Why are products always placed at ΔH₁ + ΔH₂?

That is the definition of the two-step path in this toy model: the intermediate is reached after the first step, and the second step lands on products. The direct ΔH slider is what you compare to that sum.

Do the preset numbers match every textbook exactly?

They are rounded pedagogical values for forming CO and CO₂ from the elements at 298 K and 1 bar in typical general-chemistry tables; small differences can arise from allotrope, standard-state conventions, or updated data.

Is this the same as bond-energy estimation?

No. Hess cycles here use overall reaction enthalpies. Bond-energy estimates average bond strengths and are a different (more approximate) tool.

What does an inconsistent residual mean physically?

It means the three numbers you entered cannot all refer to the same pair of thermodynamic states for the overall transformation—useful as a classroom warning, not as a measured contradiction.

Hess's Law (Enthalpy Paths)

**Hess's law** is the enthalpy face of the general idea that **state functions** (here **H**) depend only on the **initial and final states**, not on how you get there. This simulator fixes a **reference level** for reactants and draws a **two-step path** through an **intermediate** with enthalpy changes **ΔH₁** and **ΔH₂**. The **products** sit at **ΔH₁ + ΔH₂** relative to reactants. A **dashed direct route** carries a **declared overall ΔH** for the same net reaction. When **ΔH₁ + ΔH₂** matches that declaration, the cycle is **thermodynamically consistent**; when it does not, the residual highlights that **you cannot pick arbitrary step and overall values** for the same states. A preset reproduces the classic **carbon monoxide oxidation ladder** (C → CO → CO₂) with rounded literature-style enthalpies whose sum matches **direct formation of CO₂** from the elements in this model. The diagram is **schematic**: it does not distinguish constant-pressure laboratory calorimetry details, phase changes of carbon allotropes, or non-standard conditions beyond the numbers you enter.

Who it's for: High school and introductory college chemistry students learning enthalpy diagrams, reaction cycles, and state-function reasoning before full thermochemical networks or computational chemistry.

Key terms

Hess's law
Enthalpy
State function
Reaction enthalpy (ΔH)
Thermochemical cycle
Intermediate
Enthalpy diagram

How it works

**Hess's law** packages the idea that **enthalpy is a state function**: as long as you start and end in the same states, the **sum of ΔH values along any path** equals **ΔH for any other path** between those states. This lab uses a **two-step** route (reactants → intermediate → products) and compares **ΔH₁ + ΔH₂** to a **declared one-step ΔH** for the same overall change.

Key equations

ΔH_overall = ΔH₁ + ΔH₂ (same initial & final states)

Enthalpy H is a state function — path-independent.

Frequently asked questions

Why are products always placed at ΔH₁ + ΔH₂?: That is the definition of the two-step path in this toy model: the intermediate is reached after the first step, and the second step lands on products. The direct ΔH slider is what you compare to that sum.
Do the preset numbers match every textbook exactly?: They are rounded pedagogical values for forming CO and CO₂ from the elements at 298 K and 1 bar in typical general-chemistry tables; small differences can arise from allotrope, standard-state conventions, or updated data.
Is this the same as bond-energy estimation?: No. Hess cycles here use overall reaction enthalpies. Bond-energy estimates average bond strengths and are a different (more approximate) tool.
What does an inconsistent residual mean physically?: It means the three numbers you entered cannot all refer to the same pair of thermodynamic states for the overall transformation—useful as a classroom warning, not as a measured contradiction.