Transcript Electricity from chemical reactions

Chapter 26
ELECTRICITY FROM CHEMICAL
REACTIONS
GALVANIC CELLS
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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.
This device was used almost exclusively to
power the early English and American telegraph
systems because of its reliable output
THE DANIELL CELL
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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.
WHAT IS HAPPENING IN THE DANIELL CELL
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The current flows because a chemical reaction is taking
place in the cell.
Initially there is little indication that a reaction is taking
place in the beakers.
However if we leave the cell with a light globe connected
for several hours a number of changes can be observed:
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The zinc metal corrodes
The copper metal becomes covered with a furry brown-black
deposit
The blue copper(II) sulfate solutions loses some of its colour
These changes provide evidence of a chemical reaction
WHAT IS HAPPENING IN THE DANIELL CELL
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.
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THESE OBSERVATIONS ALLOW US TO DECIDE
WHAT IS HAPPENING IN THE CELL
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The reaction in the cell is a redox reaction, as electrons
are being produced and consumed.
The zinc electrode is eaten away, forming zinc ions in
solution:
Zn(s) → Zn2+(aq) + 2eThe oxidation of the zinc metal releases electrons, which
flow through the wire to the copper electrode.
Electrons are accepted by copper ions in the solution
when the ions collide with the copper electrode:
Cu2+(aq) + 2e- → Cu(s)
The salt bridge forms an essential part of the cell
OVERALL
Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)
 Copper ions act as the oxidant and zinc metal
acts as the reductant.
• What is being oxidised
• What is being reduced
WHY IS ELECTRICAL ENERGY RELEASED?
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A galvanic cell is designed so that half reactions
occur in two separate compartments of the cell.
The oxidants and reductants do not come into
direct contact with each other.
Electrons can only be transferred by travelling
through an external circuit connecting the negative
and positive electrodes.
The flow of electrons creates an electric circuit.
The chemical energy is transformed into electrical
energy.
HALF CELLS
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
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HALF CELLS
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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.
HALF CELLS
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The electrode at which oxidation occurs is called
the anode.
In galvanic cells the anode, where electrons are
produced, is described as the negative terminal
The electrode where reduction occurs is called the
cathode.
In galvanic cells this is where electrons are
consumed, it is described as the positive terminal.
In the Daniell cell the zinc electrode is the anode
and the copper electrode is the cathode.
SALT BRIDGE
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.
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THE ELECTROCHEMICAL SERIES
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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.
THE ELECTROCHEMICAL SERIES
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+.
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THE ELECTROCHEMICAL SERIES
What are the half cells?
 What is being oxidised
and what is being
reduced?
 What is the oxidant?
 What is the reductant?
 Why is copper oxidised
here but it is reduced in
the Daniell Cell?
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THE ELECTROCHEMICAL SERIES
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Remember from last year that the
electrochemical series lists the half cell
equations in order of their tendency to occur as
reduction reactions.
THE ELECTROCHEMICAL SERIES
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Is a list of various half cells.
Is only valid under conditions in which it was determined.
The one in your book applies at:
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A temperature of 25°C
Pressure of 1 atm
1 M concentrations of solutions
These are standard laboratory conditions (SLC)
We can use the electrochemical series to predict what will
happen when two specific half cells are combined to from a
cell.
WORKED EXAMPLE 26.2
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Use the electrochemical series to predict the
behaviour of a cell composed of Ag+/Ag and
Fe2+/Fe half cells.
YOUR TURN
Page 419
 Questions 1 and 2
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ANOTHER EXAMPLE
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A galvanic cell is to be constructed using magnesium
metal, silver metal and solutions of magnesium nitrate
and silver nitrate.
Draw and label a diagram with
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The direction of the flow of electrons in the external circuit
The metal electrodes and the electrolytes in the half cells
The appropriate electrolyte in the salt bridge
The polarity of each electrode
Write the reaction occurring at the anode.
POTENTIAL DIFFERENCE
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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
POTENTIAL DIFFERENCE
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.
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POTENTIAL DIFFERENCE
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
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YOUR TURN
Page 423
 Question 3
 Predict what will happen, if anything, if some
iron filings are added to a solution of tin(II)
chloride.
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PREDICTING DIRECT REDOX REACTIONS
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.
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WORKED EXAMPLE 26.4
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Consider the following equations that appear in the
order shown in the electrochemical series:
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Br2(aq) + 2eNi2+(aq) + 2eMg2+(aq) + 2e-
2Br-(aq)
Ni(s)
Mg(s)
Predict the effect of mixing
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Br2(aq) and Mg2+(aq)
Mg2+(aq) and Ni(s)
Ni2+(aq) and Mg(s)
LIMITATIONS OF PREDICTIONS
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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.
YOUR TURN
Page 425
 Question 4
 Page 436
 Question 6 – 9, 12, 14
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