This mini-lesson covers Reactivity 1 at SL: enthalpy change, measuring it by calorimetry (q = mcΔT), Hess's law, bond enthalpies, and energy from fuels.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Reactivity 1.1
Enthalpy change (ΔH)
Enthalpy change ΔH is the heat exchanged at constant pressure.
Exothermic: releases heat, ΔH negative, products lower in energy.
Endothermic: absorbs heat, ΔH positive, products higher in energy.
On an energy profile, the activation energy is the barrier that must be climbed before reactants become products.
Quick check
Exo or endo?
?A reaction releases heat to the surroundings. Its ΔH is:
Reactivity 1.1
Measuring ΔH by calorimetry
Burn or react a known amount and measure the temperature rise of water. The heat gained by the water is:
q = m c ΔTm = mass of water (g), c = 4.18 J g⁻¹ °C⁻¹, ΔT = temperature change
Then divide by the moles reacted to get ΔH per mole.
Calculate
Heat released
1Burning a fuel raises the temperature of 100 g of water by 20.0 °C (c = 4.18 J g⁻¹ °C⁻¹). Calculate q, in joules.
J
q = m c ΔT = 100 × 4.18 × 20.0.
Calculate
Enthalpy per mole
2That 8.36 kJ came from burning 0.010 mol of fuel. Calculate ΔH of combustion, in kJ mol⁻¹ (remember the sign).
kJ mol⁻¹
ΔH = −q ÷ n = −8.36 ÷ 0.010 (negative because combustion is exothermic).
Reactivity 1.2
Hess's law and energy cycles
Hess's law: the enthalpy change of a reaction is independent of the route taken — it depends only on the initial and final states. This lets us add enthalpies around a cycle to find a value we cannot measure directly, using standard enthalpies of formation or combustion.
Reactivity 1.2
Bond enthalpies
Breaking bonds absorbs energy; making bonds releases energy. For gas-phase reactions:
ΔH = Σ(bonds broken) − Σ(bonds formed)using average bond enthalpies
A fuel releases energy by combustion. Complete combustion of a hydrocarbon (plenty of O₂) gives carbon dioxide and water; incomplete combustion (limited O₂) gives toxic carbon monoxide and soot (carbon), releasing less energy.
Fossil fuels (coal, oil, natural gas) are finite and add CO₂ to the atmosphere; biofuels such as bioethanol are renewable and closer to carbon neutral. Hydrogen and methanol can power fuel cells.
Quick check
Complete or incomplete?
?Which product forms during INCOMPLETE combustion of a hydrocarbon but not during complete combustion?
Sort it
Fuels and their products
Tap an item, then the box it fits.
⛽ Fossil fuel
🌱 Biofuel
💨 Combustion product
Match it
Match the term to its meaning
Tap an item on the left, then its match on the right.
Term
Meaning
Recap
The big ideas to know
ΔH: exothermic (−, releases heat) vs endothermic (+, absorbs heat)
Calorimetry: q = mcΔT, then ΔH = q ÷ n
Hess & bonds: ΔH is route-independent; ΔH = bonds broken − bonds formed
Fuels: complete combustion → CO₂ + H₂O; incomplete (limited O₂) → toxic CO and soot
You've covered What drives reactions — energetics for IB Diploma Chemistry SL. Press Finish to see your score.
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You've worked through What drives reactions — energetics for IB Diploma Chemistry SL. 🎉