Bond Energy: Videos & Practice Problems

Bond energy, or bond enthalpy (ΔH_be), is the energy stored in a bond between atoms. It is crucial for calculating the enthalpy of reaction (ΔH_reaction). In endothermic processes, energy is absorbed, resulting in a positive ΔH, while exothermic processes release energy, leading to a negative ΔH. The enthalpy of reaction can be calculated using the formula: $\Delta Hreaction=\mathrm{reactants}-\mathrm{products}$ when using individual bond enthalpies.

Bond Energy

Bond energy, also known as bond enthalpy and represented by ΔH_be, refers to the amount of energy stored in the bonds between atoms in a molecule. This concept is crucial for calculating the enthalpy of reaction (ΔH_reaction), which is sometimes referred to as the heat of reaction. Understanding the distinction between endothermic and exothermic processes is essential in this context.

In an endothermic process, energy is absorbed to break a bond, resulting in a positive ΔH value, indicating a gain of energy. Conversely, in an exothermic process, energy is released when bonds are formed, leading to a negative ΔH value, signifying a loss of energy.

When calculating the enthalpy of reaction using individual bond enthalpies, the formula is:

ΔH_reaction = Σ(Bond Energies of Reactants) - Σ(Bond Energies of Products)

This means that you sum the bond energies of the reactants and subtract the sum of the bond energies of the products. It's important to note that if you are given the enthalpy of formation for a compound, the formula changes to:

ΔH_reaction = Σ(Bond Energies of Products) - Σ(Bond Energies of Reactants)

Thus, when working with bond enthalpies, always remember to use the reactants minus products approach for accurate calculations.

Bond Energy Example 1

The formation of ammonia (NH₃) occurs through the reaction of nitrogen gas (N₂) and hydrogen gas (H₂). To calculate the enthalpy of this reaction, we utilize bond enthalpies, which represent the energy required to break specific types of chemical bonds. In this case, the bond enthalpies provided are for the nitrogen triple bond (N≡N), hydrogen single bond (H–H), and nitrogen-hydrogen single bond (N–H), with values of 945 kJ/mol, 432 kJ/mol, and 391 kJ/mol, respectively.

First, it is essential to ensure that the chemical equation is balanced. For the reaction:

1/2 N₂ + 3/2 H₂ → NH₃

we confirm that it is balanced, as it contains one nitrogen atom and three hydrogen atoms on both sides.

Next, we calculate the total bond enthalpy for the reactants and products. For the reactants, we have:

• 1 N≡N bond: 1 × 945 kJ/mol = 945 kJ
• 3 H–H bonds: 3 × 432 kJ/mol = 1296 kJ

Thus, the total energy for the reactants is:

945 kJ + 1296 kJ = 2241 kJ

For the products, NH₃ contains three N–H bonds, and since there are two moles of NH₃ produced, we have:

• 6 N–H bonds: 6 × 391 kJ/mol = 2346 kJ

Now, we can find the enthalpy change (ΔH) for the reaction using the formula:

ΔH = (Total bond enthalpy of reactants) - (Total bond enthalpy of products)

Substituting the values gives us:

ΔH = 2241 kJ - 2346 kJ = -105 kJ

This negative value indicates that the reaction is exothermic, meaning it releases energy. Understanding the bond enthalpies and the structure of the molecules involved is crucial for calculating the enthalpy of reaction accurately. By following these steps, one can determine the enthalpy change for various chemical reactions effectively.

Consider the following equation:

Determine the bond enthalpy value for the F–S bond.

• A. -2301.0 kJ/mol
• B. 900.5 kJ/mol
• C. -1171.5 kJ/mol
• D. 1301.0 kJ/mol

Answer: (B)

Use the bond energies to estimate the enthalpy of reaction for the combustion of 5 moles of acetylene:

• A. -1074 kJ
• B. 2685 kJ
• C. -2685 kJ
• D. -430 kJ

Answer: (C)

Here’s what students ask on this topic:

What is bond energy and how is it related to enthalpy of reaction?

Bond energy, also known as bond enthalpy (ΔH_be), is the amount of energy stored in a bond between atoms in a molecule. It is crucial for calculating the enthalpy of reaction (ΔH_reaction), which is the total heat change during a chemical reaction. The enthalpy of reaction can be calculated using the formula:

${\mathrm{\Delta H}}_{\mathrm{reaction}}=\Sigma \mathrm{\Delta H}\left(\mathrm{reactants}\right)-\Sigma \mathrm{\Delta H}\left(\mathrm{products}\right)$

This formula is used when individual bond enthalpies are given. In essence, bond energy helps determine whether a reaction is endothermic (absorbs energy) or exothermic (releases energy).

How do endothermic and exothermic processes differ in terms of bond energy?

In an endothermic process, energy is absorbed to break bonds, resulting in a positive ΔH. This means the system gains energy. Conversely, in an exothermic process, energy is released when bonds are formed, leading to a negative ΔH, indicating that the system loses energy. The sign of ΔH helps determine whether a reaction is endothermic or exothermic:

${\mathrm{\Delta H}}_{\mathrm{endothermic}}>0$

${\mathrm{\Delta H}}_{\mathrm{exothermic}}<0$

What is the formula for calculating the enthalpy of reaction using bond energies?

The formula for calculating the enthalpy of reaction (ΔH_reaction) using bond energies is:

${\mathrm{\Delta H}}_{\mathrm{reaction}}=\Sigma \mathrm{\Delta H}\left(\mathrm{reactants}\right)-\Sigma \mathrm{\Delta H}\left(\mathrm{products}\right)$

This formula is used when individual bond enthalpies are provided. It helps in determining the overall energy change in a chemical reaction by considering the energy required to break bonds in the reactants and the energy released when new bonds form in the products.

Why is it important to understand bond energy in chemical reactions?

Understanding bond energy is crucial in chemical reactions because it helps predict the energy changes that occur during the reaction. By knowing the bond energies, one can determine whether a reaction is endothermic (absorbs energy) or exothermic (releases energy). This information is essential for various applications, including designing energy-efficient processes, understanding reaction mechanisms, and predicting reaction outcomes. Additionally, bond energy calculations are fundamental in fields like thermochemistry and materials science, where energy considerations are critical.

How does the sign of ΔH indicate whether a reaction is endothermic or exothermic?

The sign of ΔH (enthalpy change) indicates whether a reaction is endothermic or exothermic. If ΔH is positive, the reaction is endothermic, meaning it absorbs energy from the surroundings. This is because energy is required to break the bonds in the reactants. Conversely, if ΔH is negative, the reaction is exothermic, meaning it releases energy to the surroundings. This occurs because energy is released when new bonds form in the products. The sign of ΔH provides a straightforward way to determine the energy dynamics of a chemical reaction.