Ch7 Energy Changes Flashcards
Endothermic reacitons
temperature decreases
energy is transferred from surroundings to the reacting chemicals
Exothermic reactions
temperature increases
energy is transferred to the surroundings from the reacting chemicals
5 Examples of exothermic reactions
neutralisation precipitation displacement reaction oxidation combustion
3 examples of endothermic reactions
thermal decomposition
dissolving salts
reaction of citric acid and sodium hydrogencarbonate
Exothermic bond energy
energy released by forming bonds is greater than the energy used to break the existing bonds
Endothermic bond energy
energy released by forming bonds is less than the energy used to break the existing bonds
Overall energy change
total energy needed to break bonds - total energy released by forming new bonds
breaking bonds
energy must be supplied
forming products
energy is released
activation energy
minimum amount of energy needed by particles when they collide for a reaction to occur
from reactants energy to peak
overall energy change
energy of reactants - energy of products
Uses of exothermic reactions
self-heating cans
hand warmers
Uses of endothermic reactions
sports instant ice packs
How chemical cells work
two metals are joined together by a wire and dipped into an electrolyte
electrons flow through the wire
the greater the difference in reactivity between the two metals used, the higher the voltage produced
Batteries
two or more cells connected together in series
provide a greater voltage
Rechargeable battery
an external electrical current is supplied
reverses the chemical reactions at each electrode when the cell is discharging
Alkaline batteries
non-rechargeable
chemical reactions stop when one reactant has been used up
Fuel cells
supplied by an external source of fuel
fuel is oxidised electrochemically within the cell to produce a potential difference
hydrogen fuel cell process
Hydrogen is supplied to the negative electrode
It diffuses through the graphite electrode and reacts with hydroxide ions to form water
provides a source of electrons to an external circuit
2H2(g) + 4OH-(aq) –> 4H2O(l) + 4e-
oxygen gas is supplied to the positive electrode
it diffuses through the graphite and reacts with the electrolyte to from hydroxide ions
it accepts electrons from the external circuit
O2(g) + 2H2O(l) + 4e- –> 4OH-
The electrons and OH- ions cancel out. Left with overall change of the oxidation of hydrogen
2H2(g) + O2(g) –> 2H2O(l)
Hydrogen fuel cell advantages
doesn’t produce pollutants
alternative to conventional fossil fuels
don’t need to be electrically recharged
can be a range of sizes for different uses
Hydrogen fuel cell disadvantages
hydrogen is highly flammable
hydrogen is produced from non-renewable sources
hydrogen is difficult to store
