Enthalpy Calculator
Calculate enthalpy changes for chemical reactions and thermodynamic processes
Enthalpy Calculation
Internal Energy Change (ΔU)
Change in internal energy of the system
Pressure (p)
Constant pressure during the process
Volume Change (ΔV)
Change in volume (positive for expansion, negative for compression)
Results
Example Calculation
Combustion of Methane
Reaction: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Standard Enthalpies of Formation:
• CH₄(g): -74.87 kJ/mol
• O₂(g): 0 kJ/mol
• CO₂(g): -393.52 kJ/mol
• H₂O(l): -285.83 kJ/mol
Solution
ΔH = [1×(-393.52) + 2×(-285.83)] - [1×(-74.87) + 2×(0)]
ΔH = [-393.52 - 571.66] - [-74.87]
ΔH = -965.18 + 74.87 = -890.31 kJ
Exothermic reaction (heat released)
Key Concepts
Enthalpy (H)
Total heat content of a system
Endothermic
Positive ΔH, absorbs heat
Exothermic
Negative ΔH, releases heat
Standard Conditions
25°C, 1 bar pressure
Common ΔHf° Values
Understanding Enthalpy
What is Enthalpy?
Enthalpy (H) is a thermodynamic quantity that represents the total heat content of a system. It includes both the internal energy of the system and the energy associated with the pressure-volume work done by or on the system.
Enthalpy vs Internal Energy
While internal energy (U) accounts for the kinetic and potential energy of molecules within a system, enthalpy also includes the work done against external pressure. This makes enthalpy particularly useful for processes occurring at constant pressure.
Types of Enthalpy Changes
Formation (ΔHf°):
Energy change when 1 mol of compound forms from elements
Combustion (ΔHc°):
Energy released when 1 mol burns completely in oxygen
Fusion (ΔHfus):
Energy needed to melt 1 mol of solid at melting point
Vaporization (ΔHvap):
Energy needed to vaporize 1 mol of liquid at boiling point
Mathematical Relationships
Basic Enthalpy Formula
H = U + pV
Where H = enthalpy, U = internal energy, p = pressure, V = volume
Enthalpy Change
ΔH = ΔU + pΔV
For constant pressure processes
Reaction Enthalpy Calculation
ΔH°reaction = ∑ΔHf°(products) - ∑ΔHf°(reactants)
Used to calculate enthalpy change for chemical reactions using standard formation enthalpies