Enthalpy of Formation - Online Tutor, Practice Problems & Exam Prep

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

So at this point we have identified three ways to determine the enthalpy or heat of reaction, otherwise known as ΔHreaction. Here we have constant volume calorimetry which uses a bomb calorimeter to find the enthalpy of reaction or heat of reaction of combustion reactions. Then we have thermochemical equations which uses stoichiometry and a balanced equation to find ΔHreaction. Then finally Hess's law. If you haven't seen it yet, it uses the enthalpy of partial reactions to find ΔHreaction for the overall reaction.

Now if the first three ways are unavailable, then we use the standard enthalpy of formation for substances to find the ΔHreaction we're going to say. Recall that an element in its standard state is given an enthalpy of formation of 0. So here we can use our standard heat of reaction formula to help us find ΔHreaction. Here it's ΔHreaction in its standard state. That's what this little 0 means. Here equals the summation for the entropies of formation of products minus the summation of the enthalpies of formation for reactants.

Or broken down in its simplest way, ΔHreaction equals products minus reactants O. Here ΔHreaction with a little circle on top. This is the standard enthalpy or reaction heat of reaction in kilojoules. This Sigma sign just means a summation or sum up and we'll see how that works. Here N equals the moles of your substance. And then here we're going to say that ΔHf with the little circle equals the standard enthalpy or heat of formation for substance in kilojoules over moles or sometimes joules over moles, depending on what units they are. It's always going to be kilojoules or joules over moles O.

Now that we've seen this standard heat of form of reaction formula, we're going to see how it works and use it in the next video. So click on the next video and let's take a look at an example question.

Enthalpy of Formation Example 1

Here in this example question it says the reaction of methane with chlorine gas is illustrated by the reaction below. Here we need to calculate the enthalpy of reaction, the standard enthalpy of reaction. If the standard enthalpies of formation for methane, carbon tetrachloride and hydrochloric acid are negative 74.871, negative 139 and -92.31 kilojoules per mole respectively.

All right, so step zeros. We check to see if the chemical reaction is balanced and if not then do the necessary steps to balance it. Here we already have the equation balance, so that's great. That means we can move on to step one. Step one says, starting with the products, multiply the coefficients so the numbers in red of each substance with their enthalpy of formation value. So here we're going to say that we have one mole of carbon texture chloride and four moles of HCL. We said here that carbon tetrachloride has a value associated with -139 kilojoules per mole, and hydrochloric acid has -92.31. Here the moles will cancel out and we'll have kilojoules left. So this is -925.24 kilojoules.

Next we're going to go to the reactants. Also multiply their coefficients of each substance with their enthalpies of formation. Now realize here that for CL₂, I didn't give us an enthalpy of formation because earlier we said that if an element is in its standard or natural state, its enthalpy of formation is equal to 0. CL₂ gas is the natural form of chlorine, so it has an enthalpy of 0. Methane is -74.87 and we have one mole, and here we have formals. Moles cancel out and we'll be left with kilojoules. 4 * 0 is 0, so that doesn't matter. So this is just simply negative 74.87 kilojoules.

Now that we have the amount for products and reactants, we go to step three. We take both totals and place them into the standard heat of reaction formula to determine the standard enthalpy of reaction. So products was -925.24 kilojoules and reactants was -74.87 kilojoules. Delta H standard on enthalpy of reaction equals products minus reactants. When we plug that in, that gives us -850.37 kilojoules as our answer.

So yet this is another way that we're able to calculate the standard enthalpy of reaction. In this case, we did it through the use of the enthalpies of formation for each of the substances within the 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

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

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.