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A hand warmer heats up; a cold pack chills. Every reaction breaks bonds and makes new ones, and the balance between the two decides whether energy flows out or in.
Recommended for MYP 4 · eAssessment priority · About 4 lessons · Criteria A, B, C and D
By the end of this topic you should be able to:
You will use these skills. If any feel shaky, review them first.
Chemical reactions transfer energy to or from their surroundings. Exothermic reactions (combustion, neutralisation, respiration) release energy; endothermic reactions (thermal decomposition, photosynthesis) take it in.
overall energy change = energy to break bonds − energy released making bonds
Breaking bonds always takes in energy; making bonds always releases it. If the new bonds in the products release more energy than was needed to break the old ones, the reaction is exothermic (a negative energy change); if less, it is endothermic.
We measure temperature changes in insulated containers and convert them into energy using the heat capacity of water. Bond energies, measured for many substances, let chemists predict energy changes before doing a reaction, and the predictions agree well with experiments.
Energy from combustion powers transport and much of our electricity. Engineers design hand warmers, cold packs and self-heating meals, and choosing fuels means comparing the energy they release with their costs and emissions.
Energy changes link to rates of reaction and catalysts, to respiration and photosynthesis in biology, to specific heat capacity and energy resources in physics, and to climate change.
With bond energies: add up the energy to break every bond in the reactants, then the energy released making every bond in the products. Subtract: a negative answer means exothermic.
With calorimetry: energy transferred (J) = mass of water (g) × 4.2 × temperature change (°C). For a fuel, divide by the mass burned to get energy per gram.
Experimental values are almost always lower than data-book values, because energy is lost to the surroundings and fuels may burn incompletely.
Heat packs for sore muscles use crystallising salt solutions that release energy and can be reset in hot water. Chemical engineers must remove energy from large exothermic reactions so they do not overheat. Fire investigators use the energy released by different materials to understand how fires spread.
Calculate the energy change for H–H + Br–Br → 2 H–Br. Bond energies (kJ/mol): H–H 436, Br–Br 193, H–Br 366.
Burning 0.40 g of a fuel raises the temperature of 100 g of water by 15 °C. Calculate the energy transferred per gram.
This topic combines calculation, graph and profile interpretation, and practical evaluation. Expect:
Common ways to lose marks: saying breaking bonds releases energy; using the mass of fuel instead of the mass of water in energy = mass × 4.2 × temperature change; forgetting to multiply bond energies by the number of bonds; and describing endothermic reactions as “giving out cold”.
Quick questions on the ideas above. Try each one before using a hint.
Partially guided investigation · about 50 minutes · pairs
Safety. Wear eye protection: sodium hydroxide and hydrochloric acid are irritants and can damage eyes. Wash any splashes off skin with plenty of water. Stand the cup in a beaker so it cannot tip over.
Results from one student are used in the Criterion C questions below. Then evaluate: which reading would you repeat, and why?
Harder problems in unfamiliar contexts. Plan before you calculate.
Five questions picked at random from the whole topic. Take a new set whenever you like.
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Tracked separately for each skill, on this device only.
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