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Core Concepts

Understanding the fundamental concepts behind PHOENIX hazard evaluation.

Screening Tool

PHOENIX is for screening purposes only. All results require experimental validation.

Oxygen Balance (OB%)

Oxygen balance indicates whether a compound has excess or deficient oxygen for complete combustion.

Formula

\[ OB\% = \frac{-1600}{MW} \times \left( 2C + \frac{H}{2} + M - X - O \right) \]

Where: - \(MW\) = Molecular weight (g/mol) - \(C\) = Number of carbon atoms - \(H\) = Number of hydrogen atoms - \(M\) = Number of metal atoms (assumed monovalent) - \(X\) = Number of halogen atoms - \(O\) = Number of oxygen atoms

Interpretation

OB% Range Meaning Risk
OB% > 0 Oxygen excess Oxidizer potential
OB% ≈ 0 Balanced Maximum energy release
OB% < 0 Oxygen deficient Incomplete combustion

Example

from phoenix import Compound

# TNT has significant oxygen deficiency
tnt = Compound.from_smiles("Cc1c([N+](=O)[O-])cc([N+](=O)[O-])cc1[N+](=O)[O-]")
print(f"TNT OB%: {tnt.oxygen_balance:.1f}%")  # ~-74%

# RDX is closer to balanced
rdx = Compound.from_smiles("O=[N+]([O-])N1CN([N+](=O)[O-])CN([N+](=O)[O-])C1")
print(f"RDX OB%: {rdx.oxygen_balance:.1f}%")  # ~-22%

Compounds with OB% between -200% and +100% trigger CHETAH Criterion 3.

Enthalpy of Formation (ΔHf°)

The standard enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their standard states.

Sources

PHOENIX estimates ΔHf° using multiple methods:

  1. Benson Group Additivity (GA) - Primary method
  2. Decomposes molecules into functional groups
  3. Sums pre-tabulated group contributions

  4. Typical accuracy: ±5-10 kJ/mol for common organics

  5. Reference Data - When available

  6. NIST-JANAF thermochemical tables
  7. Experimental literature values

Usage

from phoenix import Compound

ethanol = Compound.from_smiles("CCO")

# As property (at 298.15 K)
hf = ethanol.enthalpy_of_formation
print(f"ΔHf° = {hf.value:.1f} {hf.unit}")  # kJ/mol

# At different temperature
hf_500 = ethanol.enthalpy_of_formation(T=500)
print(f"ΔHf°(500K) = {hf_500.value:.1f} kJ/mol")

Maximum Heat of Decomposition (ΔHd)

The maximum heat of decomposition represents the maximum possible energy release when a compound decomposes to its most stable products.

Calculation

\[ \Delta H_d = \Delta H_f(\text{products}) - \Delta H_f(\text{reactant}) \]

Negative values indicate exothermic decomposition (energy release).

Decomposition Hierarchy

PHOENIX uses thermodynamic priority rules to determine products:

Priority Reaction Product
1 F + H → HF Hydrogen fluoride
2 N → ½N₂ Nitrogen gas
3 P + O → ¼P₄O₁₀ Phosphorus pentoxide
4 H + O → ½H₂O Water
5 C + O → ½CO₂ Carbon dioxide
6 S + O → SO₂ Sulfur dioxide
7 C + ½O → CO Carbon monoxide
8 Cl + H → HCl Hydrogen chloride
9 Br + H → HBr Hydrogen bromide
10 C → C(s) Graphite
11 H → ½H₂ Hydrogen gas

Example

from phoenix import Compound

nitrobenzene = Compound.from_smiles("c1ccccc1[N+](=O)[O-]")
decomp = nitrobenzene.max_decomposition()

print(f"ΔHd = {decomp.delta_hd_kJ_mol:.1f} kJ/mol")
print(f"ΔHd = {decomp.delta_hd_cal_g:.1f} cal/g")

CHETAH Criteria

PHOENIX implements the CHETAH hazard screening criteria from ASTM E659.

Criterion 1: High Instability

ΔHd < -300 cal/g (-1255 kJ/kg)

Compounds meeting this criterion have high potential for violent decomposition.

Criterion 2: Medium Instability

-300 < ΔHd < -100 cal/g

Compounds in this range have moderate decomposition energy.

Criterion 3: Oxygen Balance Concern

-200% < OB% < +100%

Compounds in this OB range can participate in self-oxidation reactions.

Criterion 4: Functional Group Alerts

Presence of known reactive functional groups:

  • Nitro groups (-NO₂)
  • Peroxides (-O-O-)
  • Azides (-N₃)
  • Azo compounds (-N=N-)
  • Diazonium salts (-N₂⁺)
  • Hydrazines (-NH-NH₂)
  • Epoxides (strained rings)

Hazard Classification

Criteria Triggered Classification
Criterion 1 HIGH
Criterion 2 MEDIUM
Criteria 3 + 4 MEDIUM
Any single criterion MEDIUM
None LOW

Phase States

PHOENIX supports calculations for different phases:

Phase Code Description
Gas g Default for Benson GA
Liquid l Liquid state corrections
Solid s Solid state corrections 
from phoenix import Compound

# Gas phase (default)
water_g = Compound.from_smiles("O", phase="g")

# Liquid phase
water_l = Compound.from_smiles("O", phase="l")

Phase Limitations

Phase corrections are available for some properties but not all. Most hazard calculations assume gas-phase products.

Units

Standard Units in PHOENIX

Property Unit
Enthalpy kJ/mol
Entropy J/(mol·K)
Heat capacity J/(mol·K)
Gibbs energy kJ/mol
ΔHd (molar) kJ/mol
ΔHd (specific) cal/g
Oxygen balance %
Molecular weight g/mol
Gas volume L/g

Unit Conversions

# kJ/mol to cal/g
delta_hd_kJ_mol = -500
mw = 200  # g/mol
delta_hd_cal_g = (delta_hd_kJ_mol * 1000) / (4.184 * mw)
print(f"ΔHd = {delta_hd_cal_g:.1f} cal/g")  # -597.6 cal/g

Next Steps