Hess's law: add the steps
"Hess's law: add the steps" is a free, interactive Class 11 Chemistry simulation for NEET and JEE, built on the NCERT section "Hess's law" in Thermodynamics. An enthalpy level diagram where given thermochemical equations can be flipped and scaled as arrows; the target arrow C(graphite) + ½O2 → CO closes the loop when the steps add up. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 51 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Hess's law in Thermodynamics. Change the values and watch what happens.
What it shows
NCERT § "Hess's Law of Constant Heat Summation"
An enthalpy level diagram where given thermochemical equations can be flipped and scaled as arrows; the target arrow C(graphite) + ½O2 → CO closes the loop when the steps add up.
Controls
3 to try
- Togglereverse each given equation
- Segmentedmultiply each by ×½ / ×1 / ×2
- Segmentedtarget ΔfH(CO) / ΔfH(CH4) / ΔcH(C2H4)
Try this
Combine C → CO2 (−393.5) and reversed CO → CO2 (−283.0) to close the loop and read ΔfH(CO) = −110.5 kJ/mol.
Common mistake
Forgetting to flip the sign when reversing an equation, or to scale ΔH with coefficients.
In NEET
10 years of papers
Asked 2 times in 10 years of NEET.
See the past questions: Enthalpy and Hess's Law, Thermodynamics (chapter)
In JEE Main
42 shifts analysed
51 questions from Thermodynamics across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT § "Hess's Law of Constant Heat Summation"
- Because enthalpy is a state function, the total enthalpy change of a reaction is the same whether it happens in one step or in several.
- Thermochemical equations can therefore be added, subtracted and scaled like algebraic equations to obtain an unknown ΔᵣH.
- A classic use is finding ΔfH° of CO from the combustion enthalpies of carbon to CO₂ and of CO to CO₂, because carbon cannot be converted cleanly to CO alone.
- Combustion data give formation enthalpies: for a hydrocarbon, ΔcH° = Σ ΔfH°(CO₂ and H₂O formed) − ΔfH°(hydrocarbon), since ΔfH° of O₂ is zero.
- When combining equations, carry each species' physical state; H₂O(l) and H₂O(g) have different ΔfH° values.
Questions
5 answers
- What does the "Hess's law: add the steps" simulation show?
- An enthalpy level diagram where given thermochemical equations can be flipped and scaled as arrows; the target arrow C(graphite) + ½O2 → CO closes the loop when the steps add up.
- What mistake does the "Hess's law: add the steps" simulation fix?
- A common mistake it clears up: forgetting to flip the sign when reversing an equation, or to scale ΔH with coefficients.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 2 times in 10 years of papers.
- Does JEE Main ask questions from Thermodynamics?
- Yes. JEE Main asked 51 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Hess's law" in Thermodynamics (Class 11 Chemistry).
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