This set has twelve original questions, ordered from easier to harder, covering all five lessons in energy changes and bonds. Questions 1 to 4 are warm-ups on direction of energy and profiles, 5 to 7 cover activation energy and catalysts, and 8 to 12 use bond energies and mixed reasoning.
All data are invented for practice. Bond energies are supplied values in kJ/mol, rounded, so they will not match a data booklet exactly. Use the values in each question.
Attempt each question on paper first and write your working as you would in an exam. Then open the answer and use the routing list at the end for any you missed.
Questions
1. In a fictional reaction the temperature of the surroundings rises from 20.5 °C to 27.0 °C. Is it exothermic or endothermic, and by how much did the temperature change?
Show answer
27.0 − 20.5 = 6.5, a rise of 6.5 °C. The surroundings warmed, so energy was released to them. The reaction is exothermic.
2. A different fictional change makes the temperature of the surroundings fall from 23.0 °C to 18.0 °C. Describe the direction of energy transfer and state the sign of ΔH.
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The temperature fell by 23.0 − 18.0 = 5.0 °C, so energy is absorbed from the surroundings. The change is endothermic and ΔH is positive.
3. On an energy profile the reactants are at 120 kJ/mol, the peak is at 300 kJ/mol and the products are at 50 kJ/mol. Find ΔH and say whether it is exothermic or endothermic.
Show answer
Products minus reactants = 50 − 120 = −70 kJ/mol. The products are lower, so the reaction is exothermic.
4. Another profile has reactants at 30 kJ/mol, a peak at 190 kJ/mol and products at 110 kJ/mol. Find ΔH and the direction of the ΔH arrow.
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ΔH = 110 − 30 = +80 kJ/mol. The products are higher, so the arrow points up and the reaction is endothermic.
5. Using the profile in question 3, find the activation energy.
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Activation energy = peak minus reactant level = 300 − 120 = 180 kJ/mol.
6. A catalyst lowers the peak in question 3 to 240 kJ/mol. State the new activation energy and the new ΔH.
Show answer
New activation energy = 240 − 120 = 120 kJ/mol. ΔH is still −70 kJ/mol, because a catalyst changes the peak only, not the levels of reactants and products.
7. A student says: “This reaction has a negative ΔH, so it needs no energy to get started.” Evaluate the statement.
Show answer
It is not correct. A negative ΔH describes the overall result. Bonds in the reactants must first be broken, so the colliding particles still need at least the activation energy. A fuel that releases energy overall still needs an initial input before it begins to react.
8. Name the process that absorbs energy and the process that releases energy in a chemical reaction.
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Breaking bonds absorbs energy (endothermic). Making bonds releases energy (exothermic).
9. Use H–H 436, Cl–Cl 242 and H–Cl 431 (kJ/mol) to estimate the energy change for H₂ + Cl₂ → 2HCl.
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Broken: 436 + 242 = 678. Made: 2 × 431 = 862. Energy change = broken − made = 678 − 862 = −184 kJ/mol. It is exothermic.
10. Use H–H 436, O=O 498 and O–H 464 (kJ/mol) to estimate the energy change for 2H₂ + O₂ → 2H₂O. State how many of each bond are broken and made.
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Broken: 2 H–H and 1 O=O. Made: 4 O–H (2 molecules × 2 bonds).
Broken = 2 × 436 + 498 = 872 + 498 = 1370. Made = 4 × 464 = 1856. Energy change = 1370 − 1856 = −486 kJ/mol. Exothermic.
11. Use N≡N 945, H–H 436 and N–H 391 (kJ/mol) to estimate the energy change for N₂ + 3H₂ → 2NH₃.
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Broken: 945 + 3 × 436 = 945 + 1308 = 2253. Made: 2 molecules × 3 N–H = 6 bonds, 6 × 391 = 2346. Energy change = 2253 − 2346 = −93 kJ/mol. Exothermic.
12. Use N≡N 945, O=O 498 and N=O 630 (kJ/mol) to estimate the energy change for N₂ + O₂ → 2NO. Is it exothermic or endothermic? Then state the energy change for the reverse reaction, 2NO → N₂ + O₂.
Show answer
Broken: 945 + 498 = 1443. Made: 2 × 630 = 1260. Energy change = 1443 − 1260 = +183 kJ/mol, so the reaction is endothermic (more energy absorbed breaking bonds than released making them).
The reverse reaction has the same size with the opposite sign: −183 kJ/mol, exothermic.
If you got these wrong
Match the type of error to the lesson that repairs it.
- Questions 1, 2 and 8, direction of energy: go to distinguish energy released from energy absorbed.
- Questions 3 and 4, reading levels and sign: go to interpret an energy profile.
- Questions 5, 6 and 7, barriers and catalysts: go to explain activation energy on a diagram.
- Questions 9, 10 and 11, calculations: go to estimate an energy change using supplied bond energies.
- Questions 8 and 12, breaking versus making, and reversal: go to separate bond breaking from bond making.
Record each slip in the mistake log and retest queue with the error type, so a fresh question can be retested later. The equation balance reasoning trainer gives extra practice in counting atoms and bonds correctly.
When the same error returns after a lesson, a teacher in online one-to-one Chemistry tuition can trace it through your written working and set questions that test exactly that step.