Transcript Electrochemistry 7th Nov 2015

Electrochemistry
Simple cells, formation of metal ions in varying
degrees, electric potential, electroplating,
electrolysis
Coins on a hand
 Place a few different coins
on someone’s hand so that
they are not touching one
another.
 Using a digital voltmeter
measure the voltage
difference between pairs of
coins.
 Try other metals.
 Which pair produces the
largest voltage?
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
A nail in water
 Place a steel nail in a glass of water.
 Some iron ions (Fe++) are removed
from the nail by water molecules.
 Each of these ions leaves two
electrons behind on the nail.
 As the nail becomes more and more
negatively charged it soon begins to
attract the positive ions back.
 A steady state is quickly reached in
which the nail is negative and the
solution is positive and there is no
further net reaction.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Copper wire in water
 Insert a piece of copper wire into the water.
 Some copper ions (Cu++) are removed from
the wire by water molecules, each leaving
two electrons behind.
 However this effect occurs to a smaller
extent with copper than with iron.
 So the copper is negative but not as
negative as the iron.
 Therefore there is a voltage difference
between them. Relative to nail the copper
wire is positive.
 This simple but marvellous effect was
discovered in 1800 by Allesandro Volta.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Volta (1745–1827)
 Alessandro Volta made
the first electric battery in
1800.
 He used a pile of
Zn/brine/Ag cells.
 It was an immediate
sensation. Until then
there was no way to
generate a steady flow of
charge.
 The SI unit of electric
potential, the volt (V), is
named after him.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Activity series
 Place a tissue on a tile or plastic sheet and moisten
it with salty water or dilute acid.
 Place a selection of metals on the tissue. Make
sure they are not touching.
 Get a multimeter and set it to read up to 2 V. Hold
the negative probe (black) on one of the pieces of
metal and then touch the positive probe (red) off
each of the others in turn. If any one of them gives
a negative reading move the negative probe to it
and start again. Record the final values and
arrange the metals in the order of increasing
voltage.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Gold and silver
 If you have some gold or silver jewellery you could
add them to the selection of metals and check their
place in the activity series. (Salt water or dilute acid
will not damage them.)
 You could also try a piece of magnesium ribbon.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Electrolysis of water
Electrolysis apparatus
+
+
-
-
O2
dilute H2SO4
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
H2
Electrolysis reactions
 If the solution contains H+ ions then they will
migrate towards the negative electrode where each
can pick up an electron and form a neutral
hydrogen atom.
 Pairs of hydrogen atoms bond to form H2.
 Water molecules near the positive electrode can
lose two hydrogen ions (H+) to the solution and two
electrons to the electrode, releasing oxygen atoms.
 The oxygen atoms form O2, and sometimes O3
(ozone).
 The net effect is the decomposition of water.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Some metals react with acids
Activity series (again)
 When a piece of metal is placed in water
some of its atoms go into solution as
positive ions, leaving electrons on the
metal.
 An equilibrium is soon established.
 If the voltage difference between the metal
and the solution is high enough hydrogen
ions (H+) may be discharged by it; each H+
ion picks up an electron to form a
hydrogen atom.
 As a result more metal ions go into solution
and the process continues.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Zinc and copper in acid solution
 Zinc reacts with dilute acid; bubbles of hydrogen
quickly form on its surface.
 Copper does not react with dilute acid.
 However if the copper touches the zinc then
bubbles will be seen on the copper also.
 If the reaction is allowed to continue the zinc
metal eventually disappears but the copper is
unaffected.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Zinc and copper in acid solution
 Zinc reacts with dilute acid; bubbles of hydrogen
quickly form on its surface.
 Copper does not react with dilute acid.
 However if the copper touches the zinc then
bubbles will be seen on the copper also.
 If the reaction is allowed to continue the zinc
metal eventually disappears but the copper is
unaffected.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Zinc/copper cell
 Alternatively the zinc and copper may be
connected through an external circuit.
 This is effectively the same as the cells in
Volta’s battery.
mA
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Displacement of metals
and electroplating
Displacement of metals
 If a steel nail is dipped in a
solution of copper ions (e.g. in
1M CuSO4) it is quickly
coated with copper metal.
 You should now be able to
explain why this happens?
 Note: If some H2SO4 and
ethanol are added to the
solution the copper plate will
be less likely to fall off.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
Electroplating
 By using an external voltage
source it is possible to
‘electroplate’ items.
 The item to be plated should
be negative.
 For copper plating use a
copper anode (+) and a
copper sulfate solution.
 (Note: reduce the voltage if
bubbles appear.)
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
-
+
Gonzaga College, Saturday 7th November 2015
Copper electroplating solution
 Required:
– 30 g of copper sulphate crystals
(bluestone)
– 5 ml of concentrated sulphuric acid*
– 12 ml of ethanol (or methylated spirit)
 Dissolve the above in water and dilute to
a total volume of 200 cm3 with distilled
water.
 A PURE copper anode (+) is also
required.
-
* NOTE: Add the acid to water slowly with
stirring and cooling
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015
+
Nickel electroplating solution
 Required:
– 25 g of nickel sulphate (green crystals)*
– 3g of ammonium chloride (white solid)
– 3g of boric acid (white solid)
 Dissolve the above in water and dilute to a
total volume of 250 cm3 with distilled water.
 A PURE nickel anode (+) is also required.
* NOTE: The nickel sulphate crystals are
slow to dissolve.
Objects which have been electroplated should
be washed, dried and polished.
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
-
Gonzaga College, Saturday 7th November 2015
+
The terms ‘cathode’ and ‘anode’
 In electrolysis or electroplating the cathode is the
negative terminal.
 In the case of an electrochemical cell, such as a
zinc/copper cell, the cathode is the positive terminal
and the anode is the negative.
 Rationale…
 In all cases, cations (positive ions) in the electrolyte
move away from the anode or towards the cathode.
 (Mnemonic: The conventional current carries
cations towards the cathode.)
PDST – AmgenTeach : Electricity, Force and Heat – The Essentials
Gonzaga College, Saturday 7th November 2015