Enthalpy Calculator
Compute an enthalpy change two ways. Use the definition ΔH = ΔU + pΔV with internal energy in kJ, pressure in kPa, and volume change in liters — or switch modes for sensible heat, q = m·c·ΔT, with mass in kg, specific heat in J/(kg·K), and a temperature change in K or °C. Results in kJ, kcal, and BTU.
Example: with Mode Definition: ΔH = ΔU + pΔV · Internal energy change ΔU (kJ) 100 · Pressure p (kPa, constant) 101.325 · Volume change ΔV (liters) 5 · Mass (kg, for q = mcΔT) 2 → Enthalpy change ΔH: 100.51 kJ.
- In kcal and BTU24.02 kcal (95.26 BTU)
- BreakdownpΔV flow-work term = 0.51 kJ of the total
Computed by the calculator below using its default values. Change any input to see your own numbers.
H = U + pV by definition, so at constant pressure ΔH = ΔU + pΔV. For heating without phase change, ΔH equals q = m·c·ΔT — water's c is 4,186 J/(kg·K).
What enthalpy adds to plain energy
Enthalpy is defined as H = U + pV: a system's internal energy plus the work needed to make room for it against the surrounding pressure. That pV term is why chemists prefer ΔH — almost everything on a bench happens in open containers at constant atmospheric pressure, and any gas produced or consumed does push-the-atmosphere work automatically. ΔH = ΔU + pΔV counts that work so the single number matches what a constant-pressure calorimeter actually measures.
The units make the bookkeeping easy in SI: pressure in kilopascals times volume in liters gives joules directly, so 101.325 kPa pushing back 5 L costs 507 J — the small correction this calculator reports alongside the total.
The mcΔT shortcut
When nothing melts, boils, or reacts, the enthalpy change of heating is just sensible heat: q = m·c·ΔT. Warm 2 kg of water by 30 K at c = 4,186 J/(kg·K) and you have banked 251 kJ. This is the mode for sizing water heaters, cooling loads, and lab warm-ups. Its limits are the fine print: c itself drifts with temperature (water's varies under 1% across 0–100°C, so the constant-c assumption is good), and any phase change adds a latent-heat term — melting ice takes 334 kJ/kg before the first degree of warming.
How it’s calculated
Definition mode: ΔH = ΔU + pΔV, with ΔU in kJ, p in kPa, ΔV in L; since 1 kPa·L = 1 J exactly, pΔV/1,000 gives kJ. Sensible-heat mode: ΔH = q = m·c·ΔT / 1,000 kJ, defaulting c to water's 4,186 J/(kg·K). Conversions: 1 kcal = 4.184 kJ (thermochemical); 1 kJ = 0.947817 BTU.
Constant pressure and no phase change are assumed — vaporization, fusion, and reaction enthalpies must be added separately, and c is treated as constant over your ΔT.
Specific heats for the mcΔT mode
| Substance | c, J/(kg·K) |
|---|---|
| Water (liquid) | 4,186 |
| Ice (−10°C) | 2,090 |
| Ethanol | 2,440 |
| Air (constant pressure) | 1,005 |
| Aluminum | 897 |
| Iron / steel | 449 |
| Copper | 385 |
Standard engineering-reference values near room temperature (CRC Handbook / NIST), rounded.
Common mistakes
- Dropping the pΔV term and calling ΔU the enthalpy change — for gas-producing reactions at 1 atm the gap is about 2.5 kJ per mole of gas formed.
- Unit slips in pΔV: this tool takes kPa × L (= J); pascals × m³ also gives joules, but mixing the two systems is off by 1,000.
- Using mcΔT across a phase change — heating 0°C ice to 10°C water is mostly the 334 kJ/kg of melting, which cΔT alone misses entirely.
- Sign confusion: negative ΔH means the system releases heat (exothermic). Enter cooling as a negative ΔT and read the sign, don't flip it by hand.
Frequently asked questions
What is the enthalpy formula?
ΔH = ΔU + pΔV at constant pressure — internal-energy change plus pressure-volume work. For simple heating with no phase change it reduces to ΔH = q = m·c·ΔT.
What is the difference between enthalpy and internal energy?
The pV term. Internal energy is what the system contains; enthalpy adds the work of occupying volume against outside pressure. In a rigid sealed container heat equals ΔU; in an open beaker heat equals ΔH — that is the whole practical distinction.
Why is my ΔH negative?
The process releases heat (exothermic) — combustion and freezing are classic cases. Positive ΔH means heat is absorbed (endothermic), like melting ice or dissolving ammonium nitrate in a cold pack.
Can I use this for a chemical reaction's ΔH?
Use the definition mode if you know ΔU (say, from a bomb calorimeter) and the gas volume change — that is exactly how tabulated reaction enthalpies are corrected. Getting ΔH from bond energies or formation enthalpies is a separate Hess's-law bookkeeping this tool doesn't do.
How do kJ, kcal, and BTU relate?
1 kcal = 4.184 kJ and 1 BTU = 1.055 kJ. So 251 kJ of water heating is about 60 kcal or 238 BTU — the tool shows all three.