All calculatorsEnthalpy (Hess’s Law) Calculator
Enter known reaction steps with their ΔH 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 ΔH depends only on initial and final states, not the path taken. That means we can add/ subtract/ scale known reactions (adjusting their ΔH accordingly) to build a target reaction and compute its ΔH.
How to use this calculator
1) Choose Simple (quick 1–3 steps) or Advanced (add many steps).
2) For each step: enter ΔH (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 ΔH to see each step’s adjustment and the summed total.
Key points
- Flip reverses direction → ΔH changes sign.
- Multiplying a reaction by n → ΔH × 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)
- ΔHi = step enthalpy in consistent units (kJ)
Example Problems & Step-by-Step Solutions
Example 1 — Reversing a step
Given: H2 + 1/2 O2 → H2O(l), ΔH = −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), ΔH = −282.7 kJ, then doubling the reaction (2 CO + O2 → 2 CO2) multiplies ΔH by 2: −565.4 kJ.
Example 3 — Summing two steps
Step 1: C(s) + O2 → CO2(g), ΔH = −393.5 kJ
Step 2: H2(g) + 1/2 O2(g) → H2O(l), ΔH = −285.8 kJ
Sum (no flips, multiplier = 1): ΔHtotal = −679.3 kJ.
Frequently Asked Questions
Q: What does “Flip (reverse)” do?
It reverses the reaction direction and changes the sign of ΔH.
Q: How do multipliers work?
Multiplying all coefficients by n scales ΔH by n (e.g., ×2 doubles ΔH).
Q: Can I mix kJ and kcal?
Yes. The calculator converts everything to kJ and shows kcal too.
