Equilibrium direction predictor
With Q at 0.25 and K at 4, Q is below K, so the reaction shifts forward toward products. Enter the reaction quotient and the equilibrium constant to see the direction, the Q-to-K ratio and whether the mixture is already at equilibrium. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Reaction quotient Q
- 0.25
- Equilibrium constant K
- 4
- Ratio Q over K
- 0.0625
- Comparison
- Q < K
- Direction
- forward
- Shift
- toward_products
This equilibrium direction predictor answers the classic general-chemistry question: given the reaction quotient Q and the equilibrium constant K, which way does the net reaction move? When Q is less than K, the mixture is short of products relative to equilibrium, so the system shifts forward (reactants form products). When Q is greater than K, products are in excess, so the system shifts reverse. When Q equals K within a tight numeric tolerance, the system is already at equilibrium and there is no net change. You supply two non-negative finite numbers in the same expression form—Kc, Kp, or activities—and the engine returns the comparison, a clear direction label, a shift tag toward products or reactants, the optional ratio Q over K, and a short formula note. Negative Q or K values are rejected because they are not meaningful for standard mass-action expressions.
What comparing Q with K tells you
The reaction quotient Q is built from the same algebraic expression as the equilibrium constant K, but it is evaluated at the current composition rather than at equilibrium. Comparing those two scalars is the standard mass-action test for the direction of spontaneous net reaction under fixed temperature: the mixture evolves so that Q moves toward K.
If Q is below K, product activities are too low and the forward reaction dominates until Q rises; if Q is above K, product activities are too high and the reverse reaction dominates until Q falls; if they match, the rates of forward and reverse change balance and the system sits at equilibrium. This calculator implements only that comparison. Zero is allowed for either quantity—for example Q equals zero when products have not yet formed—while a negative value is chemically meaningless for concentration, pressure, or activity products and fails validation.
Where the Q versus K direction test fits in study, lab work, and software
Students meet the Q-versus-K rule when a general-chemistry unit moves from writing K expressions to predicting how a mixture responds after a stress, when an AP or A-level problem gives numerical Q and K and asks which way the reaction shifts, or when a physical-chemistry course links the sign of the reaction free energy to the sign of ln of Q over K. Teachers can pre-generate answer keys for modest Q and K pairs without trusting opaque calculator apps that silently swap Kc and Kp or invert the inequality.
Developers call the same contract from scripts to seed deterministic test vectors for equilibrium dashboards, to normalize user-entered composition checks before process demos, or to satisfy programming-challenge statements that demand a forward or reverse label rather than a hand-waved large or small claim. Lab and process work often needs a fast, offline source of direction checks so report numbers can be verified against the stated formula under time pressure.
Product and education tools use the evaluation when a learner types Q and K and the UI must show a clear shift toward products or reactants. Pair it with the equilibrium-constant-from-Gibbs helper when you need K first, and with partial-pressure Kp tools when you are assembling a small lab of free, deterministic reaction helpers.
What you can do with it
Homework and exam Q vs K drills
Plug textbook reaction quotients and equilibrium constants into the Q < K / Q > K rule and check whether the net reaction goes forward or reverse.
Lab reports and tutoring software
Normalize user-entered Q and K into equilibrium shift predictions for worksheets, plotters, and interactive equilibrium lessons.
FAQ
What does the equilibrium direction predictor cost?
It is free in your browser on this page.
How is the shift direction decided?
If Q < K the reaction shifts forward toward products; if Q > K it shifts reverse toward reactants; if Q equals K within a small relative tolerance the system is at equilibrium.
What happens if Q or K is negative?
Negative reaction quotients and equilibrium constants are not meaningful for standard concentration, pressure, or activity expressions.
Does this tool compute Q or K from concentrations?
No. You must supply both Q and K yourself in the same expression form (for example both as Kc). Use a dedicated tool if you need to build Q or K from species values first.
Why is ratio_Q_over_K sometimes null?
When K is zero the ratio Q/K is undefined, so the field is null. Direction is still classified by comparing Q and K directly.
Can I compare a Kc with a Q built from pressures?
No. Q and K must use the same expression form, both from concentrations, both from partial pressures or both from activities. Comparing a Kc with a pressure-based Q can point you to the wrong direction.
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/q-vs-k-direction \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"Q":0.25,"K":4}'const res = await fetch("https://api.kit.forhosting.com/chem/q-vs-k-direction", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"Q": 0.25,
"K": 4
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/q-vs-k-direction",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"Q": 0.25,
"K": 4
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/q-vs-k-direction", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"Q":0.25,"K":4}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"Q":0.25,"K":4}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/q-vs-k-direction", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"Q": 0.25,
"K": 4
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.q_vs_k_direction",
"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_value | 1000000000000000 |
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
The path is fully deterministic: pure arithmetic on Worker-safe numbers with no network calls, no thermo table lookups, and no date-dependent shortcuts.
What Q versus K means and what the response contains
It returns the normalized Q and K you supplied, the ratio Q over K when K is positive, a comparison string such as Q less than K, a direction tag of forward, reverse, or equilibrium, a shift tag of toward products, toward reactants, or none, a boolean at_equilibrium flag, a net_reaction label describing reactants to products or the reverse, the precision used, the method tag q_vs_k_direction, the formula summary, and a note reminding you that Q and K must share the same expression form. There is no approximate Monte Carlo path and no random sampling of conditions: the engine either classifies the pair inside the supported magnitude range or rejects the input with invalid_input. Rejected cases include missing Q or K, non-finite values, magnitudes above the documented ceiling, and any strictly negative Q or K.
How Q and K are read and stay reproducible
Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong direction label. Zero for Q or K is valid; both zero is treated as equilibrium with a null ratio because division by zero is undefined. Classification uses a relative epsilon so floating-point noise does not flip Q equals K into a spurious forward or reverse result: the absolute difference is compared against a tiny factor times the larger of one, absolute Q, and absolute K. The preferred fields are capital Q for the reaction quotient and capital K for the equilibrium constant, but aliases such as q, reaction_quotient, and quotient for Q, and k, equilibrium_constant, Kc, Kp, and Keq for K are accepted so spreadsheets, query strings, and typed forms can reuse the same contract. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for Q, K, and the ratio, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing the Q-versus-K test yourself.
Where the Q versus K direction test fits in study, lab work, and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.
Integration use cases
Deterministic test fixtures
Generate reproducible direction, shift, comparison, and ratio labels for unit tests, property checks, and CI assertions.