Redox theory
To balance complex redox equations, which atoms should be balanced first?
All atoms other than oxygen and hydrogen
cathode
What is a secondary cell?
What is the energy conversion that occurs in a fuel cell?
chemical to electrical
Are the reactions that occur in electrolytic cells spontaneous or non-spontaneous?
non-spontaneous
Where on the electrochemical series would you find the strongest oxidant?
top left hand corner
What is the polarity of the anode in a galvanic cell?
negative
When a secondary cell is recharging, which electrode is connected to the negative terminal of the power supply?
Cathode
Hydrogen fuel cells require the input of hydrogen gas as a fuel. How is hydrogen gas produced?
Usually by steam reforming of fossil fuels.
Can also be produced from water using electricity.
What is the main advantage in using molten electrolytes rather than aqueous electrolytes in electrolytic cells?
Water is not present, therefore cannot react preferentially to desired reaction.
What is the oxidation number of S in H2SO4?
+6
Explain the function of a salt bridge
provides negative and positive charged ions that can migrate into half cells to balance the charge. Without a salt bridge, a galvanic cell will cease to operate.
What energy conversion is occurring when a secondary cell is discharging?
chemical to electrical
Hydrogen fuel cells can be used to power cars. What disadvantages are there in the use of hydrogen as a fuel?
Safety and cost of storage and transport (explosive, very light gas must be pressurised or liquefied)
Are the half-reactions in electrolytic cells separated or contained within the one vessel? Explain why
Contained within the one vessel. Ions need to be able to transfer from anode to cathode
The combination of which two reactants on the electrochemical series would produce the most heat if allowed to react?
F2(g) and Li(s)
How is the potential voltage of a galvanic cell calculated?
cell potential difference = E0 of half-cell containing oxidant - E0 of half-cell containing reductant
Why does storing batteries at low temperatures when not in use extend their life?
Reduces the rate of self-discharge (side reactions occurring)
Why are porous electrodes used in fuel cells?
Increase surface area to maximise rate of reaction
Why must the products of electrolytic cells be kept separate?
They will spontaneously react
Explain the term 'conjugate redox pair'
An oxidant (reactant/product) and reductant (reactant/product) that is formed when the oxidant gains electrons or the reductant loses electrons. The conjugate redox pair differ in number of electrons. They are paired to show where electrons are transferred to and from.
Explain how standard electrode potentials (Eo values) are obtained.
By combining half-cells with the standard hydrogen half-cell.
Suggest three factors that reduce the performance of secondary cells over time
loss of active materials (become detached from electrodes or undergo side reactions)
conversion of small crystals of active material to larger crystals which increases resistance
impurities in cell that can cause side reactions
decrease in contact of electrolyte with electrodes because of leakage/reactions
corrosion or failure of internal components
Explain the function of the electrolyte in a fuel cell
Allows the transfer of ions from one electrode to another
Explain Faraday's first and second laws of electrolysis
1st law: mass of metal produced at cathode is proportional to electrical charge passed through the cell
2nd: to produce one mole of metal, 1,2,3 or another whole number of moles of electrons must be consumed.