How is the enthalpy change for a reaction measured experimentally?

The enthalpy change for a reaction is measured experimentally using calorimetry, specifically a calorimeter.

Calorimetry is a scientific method used to measure the heat exchanged in a chemical reaction. The device used for this purpose is called a calorimeter. There are different types of calorimeters, but the simplest one is the coffee cup calorimeter, which is essentially a polystyrene (styrofoam) cup with a lid. The cup is filled with a known amount of a substance, and the reaction is initiated within the cup. The heat produced or absorbed by the reaction changes the temperature of the substance in the cup, which can be measured with a thermometer.

The enthalpy change, ΔH, for the reaction is then calculated using the formula ΔH = -q, where q is the heat exchanged. The heat exchanged can be calculated using the formula q = mcΔT, where m is the mass of the substance in the cup, c is the specific heat capacity of the substance, and ΔT is the change in temperature. The negative sign in the formula for ΔH indicates that heat is released in an exothermic reaction (ΔH is negative) and absorbed in an endothermic reaction (ΔH is positive).

In a more sophisticated type of calorimeter, known as a bomb calorimeter, the reaction takes place in a sealed metal container, which is immersed in a known volume of water. The heat produced by the reaction increases the temperature of the water, and the enthalpy change for the reaction is calculated from the temperature change of the water.

It's important to note that calorimetry measures the enthalpy change under constant pressure conditions. Therefore, the enthalpy change measured by calorimetry is the change in enthalpy for the reaction at constant pressure, which is the definition of the enthalpy change for a reaction.

In summary, the enthalpy change for a reaction is measured experimentally by initiating the reaction in a calorimeter and measuring the resulting temperature change. The enthalpy change is then calculated from the temperature change using the formula ΔH = -q.