Bond Breaking & Bond Forming
Section: 5. Chemical Energetics | Syllabus: Cambridge AS Level Physics 9702
Energy and Chemical Bonds During chemical reactions, bonds in reactants are broken and new bonds in products are formed. These processes involve energy changes. Understanding bond breaking and bond forming is key to understanding why reactions are exothermic or endothermic.
Bond Breaking - Endothermic Process Breaking bonds REQUIRES energy - it is an endothermic process. Why does bond breaking need energy? Bonds hold atoms together with attractive forces Energy is needed to overcome these attractive forces The stronger the bond, the more energy is required to break it Energy is ABSORBED (taken in) when bonds break Example: H-H → H + H (requires +436 kJ/mol) Bond Forming - Exothermic Process Making (forming) bonds RELEASES energy - it is an exothermic process.
Why does bond forming release energy? When atoms come together to form bonds, they reach a more stable, lower energy state The excess energy is released to the surroundings The stronger the bond formed, the more energy is released Energy is RELEASED (given out) when bonds form Example: H + H → H-H (releases -436 kJ/mol) Quick Memory Aid Breaking bonds = Building up energy (endothermic) Making bonds = Making energy available (exothermic) Process Energy Change Type Bond Breaking Energy IN (absorbed) Endothermic (+) Bond Forming Energy OUT (released) Exothermic (-) What is Bond Energy?
Bond energy is the energy required to break one mole of a particular bond in gaseous molecules. Bond energies are measured in kJ/mol (kilojoules per mole). Common bond energies: Bond Bond Energy (kJ/mol) H-H 436 C-H 413 C-C 347 C=C 612 O=O 498 C=O 805 O-H 464 N≡N 945 Key point: Double bonds (C=O) are stronger than single bonds (C-O), and triple bonds (N≡N) are even stronger.
Overall Energy Change in Reactions In any chemical reaction: Energy is absorbed to break bonds in the reactants Energy is released when new bonds form in the products Overall energy change (ΔH) = Energy to break bonds - Energy from making bonds ΔH = Total energy IN - Total energy OUT Why Reactions are Exothermic A reaction is exothermic when: Energy released from bond forming > Energy needed for bond breaking Net result: More energy is released than absorbed Excess energy goes to the surroundings (gets hot) ΔH is negative Example: Combustion of methane releases energy because forming C=O and O-H bonds releases more energy than breaking C-H and O=O bonds requires.
Why Reactions are Endothermic A reaction is endothermic when: Energy needed for bond breaking > Energy released from bond forming Net result: More energy is absorbed than released Energy must be taken from the surroundings (gets cold) ΔH is positive Example: Thermal decomposition requires continuous heat input because breaking bonds in the reactant requires more energy than is released forming bonds in the products.
Calculating Energy Changes - Step by Step Step 1: Write the balanced equation Step 2: Draw out all the bonds in reactants and products (structural formulas) Step 3: Calculate energy to BREAK all bonds in reactants (energy IN) Step 4: Calculate energy RELEASED from making all bonds in products (energy OUT) Step 5: Calculate overall energy change: ΔH = Energy IN (breaking) - Energy OUT (making) Or: ΔH = Σ(bonds broken) - Σ(bonds formed) Worked Example: Combustion of Hydrogen Equation: 2H₂ + O₂ → 2H₂O Step 1: Draw the bonds Reactants: 2(H-H) + 1(O=O) Products: 2 water molecules, each with 2(O-H), total = 4(O-H) Step 2: Energy to break bonds (IN) 2 × H-H = 2 × 436 = +872 kJ 1 × O=O = 1 × 498 = +498 kJ Total IN = +1370 kJ Step 3: Energy from making bonds (OUT) 4 × O-H = 4 × 464 = -1856 kJ Total OUT = -1856 kJ Step 4: Overall energy change ΔH = +1370 - 1856 = -486 kJ Negative ΔH → Exothermic reaction Worked Example: Formation of Hydrogen Chloride Equation: H₂ + Cl₂ → 2HCl Bonds: Reactants: H-H and Cl-Cl Products: 2(H-Cl) Given bond energies: H-H = 436 kJ/mol Cl-Cl = 243 kJ/mol H-Cl = 432 kJ/mol Breaking bonds (IN): 1 × H-H = +436 kJ 1 × Cl-Cl = +243 kJ Total IN = +679 kJ Making bonds (OUT): 2 × H-Cl = 2 × 432 = -864 kJ Total OUT = -864 kJ Overall energy change: ΔH = +679 - 864 = -185 kJ Exothermic (ΔH negative) Common Mistakes to Avoid Mistake 1: Forgetting that bond breaking requires energy (endothermic) Correct: Breaking bonds is always endothermic (+) Mistake 2: Getting the signs wrong in calculations Correct: Energy IN is positive, energy OUT is negative Mistake 3: Not counting ALL the bonds Correct: Draw structural formulas and count every bond Mistake 4: Confusing bond energy values with ΔH Correct: Bond energies are always positive (energy to break), but ΔH can be positive or negative Summary Comparison Aspect Bond Breaking Bond Forming Energy change Requires energy (IN) Releases energy (OUT) Process type Endothermic Exothermic Sign in calculations Positive (+) Negative (-) Happens when At start of reaction (reactants) At end of reaction (products) Stronger bond More energy needed to break More energy released when formed Connec…
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