T5 energy changes Flashcards
what is the conservation of energy principle
energy is conserved in chemical reactions
amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes places
what is an exothermic reaction
give examples
reaction where energy is transferred to the surroundings so that the surrounding temperature increases
combustion, oxidation reactions, neutralisation
negative sign of energy change
what is an endothermic reaction
give examples
reaction where energy is taken in from the surroundings so that the surrounding temperature decreases
thermal decomposition, reaction of citric acid and sodium hydrogencarbonate
what is activation energy
minimum amount of energy that particles need to react
what is a reaction profile
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 supplied to break bonds and energy is released when bonds are made
exo: energy released from forming bonds is greater than that needed to break the bonds
endo: energy needed to break bonds is greater than energy released making them
equation to find enthalpy change in terms of bond energies
energy of reaction = sum of bonds broken - sum of bonds made
what is a cell
composed of 2 electrons dipped in an electrolyte solution
produces electricity from a chemical reaction
what is a battery
2 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
advantages and disadvantages of using cells and batteries
quite cheap, some are rechargeable, a convenient source of electrical energy
harmful chemicals
describe rechargeable and non-rechargeable cells
r - chemical reactions are reversed when an external current is supplied
n - reactants are used up, can’t be recharged
what is a fuel cell
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: 2H2 > 4H+ + 4e-
anode: O2 + 4H+ + 4e- > 2H2O
overall: > 2H2 + O2 > 2H2O
advantages and disadvantages of hydrogen fuel cells
a: no pollutants, no recharging
d: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis
