Enthalpy Change Calculator
Compute the molar enthalpy change (ΔH) of a chemical reaction from calorimetry data — mass, specific heat, temperature change, and moles of reactant. Used in thermochemistry labs to characterize exothermic and endothermic reactions.
About this calculator
Enthalpy change (ΔH) quantifies the heat exchanged between a chemical reaction and its surroundings at constant pressure. In a calorimetry experiment, the heat absorbed or released by the solution is measured and then converted to a per-mole value. The core relationship is: q = m × c × ΔT, where m is the mass of the solution, c is the specific heat capacity, and ΔT is the temperature change. The molar enthalpy change is then ΔH = −q / n, where n is the moles of reactant and the negative sign accounts for sign convention (heat released by the reaction is absorbed by the solution). A calorimeter-type factor is applied as a multiplier to account for constant-pressure (coffee-cup) versus constant-volume (bomb) calorimetry conditions. The full formula used here is: ΔH = −(m × c × ΔT × calorimeter factor) / n, expressed in J/mol or kJ/mol.
How to use
Suppose 100 g of water (c = 4.18 J/g·°C) warms by 5.0 °C when 0.5 mol of NaOH dissolves. Using a coffee-cup calorimeter factor of 1: ΔH = −(100 × 4.18 × 5.0 × 1) / 0.5 = −(2090) / 0.5 = −4180 J/mol = −4.18 kJ/mol. The negative sign confirms this is an exothermic process — heat flows from the reaction into the water. Enter mass = 100 g, specific heat = 4.18, ΔT = 5.0, moles = 0.5, and calorimeter type = 1 to reproduce this result instantly.
Frequently asked questions
What is the difference between enthalpy change and heat of reaction?
Enthalpy change (ΔH) and heat of reaction are effectively the same quantity when a process occurs at constant pressure, which is the condition in most laboratory and industrial settings. ΔH represents the total heat exchanged with the surroundings per mole of reaction. When ΔH is negative, the reaction is exothermic and releases heat; when positive, it is endothermic and absorbs heat. The distinction only becomes meaningful at constant volume, where the measured quantity is internal energy change (ΔU) rather than enthalpy, and a small pressure-volume correction (ΔngRT) converts one to the other.
How does calorimeter type affect the enthalpy calculation?
A coffee-cup calorimeter operates at constant pressure (open to the atmosphere), so it measures enthalpy change directly. A bomb calorimeter operates at constant volume, measuring internal energy change (ΔU) instead. To obtain ΔH from bomb calorimeter data, you must apply the correction ΔH = ΔU + ΔngRT, where Δng is the change in moles of gas. This calculator uses a calorimeter-type multiplier to adjust for these differences. Always select the correct calorimeter type to ensure your ΔH value carries the right physical meaning.
Why is the negative sign used in the enthalpy formula for calorimetry?
The negative sign arises from conservation of energy and sign convention. In calorimetry, the heat gained by the solution (q_solution = m × c × ΔT) is positive when temperature rises. By the first law of thermodynamics, the heat released by the reaction is equal and opposite: q_reaction = −q_solution. Since ΔH of the reaction equals q_reaction per mole, we write ΔH = −(m × c × ΔT) / n. This ensures that an exothermic reaction — one that heats the surroundings — carries a negative ΔH, consistent with IUPAC thermodynamic conventions used worldwide.