Redox reaction in batteries (summary)
To define what redox is
- Reduction: gain of electrons
- oxidation: loss of electrons
A battery works using redox reaction. Reduction and oxidation occurs and electrons are being transferred between atoms. In a battery, one generates electrons at one electrode, while the other uses up at the other electrode. Electrons moving along a conductor is electric current (electrons are negatively charged particles in an atom) The electrode where oxidation takes place is called an anode, and it is the “-“ side of a battery; whilst the electrode where reduction takes place is called a cathode, and is marked by a “+” sign. When both the electrodes are connected outside the cell in a closed circuit, electrons flow.
The different reactivities of metals
When metals react, they give away electrons and form positive ions. The reactivity series is an order arranging metals by the ease with which it does this to form hydrated ions in an aqueous solution.
Example: magnesium and copper, Mg2+(aq) or Cu2+(aq),
The first reaction happens much more readily than the second one as magnesium is more reactive than copper.
Equilibrium
Magnesium:
When in placed water, there is a tendency for the magnesium atoms to remove its electrons and go into the solution as magnesium ions (Mg2+). Hence, electrons will be left behind on the piece of magnesium. The result would be a magnesium piece that is full of electrons and a solution would have many positive magnesium ions.
After a while, there will be an accumulation of electrons on the magnesium, and the water will have more and more positive ions. The positive ions in the solution gets attracted to the negative charge (electrons are negative) on the magnesium and hence will stay very closely to the magnesium metal. It is so close that some of the positive ions will reclaim their electrons and stick onto the magnesium piece like before.
When the rate of ions leaving the surface is exactly equal to the rate at which they are joining it again, it is said that dynamic equilibrium have been established. At that point, the magnesium will have an unchanged negative charge and a fixed number of magnesium ions in the solution surrounding it.
Copper:
Copper is a less reactive metal and hence it forms its ions less readily. Any ions which do break away are more likely to reclaim their electrons and stick back on to the metal again. There will be a point of which this reaction is an equilibrium, but in the case of copper, there will be a lower charge on the metal, and fewer ions in solution.
This potential difference could be recorded as a voltage. This is how batteries are of different voltage. The bigger the difference between the positiveness and the negativeness, the bigger the voltage.
However, do take note that the exact voltage of a battery is immposible to measure.
Comparing these 2 equations, magnesium sides more to the left than the copper one.Copper sheds electron less readily than magnesium since it is less reactive. To compare the different voltages, where is a standardized system called a reference electrode.
The system used is called a standard hydrogen electrode.
standard hydrogen electrode
This diagram shows a standard hydrogen electode set up. As the hydrogen gas flows over the porous platinum, there is an equilibrium between the hydrogen molecules(H2) and the hydrogen ions (H+) in the solution. The platinum catalyses this reaction.
Inside a battery cell
This whole set up is a cell. It is a simple system (representative of that in a battery) that generates a voltage. Each of the two beakers and their contents are half cells.
salt bridge
The salt bridge is included to close the electrical circuit without adding in any new forms of metal (so as not to interrupt the reaction) into the system. The salt bridge is a glass tube filled with an electrolyte. The ends are covered by cotton wool so as to stop too much mixing of the contents of the salt bridge with the contents of the two beakers.
The electrolyte in the salt bridge is chosen so that it doesn't react with the contents of either beaker, thus not messing up the whole system.
Process:
These two equilibria are set up on the two electrodes (the magnesium and the porous platinum):
Magnesium forms ions more readily than that of hydrogen as it is more reactive, hence the position of the magnesium equilibrium will be more on the left than the hydrogen equilibrium. Hence, there will be few electron build up on the platinum from the ionization of the hydrogen, as compared to the great number of electrons build up from the ionization of the magnesium.
There is a very huge difference in the charge of the 2 electrodes. Hence, the potential difference, measured with a voltmeter, is great. The voltage measured would be 2.37 volts and the voltmeter would show the magnesium as the negative electrode (anode) and the hydrogen electrode as being positive (cathode). As the magnesium has the greater amount of negativeness.
To compare, use copper as another example:
Copper is less reactive than hydrogen and hence it losses ion less readily than that of hydrogen. Also, the hydrogen equilibria lies further left than the copper one. There is less difference between the electrical charges on the two electrodes, so the voltage measured will be less. This time it is only 0.34 volts.
The other major change is that this time the copper is the more positive (less negative) electrode. The voltmeter will show the hydrogen electrode as the negative one and the copper electrode as positive.
There is less difference between the electrical charges on the two electrodes, so the voltage measured will be less. This time it is only 0.34 volts.
The other major change is that this time the copper is the more positive (less negative) electrode. The voltmeter will show the hydrogen electrode as the negative one and the copper electrode as positive.
Acknowledgement:
http://employees.csbsju.edu/hjakubowski/classes/ch112/organicchem/redoxreview.htm
http://www.chemguide.co.uk/physical/redoxeqia/introduction.html
http://www.science.uwaterloo.ca/~cchieh/cact/c123/battery.html
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