C7 Flashcards
which would have a larger potential difference:
a cell containing magnesium and copper
or
a cell containing zinc and tin
magnesium and copper- because there is a greater difference in reactivity
making bonds is:
exothermic
breaking bonds is:
endothermic
what is an electrolyte?
A solution that can conduct electricity
Why do cells only produce electricity for a set time?
Eventually the chemicals will be used up and the reaction will stop
Greater difference in reactivity between the electrodes =
greater potential difference
How comes some batteries are rechargeable?
The chemical reactions can be reversed
Fuel cells: negative electrode =
2H2 –> 4H+ + 4e-
Fuel cells: positive electrode =
O2 + 4H+ + 4e- —> 2H2O
Fuel cells: overall equation
2H2 +O2 —> 2H2O
compare hydrogen fuel cells and rechargeable batteries in terms of working/ energy being provided
HFC- will provide electricity as long as hydrogen is provided
RB- will run out and need to be recharged
compare hydrogen fuel cells and rechargeable batteries in terms of: how long they work
HFC- doesn’t become any less efficient
RB- store less electricity the more they’re charged so will need replacing
compare hydrogen fuel cells and rechargeable batteries in terms of: products
HFC - produces drinkable water
compare hydrogen fuel cells and rechargeable batteries in terms of: safety
HFC - Hydrogen is highly flammable and explosive
RB - no dangerous fuels, though can catch fire if manufactured incorrectly
compare hydrogen fuel cells and rechargeable batteries in terms of: potential difference
HFC - low p.d produced so several needed
RB- produces a greater pd
Law of conservation of energy
Energy is conserved
Exothermic
- gives out energy
- temperature of surroundings increases
- energy in bonds of reactants will be HIGHER than energy in bonds of products
Endothermic
- takes in energy
- temperature of surroundings decreases
- energy of bonds of reactants is LOWER than energy of bonds of products
Exothermic reaction profile–
Product lower than reactants
Endothermic reaction profile–
Product energy higher than reactant
Activation energy on reaction profile
Arrow from reactants to top of curve
Overall energy change on reaction profile
Arrow from reactants to products (points down on exothermic, up on endothermic)
Endothermic reaction, energy from ___ bonds is bigger
Breaking
Exothermic reaction, energy from ___ bonds is bigger
Making
Cells are made from
Two different metals and electrolyte
In cells, electrons flow from one metal to the other to produce electric current. Why?
They are different metals
Bigger difference in reactivity between the two metals in the cell,
Bigger voltage produced
A battery is made of?
Two or more cells in series which increases the potential difference
Alkaline batteries can be used how many times?
Once
Rechargeable batteries can be recharged. Why?
Charging the battery reverses the chemical process
A fuel cell is provided with…
Fuel [hydrogen]
Air//oxygen
Fuel is oxidised electrochemically to produce…
Potential difference
Benefits of fuel cell over a combustion engine(petrol)
- Doesn’t release thermal energy as waste
- Only product is water rather than many harmful gases
Fuel cell half equation: negative electrode
2H2 + 4OH- —> 4H2O + 4e-
Fuel cell half equation: positive electrode
O2 + 2H2O + 4e- –> 4OH-
Fuel cell half equation: ultimately
2H2(g) + O2(g) –> 2H2O(l)
