Welcome to Chemical Energetics! This chapter studies energy changes during chemical reactions. The key concept is enthalpy change, represented by delta H, which measures heat energy change at constant pressure. There are two main types: exothermic reactions that release heat with negative delta H, and endothermic reactions that absorb heat with positive delta H.
Standard enthalpy changes are measured under specific standard conditions: 298 Kelvin or 25 degrees Celsius temperature, and 1 atmosphere or 101 kilopascals pressure. There are several important types: delta H formation measures enthalpy change when one mole of compound forms from its elements. Delta H combustion is for complete combustion reactions. Delta H neutralisation is for acid-base reactions. And delta H atomisation is for forming gaseous atoms from elements.
Hess's Law is a fundamental principle stating that the total enthalpy change for a reaction is independent of the route taken. This means whether a reaction occurs in one step or multiple steps, the overall energy change remains the same. We can use this law to calculate unknown enthalpy changes by constructing enthalpy cycles, often using standard enthalpy of formation or combustion data. The mathematical expression shows that delta H for the direct route equals the sum of delta H values for the indirect route.