Interpret and label energy changes and activation energy.
| English | Chinese |
|---|---|
| exothermic | 放热的 |
| endothermic | 吸热的 |
| enthalpy change | 焓变 |
| activation energy | 活化能 |
Try each question before opening its hint or answer. Use paper for calculations and diagrams.
An exothermic reaction has no activation energy. Explain why this is wrong.
Mark reactants, peak and products before subtracting energy levels.
It may still require an initial energy input; negative ΔH does not mean Eₐ is zero.
A mixture cools its surroundings. State the reaction type and the sign of ΔH.
If surroundings cool, where has their energy gone?
Endothermic; ΔH is positive.
Reactants: 20 kJ; peak: 90 kJ; products: 55 kJ. Draw and label the pathway, ΔH and Eₐ. Calculate both changes.
Subtract reactant energy from product energy and from peak energy separately.
ΔH = +35 kJ; Eₐ = 70 kJ. Products are above reactants; Eₐ runs from reactants to the peak. Label energy and reaction progress.
Use Lesson 1 in your worksheet for written practice. Your teacher will discuss those answers in class.
Predict whether the reaction releases energy. Break the reactant bonds, then form the product bonds.
H₂ + Cl₂ → 2HCl
Average gas-phase bond energies in kJ mol⁻¹ of bonds. A simplified energy account per reaction as written; it does not show the reaction mechanism or activation energy.
Break: H–H = 436; Cl–Cl = 243. Form: 2 × H–Cl = 862.
Breaking bonds always requires energy; forming bonds releases energy. Explain the overall change using both parts of the account.