Enthalpy Calculator

What an enthalpy change measures

Enthalpy change, written delta H, is the heat that flows into or out of something while the pressure stays the same. That fits almost everything in a house: a pot on the stove sits at atmospheric pressure, so the heat the water absorbs is its enthalpy change. A chemical reaction run in an open beaker works the same way.

The sign carries the direction. Heat flowing in warms the sample, melts it or boils it, and gets a positive sign; chemists call that endothermic. Heat flowing out during cooling, freezing or condensing gets a negative sign and is called exothermic. This page reports the sign so the number matches the chemistry convention.

Two kinds of change need two different formulas. Warming or cooling a substance moves its temperature, and the heat depends on how much of it there is and what it is made of. Melting, boiling, freezing and condensing happen at a constant temperature, and the heat depends on the mass and the substance's latent heat instead. The calculator switches formulas when you pick the change.

The temperature change formula

For warming and cooling, delta H = m x c x delta T. Here m is the mass, c is the specific heat in kJ per kg per degree C, and delta T is the temperature change. Degrees C and kelvins have the same size, so a change of 60 C is a change of 60 K and the formula does not care which label you use.

Worked example. Warm 2 kg of water from 20 C to 80 C. The change is 60 C, water's specific heat is 4.184, so delta H = 2 x 4.184 x 60 = 502 kJ. That is 502,000 joules, about 120 kcal, 476 BTU or 0.14 kWh.

Worked example with a metal. An empty 0.5 kg aluminum pan warms from 20 C to 180 C. Aluminum's specific heat is 0.897, so delta H = 0.5 x 0.897 x 160 = 71.8 kJ. The same temperature swing on the same mass of water would take 4.184 x 0.5 x 160 = 335 kJ, nearly five times as much, which is why an empty pan heats up fast and a pot of water does not.

Cooling runs the sign the other way. Let that 2 kg of water fall from 80 C back to 20 C and delta H = 2 x 4.184 x (20 - 80) = -502 kJ. The same number of joules leaves the water and ends up warming the kitchen.

Specific heat of common substances

Specific heat is the heat needed to raise 1 kg of a substance by 1 degree C. Water sits near the top of the everyday table, which is why it is used as a coolant and why heating it feels slow.

The third column makes comparisons instant: heating a copper pot by 10 C takes about a tenth of the heat that the same mass of water needs. The values above are typical room temperature figures; the NIST Chemistry WebBook lists measured heat capacities if you need a precise value at a specific temperature, because specific heat drifts a few percent as temperature changes.

Phase changes and latent heat

While ice melts or water boils, a thermometer pinned in the sample stops moving. The heat going in is busy pulling molecules apart instead of speeding them up, so the temperature holds at the melting or boiling point until the change finishes. That heat is called latent, meaning hidden.

For water the numbers are large. Melting 1 kg of ice at 0 C takes 334 kJ, even though the temperature never moves. Boiling 1 kg of water at 100 C takes about 2,260 kJ, more than four times the 419 kJ needed to heat that kilogram from 0 to 100 C in the first place. This is why steam burns are so severe: condensing steam dumps its entire 2,260 kJ per kg into skin on contact.

A full pot of water, then, has a long latent tail. Bring 2 L from 15 C to a boil takes 2 x 4.184 x 85 = 711 kJ, and boiling it dry adds 2 x 2,260 = 4,520 kJ, more than six times the warming stage. A covered pot, a smaller surface or a lower flame all attack that tail.

Freezing and condensing release the same quantities in reverse. That is the idea behind hot packs, salt on icy roads and district heating pipes that carry steam.

The four water changes, with the heat that goes with each:

Reading the results: kJ, kcal, BTU and kWh

The calculator reports the same heat in four units because different questions want different ones.

Kilojoules are the chemistry and physics unit, and food labels use the older Calorie, which is a kilocalorie: 1 kcal = 4.184 kJ. The 502 kJ example is 120 food calories, roughly one slice of bread.

BTU is the heating and air conditioning unit in the United States. 1 BTU raises 1 lb of water by 1 degree F, which works out to 1.055 kJ. The 502 kJ example is about 476 BTU, so a 20,000 BTU burner could supply it in about a minute and a half if it lost no heat.

Kilowatt hours are what the utility bills. 1 kWh = 3,600 kJ, so 502 kJ is 0.139 kWh. At 17 cents per kWh that is 2.4 cents of electricity.

Put together for a real kettle: 1.5 L of water from 15 C to a boil needs 4.184 x 1.5 x 85 = 533 kJ, or 0.148 kWh. On a 1,500 W element that is 0.148 / 1.5 = 0.099 hours, about 6 minutes, at perfect efficiency. Kettles run around 80 to 90 percent efficient, so 7 minutes is the realistic answer.

Why the meter reads more than the math

The formula counts the heat that lands in the sample. A kitchen sends a share of it elsewhere.

Losses. A pot radiates and convects from its sides, and a gas flame heats the room around the pot. A kettle wastes 10 to 20 percent, an electric oven around 10 percent once hot, and an open gas burner can waste half its output. Doubling the calculated energy is a reasonable guess for a slow open-burner simmer.

Heat that goes into the container. A 2 kg cast iron pan at 0.449 kJ per kg C warming 200 C takes 2 x 0.449 x 200 = 180 kJ on its own, and that lands on the bill before any food warms up.

Assumptions in the constants. Specific heat changes with temperature, water's value assumes liquid water between freezing and boiling at ordinary pressure, and latent heat is quoted at the normal boiling point. Altitude shifts the boiling point, which changes the numbers a little.

What the formula deliberately ignores. Mixtures, evaporation from the surface and heat from the food itself all bend the result. Treat the calculator as the floor: the energy that must arrive, before anything leaks.

The same 502 kJ task through four heat sources:

Enthalpy of reaction, beyond the kitchen

In chemistry, delta H also labels the heat of a reaction. The convention is the same one this page uses: positive for heat absorbed by the system, negative for heat released. Combustion is strongly negative because fuel gives heat off; the combustion of methane releases about 890 kJ per mole, so its delta H is about -890 kJ/mol.

Reaction enthalpies come from tabulated values rather than a single multiplication. The common route uses standard enthalpies of formation: delta H of reaction equals the sum of the formation enthalpies of the products minus the same sum for the reactants. A second route adds up the bonds broken and formed. Both routes live in the enthalpy chapter of OpenStax Chemistry 2e, with worked examples.

Hess's law ties it together: because enthalpy is a property of the starting and ending states, the path does not matter, so a reaction can be split into steps whose delta H values add up to the whole. The USGS Water Science School documents the water values this page uses, and they are the same numbers whether the question is a kettle, a radiator or a chemical equation.