Higher Tier: count bonds before calculating the energy change
| English | 中文 | Pinyin |
|---|---|---|
| overall energy change | 总能量变化 | zǒng néng liàng biàn huà |
| bond energy/bɒnd ˈenədʒi/ | 键能 | jiàn néng |
What would explain this observation?
- Higher Tier: Breaking a bond needs energy. An exothermic reaction releases energy overall because formation of new bonds releases more than was needed to break the old ones.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Energy must be supplied to break bonds in reactants. Energy is released when bonds form in products. Using supplied bond energies, overall energy change 总能量变化=total energy needed for bonds broken−total energy released for bonds formed. A negative answer means bond formation releases more than breaking requires, so the reaction is exothermic. A positive answer means the reverse and the reaction is endothermic.
- bond energy 键能: Energy required to break a mole of the stated bonds in the supplied model; overall energy change: Total bond-breaking energy minus total energy released by bond formation.
Breaking costs 700 kJ and forming releases 950 kJ. What is the signed overall change?
Use the balanced equation and actual bond types. A coefficient multiplies all bonds in that molecule. For H₂ + Cl₂ → 2HCl, break one H–H and one Cl–Cl and form two H–Cl bonds. For 2H₂ + O₂ → 2H₂O, break two H–H bonds and one O=O bond, and form four O–H bonds. An O=O double bond uses its supplied double-bond value once; do not double a single-bond value.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Use the balanced equation and actual bond types. A coefficient multiplies all bonds in that molecule. For H₂ + Cl₂ → 2HCl, break one H–H and one Cl–Cl and form two H–Cl bonds. For 2H₂ + O₂ → 2H₂O, break two H–H bonds and one O=O bond, and form four O–H bonds. An O=O double bond uses its supplied double-bond value once; do not double a single-bond value.
- Make separate broken and formed lists with counts, multiply each supplied value by its count, total each list, then subtract in the stated order. These values give an approximate model for the reaction quantities represented by the equation. Check units and sign and interpret the result in words. Do not infer this calculated value from a thermometer reading without a different model; AQA’s solution practical does not require that calculation.
Which two habits make the investigation or model in this case more defensible?
Make separate broken and formed lists with counts, multiply each supplied value by its count, total each list, then subtract in the stated order. These values give an approximate model for the reaction quantities represented by the equation. Check units and sign and interpret the result in words. Do not infer this calculated value from a thermometer reading without a different model; AQA’s solution practical does not require that calculation.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: supplied bond energies are H–H 436, Cl–Cl 243 and H–Cl 431 $\dfrac{\text{kJ}}{\text{mol of bonds}}$. For H₂ + Cl₂ → 2HCl, breaking requires 436+243=679 kJ; forming releases 2×431=862 kJ. Overall change=679−862=−183 kJ for the equation amounts: exothermic. The released magnitude is 183 kJ, but the signed change is negative. The balanced equation is essential to the count.
For H₂ + Cl₂ → 2HCl, use H–H 430, Cl–Cl 240, H–Cl 420 $\dfrac{\text{kJ}}{\text{mol of bonds}}$. Find the signed change for the equation amounts. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
For H₂ + Cl₂ → 2HCl, use H–H 430, Cl–Cl 240, H–Cl 420 kJ per mole of bonds. Find the signed change for the equation amounts.
The result is -170 kJ. Known: supplied bond energies are H–H 436, Cl–Cl 243 and H–Cl 431 kJ per mole of bonds. For H₂ + Cl₂ → 2HCl, breaking requires 436+243=679 kJ; forming releases 2×431=862 kJ. Overall change=679−862=−183 kJ for the equation amounts: exothermic. The released magnitude is 183 kJ, but the signed change is negative. The balanced equation is essential to the count.
Check the conclusion and its limits
- Do not call breaking bonds exothermic, subtract formed from broken in the reverse order, or count atoms as if each were a bond. A negative energy change is not negative activation energy. This entire source section is Higher-only, while drawing the profiles remains common-tier content.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Breaking a chemical bond releases energy. This claim is false: Do not call breaking bonds exothermic, subtract formed from broken in the reverse order, or count atoms as if each were a bond. A negative energy change is not negative activation energy. This entire source section is Higher-only, while drawing the profiles remains common-tier content.
Higher Tier: count bonds before calculating the energy change: Use the balanced equation and actual bond types. A coefficient multiplies all bonds in that molecule. For H₂ + Cl₂ → 2HCl, break one H–H and one Cl–Cl and form two H–Cl bonds. For 2H₂ + O₂ → 2H₂O, break two H–H bonds and one O=O bond, and form four O–H bonds. An O=O double bond uses its supplied double-bond value once; do not double a single-bond value.
Breaking a chemical bond releases energy.
Do not call breaking bonds exothermic, subtract formed from broken in the reverse order, or count atoms as if each were a bond. A negative energy change is not negative activation energy. This entire source section is Higher-only, while drawing the profiles remains common-tier content.
Energy required to break a mole of the stated bonds in the supplied model: write the technical term.
bond energy means Energy required to break a mole of the stated bonds in the supplied model.