Year 12 Chemistry: ~ Chapter 26 Electricity from Chemical Reactions
26.1 Galvanic Cells
Daniell cell
The external circuit is the write and light globe, where an electrical current can flow through. The globe converts the electrical energy into light and heat (thermal).
The current flows because of a chemical reaction taking place in the cell. If you leave the cell with a light globe connected for several hours a number of changes can be observed:
These changes provide evidence of a chemical reaction occurring.
A galvanometer can be connected into the circuit (an instrument for detecting electric current) and indicates that electrons are flowing from the zinc electrode, through the wire and to the copper electrode. Current will only flow if there is a ______present.
J The reaction in the cell is a redox reaction, as electrons are being produced and consumed.
J The zinc electrode is eaten away (corroded) forming zinc ions in solution:
Zn (s) ® Zn2+(aq) + 2e-
J The oxidation of the zinc metal releases electrons, which flow through the wire to the copper electrode.
J Electrons are accepted by copper ions in the solution when the ions collide with the copper electrode:
Cu2+(aq) + 2e- ® Cu(s)
Copper atoms are insoluble and deposit on the electrode producing a brown-black coating.
J The salt bridge forms an essential part of the cell.
J Overall equation: Zn (s) + Cu2+(aq) ® Zn2+(aq) + Cu(s)
A chemical that oxidised another substance is called an oxidant and is itself reduced in the process. A substance that reduces another substance is called a reductant. Reductants are oxidised in redox reaction.
Why is electrical energy released?
A galvanic cell is designed so that half reactions occur in two separate compartments of the cell. Because the oxidant and reductant do not come in contact with each other, electrons can only be transferred by travelling through an external circuit connecting the negative and positive electrodes. Chemical energy of the reactants is transformed into electrical creating a current.
Half Cells
In the Daniell cell, one half cell contains Cu and Cu2+, the other contains Zn and Zn2+. 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. 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.
J The electrode at which oxidation occurs is called the anode. In a galvanic cell the anode, where electrons are produced, is described as the ______terminal.
J The electrode at which reduction occurs is called the cathode. The cathode where electrons are consumed, is the ______terminal.
J In a Daniell cell the zinc electrode is the ______and the copper electrode is the ______.
The Purpose of the Salt Bridge
If no salt bridge were present, the solution in one half cell would accumulate ______charge and the solution in other half cell would accumulate a ______charge as the reaction proceeded.
The salt bridge contains ions that are free to move so that they can balance charges formed in the half cells. Cations (______ions) migrate to the cathode and anions (______ions) migrate to the anode
Cations à cathode
Anions à anode
Writing half cell equations
Knowing that the conjugate redox pair Zn2+ ions and Zn:
Zn2+(aq) + 2e- Zn(s)
Half cell equations involving polyatomic ions. The equation for a half cell containing the dichromate ion (Cr2O72-) and Cr3+ ion redox pair.
Cr2O72(aq) + 14H+(aq) + 6e- ® 2Cr3+(aq) + 7H2O(l)
26.2 The Electrochemical series
Why does the zinc metal in the Daniell cell react with Cu2+ ions, rather than the copper metal reacting with Zn2+ ions? Zinc loses its electrons much more readily than copper, zinc is more reactive than copper. In a Zn2+/Zn and Cu2+/Cu combination the electrons flow from the zinc metal in one half cell to the copper ions in the other half cell.
By looking at the electrochemical series (you have a laminated one), zinc is a stronger reductant (lower in the series) than copper, therefore losing electrons more readily. Cu2+ is a stronger oxidant than Zn2+, and accepts electrons more readily.
Example:
J Highlight the charges on the anode and cathode
J Write an overall reaction, combining the two half equations.
The electrochemical series is used to predicted half equations.
The electrochemical series is strictly valid only for the conditions under which it was determined:
J A temperature of ______
J Pressure of ______
J Concentration of solutions.
Example: Use the electrochemical table to predict the behaviour of a cell composed of Ag+/Ag and Fe2+/Fe.
J Find the two equations in the electrochemical series
J Ag+ is higher on the left side of the table than Fe2+, it is a stronger oxidant.
J Iron is lower on the right hand side and is a stronger reductant.
J The strongest oxidant reacts with the strongest reductant.
J Half equations:
(Ag+(aq) + e- Ag(s)) x2 – so electrons balance
Fe(s) Fe2+(aq) + 2e-
J Overall equation: 2Ag+(aq) + Fe(s) 2Ag(s) + Fe2+(aq)
J ANOILRIGCAT: using this and the above information draw and label the below diagram of the cell Ag+/Ag and Fe2+/Fe.
Label:
J Anode / cathode
J Positive / negative
J ½ equations and overall equations
J electrodes
J Salt bridge and draw in the direction the cations and anions would migrate
J Direction of electrons flowing in wire by using arrows.
Questions: 1, 2, 5, 8 & 12.
Year 12 Chemistry: Chapter 26.3 Predicting Redox
26.3 Potential difference
A current flows in a galvanic cell because one half cell has a greater tendency to push electrons into the external circuit than the other half cell. A potential difference exists between the two half cells. The potential difference of a cell is sometimes called the emf.
A rough indication of cell voltage can be obtained from the electrochemical series. Large potential differences are obtained when a half cell containing a strong oxidant is connected to a half cell with a strong reductant.
Potential differences of cells at standard conditions can be calculated using the E values found on the electrochemical series. Reduction reactions are most likely to occur in half cells with high E values, eg F2(g) / F-(aq) (E= 2.87 V), whereas oxidation reactions are most likely to occur in half cells with low E values, eg Li+(aq) / Li(s) (E= -3.05).
E values for half cells are measured by connecting the cells to a reference half cell, a hydrogen half cell, H+(aq) / H2(g).
The potential difference of a cell at standard conditions is the difference between the E values of its two half cells:
26.4 Predicting direct redox reactions
In a galvanic cell, the higher half reaction in the electrochemical series occurs in the forward direction (as reduction) and the lower reaction occurs in reverse (as oxidation). For an appreciable 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.
Oxidants Reductants
Example:
Consider the following equations that appear in the order shown in the electrochemical series.
Br2(aq) + 2e- 2Br-(aq)
Ni2+(aq) + 2e- Ni(s)
Mg2+(aq) + 2e- Mg(s)
Predict the effect of mixing:
a) Br2(aq) and Mg2+(aq)
b) Mg2+(aq) and Ni(s)
c) Ni2+(aq) and Mg(s)
Limitations of predictions
The standard half cell potentials are measured under standard conditions.
J Temperature:
J Pressure:
J Concentration:
It is important to bear in mind that the electrochemical series gives no information about the rate at which reactions occur.
Questions: 3, 4, 6 & 14.