Galvanic Cell
General Galvanic cell
Galvanic cells are exothermic reactions!
The fundamental aspect of galvanic cell is that the reactions occurring are spontaneous redox reactions meaning that they will occur without a net input of energy and will convert chemical energy into electrical energy. As we are aware that redox reactions involve oxidation and reduction occurring simultaneously, the galvanic cell is then made up of two distinctive compartments or half-cells. In one half-cell, oxidation occurs and in another reduction occurs however they occur at the same time. Half cells are made up of a metal electrode which allow for electrical currents to flow into or out of the half-cell. Each half-cell also contains in an aqueous form the the metal ions and with these the half-cells are complete. The half-cell that undergoes oxidation we refer to it as being the stronger reductant because it reduces the other other half-cell and itself is oxidized. Likewise the half-cell that undergoes reduction we refer it to as being a stronger oxidant because it oxidizes the other half-cell and itself is reduced. (Nelson Chemistry, 2nd Edition, pp 415-422)
Note: Redox reactions are spontaneous because compound like to be in a lower energy state then a high one, this is why redox reactions occur because this way they release their energy and go to a lower energy level hence they are stable. In nature you will never find raw copper metal or any other raw metal for that matter, rather you will find it as in a compound generally speaking its in oxide of some form. This is because it has long underwent a redox reaction and is now in a lower energy state hence if we want to extract the metal we need to put in energy (electrolysis) to do so. (Ollie Grodon, chem teacher)
Note: Redox reactions are spontaneous because compound like to be in a lower energy state then a high one, this is why redox reactions occur because this way they release their energy and go to a lower energy level hence they are stable. In nature you will never find raw copper metal or any other raw metal for that matter, rather you will find it as in a compound generally speaking its in oxide of some form. This is because it has long underwent a redox reaction and is now in a lower energy state hence if we want to extract the metal we need to put in energy (electrolysis) to do so. (Ollie Grodon, chem teacher)
Summery
oxidation= the loss of electrons (reductants undergo oxidation)
Reduction= gain of electrons (oxidants undergo reduction)
(Heinemann Chemistry 2, pp 414-424)
Reduction= gain of electrons (oxidants undergo reduction)
(Heinemann Chemistry 2, pp 414-424)
When an oxidant and reductant that are in direct contact react spontaneously with one another, the electron transfer cannot be detected. The energy released in the reaction is in the form of heat energy however the chemical energy from the redox reaction can also be converted into electrical energy which is the main purpose of galvanic cells. (Nelson Chemistry p 416)
External Circuit
Every galvanic cell has an external circuit and an internal circuit. The external circuit is the wires that are attached to each electrode which allows the passage of electrons from one electrode to another. This means that each electrode has a specific polarity. In all galvanic cells the oxidation half-cell is the anode and is assigned as the negative terminal since electrons are produced here and the reduction half-cell is the cathode which is assigned as the positive terminal since the electrons are used up in here.
A voltmeter is just an equipment used to measure voltage or potential difference.
Note: by definition anode is where oxidation occurs and cathode is very reduction occurs.
A voltmeter is just an equipment used to measure voltage or potential difference.
Note: by definition anode is where oxidation occurs and cathode is very reduction occurs.
Internal Circuit
The internal circuit is made by the salt bridge which is ionic in nature and simply has two functions:
In order for the salt bridge to function like one, it must follow some guideline otherwise the galvanic cell will not simply work. These are:
((Heinemann Chemistry 2, pp 416)
- Completes the circuit by allowing the charged particles to move through it, while still keeping the oxidant and reductants apart.
- Supplies anions (-) to the anode and cations (+) to the cathode to maintain electrical neutrality. If the salt bridge is not in place then the galvanic cell will function for a limited period of time and then stop. This is because of the fact that positive charges build in the anode half-cell while the cathode half-cell is being depleted of positive ions hence a charge difference will be created which will prevent the flow of further electrons.
In order for the salt bridge to function like one, it must follow some guideline otherwise the galvanic cell will not simply work. These are:
- It must be highly soluble in water and form a solution that conducts electricity well, in another words it must be a strong electrolyte. An electrolyte is a solution that contains ions which are free to move around and so conduct electricity.
- It must not undergo any reactions with the solution or electrode e.g. it must not undergo a precipitation reaction with either electrolyte solution.
((Heinemann Chemistry 2, pp 416)
The Electrochemical Series
Electrochemical series
This series is crucial in predicting any reaction that may or may not occur. The electrochemical series basically list many oxidants and its conjugate reductants according to their relative strength.
For a spontaneous reaction to occur there has to a strong oxidant reacting with a strong reductant.
Everything in the electrochemical series is take from the reference point of Hydrogen gas and its potential EMF is hence derived from there. The EMF in the electrochemical series occurs only under the standard condition of:
Under standard conditions the EMF of a galvanic cell from the electrochemical series is given by the equation:
- The oxidants are written on the left and the reductants are written on the right.
- Going down the electrochemical series, the oxidising strength of oxidants decreases
- Going down the electrochemical series, the reducing strength of reductants increases.
For a spontaneous reaction to occur there has to a strong oxidant reacting with a strong reductant.
Everything in the electrochemical series is take from the reference point of Hydrogen gas and its potential EMF is hence derived from there. The EMF in the electrochemical series occurs only under the standard condition of:
- 1M solution
- 1 atm pressure
- 25 oc
Under standard conditions the EMF of a galvanic cell from the electrochemical series is given by the equation:
Specific Galvanic cell - Lead acid battery
Lead acid battery.
Lead acid battery is a rechargeable galvanic cell which provides around 2V however six of these cells are connected to form a 12v battery. Lead acid batteries are relatively cheap and reliable, provide high currents and have a long lifetime. The battery is used to start the car's engine, or to operate the car's electrical accessories when the engine is not running. Once the engine starts, an alternator which is run by the engine provides electrical energy to recharge the battery.
Physical structure
In standard car battery each cell contains three positive electrodes sandwiched between four negative electrodes. Contact between the electrodes is prevented by the the presence of a porous separator. The positive electrodes consist of a lead grid packed with lead(IV) oxide (Pb02) while the negative electrode consists of a lead grid packed with powdered lead. A solution of sulfuric acid of about 4M concentration acts as the electrolyte.
(Chemistry 2, pp 211-213)
(Chemistry 2, pp 211-213)
Discharge
Chemical reactions: Anode (-)
At the anode the reaction that occurs:
In here the lead is oxidised to lead ions. These ions react with the sulfate ions from the electrolyte to form a coating of lead (II) sulfate on the electrode.
Discharge
Chemical reactions: Cathode (+)
At the cathode the reaction that occurs:
In here the lead oxide is reduced to lead ions. These then react with the sulfate ions to form lead sulfate and water.
Figure 6. An animation of what's happening in a lead acid battery.
Discharge
Overall equation
The overall equation is as follows:
Recharge
Chemical reaction: Anode(-)
As we know that lead acid battery is a secondary cell, we can recharge it by forcing the electrons in the backward direction to the discharge however the voltage applied must be greater than the EMF generated by the battery hence 14V is used to recharge the a standard car battery. The factor that makes secondary cell rechargeable is that the products should be in close proximity to the electrode. Since we are forcing the reverse of the anode reaction, its necessary that we connect the negative terminal (the anode) to the negative terminal of the battery. The energy transformation occurring in this recharge process is electrical energy being converted back into chemical energy. The reaction occurring is:
Recharge
Chemical reaction: Cathode (+)
This is also the reverse of the cathode reaction during discharge hence the reaction is:
Recharge
Overall reaction:
The overall recharge reaction is basically the reverse of the discharge reaction hence: