The symbol \(\Delta H\) represents the change in enthalpy, a fundamental concept in the study of energy transfer in chemical and physical processes. Enthalpy, symbolized by \(H\), is a thermodynamic property that measures the total heat content of a system. The Greek letter delta (\(\Delta\)) signifies “change in,” so \(\Delta H\) refers to the difference in heat content between the final and initial states. This concept is relevant in chemistry because most reactions occur under constant atmospheric pressure, where \(\Delta H\) directly measures the heat exchanged.
What Enthalpy Represents
Enthalpy (\(H\)) is a state function, meaning its value depends only on the current state of the system (pressure, temperature, and composition), not on the path taken to reach that state. Mathematically, it is defined as the sum of a system’s internal energy (\(E\)) and the energy required to displace its environment, represented by the product of pressure (\(P\)) and volume (\(V\)). The relationship is \(H = E + PV\). Internal energy includes all energy stored within the system, such as the kinetic energy of molecules and the potential energy in chemical bonds.
The \(PV\) term, known as pressure-volume work, accounts for the energy associated with the system’s volume under pressure. Enthalpy is practical for chemists because most reactions occur in open containers at constant atmospheric pressure. Under this condition, the change in enthalpy (\(\Delta H\)) simplifies to the heat transferred into or out of the system. This makes enthalpy a convenient measure for tracking energy changes in chemical reactions.
The Significance of the Change in Enthalpy
The change in enthalpy, \(\Delta H\), quantifies the heat flow during a process occurring at constant pressure. It is calculated as the difference between the final enthalpy of the products and the initial enthalpy of the reactants (\(\Delta H = H_{\text{products}} – H_{\text{reactants}}\)). This value represents the exact amount of heat energy absorbed or released by the chemical system during the transformation.
Scientists report \(\Delta H\) values in units of energy per amount of substance, typically kilojoules per mole (kJ/mol). Since enthalpy is a state function, the path taken from reactants to products does not influence the final \(\Delta H\) value. The absolute enthalpy (\(H\)) of a substance cannot be directly measured; only the change (\(\Delta H\)) can be determined experimentally. Therefore, \(\Delta H\) is the practical term used to describe the heat effects of reactions.
Using Delta H to Classify Chemical Reactions
The sign of the \(\Delta H\) value classifies a chemical reaction by indicating the direction of heat transfer.
A negative \(\Delta H\) indicates an exothermic reaction, meaning the system releases heat energy to the surroundings. In these processes, the products have a lower total enthalpy than the reactants, resulting in a net loss of energy from the system. This energy release is felt as a temperature increase in the surroundings. A common example is combustion, such as the burning of natural gas, which releases significant heat energy.
Conversely, a positive \(\Delta H\) signifies an endothermic reaction, where the system absorbs heat energy from the surroundings. The products possess a higher total enthalpy than the reactants, and the system gains energy. The surrounding environment experiences a temperature drop as its heat is drawn into the reacting system. Examples include the melting of ice or the chemical reactions utilized in instant cold packs.
How Enthalpy Changes Are Measured
Enthalpy changes are most commonly measured experimentally using calorimetry, the science of measuring heat flow. This method involves monitoring the temperature change in a controlled environment, often a device called a calorimeter, as a reaction takes place. By knowing the mass, specific heat capacity, and temperature change of the surrounding medium, the heat absorbed or released by the reaction (\(\Delta H\)) can be calculated.
For reactions difficult to measure directly, scientists rely on tabulated values and Hess’s Law. This law states that the total enthalpy change for a reaction is the same whether it occurs in one step or a series of steps. This allows the \(\Delta H\) of a complex reaction to be calculated by adding the known enthalpy changes of simpler reactions that sum up to the overall process. Standard Enthalpy Change (\(\Delta H^\circ\)) is the value reported in databases, measured under defined standard conditions, often 25°C and 1 atmosphere of pressure.