Chapter 14 - Further Redox (L) Flashcards
How are storage cells (batteries) recharged?
An external potential difference is applied that reverses the reactions of the storage cell
How do fuel cells work?
Fuel cells produce a voltage from the chemical reaction of a fuel with oxygen
What is the most common fuel cell?
Hydrogen-oxygen fuel cell
What is an advantage of using a methanol fuel cell compared to a hydrogen fuel cell?
Methanol is a liquid so is easier to store and transport than hydrogen (a gas)
Give 2 advantages of using fuel cells compared with the combustion of petrol or diesel
Less CO2 produced
Greater efficiency
Give 3 disadvantages of hydrogen fuel cells
Explosive
Hard to transport as it is a gas
Difficult supply
Give 2 ways in which hydrogen could be stored
Compressing the gas
Absorbed onto or into a solid
Why do some consider hydrogen as a fuel that uses more energy than petrol or diesel as a fuel?
Because energy is needed in order to produce the hydrogen
What is the difference between standard redox potential and standard electrode potential?
They are the same thing
What can you work out when comparing the standard electrode potentials of two reactions?
The relative positions of equilibrium. In reaction with the more smaller E value the equilibrium will shift to the left (releasing electrons), and in the higher E value the equilibrium will shift to the right (accepting electrons)
How can you work out if a reaction is thermodynamically feasible from electrode potentials?
If the positions of equilibrium which can be worked out from the relative electrode potentials match the proposed reaction equation, the reaction is feasible.
For example, is Zn + Cu2+ -> Zn2+ + Cu feasible.
The electrode potential of Zn2+ + 2e- -> Zn is lower, meaning more Zn2+ ions will be produced in the reaction.
The electrode potential of Cu2+ +2e- -> Cu is higher, meaning more Cu will be produced.
These match the products in the proposed equation, therefore the reaction is feasible.
In the reaction between manganese oxide and hydrochloric acid, why is it important to use concentrated HCl?
Because if you use 1 mol dm-3 HCl (standard conditions), the reaction is not thermodynamically feasible, but if you use conc HCl it is feasible.
Why does using conc HCl make the reaction between manganese oxide and hydrochloric acid feasible?
Because if you don’t use conc HCl, the E value of the Cl2 -> 2Cl- will be higher than the other one, meaning the equilibrium will shift to the right for Cl2 -> 2Cl-, which is not what we want.
However if you use conc HCl, the E value of the Cl2 -> 2Cl- is lower than the other one, meaning the equilibrium will shift to the left for Cl2 -> 2Cl-, making the reaction feasible
What are two reasons why reactions may not take place despite the electrode potentials indicating that they are thermodynamically feasible?
- The reactants may be kinetically stable because the activation energy is very larger
- The reaction may not be taking place under standard conditions
