Electrochem I Flashcards
What is electrochemistry?
The study of the interconversion of electrical and chemical energy via the movement of electrons from one chemical species to another
What is galvanic cell?
Redox reaction which is spontaneous and the chemical energy is converted to electrical energy
What is electrolytic cell?
Redox reaction which is non-spontaneous and electrical energy is converted to chemical energy
What occurs at the anode?
Oxidation (an - ox)
What is the sign of anode of galvanic cell and why?
negative
Anode, oxidation, e- are produced and build up, resulting in negative electrode
What occurs at the cathode?
Reduction (red - cat)
What is the sign of cathode of galvanic cell and why?
positive
Cathode, reduction, gains electrons thus to receive electrons, cathode must be positive
What does salt bridge consist of?
- inverted U-tube consisting of a salt solution, eg. NaNO3 that does not react with ions in either half-cell
- ends of the salt bridge stuffed with porous plugs to prevent direct mixing of two solutions but allow the passage of ions through them
What are the functions of a salt bridge?
- Prevents the direct mixing of two solutions but allows the passage of ions between the two half-cells to complete the electrical circuit
- Helps to maintain the electrical neutrality of each half-cell
What is emf or cell potential?
potential difference between two electrodes in a galvanic cell that provides a driving force that pushes electrons through an external circuit
What is electrode potential?
- When metal is placed in a solution of its ions, some metal atoms lose electrons to form hydrated ions and electrons will be left behind on the metal
- Over time, there is build-up of negative charge on the metal surface which will attract positive metal ions in the solution which may recombine to form metal ions
- At equilibrium, there will be a constant negative charge on the metal and a constant number of positive metal ions present in the solution around the metal
- Absolute electrode potential is the potential difference due to the separation of charge between metal strip and metal ions in the solution
By convention, what is the forward reaction of the half equation of electrode potential?
reduction
How to write electrode potential or reduction potential?
E(M n+(aq)/M (s))
What does a positive electrode potential mean?
Position of equilibrium lies more to the right, reduction favoured
What does a negative electrode potential mean?
Position of equilibrium lies more to the left, oxidation favoured
What would affect the value of electrode potential?
Factors that affect the position of equilibrium
What are the factors affecting electrode potential?
- Nature of metal
- Concentration of M n+ ions
- Temperature
- Pressure of gaseous species
How does the nature of metal affect electrode potential?
When a metal is highly electropositive, the metal atoms have a higher tendency to lose electrons and become positive ions, the position of equilibrium lies more to the left and the electrode potential therefore becomes more negative
How does the concentration of M n+ ions affect electrode potential?
If the concentration of the hydrated metal ions is decreased in the equilibrium system, the position of the equilibrium will shift to the left to replenish some of metal ions removed, thus electrode potential becomes more negative
How does temperature affect electrode potential?
If the temperature increases, there is an increase in tendency of metal to dissolve in the solution to form hydrated ions, the position of equilibrium shifts to the left and electrode potential becomes more negative
How does pressure of gaseous species affect electrode potential?
When the pressure of gas increased, the position of equilibrium shifts to the right to reduce the number of gaseous molecules and hence the electrode potential becomes more positive
What is the conditions for standard electrode potential?
- 298 K and 1 bar
- All solutions have a concentration of 1 mol dm-3
- Any gas involved has a pressure of 1 bar
- Platinum is used as electrode when half-cell does not include a metal
- Voltmeter has high resistance so negligible current flows and maximum potential difference can be measured
Can the absolute electrode potential of a single half-cell be measured?
No, only the difference in potentials of two electrodes in a complete circuit is measurable
What is standard electrode potential of an electrode?
Relative potential of this electrode under standard conditions compared with the standard hydrogen electrode whose electrode potential is assigned as 0.00V
What does standard hydrogen electrode consist of?
H2 (g) at 1 bar bubbling over platinum electrode coated with finely divided platinum which is dipped into 1 mol dm-3 H+ (aq) at 298K
What is the three most common types of half-cells used to measure standard electrode potential?
- metal in contact with its ions in aqueous solution
- non-metal in contact with its ion in aqueous solution
- ions of the same element in different oxidation states
Is electrode potential dependent on the number of electrons being transferred?
It is independent
Are species on the left or right of the equation oxidising agents?
Oxidising agents itself is always reduced, therefore it is on the left side
Are species on the left or right of the equation reducing agents?
Reducing agents itself is always oxidised, therefore it is on the right side
Does a stronger oxidising agent have a more positive or more negative standard electrode potential?
Stronger oxidising agents have a greater tendency for itself to be reduced, therefore it has a more positive standard electrode potential as the forward reaction is favoured
Does a stronger reducing agent have a more positive or more negative standard electrode potential?
Stronger reducing agents have a greater tendency for itself to be oxidised, thus the backward reaction is favoured and it has a more negative standard electrode potential
How to determine the relative stabilities of metal ions in different oxidation states?
If it is reduction, standard electrode potential more negative, more stable
If it is oxidation, standard electrode potential more positive, more stable
What is standard cell potential?
Electromotive force of a galvanic cell which consists of two half-cells connected under standard conditions. It is the difference between the two standard electrode potentials
What is the equation to calculate standard cell potential?
E cell = E (red) - E (ox)
*note that both values are standard reduction potential, meaning that you don’t flip the sign when you flip the equation
How to predict the spontaneity of redox reaction with standard cell potential?
- If E(cell) > 0, forward reaction is energetically feasible, spontaneous under standard conditions
- If E(cell) < 0, forward reaction is not energetically feasible, the reverse reaction is expected to be spontaneous under standard conditions
- If E(cell) = 0, equilibrium is established
What is the equation linking Gibbs free energy and electrode potential?
delta G naught = - nFE(cell) naught
n: amount of electrons transferred
What are the limitations of standard electrode potentials?
- E(cell) value indicates whether a reaction is possible (i.e. energetic stability) but not the rate of reaction (i.e. kinetic stability), eg. high activation energy
- E(cell) value relate only to standard conditions, changes in temperature, pressure and concentration will affect the values of electrode potential
How does fuel cell differ from conventional electrochemical cells?
- Fuel cells have a steady supply of reactants to produce an electric current
- Fuel cell (reactants) are continuously replaced as they are consumed and the products are continuously removed as they are formed
What are the surface of electrodes of fuel cells coated with?
Surface of porous graphite electrodes are coated with Ni and NiO which act as catalysts (Pt and Pd can also be used)
What are the advantages of fuel cell?
- Pollution free
- High power to mass ratio
- Highly efficient, with at least 70% of chemical energy converted into electricity
What are the disadvantages of fuel cell?
- Expensive due to large amounts of Ni and Pt required as catalysts
- catalyst are easily poisoned by impurities and are unable to function efficiently for long periods of time
- A high temperature is required to initiate a reaction in a fuel cell
