C.7 Energy changes
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what is an exothermic reaction
reaction where energy is transferred from the reacting substances to their surroundings
what is an endothermic reaction
reaction where energy is transferred fto the reacting substances from their surroundings
examples of exo reactions (what happens in reusable ones) eg for endo
hand warmers in the reusable crystallisation of a supersatured solution is used. instant cold packs
how do you recognise a exothermic reaction from a graph
line of products is belowe reactants, difference is the energy change
how do you recognise a endothermic reaction from a graph
line of products is above reactants
what is activation energy
minimum amount of energy needed before colliding particles of reactants have enough energy to cause a reaction
what happens when new bonds are formed
energy is released(exo)
what happens when bonds are destroyed
energy gets used(endo)
equation for energy change
energy for breaking -making
what is a simple electrical cell
2 metals are dipped in a salt solution and joined by a wire, more reactive will donate electrons to the less. the higher the difference in reactivity series between the metals the higher the voltage produced
how can hydrogen be used in vehicles
as a fuel, burned in combustion engines or used in fuel clls to power the car.
how does hydrogen act like a fuel
is oxidised and provides a source of electrons in the hydrogen fuel cell, only waste is water
half equqations for hydrogen cells
2H2 + 4OH-=4H2O + 4e- O2+2H2O+4e-=4OH-
disadvantaged of hydrogen fuel cells
flmmable, difficult to store
why cant batteries be put in household waste
can be toxic could cose pollution in landfill sites
what combination of electrodes would give highest volatef
2 metals with largest difference in voltage
factors that could affect voltage produced in batteries
size of electrodes tempo/conc of solution
adv of using H in cars
takes less time to refuel grater range