Thermochemical Equations: Videos & Practice Problems

Stoichiometry involves the numerical relationships in balanced chemical equations, while thermochemical equations incorporate the enthalpy of reaction (ΔH_rxn). In these equations, the focus shifts from mole-to-mole comparisons to ΔH-to-mole comparisons. By using a thermochemical stoichiometric chart, one can convert given ΔH to moles of a compound, then make a mole-to-mole comparison using coefficients from the balanced equation to find unknown quantities. This process highlights the connection between enthalpy and various forms of measurement, such as grams or molecules, essential for understanding thermochemistry.

Thermochemical Equations

Stoichiometry is essential for understanding the numerical relationships between compounds in a balanced chemical equation. When we introduce thermochemical equations, we focus on chemical reactions that involve the enthalpy of reaction, denoted as ΔH_rxn. This value represents the heat change associated with the reaction.

In thermochemical equations, we utilize a thermochemical stoichiometric chart, which helps us relate the given quantity of one compound to the unknown quantity of another. The chart is paired with a balanced chemical equation, where the ΔH_rxn is indicated alongside the reaction. The primary goal is to connect the enthalpy of reaction with various quantities such as moles, grams, or molecules, depending on the context of the chemical reaction.

It is crucial to note that in thermochemical equations, we are not merely comparing moles of reactants and products; instead, we are establishing a relationship between ΔH and the number of moles involved. This distinction is vital for accurately interpreting the energy changes that occur during chemical reactions.

Thermochemical Equations

Understanding thermochemical equations is essential in the study of stoichiometry, particularly when dealing with energy changes in chemical reactions. A key component of these equations is the enthalpy change, denoted as ΔH, which indicates the heat absorbed or released during a reaction. When analyzing a thermochemical equation, the first step is to identify the given ΔH value associated with the reaction.

From the ΔH of the reaction, you can convert this value into moles of a specific reactant or product. This conversion can also be made from grams of the substance, or even from the number of ions, atoms, formula units, or molecules. The transition from known quantities to unknown quantities is often referred to as "the jump." This jump requires careful consideration of the mole-to-mole relationships defined by the coefficients in the balanced chemical equation.

Once you have determined the moles of the unknown substance, you can further convert this information into various forms, such as ions, atoms, formula units, or molecules, or even back to grams. Additionally, you can calculate a new ΔH for the unknown substance if needed. It is crucial to remember that a thermochemical equation not only includes a balanced chemical equation but also incorporates the ΔH of the reaction, linking stoichiometry with thermodynamics.

Thermochemical Equations Example 1

In the context of thermochemical reactions, consider the reaction where 2 moles of solid magnesium react with 1 mole of oxygen gas to produce 2 moles of solid magnesium oxide, with an enthalpy change (ΔH) of -1204 kilojoules. This negative value indicates that the reaction is exothermic, meaning it releases energy.

To determine how many grams of magnesium oxide are produced when the enthalpy change is -375 kilojoules, we first need to establish a relationship between the energy change and the amount of magnesium oxide produced. Using the balanced equation, we can perform a mole-to-mole comparison. The enthalpy change of -1204 kilojoules corresponds to the formation of 2 moles of magnesium oxide. Therefore, we can set up a proportion to find the moles of magnesium oxide produced for -375 kilojoules:

For every 2 moles of magnesium oxide, the energy involved is -1204 kilojoules. Thus, we can express this relationship as:

\[\frac{2 \text{ moles MgO}}{-1204 \text{ kJ}} = \frac{x \text{ moles MgO}}{-375 \text{ kJ}}\]

Solving for \(x\) gives us the moles of magnesium oxide produced:

\[x = \frac{2 \times -375}{-1204} \approx 0.622 \text{ moles MgO}\]

Next, we need to convert moles of magnesium oxide into grams. The molar mass of magnesium oxide (MgO) can be calculated as follows: magnesium (Mg) has a molar mass of approximately 24.31 grams per mole, and oxygen (O) has a molar mass of 16 grams per mole. Therefore, the molar mass of magnesium oxide is:

\[\text{Molar mass of MgO} = 24.31 \text{ g/mol} + 16 \text{ g/mol} = 40.31 \text{ g/mol}\]

Now, we can convert the moles of magnesium oxide to grams:

\[\text{Mass of MgO} = 0.622 \text{ moles} \times 40.31 \text{ g/mol} \approx 25.11 \text{ grams MgO}\]

Thus, when -375 kilojoules of energy is released, approximately 25.11 grams of magnesium oxide are produced. This calculation illustrates the relationship between energy changes in a thermochemical reaction and the quantities of products formed.

Nitromethane (CH₃NO₂), sometimes used as a fuel for drag racing, burns according to the following reaction: 


4 CH₃NO₂ (l) + 7 O₂ (g) → 4 CO₂ (g) + 6 H₂O (g) + 4 NO₂ (g)      ∆Hº = – 2441.6 kJ


How much heat is released by burning 125.0 g of nitromethane (MW:61.044 g/mol)? 

• A. 1.250 x 10³ kJ
• B. -1.250 x 10³ kJ
• C. 2.440 x 10³ kJ
• D. -2.440 x 10³ kJ

Answer: (B)

Consider the following reaction: 
2 C₆H₆ (l) + 15 O₂ (g) → 12 CO₂ (g) + 6 H₂­O (g)  ∆Hº = – 6278 kJ
What volume of benzene (C₆H₆, d = 0.880 g/mL, molar mass = 78.11 g/mol) is necessary to evolve 5.19 x 10⁹ kJ of heat? 

• A. 1.47 x 10⁸ mL
• B. 5.19 x 10⁹ mL
• C. 4.37 x 10⁷ mL
• D. 9.51 x 10⁹ mL

Answer: (A)

The creation of liquid methanol is accomplished by the hydrogenation of carbon monoxide: 
CO (g) + 2 H₂ (g) → CH₃OH (l)       ∆Hº = – 128.1 kJ
How much heat (in kJ) is released when 125.0 g CO reacts with 2.32 x 10² g H₂?

• A. 275 kJ
• B. -572 kJ
• C. -128 kJ
• D. 281 kJ

Answer: (B)

Here’s what students ask on this topic:

What is a thermochemical equation?

A thermochemical equation is a balanced chemical equation that includes the enthalpy change (ΔH) of the reaction. This ΔH value represents the heat absorbed or released during the reaction. Unlike standard stoichiometric equations that focus on mole-to-mole comparisons, thermochemical equations emphasize the relationship between ΔH and the moles of reactants or products. For example, in the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -890 kJ, the ΔH value indicates that 890 kJ of energy is released per mole of CH₄ combusted.

How do you use a thermochemical stoichiometric chart?

A thermochemical stoichiometric chart helps convert given quantities (like ΔH, grams, or molecules) to moles and then to other desired quantities. Start with the given ΔH and convert it to moles of the given substance. From there, use the balanced chemical equation to perform a mole-to-mole comparison to find the moles of the unknown substance. Finally, convert the moles of the unknown to the desired unit (grams, molecules, etc.). This process ensures accurate calculations involving enthalpy changes in chemical reactions.

What is the difference between a thermochemical equation and a regular stoichiometric equation?

The main difference between a thermochemical equation and a regular stoichiometric equation is the inclusion of the enthalpy change (ΔH) in thermochemical equations. While regular stoichiometric equations focus on the mole-to-mole relationships between reactants and products, thermochemical equations emphasize the relationship between ΔH and the moles of substances involved. This allows for the calculation of energy changes during chemical reactions, which is crucial for understanding thermochemistry.

How do you calculate the enthalpy change (ΔH) for a reaction using a thermochemical equation?

To calculate the enthalpy change (ΔH) for a reaction using a thermochemical equation, follow these steps: 1) Write the balanced chemical equation with the ΔH value. 2) Identify the moles of reactants and products involved. 3) Use the stoichiometric coefficients to relate the moles of substances to the given ΔH. For example, if the equation is CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -890 kJ, and you have 2 moles of CH₄, the total ΔH would be 2 × -890 kJ = -1780 kJ.

Why is it important to include ΔH in a thermochemical equation?

Including ΔH in a thermochemical equation is important because it provides information about the energy changes during a chemical reaction. This helps in understanding whether the reaction is exothermic (releases heat) or endothermic (absorbs heat). Knowing the ΔH value is crucial for applications in energy management, industrial processes, and predicting reaction behavior. It also allows for accurate calculations involving the conversion of energy to moles, grams, or other units, which is essential for practical and theoretical chemistry.