Topic 5 - energy changes Flashcards
what is an exothermic reaction?
one that transfers energy to the surroundings
give some everyday uses of exothermic reactions
handwarmers- use oxidation of iron in air to release energy
self heating cans of hot chocolate or coffeee
what is an endothermic reaction?
one that takes in energy from the surroundings
what is an everyday use of endothermic reactions?
sports injusry packs
the chemical reaction allows the pack to become instantly cool without putting it in the freezer
what is activation energy?
the minimum amount of energy the reactants need to collide with eachother and react.
is bond braking exothermic or endothermic?
why
endothermic
energy must be supplied to break existing bonds
is bond formation exothermic or endothermic
why
exothermic
energy is released when new bonds are formed
when in an exothermic reacction is the energy greatest?
the energy released by forming bonds is greater than the energy used to break them
when in an endothermic reaction is energy the greatest?
energy used to break bonds is greater than the energy released by forming them
what is the equation for overall energy change?
overall energy change = energy required to break bonds-energy released by forming bonds
what factors affects the voltage of a cell?
type of electrode - different metals will react differemtly with the same electrolyte
the bigger the reactivity difference of the electrodes , the bigger the voltage of the cell
electrolyte used - different ons in solution will react differently with the metal electrodes used
how can you predict the voltage of a cell using info about the voltages of other cells?
if the metal used for electrode a is less reactive than the other metal electrode, the voltage us ppositive or vice versa.
what is a battery and describe the votltage
a battery is formed by connecting two oe more cells together in series.
the voltages of the cells in the battery are combined so there is a bigger voltage overall
explain why non rechargable batteries eventually loose their charge.
the reaction is irreversible
over time the reacting particles(the ions in the electrolyte and the metal ions on the electrode) get used up and turned into the products of the reaction
once any one of the reactants is used up , the reaction cant happen and so no electricity is produced .
the products cant be turned back into the reactants so the cell cant be recharged
in a rechargable cell , how can the reaction be reversed?
by connecting it to an external electric current
what is a fuel cell?
an electrical cell that is supplies with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently.
in a fuel cell , is the anode positive or negative?
negative
describe how hydfrogen oxygen fuel cells work.
hydrogen goes into the anode comparrtment and oxygen goes into the cathode compartment
at the -ve electrode,hydrogen loses electrons to produce H+ ions . this is oxidation
H+ions in the electrolyte move to the cathode
at the +ve electrode , oxygen gains electrons from the cathode and reacts with H+ ions to make water. this is reduciton
the electrons flow through an external circuit from the anode to the cathode -this is the electric current
what is the equation forthe overall reaction in a fuel cell
oxygen + hydrogen makes water
name the electrolyte commonly used in hydrogen oxygen fuel cells
an acid
such as phosphoric acid
what is the half equation for the reaction of oxygen in a hydrogen fuel cell?
o2 + 4H+ + 4e- -> 2H20
wht is the half equaiton for the reaction of hydrogen in a hydrogen oxygen fuel cell?
H2 -> 2H+ + 2e-
what are the advantages of fuel cell powered cars over battery powered cars?
fuel cell vehicles dont produce as many pollutants as other fuels-no greenhouse gases
the only by products are water and heat
battery powered vehicles dont produce pollutants but their batteries are more poouting to dispose of than fuel cels.
batteries in electric vehicles are rechargable but there is a limit to how many times they can be recharged before they need replacing.
batteries are more expensive than fuel cells
batteries store less energy than fuel cells and so would need to be recharged more often
what are the disadvantages of hydrogen oxygen fuel cells?
hydrogen is a gas so it takes up loads more space to store than a rechargeable battery.
hydrogen is explosive when mixed with air so its hard to store safely
the hydrogen fuel is often made either from hydrocarbons or by electrolysis of water which uses electricity
