Enthalpy of Formation: Videos & Practice Problems

Understanding the heat of reaction, or enthalpy change (ΔH), is essential in thermochemistry. There are several methods to calculate ΔH of a reaction: using a bomb calorimeter for constant volume calorimetry, employing stoichiometry with balanced chemical equations, and applying Hess's law with enthalpies of partial reactions. When these methods are not viable, the standard enthalpy of formation (ΔH_f°) is used, where elements in their standard state have a ΔH_f° of zero. The standard enthalpy of reaction (ΔH°_rxn) is determined by the formula:

ΔH°rxn=Σ(ΔHf° products )-Σ(ΔHf° reactants ) signifying the sum of the standard enthalpies of formation of products minus reactants. The units for ΔH_f° are typically kilojoules per mole (kJ/mol), and this equation is pivotal for predicting reaction energetics.

Enthalpy of Formation

In the study of thermodynamics, understanding how to determine the enthalpy change, or heat of reaction (ΔH), is crucial. There are three primary methods to calculate ΔH: constant volume calorimetry, thermochemical equations, and Hess's law. Constant volume calorimetry employs a bomb calorimeter specifically for combustion reactions, allowing for precise measurement of heat changes. Thermochemical equations utilize stoichiometry and balanced chemical equations to derive ΔH, while Hess's law enables the calculation of ΔH for an overall reaction by summing the enthalpies of individual partial reactions.

When these methods are not available, the standard enthalpy of formation can be used to find ΔH. It is important to note that the enthalpy of formation for an element in its standard state is defined as zero. The formula for calculating the standard enthalpy of reaction is expressed as:

\[\Delta H^\circ_{reaction} = \sum \Delta H_f^\circ (products) - \sum \Delta H_f^\circ (reactants)\]

This equation indicates that the standard enthalpy of reaction (ΔH°) is the difference between the total enthalpies of formation of the products and the reactants. The symbol Σ represents the summation of the enthalpies, while n denotes the moles of the substances involved. The standard enthalpy of formation (ΔH_f°) is measured in kilojoules per mole (kJ/mol) or sometimes in joules per mole (J/mol), depending on the units used.

By applying this formula, one can effectively calculate the heat of reaction for various chemical processes, setting the stage for deeper exploration of thermodynamic principles in subsequent examples.

Enthalpy of Formation Example 1

In the reaction of methane with chlorine gas, calculating the standard enthalpy of reaction involves several systematic steps. First, ensure that the chemical equation is balanced. In this case, the equation is already balanced, allowing us to proceed to the next steps.

To find the standard enthalpy of reaction, we start by determining the enthalpy of formation for the products. The relevant substances here are carbon tetrachloride (CCl₄) and hydrochloric acid (HCl). The standard enthalpy of formation values are as follows: CCl₄ has a value of -139 kJ/mol, and HCl has a value of -92.31 kJ/mol. For the balanced reaction, we have 1 mole of CCl₄ and 4 moles of HCl. Thus, the total enthalpy for the products is calculated as:

Products = (1 mol × -139 kJ/mol) + (4 mol × -92.31 kJ/mol) = -139 kJ - 369.24 kJ = -508.24 kJ.

Next, we calculate the enthalpy of formation for the reactants, which include methane (CH₄) and chlorine gas (Cl₂). The standard enthalpy of formation for methane is -74.87 kJ/mol, while chlorine gas, being in its standard state, has an enthalpy of formation of 0 kJ/mol. For the balanced reaction, we have 1 mole of CH₄ and 2 moles of Cl₂. Therefore, the total enthalpy for the reactants is:

Reactants = (1 mol × -74.87 kJ/mol) + (2 mol × 0 kJ/mol) = -74.87 kJ.

Now, we can apply the standard heat of reaction formula:

ΔH_reaction = Total Enthalpy of Products - Total Enthalpy of Reactants.

Substituting the values we calculated:

ΔH_reaction = -508.24 kJ - (-74.87 kJ) = -508.24 kJ + 74.87 kJ = -433.37 kJ.

Thus, the standard enthalpy of reaction for the combustion of methane with chlorine gas is -433.37 kJ. This process illustrates how to utilize the enthalpies of formation to determine the overall energy change in a chemical reaction.

The oxidation of ammonia is illustrated by the following equation:
4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g)
Calculate the enthalpy of reaction,ΔH_Rxn, based on the given standard heats of formation.

• A. -906 kJ
• B. -1273.2 kJ
• C. 1089.6 kJ
• D. -183.6 kJ

Answer: (A)

Consider the following equation:
2 ClF₃(g) + 2 NH₃(g) → 1 N₂(g) + 6 HF (g) + 6 Cl₂(g)ΔH_rxn = –1196 kJ
Determine the standard enthalpy of formation for chlorine trifluoride, ClF₃.

• A. -175.1 kJ
• B. 350.2 kJ
• C. 442.0 kJ
• D. -1638 kJ

Answer: (A)

Here’s what students ask on this topic:

How do you calculate the enthalpy of formation?

To calculate the enthalpy of formation, you need to understand that it is defined as the heat change that results when one mole of a compound is formed from its elements in their standard states. The standard state of an element is its physical state at 1 atm pressure and a specified temperature, typically 25°C.

Here's the general process to calculate the enthalpy of formation:

1. Write the balanced chemical equation for the formation of one mole of the compound from its elements in their standard states.
2. Use Hess's Law, which states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in. This means you can sum the enthalpy changes of individual steps that lead to the overall reaction.
3. If the enthalpies of formation for the reactants and products are known, you can calculate the enthalpy of formation of the compound using the equation:

$∆H_f°=\sum {∆}_{H_f}°\left(\text{products}\right)-\sum {∆}_{H_f}°\left(\text{reactants}\right)$

where ∆Hf° is the standard enthalpy of formation for each substance, and the summation is done over all reactants and products, taking into account their stoichiometric coefficients in the balanced equation.

Remember, the standard enthalpy of formation for a pure

How can one find the enthalpy of formation?

The enthalpy of formation, or standard enthalpy of formation, is the change in enthalpy when one mole of a substance is formed from its elements in their standard states. To find the enthalpy of formation, you can use the following methods:

1. Direct Measurement: In a laboratory, the enthalpy of formation can be determined directly by measuring the heat exchange in a calorimeter during the reaction of elements to form the compound.
2. Thermochemical Tables: More commonly, you can look up the values in standard reference tables. These tables list the standard enthalpy of formation for a wide range of substances at a reference temperature, usually 25°C (298 K).
3. Hess's Law: If the enthalpy of formation cannot be measured directly or found in tables, it can be calculated using Hess's Law. This law states that the total enthalpy change for a reaction is the same, regardless of the number of steps in the reaction. By combining known enthalpies of formation for other reactions, you can calculate the enthalpy of formation for the reaction of interest.
4. Born-Haber Cycle: For ionic compounds, the Born-Haber cycle can be used. This is an application of Hess's Law that uses lattice energy, ionization energy, electron affinity, and other factors.

What is the enthalpy of formation?

The enthalpy of formation, also known as the standard enthalpy of formation, is a thermodynamic quantity that measures the change in enthalpy when one mole of a substance is formed from its elements in their most stable states under standard conditions (usually at a pressure of 1 bar and a temperature of 298.15 K). It is denoted by ${\mathrm{\Delta H}}_f°$.

For example, the enthalpy of formation for carbon dioxide (CO₂) involves the reaction of carbon in its standard state (graphite) and oxygen gas (O₂) to form one mole of CO₂. The reaction is as follows:

$C\left(\text{graphite}\right)+O_2\left(g\right)\to {\mathrm{CO}}_2\left(g\right)$

The enthalpy change for this reaction would represent the enthalpy of formation for carbon dioxide. If the enthalpy of formation is negative, it means that the formation of the compound from its elements releases energy, indicating an exothermic reaction. Conversely, a positive value indicates that energy is absorbed, and the reaction is endothermic. Enthalpy of formation values are fundamental for calculating the enthalpies of other chemical reactions using Hess's law.

When is the enthalpy of formation zero?

The enthalpy of formation is defined as the heat change that occurs when one mole of a compound is formed from its elements in their standard states. The standard state of an element is its most stable form at 1 atmosphere of pressure and a specified temperature, typically 298.15 K (25°C).

The enthalpy of formation is zero for all elements in their standard states because there is no heat change involved in forming a substance from itself. For example, the enthalpy of formation for elemental oxygen ($O_2$), nitrogen ($N_2$), and carbon (graphite) is zero because these are the forms in which these elements exist naturally under standard conditions. In other words, no energy is required to form an element from itself, so the enthalpy change for this process is zero by definition. This serves as a reference point for measuring the enthalpy changes in the formation of compounds from these elements.