Calorimetry equation (calorimetry formula)

Definition: Calorimetry is the part of chemistry which study the quantity of heat that is absorbed or released with the surroding during a chemical reaction. It is measured using a equipment called calorimetric that has a thermometer and measure a variable called heat of combustions ΔHcombustion.

General formula: In calorimetry, the value of the heat of combustion is constant for each substance (it is commonly known as heat of combustion standard) and it is measured with the calorimetry (as discussed above) and this value corresponds to the heat released during the combustion of 1 mol of certain substance. Thus, if it is needed to calculate the heat of combustion of 2 g of a substance, it should be apply the equation:

ΔH_combustion = ΔH_combustion^° (KJ/mol)* mass of the compound (g) * Molecular weight (g/mol)

A second important equation in calorimetry is the heat of reaction equation, which corresponds to the heat released or produced during a chemical reaction:

ΔH_reaction = (heat capacity of the calorimeter) * ΔT where, the heat capacity of the calorimeter is an experimental variable.

Use: The heat of combustion (ΔH_combustion) is used for determining the enthalpies of formation of compounds and thus, it can be determined if a reaction will result in the formation of the desired product or not. Before the chemists try experimentally the synthesis of new compounds, they calculate theoretically the enthalpy of formation using the heat of combustion of all the reactants involved in the reaction.

Example: 1 g of glucose is submitted to combustion in a calorimetry and the temperature increase from 25 ºC to 27 ºC. If the heat of combustion standard is 3,85 kJ/mol, what is the heat of combustion for this amount of glucose?

First step, calculate the molecular weight of glucose (C₆H₁₂O₆), which is 180 g/mol.

Second step, apply the next equation:

ΔH_combustion = ΔH_combustion^° (Kj/mol)*mass of the compound (g) / Molecular weight (g/mol)

ΔH_combustion = (3,85 kJ/mol*1 g)/ 180 g/mol = 0.0213 kJ

Considerations: In calorimetry, the positive values correspond to energy that was absorbed by the system for performing a chemical reaction and these type of reactions are called endothermic reactions. On the other hand, if the system releases energy, the enthalpy of reaction is negative and the reactions are called exothermic reactions.