chem energy Y10

exothermic reaction + examples

releases energy (creating bonds > breaking them), eg self heating cans or combustion or respiration or neutralisation or rust

endothermic reaction + examples

takes in energy (breaking bonds > creating them), eg instant ice packs or photosynthesis or thermal decomposition

hydrogen fuel cell facts

harder to find places to refuel and far more expensive to refuel and manufacture, but no harmful waste products (only produces water) and more efficient

regular battery facts

more expensive than traditional petrol cars, but cheaper than hydrogen fuel cells. easy to recharge and not too expensive. relatively efficient

reactions in a hydrogen fuel cell

positive electrode: O2 + 2H2O + 4e- → 4OH-

negative electrode: 2H2 + 4OH- → 4H2O + 4e-

why are some batteries impossible to recharge

reversing current through battery does not reverse the reaction

reaction diagrams

initial energy as a straight line, curve upwards for activation energy and downwards for final energy. if final is higher than initial, endothermic. if initial is higher than final, exothermic. energy change is difference between initial and final, activation is difference between initial and the highest point

voltage + battery notes

difference in reactivity between metals at either end of the battery. electrons flow from more reactive → less reactive. the less reactive terminal is negative because electrons flow to it, so flow positive → negative

rechargable vs nonrechargable batteries

non are cheaper but worse for the environment, they fail when one reactant runs out and should be recycled but mostly go to landfill.

rechargables are more expensive, better for the environment, recharged by reversing current flow. they run out eventually. should be recycled