Thermochemical Stoichiometry and Thermochemical Equations: Study Notes

Thermochemical Stoichiometry

Introduction to Thermochemical Equations

Thermochemical equations are chemical equations that include the enthalpy change (ΔH) of a reaction. These equations allow chemists to relate the quantities of reactants and products to the energy changes that occur during chemical reactions.

• Stoichiometry deals with the numerical relationship between compounds in a balanced chemical equation.

• Thermochemical Equations show both the reactants/products and the enthalpy change associated with the reaction.

• ΔH (enthalpy change) is typically given in kJ and indicates whether the reaction is exothermic (releases heat, negative ΔH) or endothermic (absorbs heat, positive ΔH).

Thermochemical Stoichiometric Chart

The chart below outlines the steps for converting a given quantity of a compound to the unknown quantity of another compound using a thermochemical equation.

• Step 1: Convert the given quantity into moles of the given substance.

• Step 2: Use the coefficients in the balanced equation to perform a mole-to-mole comparison between the given and unknown substances.

• Step 3: Convert the moles of the unknown substance into the desired final units (grams, molecules, atoms, etc.).

Example Thermochemical Equation:

• $2\ \text{Mg} (s) + \text{O}_2 (g) \rightarrow 2\ \text{MgO} (s)\ \Delta H = -1204\ \text{kJ}$

This equation shows that when 2 moles of magnesium react with 1 mole of oxygen, 2 moles of magnesium oxide are produced and 1204 kJ of heat is released.

Stepwise Approach to Thermochemical Calculations

• STEP 1: Convert the given quantity into moles of the given substance using its molar mass.

• STEP 2: Use the mole ratio from the balanced equation to find moles of the unknown substance.

• STEP 3: Convert moles of the unknown substance into the desired units (grams, volume, energy, etc.).

Practice Problems in Thermochemical Stoichiometry

Example 1: Nitromethane Combustion

Reaction:

• $4\ \text{CH}_3\text{NO}_2 (l) + 7\ \text{O}_2 (g) \rightarrow 4\ \text{CO}_2 (g) + 6\ \text{H}_2\text{O} (g) + 4\ \text{NO}_2 (g)\ \Delta H = -2441.6\ \text{kJ}$

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

• Key Steps:

  1. Convert grams of nitromethane to moles: $\text{moles} = \frac{125.0\ \text{g}}{61.044\ \text{g/mol}}$

  2. Use the stoichiometric ratio from the equation to relate moles of nitromethane to kJ released.

  3. Calculate total heat released using $\Delta H$.

Example 2: Benzene Combustion

Reaction:

• $2\ \text{C}_6\text{H}_6 (l) + 15\ \text{O}_2 (g) \rightarrow 12\ \text{CO}_2 (g) + 6\ \text{H}_2\text{O} (g)\ \Delta H = -6278\ \text{kJ}$

Practice Question: What volume of benzene (C6H6, d = 0.880 g/mL, molar mass = 78.11 g/mol) is necessary to evolve 5.19 × 103 kJ of heat?

• Key Steps:

  1. Determine moles of benzene needed to produce the specified heat using $\Delta H$.

  2. Convert moles to grams using molar mass.

  3. Convert grams to volume using density.

Example 3: Methanol Synthesis

Reaction:

• $\text{CO} (g) + 2\ \text{H}_2 (g) \rightarrow \text{CH}_3\text{OH} (l)\ \Delta H = -128.1\ \text{kJ}$

Practice Question: How much heat (in kJ) is released when 125.0 g CO reacts with 2.32 × 102 g H2?

• Key Steps:

  1. Convert grams of CO and H2 to moles.

  2. Identify the limiting reactant.

  3. Use the stoichiometric ratio and $\Delta H$ to calculate heat released.

Summary Table: Thermochemical Stoichiometry Steps

Step	Description	Formula/Equation
1	Convert given quantity to moles	$\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$
2	Mole-to-mole comparison using coefficients	Use coefficients from balanced equation
3	Convert moles of unknown to desired units	Use molar mass, density, or other conversion factors
4	Calculate energy change	Multiply moles by $\Delta H$ per mole

Key Terms and Definitions

• Stoichiometry: The calculation of reactants and products in chemical reactions.

• Thermochemical Equation: A balanced chemical equation that includes the enthalpy change ($\Delta H$).

• Enthalpy Change ($\Delta H$): The heat change at constant pressure during a chemical reaction.

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

Applications

• Thermochemical stoichiometry is essential in calculating energy changes in industrial chemical processes, combustion reactions, and synthesis reactions.

• Understanding these calculations helps in designing energy-efficient chemical processes and in predicting the environmental impact of chemical reactions.