Question

Estimate the enthalpy change for this reaction. Start by drawing the Lewis electron dot diagrams for each substance. \(4 \mathrm{NH}_{3}+3 \mathrm{O}_{2} \rightarrow 2 \mathrm{~N}_{2}+6 \mathrm{H}_{2} \mathrm{O}\)

Short answer

Subtract the total bond energy of products from reactants to estimate enthalpy change.

Step-by-step solution

Draw Lewis Structures

First, draw the Lewis structures for each molecule involved in the reaction. For \( \text{NH}_3 \), draw a nitrogen atom with three hydrogen atoms and a lone pair of electrons on the nitrogen. For \( \text{O}_2 \), draw two oxygen atoms with a double bond between them. \( \text{N}_2 \) has a triple bond between the two nitrogen atoms. For \( \text{H}_2\text{O} \), draw an oxygen atom with two hydrogen atoms and two lone pairs of electrons on the oxygen.

Determine Bond Energies

List the bond energies for each type of bond involved. Typical approximate bond energies are: N-H (391 kJ/mol), O=O (498 kJ/mol), N≡N (945 kJ/mol), O-H (463 kJ/mol).

Calculate Total Energy of Reactants

Calculate the total energy required to break all bonds in the reactants. For 4 molecules of \( \text{NH}_3 \), we have 12 N-H bonds. Total energy for N-H bonds is \( 12 \times 391 \). For 3 molecules of \( \text{O}_2 \), we have 3 O=O bonds, which equals \( 3 \times 498 \). Sum these energies to find the total energy for breaking the reactant bonds.

Calculate Total Energy of Products

Calculate the energy released when forming the bonds in the products. For 2 molecules of \( \text{N}_2 \), there are 2 N≡N bonds: \( 2 \times 945 \). For 6 molecules of \( \text{H}_2\text{O} \), there are 12 O-H bonds: \( 12 \times 463 \). Sum these values to find the total energy released by forming the product bonds.

Calculate Enthalpy Change

To find the enthalpy change, subtract the total bond energy of the products from that of the reactants. This will be given by the equation: \( \Delta H = \text{Energy of Reactants} - \text{Energy of Products} \). Evaluate this expression to find the estimated change in enthalpy for the reaction.

Key concepts

Lewis electron dot diagrams

Understanding Lewis electron dot diagrams is essential when exploring the estimation of enthalpy change in chemical reactions. These diagrams are a simple representation of molecules showing how atoms bond and exist together. Generally, they display the valence electrons around the atoms within a molecule. For example, ammonia (\( \text{NH}_3 \)) involves drawing nitrogen in the center with three hydrogen atoms bonded around it, and a lone pair of electrons on nitrogen.
Oxygen gas (\( \text{O}_2 \)) is represented by two oxygen atoms connected by a double bond. Nitrogen gas (\( \text{N}_2 \)) is shown with a triple bond between two nitrogen atoms. Finally, for water (\( \text{H}_2\text{O} \)), the oxygen atom is centered with two hydrogen atoms bonded to it, alongside two lone pairs of electrons on oxygen.

• Lewis structures help visualize the structure of reactants and products.
• They assist in identifying the bonds that break and form during chemical reactions.
• Each bond and lone pair is linked to the enthalpy changes in the reaction.

Through these representations, we understand which bonds are involved and how they contribute to the overall energy calculations needed to estimate enthalpy change.

bond energies

Bond energies provide invaluable insight into the energy involved in breaking and forming chemical bonds during reactions. These energies are average values representing the strength of particular bonds in a molecule and differ from bond to bond. For example, the bond energy for N-H is typically around 391 kJ/mol. Oxygen double bonds (O=O) are stronger with approximately 498 kJ/mol, while the triple bond in nitrogen (N≡N) has even higher bond energy at 945 kJ/mol.
Meanwhile, the bond energy for O-H bonds is around 463 kJ/mol. These values illustrate that stronger bonds require more energy to break, while forming them releases the same amount.

• Bond energies help calculate the total energy required to break bonds in reactants.
• They also determine the energy released when bonds form in products.
• Accurate bond energy data is crucial for precise enthalpy change estimates.

By understanding bond energies, we can perform essential calculations to determine how energy changes during the reaction, aiding the calculation of the overall enthalpy change.

chemical reactions

Chemical reactions involve transforming reactants into products, resulting in breaking and forming chemical bonds. Understanding the nature of these transformations is key when estimating enthalpy change. This involves calculating the energy required to break bonds in the reactants and the energy released when forming new bonds in the products.
In our example reaction of ammonia with oxygen producing nitrogen gas and water, bonds in ammonia and oxygen need to be broken. Then, the formation of new bonds in nitrogen gas and water releases energy.

• Enthalpy change estimates involve a balance between breaking and forming bonds.
• The sum of energy required for bond-breaking provides a figure for reactants.
• Energy released by new bond formation gives us product energy values.

The key idea is that if the energy released from product formation exceeds the energy required to break reactant bonds, the reaction is exothermic, resulting in a negative enthalpy change. Conversely, if more energy is needed to break bonds than is released, the reaction is endothermic with a positive enthalpy change.