔH and ΔG?
Entropy (S) is typically measured in J/mol·K, while Enthalpy (ΔH) and Gibbs Energy (ΔG) are measured in kJ/mol. Always remember to convert units (divide S by 1000) when using them in the same equation!
Does temperature affect spontaneity?
Yes! The term -TΔS in the Gibbs equation shows that temperature plays a crucial role. A reaction that is non-spontaneous at low temperatures can become spontaneous at high temperatures if entropy increases.
What are standard conditions?
Standard thermodynamic conditions are defined as a pressure of 1 atm (or 1 bar) and a specified temperature, usually 25°C (298.15 K). Solutions are at 1 M concentration.
What is Hess's Law?
Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps or the path taken. This allows us to calculate reaction enthalpies by adding up standard enthalpies of formation.
Why is diamond's ΔH°f not zero?
Carbon's standard state is graphite, which is its most stable form at 25°C. Diamond is a less stable allotrope, so it has a positive enthalpy of formation (1.9 kJ/mol).