AQA · Chemistry · 8462
GCSE Chemistry (AQA): Bond Energies
Topic 5 — Energy Changes. Generated from the caption track of Cognito's lesson video "GCSE Chemistry - Bond Energies - Determining if Reactions are Exothermic or Endothermic"; each card deep-links to the moment in the video it came from. Lesson 5.02 of Cognito's AQA GCSE Combined Science (Trilogy) and Chemistry course (Higher tier), filed under AQA Chemistry 8462. Teaching by Cognito — https://cognitoedu.org/coursesubtopic/c2-gcse-aqa-h-c_5.02.
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Why is it useful to draw displayed formulae before a bond energy calculation?
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It makes every bond that must be broken or formed visible, so none are miscounted.
What is the overall energy change for N₂ + 3H₂ → 2NH₃, given N≡N 941, H–H 436, N–H 391 kJ/mol?
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−97 kJ/mol: (941 + 3 × 436) − (6 × 391) = 2249 − 2346.
Is bond formation exothermic or endothermic?
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Exothermic — energy is released to the surroundings when bonds form.
What does a positive overall energy change tell you about a reaction?
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The reaction is endothermic — more energy is taken in breaking bonds than is released forming them.
Is breaking bonds exothermic or endothermic?
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Endothermic — energy must be taken in from the surroundings to break bonds.
How is the overall energy change of a reaction calculated from bond energies?
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Energy required to break bonds − energy released forming bonds.
What is the value of Avogadro's constant?
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6.02 × 10²³ per mole.
Define bond energy.
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The energy needed to break one mole of a particular covalent bond.
What does a negative overall energy change tell you about a reaction?
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The reaction is exothermic — the chemicals lose energy to the surroundings.
How much energy is released when one mole of H–Cl bonds forms, if the H–Cl bond energy is 431 kJ/mol?
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431 kJ — bond formation releases the same amount of energy that breaking the bond requires.
In N₂ + 3H₂ → 2NH₃, which bonds must be broken?
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One N≡N triple bond and three H–H bonds.
How many N–H bonds are formed in N₂ + 3H₂ → 2NH₃?
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Six — each of the two ammonia molecules contains three N–H bonds.
What is the overall energy change for H₂ + Cl₂ → 2HCl, given bond energies H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol?
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−184 kJ/mol: (436 + 242) − (2 × 431) = 678 − 862.
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