Calculate heat q = m c ΔT for a calorimetry sample and the molar heat of reaction in kJ/mol from mass, specific heat, both temperatures and moles reacted.
g
J/(g·°C)
°C
°C
mol
Results
Calculated
Heat absorbed q
—
J, q = m·c·ΔT
Heat absorbed q
—
kJ
Temperature change ΔT
—
°C, final minus initial
Heat of reaction ΔH
—
kJ/mol, −q/n (needs moles)
Ready
Enter mass, specific heat and both temperatures (moles optional), then press Calculate.
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What this calculator finds
Thermochemistry tracks the heat that flows during a chemical or physical change. In a calorimetry experiment you measure the temperature change of a known mass of water or solution and convert it to heat. This calculator does that with q = m · c · ΔT, then divides by the moles of reactant to give the heat of reaction per mole.
It suits coffee-cup calorimetry, specific-heat lab problems, and estimating the heat needed to warm or cool a sample.
The equations
q = m × c × ΔT, with m in grams, c in J/(g·°C) and ΔT = Tfinal − Tinitial.
ΔHrxn = −q / n, in kJ/mol when q is converted to kJ. The minus sign appears because the solution's heat gain equals the reaction's heat loss.
A positive q means the sample absorbed heat; a negative q means it released heat.
Worked example
100 g of water warms from 22.0 °C to 25.5 °C when 0.05 mol of a salt dissolves in it, the default inputs.
ΔT = 25.5 − 22.0 = +3.50 °C, so q = 100 × 4.184 × 3.50 = 1,464.4 J = 1.464 kJ. The heat of solution is −1.4644 kJ / 0.05 mol = −29.29 kJ/mol, an exothermic process. The calculator shows +1464.4 J, +1.464 kJ, +3.50 °C and −29.29 kJ/mol.
Common mistakes and how to read the result
Using the mass of the solute only. Use the mass of the water or solution that changed temperature.
Sign errors. The water's q and the reaction's ΔH have opposite signs.
Ignoring heat losses. Real cups leak heat, so measured values are usually slightly low in magnitude.
Wrong specific heat. Solutions are close to water, but metals and organic liquids differ a lot.
Frequently Asked Questions
What is the difference between q and Delta H?
q is the heat that crossed the boundary at constant pressure and, for a coffee-cup calorimeter, equals the enthalpy change. Delta H per mole is that heat divided by moles of reactant, with the sign reversed because the water gained what the reaction lost.
Why is the molar enthalpy the opposite sign of q?
The q in this calculator is heat gained by the water. The reaction lost exactly that much (if the calorimeter itself absorbs nothing), so Delta H for the reaction is minus q divided by the moles reacted.
Does it include the calorimeter's heat capacity?
No. It assumes all heat goes into the mass and specific heat you entered. For a bomb calorimeter, add the calorimeter constant times the temperature change to q before dividing by moles.
Which temperature units does it use?
Celsius or kelvin both work for the difference, because a change of 1 degree C equals a change of 1 K. Just enter both temperatures in the same scale.
Thermochemistry Calculator is most useful when the inputs reflect the situation you are actually planning around, not a best-case estimate. Treat the result as a decision aid: it gives you a structured way to compare assumptions, spot outliers, and decide what to verify next. For Chemistry work, the most important review lens is units, concentration, limiting assumptions, temperature, precision, and significant figures.
Start with a baseline run using values you can defend. Then change one assumption at a time and watch which output moves the most. If one input dominates the result, spend your verification time there first. If several inputs have similar influence, use a conservative scenario and an optimistic scenario to create a practical range instead of relying on a single exact number.
Before acting on the result, verify inputs against lab notes, reagent labels, and the expected reaction or solution model. This is especially important when the calculator supports a purchase, project plan, performance target, or operational decision. The calculator can make the math consistent, but the quality of the conclusion still depends on current data, clear units, and assumptions that match your real constraints.
When the output looks surprising, slow down and inspect each input in order. A small change in one high-leverage field can move the final number more than several low-leverage fields combined. For Thermochemistry Calculator, that means you should first confirm the value with the greatest scale, then confirm the value with the greatest uncertainty, then rerun the calculator with conservative and optimistic assumptions. This sequence turns the calculator from a single answer into a practical decision range.
Review Checklist
Confirm every input uses the unit and time period requested by the calculator.
Run a low, expected, and high scenario so the answer has a useful range.
Check whether rounding or a missing decimal place changes the decision.
Update the calculation for every new mixture, batch, reaction, or homework data set.
How to Validate the Result
Use Thermochemistry Calculator as a repeatable checkpoint rather than a one-time answer. The safest workflow is to record the original inputs, save the output, and write down which assumption you are testing. Then rerun the calculator with one changed value. If the result changes sharply, that input deserves more attention before you act on the number.
For this topic, the main validation lens is units, concentration, limiting assumptions, temperature, precision, and significant figures. That means a result can be mathematically correct and still be misleading if the inputs come from the wrong time period, use inconsistent units, or mix expected values with best-case values. Keep baseline, conservative, and optimistic runs separate so the final decision is easier to explain later.
When you share the result with someone else, include the assumptions and the date of the calculation. Many calculator outputs become stale after prices, schedules, measurements, or constraints change. A short note about the source of each input makes the calculation auditable and prevents later confusion about why the answer moved.
Label the source for each input before comparing scenarios.
Use the same rounding method across every run.
Flag any input that is estimated rather than measured.
Recalculate for every new mixture, batch, reaction, or homework data set.