Chemical reactions represent a fundamental process of energy exchange. The study of these energy relationships is known as thermochemistry, a specialized branch of thermodynamics. All chemical transformations involve a change in energy as bonds are rearranged to form new substances. This energy transfer occurs as heat, which can either be released into or absorbed from the surroundings. Understanding the direction and magnitude of this heat flow is a central goal of chemical science.
Defining Enthalpy and Enthalpy Change
Enthalpy, symbolized by the letter \(H\), represents the total heat content of a system measured at a constant pressure. This thermodynamic property includes the system’s internal energy plus the energy required to displace its surroundings. Because most chemical reactions occur under the constant pressure of the atmosphere, enthalpy is a highly useful measure for tracking energy changes.
Enthalpy is considered a state function, meaning its value depends only on the current state of the system (such as its temperature, pressure, and composition), not on the path taken to reach that state. The change in enthalpy, designated as \(\Delta H\), is simply the difference between the final heat content of the system and its initial heat content. This change, at constant pressure, is equal to the heat that is either absorbed or released during the process.
The Standard Convention: Products Minus Reactants
Calculating the change in heat content for a reaction is determined by a universal scientific convention. The change in any property in science is always calculated by taking the value of the final state and subtracting the initial state. In a chemical reaction, the final state is represented by the products, and the initial state is represented by the reactants.
Therefore, the change in enthalpy is calculated by subtracting the heat content of the reactants from the heat content of the products. This is expressed mathematically as \(\Delta H = H_{\text{products}} – H_{\text{reactants}}\). This final-minus-initial approach ensures that the resulting sign of the calculated value accurately reflects the direction of the energy flow.
Interpreting the Result: Endothermic and Exothermic Processes
The sign of the calculated \(\Delta H\) value communicates whether a chemical process releases heat or absorbs heat. A negative \(\Delta H\) value signifies an exothermic process, where the final heat content of the products is less than the initial heat content of the reactants. This energy difference is released from the reacting system into its surroundings, often causing the surroundings to feel warmer.
Conversely, a positive \(\Delta H\) value indicates an endothermic process, meaning the final heat content of the products is greater than that of the reactants. For the system to gain this energy, it must absorb heat from its surroundings, which often results in the surroundings feeling cooler. The system’s perspective governs the sign: losing energy means a negative sign, and gaining energy means a positive sign.
Practical Application: Calculating Enthalpy of Reaction
In practice, determining the heat content of every substance is impossible, so scientists rely on a related value called the standard enthalpy of formation, denoted as \(\Delta H_f^\circ\). This tabulated value is the heat change that occurs when one mole of a compound is formed from its constituent elements in their most stable form under standard conditions. Standard conditions are typically defined as a pressure of 1 bar and a specific temperature, usually 25°C.
To find the standard heat change for an entire reaction, or \(\Delta H^{\circ}_{\text{rxn}}\), chemists use the products-minus-reactants rule with these tabulated formation values. The calculation involves summing the standard enthalpies of formation for all products and subtracting the sum of the standard enthalpies of formation for all reactants. This method is a direct application of the principle that heat content is a state function.
The full formula for this calculation is \(\Delta H^{\circ}_{\text{rxn}} = \Sigma n \Delta H^{\circ}_{f} (\text{products}) – \Sigma m \Delta H^{\circ}_{f} (\text{reactants})\). The coefficients \(n\) and \(m\) are the stoichiometric coefficients from the balanced chemical equation. These coefficients are used to multiply each substance’s formation value to account for the number of moles involved.
A key simplification is that elements in their standard state, such as oxygen gas (\(\text{O}_2\)) or solid carbon (graphite), have a \(\Delta H_f^\circ\) value of zero by definition. This standard methodology allows for the calculation of the energy change for virtually any chemical reaction, provided the formation data are available.