🔥Hess's Law Calculator
e.g. -285.8, -393.5
Hess's Law states that the total enthalpy change of a reaction is independent of the pathway taken — it depends only on the initial and final states. This allows calculation of ΔH for reactions that are difficult to measure directly.
- 1Write the target reaction
- 2Identify a series of known reactions that sum to the target
- 3Reverse any reaction that needs to be reversed (change sign of ΔH)
- 4Multiply reactions by appropriate stoichiometric factors
- 5Sum all ΔH values
C + O₂ → CO₂ (ΔH=−393kJ) and 2CO + O₂ → 2CO₂ (ΔH=−566kJ)=C + ½O₂ → CO: ΔH = −110kJSubtract half of second equation from first
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Fun Fact
Hess's Law (1840) was derived before the first law of thermodynamics was formally stated. It is essentially a special case of energy conservation applied to chemical reactions.
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