Redox Flashcards
A redox reaction is one where
Both oxidation and reduction take place
Oxidation number of an uncombined element
0
Oxidation number of combined oxygen
-2
Unless in H2O2
Oxidation number of combined hydrogen
+1
Define oxidation number
A measure of the number of electrons that an atom uses to bond with atoms of another element
Define oxidising agent
A reagent that oxidises another species
Define reducing agent
A reagent that reduces another species
Half reactions show…
Reduction or oxidation only
Define oxidation
A loss of electrons, or an increase in oxidation number
Define reduction
A gain of electrons, or a decrease in oxidation number
Define standard electrode potential
The emf of a half cell compared with the standard hydrogen half cell, measured at 298K with solution concentrations of 1moldm^-3 and a gas pressure 1atm
Describe a metal/metal ion half cell eg Zn/Zn2+
A zinc strip is placed in a solution of Zn2+ (aq) (1moldm^-3)
Describe a standard hydrogen half cell
A platinum electrode, which is surrounded by a glass tube with holes in to allow bubbles of H2(g) to escape, is placed in a beaker of acid solution containing 1moldm^-3 H+(aq)
H2(g) is pumped into the glass tube from the top at 298K and 1atm
Describe a metal ion/metal ion half cell
A platinum electrode is placed in a solution containing equal concentrations of Fe2+ (aq) and Fe3+ (aq)
What does a negative electrode potential tell you about the half cell?
It undergoes oxidation (releases electrons) when connected to a hydrogen half cell
What does a positive electrode tell you about a half cell?
The half cell undergoes reduction (accepts electrons) when connected to a hydrogen half cell
Which half equation gets reversed when working out full equations of reactions from half equations and their electrode potentials
The equation with the more negative electrode potential
What are the limitations of predictions made using standard electrode potentials?
Non-standard conditions alter the value of the electrode potential:
Changes in concentration
Many reactions may not take place in the aqueous state, or at room temperature
The rate of reaction may be extremely slow because of a high activation energy
What are the 3 main types of modern-day cells and batteries?
Non-rechargeable cells
Rechargeable cells
Fuel cells
A fuel cell…
Uses the energy from the reaction of a fuel with oxygen to create a voltage
What half equation takes place at the negative terminal of a hydrogen-oxygen fuel cell?
2H2O(l) + 2e- H2(g) + 2OH-(aq)
Scientists in the car industry are developing fuel cell vehicles fuelled by…
Hydrogen gas and hydrogen-rich fuels
What are the advantages to using fuel cell vehicles?
Less pollution and less CO2
Greater efficiency
What half equation takes place at the positive terminal of a hydrogen-oxygen fuel cell?
1/2O2(g) + H2O(l) + 2e- 2OH-(aq)
How might hydrogen be stored in fuel cell vehicles?
As a liquid under pressure
Adsorbed on the surface of a solid material
Adsorbed within a solid material
What are the limitations of hydrogen fuel cells?
The feasibility of storing a pressurised liquid
Current absorbers and adsorbers of hydrogen have a limited lifetime
Fuel cells use toxic chemicals in their productions
Storing and transporting hydrogen is expensive and requires a lot of energy
Current fuel cells have a limited lifetime, requiring regular replacement and disposal following high production costs
What are the limitations of a hydrogen economy?
Public and politic acceptance of hydrogen as a fuel
Handling and maintenance of hydrogen systems
Initial manufacture of hydrogen, requiring energy
