Chemistry · Rung 16 of 24

Reactions that give and take heat

Breaking a bond always costs energy and forming one always gives it back; what decides whether a reaction heats or cools is which of the two wins. If what comes back forming the new bonds beats what it cost to break the old ones, there is energy left over and it comes out as heat: the reaction is exothermic and the container warms, as when wood burns, when you breathe, or when water is poured on quicklime. If it falls short, the energy has to come from outside: it is endothermic and the container cools, which is what an instant cold pack does and, on another scale, photosynthesis. The words come from the Greek and say which way the heat goes, so there is nothing to memorise. The sum is a subtraction: what comes back minus what it cost. And it is worth being clear that the atoms are not used up in the process, they only change partners; mass disappearing is nuclear physics and does not happen here.

Where this sits

Comes after
Reactions and balancing
Opens up
Reaction rate

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What is a reaction that releases heat called?

    • Exothermic
    • Catalytic
    • Endothermic
    • Isothermic

    Why Exothermic, from the Greek for “outwards”; the one that takes heat in is endothermic, “inwards”. The word says which way the heat goes, and once you see that there is nothing to memorise. Almost everything that burns or rusts is exothermic, and almost everything you have to cook or charge is endothermic.

    easier

  2. Where does a reaction's energy come from?

    • From breaking some bonds and forming others
    • From mass that disappears
    • From the atoms, which are used up
    • From the heat outside

    Why From the difference between what it costs to break the starting bonds and what is given back forming the ending ones. The atoms are not used up: they are the same before and after, they only change partners. And no mass disappears either — that is nuclear physics and it does not happen here.

    in between

  3. Breaking the bonds costs 400 kJ and forming the new ones gives back 100 kJ. How much energy is left over?

    Answer -300

    Why 100 − 400 = -300 kJ. Energy is short, so it has to come from outside: the reaction is endothermic. It is a subtraction, not a formula to memorise: what comes back minus what it cost.

    harder