R1.2 Energy cycles in reactionsIB Chemistry SL: Flashcards

Card 1 of 120 of 12 known

Question

Is bond breaking endothermic or exothermic?

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

Is bond breaking endothermic or exothermic?: Endothermic: energy is absorbed to overcome the attraction between the bonded atoms.
Is bond forming endothermic or exothermic?: Exothermic: energy is released.
Define bond enthalpy.: The energy needed to break one mole of a given bond in gaseous molecules.
Why is it called an average bond enthalpy?: It is the mean value for that bond across a range of different compounds.
Formula for ΔH from bond enthalpies?: ΔH = Σ(bonds broken) − Σ(bonds formed).
Why are ΔH values from bond enthalpies approximate?: Average values are not exact for a given molecule, and they apply only to gases.
Why does H₂ + Br₂(l) → 2HBr(g) differ from the bond-enthalpy value?: Bromine is a liquid, so energy is also needed to vaporise it.
State Hess's law.: The enthalpy change for a reaction is independent of the pathway between the initial and final states.
What happens to ΔH when you reverse an equation?: Its sign changes.
What happens to ΔH when you double an equation?: It doubles.
S + O₂ → SO₂ is −297 kJ and 2SO₂ + O₂ → 2SO₃ is −198 kJ. ΔH for S + 1½O₂ → SO₃?: −297 + ½(−198) = −396 kJ.
Why is Hess's law useful?: It gives enthalpy changes for reactions that cannot be measured directly.