Vapor Pressure Calculator

Vapour pressure data for common solvents (Clausius-Clapeyron parameters)

Vapor Pressure Calculator: Complete Guide

The Clausius-Clapeyron equation describes how vapour pressure changes with temperature: ln(P2/P1) = -(delta-Hvap/R) x (1/T2 - 1/T1), where delta-Hvap is the molar enthalpy of vaporisation (J/mol), R = 8.314 J/mol/K, and T is in Kelvin. This equation allows calculation of vapour pressure at any temperature from a single reference point and delta-Hvap, or determination of delta-Hvap from two vapour pressure measurements.

Clausius-Clapeyron equation derivation and use

The full Clausius-Clapeyron equation: dP/dT = delta-H_vap x P / (RT^2). Assuming delta-H_vap is constant with temperature and integrating between two points: ln(P2/P1) = -(delta-H_vap/R) x (1/T2 - 1/T1). Example: water has vapour pressure 101.325 kPa at 100 deg C (373.15 K) and delta-H_vap = 40.7 kJ/mol. At 50 deg C (323.15 K): ln(P2/101.325) = -(40700/8.314) x (1/323.15 - 1/373.15) = -4895 x (3.095 x 10^-3 - 2.680 x 10^-3) = -4895 x 4.15 x 10^-4 = -2.032. P2 = 101.325 x e^-2.032 = 101.325 x 0.131 = 13.3 kPa. Experimental value: 12.35 kPa. The Clausius-Clapeyron equation gives a good approximation; the slight discrepancy arises because delta-H_vap is not perfectly constant with temperature.

Boiling point as a function of pressure

The normal boiling point is the temperature at which vapour pressure equals 1 atm (101.325 kPa). At higher altitude (lower atmospheric pressure), water boils at a lower temperature. At altitude 2000 m, P_atm approximately 79.5 kPa. Using Clausius-Clapeyron with water (delta-H_vap = 40.7 kJ/mol, normal BP = 373.15 K): 1/T2 = 1/373.15 + (8.314/40700) x ln(79.5/101.325) = 2.680 x 10^-3 + 2.043 x 10^-4 x (-0.2415) = 2.680 x 10^-3 - 4.93 x 10^-5 = 2.631 x 10^-3. T2 = 380.1 K... wait -- that is higher. Check signs: ln(79.5/101.325) = -0.2415 (negative, lower pressure). 1/T2 = 1/373.15 - 8.314/40700 x 0.2415 = 2.680e-3 - 4.93e-5 = 2.631e-3 -- no, this gives T2 = 380 K which is higher. Sign check: lower P should give lower T. The correct rearrangement: 1/T2 = 1/T1 - (R/delta-H) x ln(P2/P1). With P2 < P1: ln(P2/P1) < 0. So 1/T2 = 1/T1 - (R/dH) x (negative) = 1/T1 + positive number, meaning 1/T2 > 1/T1, so T2 < T1. Let us recalculate: 1/T2 = 1/373.15 + (8.314/40700) x ln(79.5/101.325) = 2.680e-3 + 2.043e-4 x (-0.2415) = 2.680e-3 - 4.93e-5 = 2.631e-3; T2 = 1/2.631e-3 = 380.1 K -- still wrong. The correct form: 1/T2 = 1/T1 + (R/dH)*ln(P2/P1). With P2 < P1: ln = negative; 1/T2 < 1/T1; T2 > T1. This is wrong. At lower pressure, boiling point should be lower. The formula must be: ln(P2/P1) = -(dH/R)*(1/T2-1/T1). So 1/T2 = 1/T1 - (R/dH)*ln(P2/P1). ln(79.5/101.325)=-0.2415. 1/T2=2.680e-3-2.043e-4*(-0.2415)=2.680e-3+4.93e-5=2.729e-3. T2=1/2.729e-3=366.4 K=93.3 deg C. Correct -- water boils at about 93 deg C at 2000 m altitude.

Vapour pressure in engineering applications

Vapour pressure data is essential in chemical engineering for: distillation column design (relative volatility = ratio of vapour pressures; determines number of theoretical plates); vacuum system design (choosing pump capacity for a given process temperature); environmental assessment (vapour pressure determines atmospheric concentration and exposure limits for chemicals -- Henry's law constant = P_vap x M_r / rho_liquid); refrigeration system design (refrigerant vapour pressure at operating temperatures determines system pressure and compressor specifications); fire hazard assessment (flash point correlates with the temperature at which vapour pressure reaches the lower explosive limit concentration). The Antoine equation (log10(P) = A - B/(C+T)) is a more accurate empirical representation of vapour pressure vs temperature than the Clausius-Clapeyron equation, using three substance-specific constants A, B, C tabulated in databases (NIST, Perry's Chemical Engineers' Handbook).

Step-by-step worked example

A chemical engineer is designing a process to produce liquid ammonia from nitrogen and hydrogen gases at 25 deg C and 1 bar. The reaction is: N2(g) + 3H2(g) -> 2NH3(g). Standard thermodynamic data at 298 K: delta-Hf(NH3) = -46.11 kJ/mol; S(N2) = 191.6 J/mol/K; S(H2) = 130.7 J/mol/K; S(NH3) = 192.8 J/mol/K. Step 1 -- calculate delta-H: delta-H = 2 x (-46.11) - (0 + 3 x 0) = -92.22 kJ. Step 2 -- calculate delta-S: delta-S = 2 x 192.8 - (191.6 + 3 x 130.7) = 385.6 - (191.6 + 392.1) = 385.6 - 583.7 = -198.1 J/K = -0.1981 kJ/K. Step 3 -- calculate delta-G at 298 K: delta-G = delta-H - T x delta-S = -92.22 - 298 x (-0.1981) = -92.22 + 59.03 = -33.19 kJ. Step 4 -- check sign: delta-G < 0 -- reaction is spontaneous at 25 deg C. Step 5 -- calculate Kc: delta-G = -RT ln(K); K = exp(-delta-G/RT) = exp(33190/(8.314 x 298)) = exp(13.39) = 6.6 x 10^5. Very large K -- products strongly favoured thermodynamically. Step 6 -- note the kinetic problem: despite favourable thermodynamics (large K, negative delta-G), the reaction is kinetically very slow at 25 deg C. This is why the Haber process operates at 400 to 500 deg C with an iron catalyst -- kinetics are too slow at low temperature even though thermodynamics are more favourable there. At 500 deg C (773 K): delta-G = -92.22 - 773 x (-0.1981) = -92.22 + 153.1 = +60.88 kJ. Now delta-G > 0 and K = exp(-60880/(8.314 x 773)) = exp(-9.47) = 7.7 x 10^-5. K is small at high T -- only 15 to 25% conversion per pass. High pressure is used to compensate (shifts equilibrium toward fewer gas moles, increasing ammonia yield). This full analysis -- delta-H, delta-S, delta-G, K at two temperatures, and qualitative kinetic reasoning -- integrates the complete Chemical Thermodynamics suite.

Connections to the thermodynamics suite

The twelve Chemical Thermodynamics calculators in LazyTools cover every major thermodynamic calculation needed in chemistry and chemical engineering. The Gibbs Free Energy Calculator computes delta-G from delta-H and delta-S and predicts spontaneity. The Entropy Calculator sums standard molar entropies from NIST or textbook tables. The Equilibrium Constant Calculator connects K to delta-G via delta-G = -RT*ln(K). The Arrhenius Equation Calculator predicts k at any temperature from Ea and A, bridging thermodynamics and kinetics. The Vapor Pressure Calculator uses the Clausius-Clapeyron equation to find vapour pressure at any temperature from the enthalpy of vaporisation. The Boiling Point Calculator finds the normal boiling point from vapour pressure data. The Boiling Point Altitude Calculator adjusts boiling point for atmospheric pressure at altitude. The Boiling Point Elevation Calculator gives delta-Tb = i*Kb*m for solutions. The Freezing Point Depression Calculator gives delta-Tf = i*Kf*m. The STP Calculator converts between STP and SATP volumes. The Q10 Calculator gives the temperature sensitivity ratio for biochemical reactions. The Gibbs Phase Rule Calculator applies F = C - P + 2 to phase diagrams. Together these twelve calculators span reaction thermodynamics, phase equilibria and colligative properties -- the core quantitative content of undergraduate physical chemistry.

Thermodynamics in industry and environment

Chemical thermodynamics calculations are fundamental to engineering design. Delta-G determines whether a reaction is thermodynamically feasible under proposed conditions before any experimental work is done -- saving enormous amounts of laboratory time and resources. Process engineers use delta-H data to design heat exchangers (heat integration across exothermic and endothermic reaction stages). Entropy calculations guide understanding of process irreversibility and efficiency losses. The Clausius-Clapeyron equation is used in distillation column design (vapour pressure at every stage), in refrigeration system design (refrigerant properties), and in predicting the boiling point of mixtures. Colligative property calculations (boiling point elevation, freezing point depression) are used in antifreeze formulation, food preservation, pharmaceutical parenteral formulation (osmolarity of IV fluids), and polymer solution characterisation. The Gibbs phase rule constrains the number of independent variables in multi-component phase systems -- essential for alloy phase diagram interpretation, extraction process design, and supercritical fluid applications. All results in this suite display units and formulas explicitly, enabling straightforward verification and documentation for regulated engineering and pharmaceutical applications.

Worked numerical example

A chemical engineer is evaluating the feasibility of a new industrial process at 600 K. The proposed reaction is: CO2(g) + 4H2(g) -> CH4(g) + 2H2O(g) (Sabatier reaction for methane production from CO2 and green hydrogen). Standard thermodynamic data at 298 K: delta-Hf values -- CO2(g) -393.5, H2(g) 0, CH4(g) -74.8, H2O(g) -241.8 kJ/mol. Standard molar entropies -- CO2 213.8, H2 130.7, CH4 186.3, H2O(g) 188.8 J/mol/K. Step 1 -- calculate delta-H_rxn: delta-H = [(-74.8) + 2(-241.8)] - [(-393.5) + 4(0)] = (-74.8 - 483.6) - (-393.5) = -558.4 + 393.5 = -164.9 kJ. Exothermic. Step 2 -- calculate delta-S_rxn: delta-S = [186.3 + 2(188.8)] - [213.8 + 4(130.7)] = [186.3 + 377.6] - [213.8 + 522.8] = 563.9 - 736.6 = -172.7 J/K = -0.1727 kJ/K. Entropy decreases (5 mol gas -> 3 mol gas). Step 3 -- delta-G at 298 K: delta-G = -164.9 - 298(-0.1727) = -164.9 + 51.46 = -113.4 kJ. Spontaneous at 298 K; K = exp(113400/(8.314 x 298)) = exp(45.8) = 7.4 x 10^19. Very product-favoured thermodynamically. Step 4 -- delta-G at 600 K: delta-G = -164.9 - 600(-0.1727) = -164.9 + 103.6 = -61.3 kJ. Still spontaneous at 600 K, K = exp(61300/(8.314 x 600)) = exp(12.3) = 2.2 x 10^5. Still large but smaller -- lower temperature is thermodynamically preferred. Step 5 -- crossover temperature (delta-G = 0): T = delta-H/delta-S = -164900/(-172.7) = 955 K. Above 955 K the reaction becomes non-spontaneous. Process engineering conclusion: operate below 955 K with a catalyst (Ni or Ru) to achieve reasonable reaction rates. The Sabatier process is commercially operated at 300 to 400 deg C (573 to 673 K) with Ni catalyst, giving high conversion and good selectivity to methane.

Chemical thermodynamics in industrial and environmental contexts

Thermodynamic calculations of delta-G, delta-H and delta-S underpin every large-scale chemical process. Carbon capture and utilisation (CCU) processes like the Sabatier reaction and Fischer-Tropsch synthesis use thermodynamic feasibility calculations to screen reactions before committing to experimental and pilot plant work. The Haber-Bosch process (N2 + 3H2 -> 2NH3, delta-G = -33 kJ/mol at 298 K, delta-S = -198 J/K) operates below the thermodynamic crossover temperature of 467 K (194 deg C) to maintain negative delta-G, but uses elevated temperature (450 deg C) for acceptable kinetics -- at significant thermodynamic cost in equilibrium yield. Environmental chemistry uses Gibbs energy to predict which pollutants will persist in the environment (delta-G for aerobic degradation), whether metals will dissolve in groundwater (delta-G for dissolution vs precipitation), and whether greenhouse gases will react with atmospheric species (very negative delta-G values for OH radical reactions drive atmospheric chemistry). Biochemical thermodynamics: ATP hydrolysis (delta-G approximately -30 kJ/mol under cellular conditions) drives biosynthesis, active transport and mechanical work. Coupled reactions with negative delta-G drive unfavourable reactions with positive delta-G -- the universal biological energy currency.

Precision and limitations of thermodynamic calculations

Standard thermodynamic data (delta-Hf, S) are measured at 298 K and 1 bar. Using these values to predict delta-G at other temperatures involves two approximations: (1) delta-H is assumed constant with temperature (Kirchhoff's law: d(delta-H)/dT = delta-Cp, where delta-Cp is the heat capacity difference; for reactions without phase changes, delta-Cp is typically 5 to 50 J/mol/K, causing delta-H to change by 0.5 to 5 kJ per 100 K). (2) delta-S is assumed constant with temperature (similarly, d(delta-S)/dT = delta-Cp/T). For temperature extrapolation beyond 200 to 300 K from the reference temperature, these errors accumulate and more accurate calculations require integrating heat capacity data (Shomate equation or NASA polynomial fits). For engineering design, the JANAF tables (National Institute of Standards and Technology), HSC Chemistry software, and the Dortmund Data Bank provide temperature-dependent thermodynamic data. For regulatory submissions to the FDA or EMA for pharmaceutical manufacturing processes, thermodynamic calculations must be documented, justified, and accompanied by experimental validation at the intended process conditions. All calculations in this suite display the formula and inputs explicitly to enable straightforward documentation and verification.

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