BackEnthalpy (Hess’s Law) Calculator
Enter known reaction steps with their Δ values, flip or scale any step, and I’ll sum them to find the overall enthalpy change using Hess’s Law.
Background
Hess’s Law says enthalpy is a state function: the overall Δ depends only on initial and final states, not the path taken. That means we can add/ subtract/ scale known reactions (adjusting their Δ accordingly) to build a target reaction and compute its Δ.
How to use this calculator
1) Choose Simple (quick 1–3 steps) or Advanced (add many steps).
2) For each step: enter Δ (kJ or kcal). If you reverse a reaction, check Flip.
If you multiply a reaction, set the Multiplier (e.g., 2 or 1/2).
3) Click Calculate Δ to see each step’s adjustment and the summed total.
Key points
- Flip reverses direction → Δ changes sign.
- Multiplying a reaction by n → Δ × n.
- Units are auto-converted (1 kcal = 4.184 kJ); results shown in kJ (and kcal).
Formula & Equation Used
For steps i = 1…N:
- si = +1 (as written) or −1 (if flipped)
- mi = multiplier (e.g., 2, 1/2)
- Δi = step enthalpy in consistent units (kJ)
Example Problems & Step-by-Step Solutions
Example 1 — Reversing a step
Given: H2 + 1/2 O2 → H2O(l), Δ = −285.8 kJ.
Reverse it to form H2O(l) → H2 + 1/2 O2. Flipping changes the sign: +285.8 kJ.
Example 2 — Scaling a step
If CO(g) + 1/2 O2 → CO2(g), Δ = −282.7 kJ, then doubling the reaction (2 CO + O2 → 2 CO2) multiplies Δ by 2: −565.4 kJ.
Example 3 — Summing two steps
Step 1: C(s) + O2 → CO2(g), Δ = −393.5 kJ
Step 2: H2(g) + 1/2 O2(g) → H2O(l), Δ = −285.8 kJ
Sum (no flips, multiplier = 1): Δtotal = −679.3 kJ.
Frequently Asked Questions
Q: What does “Flip (reverse)” do?
It reverses the reaction direction and changes the sign of Δ.
Q: How do multipliers work?
Multiplying all coefficients by n scales Δ by n (e.g., ×2 doubles Δ).
Q: Can I mix kJ and kcal?
Yes. The calculator converts everything to kJ and shows kcal too.
