energy changes 5 Flashcards
what happens to energy in a reaction
energy is conserved in reactions and only move around
what is an exothermic reaction
one that transfers energy to the surroundings usually by heating
examples of exothermic reactions
combustion
neutralisation reactions
oxidation reactions
what is an endothermic reaction
one that takes in energy from the surroundings
examples of endothermic reactions
thermal decomposition
reaction between citric acid and sodium hydrogencarbonate
everyday uses of endothermic reactions
sports injury packs (allow the pack to become instantly cooler)
everyday uses of exothermic reactions
hand warmers(oxidation of iron in air to release energy) self heating cans(chemicals in there base)
what happens to bonds during a reaction
old bonds break and new ones form
what reaction is bond breaking
endothermic
what reaction is bond forming
exothermic
what reaction has a minus in bond calculations
exothermic
what reaction has plus in bond calculations
endothermic
what are reaction profiles
diagrams that show the relative energies of the reactants and products in a reaction
what is the activation energy
the initial rise. and is the minimum amount of energy the reactants need to collide with each other and react.
what does the difference in height mean in reaction profiles
represents the overall energy change in the reaction
what is at higher energy by the end of an exothermic reaction. Product or reactant
reactants
what is at higher energy by the end of an endothermic reaction. Product or reactant
products
what is an eletrochemical cell
a basic system made up of two different electrodes in contact with an electrolyte
two electrodes are usually metal
what is electrolyte
a liquid that contains ions that react with the electrodes
how cells produce energy
chemical reaction between the electrodes and the electrolyte set up a charge difference between the electrodes.
If the electrodes are then connected by a wire, the charge is able to flow and produces electricity.
what factors change the voltage of a cell
type of electrodes
electrolyte
what happens in a normal cell
chemical reaction at the electrode are irreversible
the ions in the electrlyte and the metal ions on the electrode get used up
what happens in a re chargeable cell
reaction can be reversed by connecting it to an external electric current
how is a battery formed
connecting two or more cells together in series
disadvantages of using hydrogen fuel cells
Takes up lots of space
explosive so hard to store
hydrogen made from hydrocarbons which mean they still pollute
advantages of using hydrogen fuel cells
don’t produce as many pollutants
batteries are more expensive
batteries store less energy so need charging
batteries need replacing
why batteries are better than conventional fuels
conventional fuels have finite supply
batteries and fuel cells are cleaner
what is a fuel cell
is an electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy sufficiently
how do fuel cell work
fuel enters the cell it becomes oxidised and sets up a potential difference within the cell
how hydrogen oxygen fuel cell works
Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment
at the anode hydrogen loses electrons to produce H plus ions
H plus ions in the electrolyte move to the cathode
at the cathode oxygen gains electrons from the cathode and reacts with H plus ions to make water
the electrons flow through an external circuit from anode to cathode
overall reaction is hydrogen + oxygen = water
