Enthalpy Change Calculator
Calculate reaction enthalpy (ΔH) and visualize exothermic vs endothermic energy profiles.
Energy Profile Diagram
Upgoing Arrow = Endothermic (Heat Absorbed)
The Science of Chemical Energy
Whether it's the explosive power of rocket fuel or the cooling effect of an ice pack, Enthalpy Change (ΔH) is the key to understanding energy flow in the universe.
Calculating ΔH
There are two main ways to calculate Enthalpy Change: using experimental values (Products minus Reactants) or theoretical bond energies.
The Energy Account
Think of a reaction like a bank account. You must "spend" energy to break old bonds, and you "earn" energy when new bonds form.
- -ΔHExothermic (Profit)
You earned more than you spent. The "extra" energy is released as heat.
- +ΔHEndothermic (Loss)
You spent more than you earned. You had to absorb extra energy from outside.
Average Bond Energies (kJ/mol)
| Bond | Energy (kJ/mol) | Bond | Energy (kJ/mol) |
|---|---|---|---|
| H–H | 436 | C–H | 413 |
| C–C | 348 | C=C | 614 |
| O=O | 495 | O–H | 463 |
| Cl–Cl | 242 | H–Cl | 431 |
| N≡N | 941 | N–H | 391 |
Note: N≡N (Triple bond) is incredibly strong, which is why Nitrogen gas is so inert!
Combustion
Fuels like Methane (CH4) are powerful because the bonds in the products (CO2 and H2O) are much stronger than the bonds in the fuel. The formation of these stable bonds releases a massive amount of energy.
Photosynthesis
Plants do the opposite of combustion. They use energy from sunlight to drive an endothermic reaction, converting CO2 and water into glucose (stored chemical energy).
The Haber Process
Making ammonia (NH3) from Nitrogen is tricky. The N≡N triple bond is so strong (941 kJ/mol) that high energy is needed to break it, even though the overall reaction is exothermic.
Explore More Chemistry Tools
Frequently Asked Questions
What is Enthalpy Change (ΔH)?
Enthalpy change is the heat energy transferred between a chemical system and its surroundings at constant pressure. It tells you if energy is released or absorbed.
What is the difference between Exothermic and Endothermic?
Exothermic reactions release heat (negative ΔH), warming the surroundings (e.g., burning wood). Endothermic reactions absorb heat (positive ΔH), cooling the surroundings (e.g., melting ice).
How do I calculate ΔH from bond energies?
Use the formula: ΔH = Σ(Bonds Broken) - Σ(Bonds Formed). Breaking bonds requires energy (Input), while forming bonds releases energy (Output).
Why is bond breaking endothermic?
Atoms are held together by attractive forces. Energy must be supplied (absorbed) to overcome these forces and pull the atoms apart, much like needing energy to pull two magnets apart.
What is an Energy Profile Diagram?
It's a graph showing the energy levels of reactants and products. The 'hump' represents the activation energy, and the difference in height between start and finish represents ΔH.
Does a catalyst change Enthalpy?
No. A catalyst lowers the activation energy (the path), but the starting energy of reactants and ending energy of products remain the same, so ΔH is unchanged.
What is Hess's Law?
Hess's Law states that the total enthalpy change for a reaction is independent of the pathway taken. Whether you go from A to B directly or via C, the total energy change is the same. This allows us to calculate ΔH for impossible-to-measure reactions by adding up steps.
What are standard conditions?
Standard enthalpy changes (ΔH⦵) are measured at 100 kPa (1 atm) pressure and 298 K (25°C). Concentration is usually 1 mol/L.
What is Activation Energy?
It is the minimum energy required to start a reaction. On an energy profile, it's the energy difference between the reactants and the top of the 'hump'. Even exothermic reactions (like lighting a match) need this initial spark.
Why are some reactions spontaneous?
Spontaneity depends on both enthalpy (ΔH) and entropy (ΔS). Exothermic reactions generally tend to be favorable, but are not guaranteed to be spontaneous (Gibbs Free Energy ΔG = ΔH - TΔS is the true decider).
Examples of Exothermic reactions?
Combustion (burning), respiration in cells, neutralization (acid + base), and freezing water.
Examples of Endothermic reactions?
Photosynthesis (absorbs sunlight), thermal decomposition (breaking down limestone), and melting ice or evaporating water.
Why are bond energy calculations only estimates?
Bond energy values (like C-H = 413 kJ/mol) are averages taken from many different molecules. The specific strength of a C-H bond might vary slightly depending on the rest of the molecule's structure.
What is the unit of Enthalpy?
The SI unit for energy is Joules (J), but enthalpy change is typically expressed in kilojoules per mole (kJ/mol).
How does Enthalpy relate to temperature?
In an exothermic reaction, the chemical potential energy is converted to kinetic energy (heat), causing the temperature of the surroundings to rise. In an endothermic reaction, heat is taken from the surroundings, causing the temperature to fall.