Enthalpy of fusion calculator
Get the molar enthalpy of fusion from a melting measurement: 2 mol of ice absorbing 12.02 kJ give 6.01 kJ/mol, in line with tabulated values for water. Enter the moles and the heat, already prepared. Free in your browser.
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Worked example
- Moles
- 2
- Heat
- 12.02
- Molar enthalpy of fusion
- 6.01
The molar enthalpy of fusion is the heat required to melt one mole of a pure substance at constant pressure and the melting temperature, without changing its temperature. Labs and homework problems often give the total heat absorbed while a known amount of solid turns to liquid, and ask for ΔH_fus by dividing that heat by the moles melted.
What the molar enthalpy of fusion measures
Melting turns a solid into a liquid at its melting point, and the heat it takes is latent heat, so the temperature stays constant while the crystal lattice rearranges into the liquid. The molar enthalpy of fusion is that heat for one mole of a pure substance at constant pressure. In practice you melt a known number of moles, measure the total heat absorbed during melting alone, and divide the heat by the moles. Two moles of ice absorbing 12.02 kJ give 6.01 kJ/mol, which matches common tabulated values for water.
Preparing your numbers and avoiding mistakes
Moles must be greater than zero: with zero moles a per-mole value is undefined, and a negative amount of substance is not physical, so both are rejected with a clear message. Keep one energy unit throughout, because heat in kilojoules gives a result in kilojoules per mole and heat in joules gives joules per mole. The calculator does not turn grams into moles or look up tables, so if your raw data are a mass and a molar mass, work out the moles first. Count only the heat used for the melting itself, not the heat spent warming the solid or the liquid.
What molar enthalpy of fusion measures and which inputs to send
Fusion is the phase change from solid to liquid at the melting point. If you melt n moles and measure total heat Q absorbed by the sample during melting alone, the molar enthalpy of fusion is simply ΔH_fus equals Q divided by n. Textbooks tabulate ΔH_fus in kilojoules per mole for ice, metals, and organic solids; this calculator recovers that intensive value from an extensive measurement when you already know moles and heat.
Moles must be strictly positive: zero moles makes any molar quantity undefined, and negative moles is non-physical for an amount of substance. Heat may be positive for the usual endothermic melting direction; keep the same energy unit for Q and for the reported ΔH_fus so that if Q is in kilojoules then ΔH_fus is in kilojoules per mole. The engine does not convert mass to moles or look up substance tables—you supply n and Q already prepared.
FAQ
What happens if moles is zero or negative?
Molar enthalpy of fusion is undefined for zero moles and non-physical for a negative amount; those cases are rejected and not charged.
What units should I use for heat and the result?
Use any coherent energy unit for heat (commonly kJ or J). The molar result is that energy unit per mole. The engine does not convert between J and kJ for you.
Is the enthalpy of fusion calculator free?
The browser tool runs free on your device.
Does this convert mass to moles or look up table values?
No. Supply moles already computed and the measured heat absorbed.
Why doesn't the temperature change while the solid melts?
Because the heat goes into breaking up the crystal lattice, not into warming the sample. This latent heat keeps the temperature at the melting point until all of the solid has turned to liquid.
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/enthalpy-of-fusion-moles \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"moles":2,"heat":12.02}'const res = await fetch("https://api.kit.forhosting.com/chem/enthalpy-of-fusion-moles", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"moles": 2,
"heat": 12.02
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/chem/enthalpy-of-fusion-moles",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"moles": 2,
"heat": 12.02
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/chem/enthalpy-of-fusion-moles", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"moles":2,"heat":12.02}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"moles":2,"heat":12.02}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/chem/enthalpy-of-fusion-moles", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"moles": 2,
"heat": 12.02
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "chem.enthalpy_of_fusion_moles",
"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
This capability applies the standard relation ΔH_fus equals Q over n, returns structured JSON with the normalized inputs and the molar result, and rejects zero or negative moles with a clear invalid input error so pipelines never publish an undefined quotient. Keep energy units coherent across the request so the answer is energy per mole in the same unit you used for Q.
What molar enthalpy of fusion measures and which inputs to send
The heat absorbed in that step is latent heat: temperature stays constant while bonds in the crystal lattice rearrange into the liquid. Send moles and heat as JSON numbers or numeric strings. Aliases n, amount, moles_melted, and sample_moles map to moles; aliases q, Q, heat_absorbed, energy, and enthalpy map to heat so lab spreadsheets and worksheet column names map cleanly. Identical inputs always produce identical JSON because the path is pure arithmetic with no network, no random seed, and no wall clock.
Reading molar_enthalpy_of_fusion, formula, and method fields
For a typical classroom ice example with two moles of water ice absorbing twelve point zero two kilojoules, the molar result is six point zero one kilojoules per mole, matching common tabulated values for water. Pair it with a moles-from-mass tool when your raw data are grams and molar mass, then feed n and Q here for the fusion step. On success the response echoes the normalized moles and heat used in the solve after finite-number parsing and stable rounding. The headline field molar_enthalpy_of_fusion is heat divided by moles, equal to Q over n. The formula label is always the Unicode string for ΔH_fus equals Q over n, and method is always enthalpy_of_fusion_moles so downstream code can branch without sniffing shapes. Use moles and heat alone when grading intermediate steps; use molar_enthalpy_of_fusion when you need the per-mole answer for a report or comparison to a table. Missing required fields, non-finite numbers, zero moles, and negative moles all fail with invalid_input and are not charged.
API usage, pricing, lab workflows, and practical limits
Call the free in-browser enthalpy of fusion calculator for pre-lab checks, lecture demos, and homework verification without installing a spreadsheet macro or CAS. Authors shipping SDKs can map the three numeric fields into UI tables that show moles melted, heat absorbed, and molar enthalpy of fusion—the same intermediate steps students show on paper when they grade a melting-point calorimetry or ice-calorimeter worksheet by hand. Send a JSON body with required moles and heat. Use the prepaid API when a learning platform, LIMS helper, curriculum generator, or thermochemistry pipeline 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 mass-to-moles or density tools when the sample mass and molar mass are known upstream, or with heating-curve exercises when the next step is summing sensible and latent segments outside this product. This calculator does not convert units, subtract container heat capacity, or identify which solid you melted; send already-corrected Q and n when your lab protocol requires those adjustments.
Integration use cases
Chemistry homework and exam checks
Students enter moles melted and heat absorbed from a fusion worksheet, verify ΔH_fus = Q/n, and get a clear invalid_input error if moles is zero or negative by mistake.
Tutoring and curriculum generators
Lesson builders produce worked enthalpy-of-fusion examples with structured JSON for flashcards, step sheets, and auto-graded drills without embedding a full chemistry CAS.
Lab and materials screening scripts
Lightweight pipelines convert measured melting heats and sample mole amounts into molar ΔH_fus via a stable prepaid endpoint instead of reimplementing the divide in every language.
Technical questions
How do I calculate molar enthalpy of fusion with this tool?
Send JSON with moles set to the amount melted and heat set to the heat absorbed during melting (for example moles: 2, heat: 12.02). The response includes molar_enthalpy_of_fusion equal to Q/n.