Combined equilibrium constant calculator
Get the overall equilibrium constant of a reaction built from steps: a step with K of 2.5 plus a reversed step with K of 0.4 gives 6.25, because constants multiply and a reversed step counts as one over K. Each step's share is shown. Free in your browser.
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When you build a net chemical equilibrium from tabulated half-reactions, the overall equilibrium constant is not a sum of the step constants—it is a product, with reverses written as reciprocals and scaled equations raised to the stoichiometric power. Students rearranging multi-step problems, tutors checking Hess-style equilibrium worksheets, and lab scripts that chain literature K values all need the same deterministic contract: list each step’s K, mark which steps run backwards, optionally multiply a half-reaction by an integer, and read back a single overall K with every intermediate contribution spelled out.
What overall K means when reactions are added or reversed
Start from the textbook rule that equilibrium constants transform exactly like free-energy changes under Hess manipulations, except the algebra is multiplicative rather than additive. If reaction one has constant K1 and reaction two has constant K2 and you add them after cancelling intermediate species, the net equilibrium constant is K1 times K2. If you reverse a step so products become reactants, its contribution becomes the reciprocal one over K.
If you multiply a balanced equation by a positive integer n so that stoichiometry matches the species you need to cancel, the contribution becomes K to the n—or open parenthesis one over K close parenthesis to the n when that scaled step is also reversed. Search intent for this landing clusters around phrases such as combined equilibrium constant calculator, overall K from two steps, reverse reaction K inverse, multiply equilibrium constant by two, and Hess law for K values.
People want a numeric overall constant they can check against a worksheet answer key, embed in a lab report, or feed into a larger process model—not only a reminder that the product rule exists.
What you can do with it
Lab reports chaining literature constants
Teaching assistants and technicians combine published step constants into a net process K for reports without rebuilding product-and-reciprocal arithmetic in every spreadsheet.
FAQ
How do I calculate the overall equilibrium constant from several steps?
Each step needs positive k; set reversed true to use 1/k, and optional multiplier n to raise that base to the n.
What happens if a step constant is zero or negative?
Zero or negative K is not allowed for this product rule, so the overall constant would be undefined or non-physical; the error is not charged as a successful item.
How does reversing a reaction change K?
If reversed is true for a step, that step contributes 1/k instead of k. After any optional multiplier, the overall constant multiplies all step contributions together.
Is the combined equilibrium constant calculator free?
The browser tool runs free on your device.
Does this compute Kc from concentrations?
No. This capability only multiplies already-known step constants (with reverse and scale rules). Evaluating mass-action Kc from equilibrium concentrations is a separate calculator.
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/combined-equilibrium-constant \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"reactions":[{"k":2.5,"reversed":false},{"k":0.4,"reversed":true}]}'const res = await fetch("https://api.kit.forhosting.com/chem/combined-equilibrium-constant", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"reactions": [
{
"k": 2.5,
"reversed": false
},
{
"k": 0.4,
"reversed": true
}
]
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/combined-equilibrium-constant",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"reactions": [
{
"k": 2.5,
"reversed": false
},
{
"k": 0.4,
"reversed": true
}
]
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/combined-equilibrium-constant", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"reactions":[{"k":2.5,"reversed":false},{"k":0.4,"reversed":true}]}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"reactions":[{"k":2.5,"reversed":false},{"k":0.4,"reversed":true}]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/combined-equilibrium-constant", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"reactions": [
{
"k": 2.5,
"reversed": false
},
{
"k": 0.4,
"reversed": true
}
]
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.combined_equilibrium_constant",
"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.
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
What overall K means when reactions are added or reversed
Delivering k_overall, each step’s contribution, reversed flags, and multipliers as JSON makes the result easy to paste into notes, diff in automated tests, or reuse across languages without reimplementing reciprocal-and-power arithmetic. Keep every step constant strictly positive; a zero or negative K is unphysical for this product contract and fails with invalid_input so pipelines never publish Infinity or a silent sign error.
How to send steps and how to read the response
Validation failures such as zero or negative k mention that those constants are not allowed so learners can fix a table entry without reading a stack trace. Send a JSON object with the required field reactions as a non-empty array of step objects. Each step needs a positive finite equilibrium constant under the key k; aliases K, Kc, Kp, constant, and value are accepted so client code can speak concentration, pressure, or generic laboratory English without renaming fields. Optional reversed (aliases reverse, invert) is a boolean: true means the step is written backwards and contributes one over k; false or omitted means the step is written as tabulated and contributes k itself. Optional multiplier (aliases mult, n, scale, coefficient) is a positive integer defaulting to one that raises the base contribution to that power after the reverse rule is applied. Array aliases steps, constants, and reactions_list map to the same list so SDKs can match existing schema names. On success the response includes k_overall together with aliases K_overall and K for clients that prefer textbook casing, steps as the normalized list of k, reversed, multiplier, and contribution for each row, step_count, formula restating the product rule, and method always combined_equilibrium_product so routers can branch without sniffing shapes. Use k_overall alone when a form only needs the answer cell; use the steps array when grading software must verify that a student marked reverse correctly or scaled a half-reaction by two. Missing arrays, empty arrays, non-finite numbers, non-boolean reversed flags, and non-positive multipliers also raise invalid_input and are not charged after the engine rejects the request.
API usage, pricing, classroom workflows, and practical limits
Call the free in-browser combined equilibrium constant calculator for homework checks, lecture demos, and quick verification of multi-step net equilibria without installing a spreadsheet of reciprocal and power formulas or a chemistry CAS. Source inputs follow the same retention rules as other KIT parse items. Send a JSON body with a reactions array of step objects, each carrying k and optionally reversed and multiplier as numbers, booleans, or numeric strings where appropriate. Use the prepaid API when a tutoring platform, coding-assessment sandbox, LIMS integration, or curriculum generator needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with the mass-action equilibrium constant tool when the next step is evaluating Kc from measured concentrations for a single balanced equation, and with the Kp from Kc converter when temperature and gas-mole change must translate a concentration constant into a pressure form—this capability intentionally stops at combining already-known step constants so each SEO intent stays focused. It does not invent missing literature values or temperature corrections; supply the K you already tabulated or measured for each elementary write-up. Step lists are capped for Worker isolate safety while still covering multi-step classroom and process cases, and multipliers are bounded so large powers cannot overflow isolate arithmetic into non-finite noise. Authors shipping SDKs can map reactions and k_overall into labeled tables Step K, Reversed, Multiplier, Contribution, and Overall K—the same columns students write when they rearrange Hess-style equilibrium problems by hand on paper.
Integration use cases
Homework and multi-step equilibrium worksheets
Students enter tabulated K values, mark which half-reactions run reverse, apply integer multipliers to match stoichiometry, and compare overall K against structured JSON instead of hand-multiplying reciprocals.
Tutoring apps and assessment pipelines
Curriculum tools call a stable prepaid endpoint instead of reimplementing Hess-style K combination in every language, grading harness, and worksheet template.