C7: Energy Changes Flashcards
reactions that transfer energy from the chemicals to their surroundings are
exothermics
what are exothermic reactions often associated with
a rise in temperature
reactions that transfer energy from the surroundings to the reacting chemicals are
endothermic
what are endothermic reactions often associated with
fall in temperature
examples of exothermic reactions
respiration, combustion, neutralisation
examples of endothermic reactions
thermal decomposition
products that use exothermic reactions
hand and body warmers, self-heating cans
products that use endothermic reactions
chemical cold packs
how do cooling packs work
when the ammonium nitrate in it dissolves, it absorbs energy from its surroundings, making it colder.
how do hand and body warmers usually work
small metal disc makes crystals of salt that release heat. can be done again if the crystals are evaporated with boiling water.
what does an exothermic reaction profile look like
reactants have more energy than products
what does an endothermic reaction profile look like
products have more energy than reactants.
what is activation energy
the amount of energy needed to cause colliding particles to form a reaction
what are bond energies measured in
kJ/mol
what do electrical cells and batteries rely on
differing reactivity of metals
how does it work creating an electrical cell
join to metals together by a wire and dip them in an electrolyte, electrons flower through the wire from one metal to another.
what’s the general rule for reactivity in electrical cells
the greater the difference in reactivity between the two metals used, the higher the voltage produced.
what gas are used as fuel
hydrogen
pros of hydrogen powered vehicles
no co2
cons of hydrogen powered vehicles
need non-renewable fossil fuels to be made, can be difficult to store
what is a fuel cell
cells fed with hydrogen and oxygen that produce water.
pros of fuel cells
-no electrical recharge
-no pollutants
-range of sizes
cons of fuel cells
-hydrogen is flammable
-hydrogen difficulty storing
-hydrogen is produced for the cell by non-renewable sources.
how does a hydrogen fuel cell work?
-hydrogen gas supplied as fuel to the cathode, diffusing through graphite electrode. provides electrons to external circuit.
-oxygen gas supplied to the anode, diffusing through graphite. accept electrons from outer circuit
-add the electrodes together and the electrons and hydroxide cancel out.
LEFT WITH OXIDATION OF HYDROGEN
