5 Energy Changes Flashcards
what is the conservation of energy principle?
energy is conserved in chemical reactions. the amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
what is an endothermic reaction? give examples.
a reaction where energy is taken in from the surroundings so the surroundings temperature decreases - thermal decomposition, reaction of citric acid and sodium hydrogencarbonate. positive sign of energy change.
what is an exothermic reaction? give examples.
a reaction where energy is transferred to the surroundings so that the surroundings temperature increases - combustion, oxidation reactions and neutralisation (acid + alkali) reactions. negative sign of energy change.
what is a cell?
a cell is composed of two electrodes dipped in an electrolyte solution. it produces electricity from a chemical reaction.
what is activation energy?
minimum amount of energy that particles need to react.
what is a reaction profile?
reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.
what occurs in a chemical reaction in terms of bond energies? describe exothermic and endothermic reactions in terms of bond breaking/forming.
energy is supplies to break bonds and energy is released when bonds are made; exothermic - energy released from forming bonds is greater than that needed to break the bonds; endothermic - energy needed to break bonds is greater than energy released making them.
describe rechargeable and non-rechargeable cells.
rechargeable - chemical reactions are reversed when an external current is supplied
non-rechargeable - reactants are used up, cannot be recharged
what is the equation to find enthalphy change in terms of bond energies?
energy of reaction = sum of bonds broken - sum of bonds made
what is a battery?
a battery consists of two or more cells connected in series.
what determines the voltage obtained from a cell?
identities of metals used as electrodes and the identity and concentration of an electrolyte.
what is a fuel cell?
fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
what is the overall reaction in a hydrogen fuel cell? what are the half equations?
cathode: 2 H2 –> 4 H+ + 4 e-
anode: O2 + 4 H+ + 4 e- –> 2-H2O
overall: –> 2 H2 + O2 –> 2 H2O
state the advantages and disadvantages of using cells and batteries.
(+) more or less cheap, some are rechargeable, a convenient source of electrical energy
(-) harmful chemicals
