Chapter 8 Electrochemistry in Action ✓ Flashcards
(70 cards)
What are galvanic cells?
An electrochemical cell that converts the chemical energy of spontaneous redox reactions into electrical energy
How do galvanic cells generate an electric current?
-Galvanic cells generate an electric current by physically separating the oxidation and reduction half-equations into two half-cells.
-Electrons released in the oxidation reaction travel through an external circuit to the reduction half-cell, harnessing the spontaneous redox reaction to produce electricity.
What are other words for galvanic cells?
Voltaic cells or electrochemical cells
What are some significant applications of electrochemistry?
Electrochemistry is used in electrolysis processes such as:
-Electrorefining metals like copper
-Electroplating to create durable and decorative metal coatings.
What are primary cells?
-Primary cells, such as dry cells, are non-rechargeable galvanic cells that contains a fixed amount of oxidant and reductant.
-Once these reactants are used up, they cannot be replaced or regenerated, so the cell becomes ‘flat’ and is discarded after a single use
What are secondary cells?
-Secondary cells, such as the lead-acid cell (car battery), are rechargeable galvanic cells.
-When galvanic cells like this go flat, they can be recharged by applying a DC voltage that forces current through the cell in an opposite direction to that which occurs spontaneously (normally) during its discharge (use)
Why do secondary cell have a limited number of recharge cycles?
Because their electrodes degrade physically over time. After a certain number of cycles, they can no longer effectively reverse the reactions, so the cell can no longer be recharged
Why is recharging essential in secondary cells?
It uses electrical energy to reverse the spontaneous discharge (normally used) reactions and regenerate the oxidant and reductant originally present in the charged cell, allowing the secondary cell to be used again.
What are the three types of galvanic cells?
-Primary cells
-Secondary cells
-Fuel cells
What are fuel cells?
Fuel cells are galvanic cells where the oxidant and reductant are continuously fed into the cell.
The reductant is usually a fuel e.g. hydrogen, methane or methanol and the oxidant is usually oxygen gas
How do fuel cells operate continuously?
Fuel cells (theoretically) operate continuously because the fuel (reductant) and oxygen (oxidant) are constantly supplied to the cell, allowing the reactions to keep going
What are the advantages and disadvantages of dry cells?
Advantages
-Dry cells are simple and inexpensive
-Ideal for devices requiring low currents (e.g. flashlight, calculator)
-Materials used have a negligible environmental impact, so spent dry-cells are considered non-hazardous waste
Disadvantages
-They produce a maximum voltage of 1.5 V
-The voltage slowly decreases over the life of the cell
-They have a very low energy-to-mass ratio (low energy density)
-Cannot be recharged and must be discarded when flat
-No current economical process for recycling spent dry cells
What is an anode?
An electrode where an oxidation reaction occurs (loss of electrons for the electroactive species).
What is a cathode?
A cathode is an electrode where a reduction reaction occurs (gain of electrons for the electroactive species)
What is an alkaline cell?
It is a type of primary battery that uses Zn as the reducing agent and MnO2 as the oxidising agent, with a potassium electrolyte
What materials do alkaline cells use as the reducing and oxidising agents?
Zinc (reducing agent) and manganese dioxide (oxidising agent), just like dry cells
What electrolyte does an alkaline cell use and how does it improve shelf life?
It uses potassium hydroxide (KOH) electrolyte paste instead of ammonium chloride (NH4Cl), prevent zinc anode corrosion and improving shelf life.
Why doesn’t corrosion occur in alkaline cells?
-Because alkaline cells use potassium hydroxide electrolyte, which is alkaline and contains no acidic ammonium ions, like dry cells do.
-This prevents the zinc anode from dissolving, so corrosion dose not occur, leading to better shelf life and performance
How does corrosion occur in dry cells?
-Corrosion occurs because the ammonium chloride electrolyte produces acidic ammonium ions that react with and dissolve the zinc anode over time.
-This corrosion reduces the zinc anode’s effectiveness, shortening the cell’s shelf life and performance
What are dry cells?
Dry cells are primary batteries that use zinc as the anode (reducing agent), manganese dioxide as the cathode (oxidising agent), and an ammonium chloride paste as the electrolyte.
Why do dry cells use ammonium chloride paste?
Because it is cheap and effective electrolyte that helps the cell produce electricity, even though it can cause zinc corrosion
What is the overall equation as well as the half equations for dry cells?
Dry cells contains zinc and manganese dioxide and a ammonium chloride electrolyte paste
Balanced equation:
Zn(s)+ 2MnO2(s) + 2NH4Cl(aq) -> ZnCl2(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)
Half equations
Anode: Zn(s) -> Zn2+(aq) +2e-
Cathode: 2MnO2 + 2NH4+ + 2e- -> Mn2O3(s) + 2NH3(aq) + H2O(l)
Overall redox reaction
2MnO2 + 2NH4+ + Zn(s) -> Mn2O3(s) + 2NH3(aq) + H2O(l) + Zn2+(aq)
What is the overall equation as well as the half equations for alkaline cells?
Alkaline cells contain zinc and manganese dioxide and potassium hydroxide electrolyte (not present in equation)
Balanced equation:
Zn(s) + 2MnO2(s) + 2H2O(l) -> Zn(OH)2(s) + 2MnOOH(s)
Half equations
Anode: Zn(s) + 2OH-(aq) -> Zn(OH)2(s) + 2e-
Cathode: 2MnO2(s) + H2O(l) + 2e- -> Mn2O3(s) + 2OH-(aq)
Overall redox reaction
Zn(s) + 2MnO2 ->ZnO(s) + Mn2O3(s)
What devices are alkaline cells ideal for?
Devices that require a higher current flow e.g. toys, portable radios, CD players, electronic games, torches
