Question

What two conditions must be met by a thermochemical equation so that its standard enthalpy change can be given the symbol \(\Delta H_{\mathrm{f}}^{\circ}\) ?

Short answer

The two conditions are: 1) The equation must describe the formation of exactly one mole of a single product, and 2) The reactants must be the elements in their standard states.

Step-by-step solution

Understanding Standard Enthalpy of Formation

Standard enthalpy of formation, denoted as \(\Delta H_{\mathrm{f}}^{\circ}\), is defined as the heat change that results when one mole of a compound is formed from its elements in their standard states.

Identifying the Required Conditions

In order for a thermochemical equation to represent the standard enthalpy of formation, it must satisfy two conditions. First, the equation must describe the formation of exactly one mole of a single product. Second, the reactants must be the elements in their standard states.

Key concepts

Thermochemical Equations

In chemistry, thermochemical equations are representations of chemical reactions that also quantify the energy changes, specifically heat, involved. These equations are critical for predicting how much energy is released or absorbed during chemical processes. A unique feature of thermochemical equations is that they not only specify the substances involved but also integrate the heat change as a part of the equation.

For instance, when we write the thermochemical equation for the formation of water, we denote not just the reaction of hydrogen and oxygen to form water but also the associated heat change, which is released in the process. The equation would include the enthalpy change symbol, \( \Delta H \), which reflects this heat change and is measured in joules or kilocalories.

It's important for students to remember that the coefficients in thermochemical equations reflect not just molar amounts but also molar enthalpies. When interpreting these equations, the physical states of reactants and products (solid, liquid, gas, or aqueous) are indicated because they affect the enthalpy change. A proper understanding of these factors helps in accurately calculating energy requirements or releases in chemical reactions.

Standard States

The term standard states refers to a set of specific physical conditions chosen as a reference point to report thermodynamic properties, like enthalpy (\( H \)). The standard states for a substance are typically defined at a temperature of 298.15 K (25°C) and a pressure of 1 atmosphere (atm). For pure substances, the standard state is the most stable physical form of the substance at these conditions.

For example, the standard state of carbon is graphite, not diamond, because graphite is more stable at 298.15 K and 1 atm. For a gas, the standard state is the hypothetical ideal gas behavior. Solutions have standard states, too, often defined for a concentration of exactly 1 molar (1 M).

It is essential to understand standard states because they provide a uniform basis for comparing different materials and their thermochemical properties. In the context of enthalpy of formation, compounds are formed from their elements in their standard states, providing a basis for tabulating and comparing such values across different substances.

Heat Change in Reactions

The concept of heat change in reactions is pivotal in understanding thermochemistry. Every chemical reaction involves a transfer of energy, primarily in the form of heat, between the system and surroundings. This transfer is measured as the enthalpy change (\( \Delta H \)) and can indicate whether a reaction is exothermic (releasing heat) or endothermic (absorbing heat).

The signs of \( \Delta H \), negative for exothermic and positive for endothermic reactions, are essential indicators of the reaction's energy dynamics. When a reaction occurs, the heat change results from the breaking and forming of chemical bonds, as bond breakage requires energy (endothermic) and bond formation releases energy (exothermic).

Enthalpy changes for reactions can be measured experimentally or estimated using standard enthalpies of formation, which list the heat change when one mole of a substance is formed from its elements in their standard states. Understanding these concepts is crucial for estimating the overall energy changes in chemical manufacturing processes, and designing reactions that are energetically favorable or tailored for specific heat requirements.