Bond breaking is endothermic and bond making is exothermic
Definition
Bond energy
The energy needed to break one mole of a particular covalent bond, which equals the energy released when that bond forms.
Definition
Covalent bond
A shared pair of electrons between two atoms.
- Breaking a covalent bond needs energy to overcome the attraction between the shared electrons and the two nuclei, so bond breaking is endothermic.
- Making a bond releases energy, so bond making is exothermic.
- Forming a bond releases the same amount of energy that is needed to break that bond.
- In a reaction, bonds in the reactants break and new bonds form in the products.
- Different bonds have different strengths, such as 436 kJ/mol436\,\text{kJ/mol}436kJ/mol for H-H\text{H-H}H-H and 243 kJ/mol243\,\text{kJ/mol}243kJ/mol for Cl-Cl\text{Cl-Cl}Cl-Cl.
Common Mistake
Do not say that breaking bonds releases energy, because energy must always be put in to pull bonded atoms apart.
Energy change equals bonds broken minus bonds made
- Calculate the energy change using: energy change=energy to break bonds−energy released making bonds\text{energy change} = \text{energy to break bonds} - \text{energy released making bonds}energy change=energy to break bonds−energy released making bonds
- A negative answer means more energy is released making bonds than is taken in breaking them, so the reaction is exothermic.
- A positive answer means more energy is taken in breaking bonds than is released making them, so the reaction is endothermic.
- The answer is in kilojoules per mole, kJ/mol\text{kJ/mol}kJ/mol, for the equation as written.
- Bond energies are average values taken from many different molecules, so the result is an estimate.
Key Idea
Whether a reaction is exothermic depends on how the energy released by the new bonds compares with the energy needed to break the old ones, not on how many bonds break or form.
Count every bond broken and made, using balancing numbers
Definition
Displayed formula
A two-dimensional formula that shows every atom in a molecule and each bond between the atoms.
- Draw the displayed formula of each substance to see every bond.
- Multiply the bonds in each molecule by its balancing number in the equation.
- In 2H2O2\text{H}_2\text{O}2H2O there are four O-H\text{O-H}O-H bonds, because each water molecule has two.
- A double bond such as O=O\text{O=O}O=O or C=O\text{C=O}C=O has its own bond energy and counts as one bond.
- Each CO2\text{CO}_2CO2 molecule, O=C=O\text{O=C=O}O=C=O, contains two C=O\text{C=O}C=O bonds.
- List the bonds broken and the bonds made in two columns, so that no bond is missed.
Example
- Hydrogen reacts with chlorine, H2+Cl2→2HCl\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}H2+Cl2→2HCl; calculate the energy change using H-H=436\text{H-H} = 436H-H=436, Cl-Cl=243\text{Cl-Cl} = 243Cl-Cl=243 and H-Cl=432 kJ/mol\text{H-Cl} = 432\,\text{kJ/mol}H-Cl=432kJ/mol.
- Energy to break bonds =436+243=679 kJ/mol= 436 + 243 = 679\,\text{kJ/mol}=436+243=679kJ/mol.
- Energy released making bonds =2×432=864 kJ/mol= 2 \times 432 = 864\,\text{kJ/mol}=2×432=864kJ/mol.
- Energy change =679−864=−185 kJ/mol= 679 - 864 = -185\,\text{kJ/mol}=679−864=−185kJ/mol, so the reaction is exothermic.
Burning methane is exothermic as bond making releases more
- Methane burns in oxygen: CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}CH4+2O2→CO2+2H2O
- The bonds broken are four C-H\text{C-H}C-H bonds in methane and two O=O\text{O=O}O=O bonds in the oxygen molecules.
- The bonds made are two C=O\text{C=O}C=O bonds in carbon dioxide and four O-H\text{O-H}O-H bonds in the water molecules.
- The bond energies are C-H=413\text{C-H} = 413C-H=413, O=O=498\text{O=O} = 498O=O=498, C=O=805\text{C=O} = 805C=O=805 and O-H=463 kJ/mol\text{O-H} = 463\,\text{kJ/mol}O-H=463kJ/mol.
Example
- Calculate the energy change when methane burns.
- Energy to break bonds =(4×413)+(2×498)=1652+996=2648 kJ/mol= (4 \times 413) + (2 \times 498) = 1652 + 996 = 2648\,\text{kJ/mol}=(4×413)+(2×498)=1652+996=2648kJ/mol.
- Energy released making bonds =(2×805)+(4×463)=1610+1852=3462 kJ/mol= (2 \times 805) + (4 \times 463) = 1610 + 1852 = 3462\,\text{kJ/mol}=(2×805)+(4×463)=1610+1852=3462kJ/mol.
- Energy change =2648−3462=−814 kJ/mol= 2648 - 3462 = -814\,\text{kJ/mol}=2648−3462=−814kJ/mol.
- The answer is negative, so burning methane is exothermic.
A positive energy change shows a reaction is endothermic
- When water is split into hydrogen and oxygen, 2H2O→2H2+O22\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_22H2O→2H2+O2, four O-H\text{O-H}O-H bonds are broken.
- Using O-H=463 kJ/mol\text{O-H} = 463\,\text{kJ/mol}O-H=463kJ/mol, the energy to break them is 4×463=1852 kJ/mol4 \times 463 = 1852\,\text{kJ/mol}4×463=1852kJ/mol.
- Two H-H\text{H-H}H-H bonds and one O=O\text{O=O}O=O bond are made.
- Using H-H=436\text{H-H} = 436H-H=436 and O=O=498 kJ/mol\text{O=O} = 498\,\text{kJ/mol}O=O=498kJ/mol, the energy released is (2×436)+498=1370 kJ/mol(2 \times 436) + 498 = 1370\,\text{kJ/mol}(2×436)+498=1370kJ/mol.
- The energy change is 1852−1370=+482 kJ/mol1852 - 1370 = +482\,\text{kJ/mol}1852−1370=+482kJ/mol, so the reaction is endothermic.
- This matches the reaction profile of an endothermic reaction, in which the products sit higher in energy than the reactants.
Exam technique
- When a question asks for an energy change from bond energies, show the total for bonds broken and the total for bonds made before you subtract.
- Always subtract in the order broken minus made, and give the sign and the unit kJ/mol\text{kJ/mol}kJ/mol with your answer.
Self review
- Why is bond breaking endothermic?
- What is the equation for the energy change of a reaction from bond energies?
- How many N-H\text{N-H}N-H bonds are made in N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3N2+3H2→2NH3?
- What does a negative energy change tell you about a reaction?
- Why is an energy change calculated from bond energies only an estimate?
