Exothermic & Endothermic Reactions

Calorimetry is the experimental technique used to measure the heat energy change of a reaction by monitoring the temperature change of a known mass of a substance (usually water or a solution).

The heat energy change ($Q$) is calculated using the formula: $Q = m \times c \times \Delta T$ where $m$ is the mass of the substance being heated (in grams), $c$ is the specific heat capacity (for water, $4.18 J/g/^\circ C$), and $\Delta T$ is the change in temperature.

For reactions in solution, a polystyrene cup is often used as a simple calorimeter because it is an excellent insulator, minimizing heat loss to the surroundings.

For combustion reactions, a known mass of fuel is burned to heat a specific volume of water in a copper container, allowing the energy released per gram or per mole to be determined.

Unit Consistency: Always ensure that mass ($m$) is in grams and temperature change ($\Delta T$) is in Celsius or Kelvin. Note that a change of $1 ^\circ C$ is identical to a change of $1 K$.

Energy vs. Enthalpy: $Q$ is usually calculated in Joules ($J$). However, molar enthalpy change ($\Delta H$) is almost always reported in kilojoules per mole ($kJ/mol$). Remember to divide $Q$ by $1000$ before calculating $\Delta H$.

The Sign Rule: In exams, you must manually assign the correct sign to $\Delta H$. If the temperature went up, the reaction is exothermic, and you must include a negative sign (e.g., $-57 kJ/mol$).

Mass Selection: In $Q = mc\Delta T$, the mass ($m$) refers to the substance that changed temperature (e.g., the $100g$ of water), NOT the mass of the solid reactant added.

Mass of Reactants: A common error is adding the mass of a solid reactant to the mass of the solution in the $Q$ calculation. Unless specified, usually only the mass of the liquid/solution is used, assuming a density of $1 g/cm^3$.

Heat Loss: Students often forget that experimental values for $\Delta H$ are usually lower than theoretical values because heat is lost to the air or absorbed by the calorimeter itself.

Incomplete Combustion: In fuel-burning experiments, soot formation indicates incomplete combustion, which results in less energy being released than the theoretical maximum.