Bond Energy: Videos & Practice Problems

Bond energy, or bond enthalpy (ΔH_BE), is the energy stored in the chemical bonds between atoms in a molecule. It is instrumental in calculating the enthalpy of reaction (ΔH of reaction) or heat of reaction. There are two types of processes based on energy changes: endothermic, where energy is absorbed to break bonds, indicated by a positive sign, and exothermic, where energy is released when bonds form, marked by a negative sign. The enthalpy of reaction can be determined using the formula:

ΔH of reaction = reactants - products when individual bond energies are provided. This contrasts with the formula:

ΔH of reaction = products - reactants when using standard enthalpy of formation values for entire compounds. Understanding these concepts is crucial for grasping the energy dynamics in chemical reactions.

Bond Energy

Now here I'm going to say that bond energy, also called bond enthalpy and it's represented by ΔHBE, is the amount of energy stored in a bond between atoms in a molecule. Now realize that bond energy values can be calculated and can be used to calculate the enthalpy of reaction, which is ΔH of reaction. Also what's called the heat of reaction at times as well. So just remember it's the enthalpy or heat of reaction.

Now we have endothermic processes versus exothermic processes. In an endothermic process, energy is absorbed to break a bond. And since we're taking in energy, we're gaining energy, it has a positive sign. In an exothermic process, energy is released, lost, and if you're losing energy you have a negative sign. Now if we take a look here, we're accustomed to seeing equations like this.

These two equations we need to remember. We're going to say when given individual bond enthalpies or bond energies, then the enthalpy of reaction formula becomes this: enthalpy of reaction equals reactants minus products. So again, if they're giving us individual bond enthalpies or bond energies, we use this version of the formula.

Now if you've looked at my videos in terms of thermochemistry, you would see that that formula seems kind of familiar. Well, when we're given the enthalpy of formation for a whole compound, then it's products minus reactants. So if you've seen my videos in thermochemistry, you will remember this. If you haven't yet, don't worry about it just yet. Realize that when we're dealing with bond enthalpies, bond energies, it's reactants minus products.

Bond Energy Example 1

Here it says the formation of ammonia is accomplished by the reaction between hydrogen and nitrogen gas. So here it says calculate the enthalpy of reaction if the bond enthalpy. So bond enthalpies of the N triple bond, NH single bond, H&N single bond, HR 940-5432 and 391 kilojoules per mole respectively.

So the steps we're going to take here is we're going to check to see if the chemical reaction is balanced. If and if not then do the necessary steps to balance it. So here they're already drawn for us. It's already balanced for us. We have one mole of this. If Lewis structures is not given, then you will have to draw them as well. Luckily I gave it to a strong now step one for the reactants and products, multiply the coefficients of each bond type with its bond enthalpy value.

So if we take a look here, we have 1 N triple bond, N bond and its energy is 945 kilojoules per mole. And here we have 3 H single bond H bonds, and each one we're told is 432. And then finally we have how many NH bonds? This one's tricky. We have 123 NH bonds within NH₃, but then it's times 2, so 3 * 2 = 6 total NH bonds. So it's going to be 6 NH bonds here. Each one is 391.

So then what we need to realize here is that this will cancel out with the moles that we have and our ants will be in kilojoules. So what we're going to do here is 1 * 945 + 3 * 432 = 2241 kilojoules. Here we're doing reactants minus products to find the delta H of reaction. Then we have 6 * 391 = 2346 kilojoules. When we subtract these two, it gives me - 105 kilojoules as the enthalpy of reaction when given those different bond enthalpies.

So here all it is is first knowing that you have a balanced equation, knowing that you've drawn your Louis dot structures, and carefully examining the different types of bonds that exist. If you can do that, you'll be able to find the enthalpy of reaction for any of these given questions.

Consider the following equation:

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

• A. -2301.0 kJ/mol
• B. 335.5 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. -5020 kJ
• D. -430 kJ

Answer: (C)

Here’s what students ask on this topic:

What is bond energy?

Bond energy is the amount of energy required to break one mole of a particular chemical bond in a gaseous substance. It's a measure of the bond's strength and stability. When a bond is formed, energy is released, and conversely, when a bond is broken, energy is absorbed. This energy is usually expressed in units of kilojoules per mole (kJ/mol).

For example, if you have a molecule of hydrogen gas (H₂), the bond energy represents the energy needed to separate the two hydrogen atoms into individual gaseous atoms. Bond energies are average values derived from a variety of molecules containing that type of bond, as the energy can vary depending on the specific molecular environment.

Understanding bond energy is crucial in fields such as chemistry and biochemistry, as it helps predict the behavior of molecules during chemical reactions. It allows scientists to estimate the energy changes involved in these reactions, which is essential for understanding reaction dynamics and the feasibility of a reaction under certain conditions.

What is bond dissociation energy?

Bond dissociation energy is the amount of energy required to break a particular chemical bond in a molecule, separating the atoms in a gas phase into isolated, gaseous atoms. It is typically measured in kilojoules per mole (kJ/mol) or sometimes in electronvolts (eV). This energy reflects the strength of the bond; the higher the bond dissociation energy, the stronger the chemical bond.

When you have a molecule with multiple bonds, the bond dissociation energy will usually be different for each bond, as it depends on the specific atoms involved and their environment within the molecule. For instance, breaking a carbon-hydrogen bond in methane will have a different energy requirement than breaking a carbon-carbon bond in ethane.

In chemical reactions, knowing the bond dissociation energies helps predict the reactivity of different molecules and the energy changes that will occur during the reaction. It's a fundamental concept in chemistry that helps us understand how and why chemical reactions happen.

How do you calculate bond energy?

Bond energy is the amount of energy required to break one mole of a bond in a chemical compound and separate the atoms to an infinite distance. To calculate bond energy, you typically use the following steps:

1. Identify the bonds: Determine which chemical bonds are being broken and formed in the chemical reaction.
2. Look up bond energies: Use a table of standard bond energies, which lists the average energy associated with different types of bonds (e.g. C-H, O-H, N=N).
3. Calculate energy changes for bonds broken: Multiply the number of each type of bond broken by the bond energy from the table.
4. Calculate energy changes for bonds formed: Multiply the number of each type of bond formed by the bond energy from the table.
5. Determine the net energy change: Subtract the total energy of bonds formed from the total energy of bonds broken. This will give you the net energy change for the reaction.

The bond energy is usually expressed in kilojoules per mole (kJ/mol). If the net energy is positive, energy is absorbed, indicating an endothermic reaction. If it's negative, energy is released, indicating an exothermic reaction.

How can the net energy exchange of a chemical reaction be determined using average bond energies?

To determine the net energy exchange of a chemical reaction using average bond energies, you need to follow these steps:

1. Identify the Bonds Broken and Formed: Look at the reactants and products of the reaction and identify which chemical bonds are broken and which are formed during the reaction.
2. Use Average Bond Energies: Find the average bond energies for each type of bond. These values are typically provided in tables in chemistry textbooks or reliable online resources. Average bond energies are given in units of kilojoules per mole (kJ/mol).
3. Calculate Energy Required to Break Bonds: Add up the energy required to break all the bonds in the reactants. This is the energy input, and it is considered positive because energy is absorbed to break bonds.
4. Calculate Energy Released in Forming Bonds: Add up the energy released when new bonds are formed in the products. This is the energy output, and it is considered negative because energy is released when bonds are formed.
5. Determine Net Energy Exchange: Subtract the total energy required to break bonds from the total energy released in forming bonds. The result is the net energy exchange for the reaction.

If the result is positive, the reaction is endothermic (absorbs energy). If the result is negative, the reaction is exothermic (releases energy). This calculation