Calorimetry at Constant Pressure: Step-by-Step Problem Method
By ChemistryIQ Team · February 28, 2026
Core Idea: Heat Lost Equals Heat Gained
In a typical coffee-cup calorimeter, energy is conserved between system and surroundings. If a reaction releases heat, the solution absorbs it; if the reaction absorbs heat, the solution loses it. The sign convention is critical: q_solution = m c ΔT, and q_reaction = -q_solution (ignoring calorimeter heat capacity unless provided).
Step 1: Organize Known Values
List mass of solution, specific heat capacity, initial and final temperatures, and moles of limiting reactant if asked for molar enthalpy. Convert units first. Most mistakes start with inconsistent mass units or incorrect ΔT direction.
Step 2: Compute q for the Solution
Calculate ΔT as T_final - T_initial, then compute q_solution = m c ΔT. If the temperature increases, q_solution is positive (solution gained heat). If temperature decreases, q_solution is negative (solution lost heat).
Step 3: Convert to Reaction Heat and ΔH
Use q_reaction = -q_solution. If needed, convert this energy to per-mole enthalpy using moles reacted: ΔH = q_reaction / n. Keep track of significant figures and convert joules to kilojoules when reporting thermochemical values.
Quality Checks
Ask whether the sign makes physical sense. Exothermic reactions should have negative ΔH and warm the solution; endothermic reactions should have positive ΔH and cool the solution. Always state assumptions (for example, solution density approximated as 1.00 g/mL when appropriate). ChemistryIQ can review your calorimetry setup and sign convention from a photo to catch mistakes before submission.
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Common questions about calorimetry at constant pressure
Because energy is conserved. Heat transferred to the solution must come from the reaction, and vice versa, so the two values have equal magnitude and opposite sign.
Include it when the problem provides a calorimeter constant. Then total absorbed heat is q_solution + q_calorimeter, and q_reaction is the negative of that total.