Hess's law calculator
Double a step of −286 and reverse a step of −572, and the contributions of −572 and +572 cancel to an overall ΔH of 0. Enter each known enthalpy with its multiplier to get the total, step by step. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Runs in your browser. Free, unlimited — your data never leaves this page.
A Hess's law calculator turns a list of thermochemical steps into the single overall reaction enthalpy you need for exams, lab reports, and process checks. Enter each known reaction enthalpy together with a multiplier—positive to scale a step, negative to reverse it—and the tool returns ΔH_overall equals the sum of n times ΔH for every step, plus per-step contributions, a short sign classification (exothermic, endothermic, or thermoneutral), and ordered substitution lines you can paste into a worksheet. Students checking multi-step formation paths, engineers stacking tabulated formation data, and coding kata authors all need that same linear combination without opening a spreadsheet.
What Hess's law says about overall reaction enthalpy
Hess's law states that the enthalpy change for a chemical process depends only on the initial and final states, not on the path taken between them. In practice that means if you can write a target reaction as a linear combination of known thermochemical equations, the overall ΔH is exactly the same linear combination of the known enthalpies.
A multiplier of two doubles both the reaction as written and its enthalpy; a multiplier of minus one reverses the reaction and flips the sign of ΔH, which is how you use a formation reaction in the opposite direction. Because enthalpy is a state function, intermediate species that appear on both sides of the assembled path cancel, and only the net stoichiometry remains—yet you never need the intermediate species names to compute the number, only the list of step enthalpies and multipliers.
Classroom problems often mix formation enthalpies, combustion data, and bond-energy approximations into three to five steps; industrial energy balances may stack more, but the arithmetic is identical. This calculator implements that sum with fixed rounding and rejects an empty step list so that a missing data path cannot silently report zero heat for a reaction that was never assembled.
Practical tips, limits, and related searches
Keep every enthalpy and multiplier inside the published absolute range so products cannot overflow into a non-finite overall enthalpy that the solver must reject. Prefer sending numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern. When reversing a tabulated reaction, set multiplier to negative one rather than negating the enthalpy by hand—both are equivalent, but the multiplier makes the direction of the path explicit in the contribution table.
Prefer this capability when you already have step enthalpies and only need the linear combination; if your source is a word problem, extract each known ΔH and its scale first.
What you can do with it
Homework checks for multi-step thermochemistry
Students enter tabulated step enthalpies and multipliers from textbook Hess-law problems and compare hand-written overall ΔH and contribution lines against a deterministic result before submitting.
Lab and process heat estimates from known paths
Engineers stack formation or combustion enthalpies with stoichiometric multipliers to get a quick overall ΔH before more detailed simulation.
FAQ
What happens when no steps are provided?
An empty or missing steps list is not a valid Hess cycle and is never charged.
How do I reverse a reaction?
Set the step multiplier to a negative number (usually −1). That flips the sign of the step's contribution, which is equivalent to writing the reaction backwards.
Do I need chemical formulas or species names?
No. This calculator only needs numeric enthalpies and multipliers.
Is the Hess's law calculator free?
The browser tool is free and runs locally.
What do exothermic, endothermic and thermoneutral mean in the result?
They describe the sign of the overall sum. A negative total is exothermic, so heat is released; a positive total is endothermic, so heat is absorbed; a total of exactly zero is thermoneutral.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/chem/hess-law \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"steps":[{"enthalpy":-286,"multiplier":2},{"enthalpy":-572,"multiplier":-1}]}'const res = await fetch("https://api.kit.forhosting.com/chem/hess-law", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"steps": [
{
"enthalpy": -286,
"multiplier": 2
},
{
"enthalpy": -572,
"multiplier": -1
}
]
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/hess-law",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"steps": [
{
"enthalpy": -286,
"multiplier": 2
},
{
"enthalpy": -572,
"multiplier": -1
}
]
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/hess-law", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"steps":[{"enthalpy":-286,"multiplier":2},{"enthalpy":-572,"multiplier":-1}]}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"steps":[{"enthalpy":-286,"multiplier":2},{"enthalpy":-572,"multiplier":-1}]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/hess-law", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"steps": [
{
"enthalpy": -286,
"multiplier": 2
},
{
"enthalpy": -572,
"multiplier": -1
}
]
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.hess_law",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs | 1000000000000 |
max_steps | 64 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
When no steps are provided the API returns a clear invalid_input error instead of inventing a zero enthalpy for an empty cycle.
What Hess's law says about overall reaction enthalpy
Algebraically, ΔH_overall equals the sum over i of n_i times ΔH_i, where each n_i is a real multiplier that scales the stoichiometry of step i.
How to call the API and what comes back
Send a JSON body with one required field steps: a non-empty array of objects. Each object must include enthalpy (aliases delta_h, dh, H)—the ΔH of the reaction as tabulated—and may include multiplier (aliases n, coeff, coefficient, factor, scale), which defaults to one when omitted. Absolute values of enthalpy and multiplier must stay within the published limit of one trillion so intermediate products remain finite in IEEE double precision on the Worker and in the browser. At most sixty-four steps are accepted per request. On success the response sets overall_enthalpy, delta_h, and total to the same rounded sum, n_steps to the count of steps used, formula to the string ΔH_overall equals sum of n_i times ΔH_i, sign to endothermic, exothermic, or thermoneutral according to the sign of the sum, message to a short English summary, contributions to an ordered array of per-step rows with index, enthalpy, multiplier, and contribution, and steps to an ordered array of substitution lines. Failed validation—missing steps, an empty array, non-object entries, missing enthalpy on a step, non-finite numbers, or values outside absolute limits—returns invalid_input and is never charged. Pricing is a flat $0.002 per successful request with unit item and zero marginal unit price.
Practical tips, limits, and related searches
Pair textbook cases: two moles of water formation at about minus two hundred eighty-six kilojoules each, then reverse the hydrogen-oxygen combination at plus five hundred seventy-two, should net near zero when the path closes; a single formation step with multiplier one should echo the tabulated value; and an empty steps array demonstrates the hard invalid_input path. This landing targets people searching for a Hess law calculator, overall reaction enthalpy, or multi-step delta H online; it is not a formation-enthalpy table lookup, not a bond-energy estimator, and not a reaction balancer. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send an empty list. Round-trip checks are easy: recompute each n times ΔH by hand, sum them, and confirm the total equals overall_enthalpy within the same twelve-decimal rounding.
Integration use cases
Unit tests for student energy-balance code
Generate known-good multi-step sums, reversed-step cases, and deliberate empty-list rejections for coding kata expected values.
Technical questions
What is Hess's law?
Hess's law says the enthalpy change of a reaction is path-independent: if a target reaction is a linear combination of known steps, ΔH_overall equals the same linear combination of the step enthalpies, ΔH = Σ(n_i · ΔH_i).