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Chemical Cells

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Matter - Hong Kong True Light College ... Chemical Cells – PowerPoint PPT presentation

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Title: Chemical Cells


1
Chemical Cells
2
Chemical Energy ? Heat Energy
  • When magnesium powder is added into copper(II)
    sulphate solution, the temperature of the mixture
    rises.
  • Displacement reaction occurs.
  • Chemical Energy ? Heat Energy

3
Chemical Energy ? Heat Energy
  • Overall equation (Redox reaction)
  • Mg(s) CuSO4(aq) ? MgSO4(aq) Cu(s)
  • Ionic equation
  • Mg(s) Cu2(aq) ? Mg2(aq) Cu(s)
  • Half equations
  • Mg(s) ? Mg2(aq) 2e- Oxidation
  • Cu2(aq) 2e- ? Cu(s) Reduction

4
Chemical Energy ? Heat Energy
  • Observable changes
  • mass of the magnesium strip decreases
  • copper deposits on the copper strip
  • blue colour of solution fades out.

5
Chemical Energy ? Electrical Energy
  • When electrons flow through an external circuit,
    a simple chemical cell is formed.
  • Chemical Energy ? Electrical Energy

6
Chemical Cell of Mg/Cu couple
  • Magnesium is more reactive than copper, it
    oxidizes and loses electrons to form positive
    ions more readily.
  • Mg(s) ? Mg2(aq) 2e-
  • Magnesium ions dissolve into the solution.
  • Thus the mass of magnesium strip decreases.

7
Chemical Cell of Mg/Cu couple
  • Electrons flow through the external circuit to
    the copper strip.
  • Voltmeter shows positive deflection.

8
Chemical Cell of Mg/Cu couple
  • Copper(II) ions in electrolyte move to the copper
    strip. They reduce and gain electrons to form
    copper atoms.
  • Cu2(aq) 2e- ? Cu(s)
  • Thus copper deposits on the copper strip.

9
Simple chemical cell
  • Basic requirements Two different metals dipped
    in a solution of electrolyte

10
Simple chemical cell
  • The more reactive metal forms ions more readily.
    It oxidizes and loses electrons. It becomes the
    negative electrode.
  • The less reactive metal becomes the positive
    electrode.
  • Positive metal ions dissolves into the
    electrolyte and electrons flow from the more
    reactive metal through the external circuit to
    the less reactive metal.

11
Simple chemical cell
  • The voltage of the cell gives a measure of how
    strongly the electrons are pushed through the
    circuit, and is measured by a voltmeter.
  • The voltmeter should be correctly connected.
  • The negative terminal should be connected to the
    more reactive metal while the positive terminal
    to the less reactive metal.
  • Positive ions in electrolyte move to the less
    reactive metal. They reduce and gain electrons
    to form metal.

12
Comparing the tendency to form ions of different
metals
  • The more reactive metal should be connected to
    the negative terminal of voltmeter.

13
Comparing the tendency to form ions of different
metals
  • The greater the difference in their tendencies to
    form ions, the higher is the voltage of the cell.

14
Chemical cell of Cu/Ag couple
  • Electrolyte is in the form of filter paper soaked
    with sodium chloride solution.

15
Chemical cell of Cu/Ag couple
  • Copper is more reactive than silver, it oxidizes
    and loses electrons to form positive ions more
    readily.
  • Cu(s) ? Cu2(aq) 2e-
  • Copper(II) ions dissolve into the electrolyte .
  • Thus the mass of copper strip decreases.
  • Copper is the negative electrode.

16
Chemical cell of Cu/Ag couple
  • Electrons flow from copper through the external
    circuit to silver. Silver is the positive
    electrode.
  • Hydrogen ions in electrolyte move to the silver
    electrode. They reduce and gain electrons to
    form hydrogen gas.
  • 2H(aq) 2e- ? H2(g)
  • Thus effervescence occurs at silver strip.
  • Overall reaction
  • Cu(s) 2H(aq) ? Cu2(aq) H2(g)

17
The electrochemical series of metals
  • Metals arranged in order of their tendencies to
    form ions.
  • The order of metals in the electrochemical series
    is the same as that in their reactivity series
    (except for the position of calcium).

18
The electrochemical series of metals
19
Modification of simple chemical cell
  • Two different metals dipped in two separate
    electrolytes.
  • The two electrolytes are connected by a salt
    bridge which can be made by soaking a piece of
    filter paper in saturated potassium nitrate
    solution.

20
Modification of simple chemical cell
21
Modification of simple chemical cell
  • Anode (oxidation)
  • Mg(s) ? Mg2(aq) 2e-
  • Cathode (reduction)
  • Cu2(aq) 2e- ? Cu(s)
  • Overall equation
  • Mg(s) Cu2(aq) ? Mg2(aq) Cu(s)

22
Modification of simple chemical cell
23
Salt bridge
  • Two main functions
  • It completes the circuit by allowing ions to move
    towards one half cell from the other.
  • It provides ions to balance the charges in the
    solutions of the two half cells.
  • Salt bridge must not be dried.
  • .

24
Chemical Energy ? Heat Energy
  • When excess FeSO4(aq) is added into a purple
    solution of acidified KMnO4(aq), the colour
    changes to yellow.

25
Chemical Energy ? Heat Energy
  • Oxidation half equation
  • Fe2(aq) ? Fe3(aq) e?
  • Green yellow
  • Reduction half equation
  • MnO4(aq) 8H(aq) 5e ? Mn2(aq) 4H2O(?)
  • Purple colourless
  • Overall equation
  • 5Fe2(aq) MnO4(aq) 8H(aq)
  • ? 5Fe3(aq) Mn2(aq) 4H2O(?)

26
Other forms of chemical cells inert electrodes
27
Other forms of chemical cells inert electrodes
  • Anode (oxidation)
  • Fe2(aq) ? Fe3(aq) e?
  • Cathode (reduction)
  • MnO4(aq) 8H(aq) 5e ? Mn2(aq) 4H2O(?)
  • Overall equation
  • 5Fe2(aq) MnO4(aq) 8H(aq)
  • ? 5Fe3(aq) Mn2(aq) 4H2O(?)

28
Other forms of chemical cells inert electrodes
  • Observable changes
  • Green iron(II) sulphate solution changes to
    yellow
  • Purple potassium permanganate solution changes to
    colourless (purple colour fades.)

29
Chemical Energy ? Heat Energy
  • When excess colourless KI(aq) is added into a
    yellow solution of Fe2(SO4)3(aq), the colour
    changes to brown.

30
Chemical Energy ? Heat Energy
  • Anode (oxidation)
  • 2I-(aq) ? I2(aq) 2e?
  • Colourless brown in KI
  • Cathode (reduction)
  • Fe3(aq) e? ? Fe2(aq)
  • yellow green
  • Overall equation
  • 2Fe3(aq) 2I(aq) ? 2Fe2(aq) I2(aq)

31
Other forms of chemical cells inert electrodes
32
Other forms of chemical cells inert electrodes
  • Anode (oxidation)
  • 2I-(aq) ? I2(aq) 2e?
  • Cathode (reduction)
  • Fe3(aq) e? ? Fe2(aq)
  • Overall equation
  • 2Fe3(aq) 2I(aq) ? 2Fe2(aq) I2(aq)

33
Other forms of chemical cells inert electrodes
  • Observable changes
  • Yellow iron(III) sulphate solution changes to
    green
  • Colourless potassium iodide solution changes to
    brown because iodine formed will combine with
    potassium iodide to form a brown compound.
  • I2(aq) KI(aq) ? KI3(aq)
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