Bond energy is an important concept in chemistry that describes the amount of energy required to break bonds between covalent bond gases. Fill-type bond energies do not apply to ionic bonds. When 2 atoms bond together to form a new molecule, the degree of bond strength between the atoms can be determined by measuring the amount of energy required to break the bond. Remember, one atom has no bond energy; this energy exists only in bonds between two atoms. To calculate bond energies, simply determine the total number of bonds broken, then subtract the total number of bonds formed.
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Part 1 of 2: Determining the Broken and Formed Bonds
Step 1. Define the equation to calculate the bond energy
Bond energy is defined as the sum of all bonds broken minus the number of bonds formed: H = H(break bond) - H(bond formed). H is the change in bond energy, also known as the bond enthalpy, and H is the sum of the bond energies for each side of the equation.
- This equation is a form of Hess's Law.
- The unit for bond energy is the kilojoule per mole or kJ/mol.
Step 2. Write a chemical equation showing all the intermolecular bonds
When the equation for the reaction in the problem is written only with chemical symbols and numbers, it is helpful to write this equation because it describes all the bonds that form between the various elements and molecules. This visual representation will allow you to calculate all the bonds that are broken and formed on the reactant and product sides of the equation.
- Remember, the left side of the equation is the reactants, and the right side is the products.
- Single, double, and triple bonds have different bond energies so be sure to draw a diagram with the correct bonds between the elements.
- For example, if you draw the following equation for the reaction between 2 hydrogens and 2 bromine: H2(g) + Br2(g) - 2 HBr(g), you will get: H-H + Br-Br - 2 H-Br. The hyphen (-) indicates a single bond between the elements in the reactants and products.
Step 3. Know the rules for counting bonds broken and formed
In some cases, the bond energies that will be used for this calculation will be the average. The same bond can have slightly different bond energies based on the molecules formed; thus, the average bond energy is commonly used..
- Single, double and triple bonds are treated as 1 break. They all have different bond energies, but count as just one break.
- The same is true for single, double, or triple formations. This will count as one formation.
- In this example, all bonds are single bonds.
Step 4. Identify the bond break on the left side of the equation
The left side of the equation contains the reactants, which will represent all the broken bonds in the equation. It is an endothermic process that requires absorption of energy to break bonds.
In this example, the left side has 1 H-H bond and 1 Br-Br bond
Step 5. Count all the bonds formed on the right side of the equation
The right side of the equation contains all the products. These are all the bonds that will form. Bond formation is an exothermic process that releases energy, usually in the form of heat.
In this example, the right side has 2 H-Br bonds
Part 2 of 2: Calculating Bond Energy
Step 1. Find the bond energy of the bond in question
There are many tables that contain information on the average bond energies of a particular bond. You can look it up on the internet or in chemistry books. It is important to note that the bond energy information in the table is always for gaseous molecules.
- For example, you want to find the bond energies of H-H, Br-Br, and H-Br.
- H-H = 436 kJ/mol; Br-Br = 193 kJ/mol; H-Br = 366 kJ/mol.
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To calculate the bond energy of a molecule in liquid form, you need to also find the enthalpy of change of vaporization for the liquid molecule. This is the amount of energy required to turn a liquid into a gas. This number is added to the total bond energy.
For example: If the question asks about liquid water, add the enthalpy change of vaporization of water (+41 kJ) to the equation
Step 2. Multiply the bond energy by the number of bonds broken
In some equations, you can have the same bond broken many times. For example, if 4 hydrogen atoms are in a molecule, the hydrogen bond energy must be calculated four times, aka times 4.
- In this example, there is only 1 bond per molecule so simply multiply the bond energy by 1.
- H-H = 436 x 1 = 436 kJ/mol
- Br-Br = 193 x 1 = 193 kJ/mol
Step 3. Add up all the bond energies of the broken bonds
After multiplying the bond energies by the number of individual bonds, you need to add up all the bonds on the reactant side.
In our example, the number of bonds broken is H-H + Br-Br = 436 + 193 = 629 kJ/mol
Step 4. Multiply the bond energy by the number of bonds formed
As when working on breaking bonds on the reactant side, you must multiply the number of bonds formed by the respective bond energies. If you find that 4 hydrogen bonds are formed, multiply the energy of those bonds by 4.
In this example, 2 H-Br bonds are formed so that the H-Br bond energy (366 kJ/mol) will be multiplied by 2: 366 x 2 = 732 kJ/mol
Step 5. Add up all the bond energies formed
Again, like breaking bonds, all bonds formed on the product side are added up. Sometimes only 1 product is formed and you can skip this step.
In our example, there is only 1 product formed so the bond energy formed is equal to the bond energy of the 2 H-Br bonds which is 732 kJ/mol
Step 6. Subtract the number of bonds formed by broken bonds
Once all the bond energies on both sides are added up, simply subtract the broken bonds by the bonds formed. Remember this equation: H = H(break bond) - H(bond formed). Plug the numbers into the formula and subtract.
In this example: H = H(break bond) - H(bond formed) = 629 kJ/mol - 732 kJ/mol = -103 kJ/mol.
Step 7. Determine whether the entire reaction is endothermic or exothermic
The final step is to calculate the bond energy to determine whether the reaction releases energy or consumes energy. An endothermic (which consumes energy) will have a positive final bond energy, while an exothermic reaction (which releases energy) will have a negative bond energy.
