Chapter 26. Electricity from chemical reactions. An electrochemical cell A device that converts chemical energy into electrical energy. A Daniell cell is a device that could supply a useful electric current.
Electricity from chemical reactions
A model of the Daniell cell can be made using a simple laboratory apparatus.
An electric current flows through the wire and light the globe.
This part of the cell is called the external circuit.
The glove converts the electrical energy of the current into light and heat.
If the light glove is replace with a galvanometer, the galvanometer indicates that electrons flow from the zinc electrode, through the wire, and to the copper electrode.
Current flows only if a salt bridge is present.
Zn(s) → Zn2+(aq) + 2e-
Cu2+(aq) + 2e-→ Cu(s)
Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)
Copper ions act as the oxidant and zinc metal acts as the reductant.
The Daniell cell can be regarded as consisting of two half cells.
Each half cell contains an electrode in contact with a solution.
The species present in each half cell form a conjugate redox pair
In general if one member of the conjugate pair in a half cell is a metal, it is usually used as the electrode.
However if no metal is present an inert electrode such as platinum or graphite is used.
In some half cells one of the conjugate pairs may be a gas.
The salt bridge connects the circuit
It contains ions that are free to move so that they can balance charges formed in the half cells.
Cations migrate to the cathode and anions migrate to the anode.
This part of the cell is called the internal circuit.
Why does the zinc metal react with the copper ions rather than the copper metal react with the zinc ions?
They both have a tendency to lose electrons and form ions, but they differ in their tendencies to do so.
Zinc loses its electrons much more readily than copper.
When the Zn2+/Zn and Cu2+/Cu half cells are connected, electrons flow from the zinc metal in one half cell to the copper ions in the other.
Zinc is described as a stronger reductant than copper.
Since zinc loses electrons more readily, it follows that the oxidised form of zinc, Zn2+, will not accept electrons as readily as Cu2+ ions.
Cu2+ is described as a stronger oxidant than Zn2+.
Remember from last year that the electrochemical series lists the half cell equations in order of their tendency to occur as reduction reactions.
Use the electrochemical series to predict the behaviour of a cell composed of Ag+/Ag and Fe2+/Fe half cells.
Questions 1 and 2
Sometimes called the emf.
It is the electrical pressure between two half cells.
It is measured in volts. It measures the amount of energy supplied by a fixed amount of charge flowing from a galvanic cell.
A Daniell cell has a potential difference of about 1 volt.
We can get a rough indication of the potential difference from the electrochemical series
We can use the E° values from the electrochemical series.
These are standard half-cell potentials and give a numerical measure of the tendency of a half cell reaction to occur as a reduction reaction.
The hydrogen half cell is the reference cell and has an E° value arbitrarily assigned as zero.
Everything else is compared to this cell.
The potential difference of a cell at standard conditions is the difference between the E° values of its two half cells.
Cell potential difference =
higher half cell E° – lower half cell E°
For example Ag+/Ag and Fe2+/Fe half cells.
Cell potential difference = E°(Ag+/Ag) – E°(Fe2+/Fe)
= 0.80 – (-0.41)
= 1.21 V
Predict what will happen, if anything, if some iron filings are added to a solution of tin(II) chloride.
In a galvanic cell, the higher half reaction in the electrochemical series occurs in the forward direction (reduction).
The lower reaction occurs in the reverse direction (oxidation)
For a reaction to occur, a chemical on the left of the electrochemical series must react with a chemical on the right that is lower in the series.
The standard half cell potentials are measured under standard conditions.
The half cell potentials vary if you move away from the standard conditions.
The order of the half cell reactions can change and the standard half cell potentials will not be relevant.
The electrochemical series gives no information about the rate at which reactions occur.
According to the electrochemical series hydrogen peroxide would form water and oxygen gas but this reaction takes years to occur unless a catalyst is present.
Question 6 – 9, 12, 14