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VOLTAIC CELL

  • A voltaic cell is a device which converts chemical energy to electrical energy. The chemical reactions that take place inside the cell cause the flow of electrons and hence, electricity is produced.

  •  A simple voltaic cell is made by placing two different metals in contact with an electrolyte as shown in figure 1. The metals act as the electrodes for the simple cell. The electrodes are connected to a voltmeter, a galvanometer or an electric bulb using connecting wires.

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  • The production of electricity in the cell will cause the bulb to light up, and make the voltmeter or galvanometer to show a reading.

  •  The voltage of the voltaic cell is the sum of the voltages of the two half-cells. It is measured by connecting a voltmeter to the two electrodes. The voltmeter has very high resistance, so the current flow is effectively negligible. When a device such as an electric motor is attached to the electrodes, a current flows and redox reactions occur in both half-cells. This will continue until the concentration of the cations that are being reduced goes to zero.

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  • For the voltaic cell, depicted in the figure 2, the two metals are zinc and copper and the two salts are sulfates of the respective metal. The reactions in the voltaic cell are as follows:

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Zinc which higher than copper in the electrochemical series releases electrons more easily than copper. Each Zinc atom donates two electrons to form a zinc ion. The zinc ions are released into the electrolyte. The zinc plate dissolves gradually.

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At the negative terminal: Zn (s)à Zn 2+ (aq) + 2 e

 

The electrons the flow through the connecting wire towards the copper plate which acts as the positive terminal. These electrons are accepted by copper(II) ions to form copper metal. The copper plate becomes thicker gradually.

 

At the negative terminal:Cu2+ (aq) + 2e à Cu (s)

 

 

The overall ionic equation:

 

Zn (s) + Cu2+ (aq) à Zn 2+ (aq) + Cu (s)

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  • The intensity of the blue colour of the copper(II) sulphate solution decreases as the concentration of copper(II) ions decreases gradually.

  • The flow of electrons from the zinc plate to the copper plate produces an electric current. The voltmeter shows a reading.

  • A voltaic cell is also called a galvanic cell.

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Unique feature from this activity

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  • Teacher will find difficulties in explaining to students about the concept of voltaic cell. In order for students to master this concept, they need to visualise the moving of electrons from positive to negative electrode. At the same time, free moving ions in the electrolyte and ions from the electrodes also need to be explained. Hence, by using computer simulation, the all of the moving of ions can be shown in 3-dimension (3-D). This will enhance the students understanding and save the time consumed.

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  • Teacher will use 3E model of simulation in order to probe student’s interest and to elicit student’s idea that will constructed by reflection before and after teaching lesson. There are phase in 3E model of simulation which is are ENGAGING, EMPOWERING and ENHANCE.

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ENGAGING PHASE

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  • Teacher shows students a video from the internet (youtube)*. They are asked to watch the video carefully.

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Questions:

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  • From the video shown, state your observations.

 

   ________________________________________________________________________________________

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  • What can be represented by the presence of lemon in the video?

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   _________________________________________________________________________________________

 

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  • What cause the voltmeter showed some reading?

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    _________________________________________________________________________________________

 

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  • Predict what will happen to the reading of the voltmeter when we add more lemon?

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     _________________________________________________________________________________________

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  • What is the conclusion from the video?

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   __________________________________________________________________________________________

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  • Can you relate the video with the topic will be discussed?

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   __________________________________________________________________________________________

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EMPOWERING PHASE

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  • Ensure that teacher has an internet connection.

  • Internet access to:

     http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

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  • Teacher is able to show the moving of electrons in voltaic cell by using the simulation above.

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Questions:

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1) From the simulation, which electrode acting as anode? Which one is cathode?

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    ________________________________________________________________________

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2) What happens at anode and cathode during the process?

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    ________________________________________________________________________

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3) In what direction do the electrons flow?

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    ________________________________________________________________________

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4) What is the function of the salt bridge?

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    ________________________________________________________________________

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5) Can you differentiate between electrolysis and the process shown in the simulation?

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    ________________________________________________________________________

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ENHANCE PHASE

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The diagram below shows the set up of the apparatus to arrange metal W, X, Y and Z based on the potential difference of the metal.

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Sources : http://www.inds.co.uk/sciencetoolbox/index.htm

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The table below shows the result of the experiment.

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Questions:

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  • Arrange metal W,X,Y and Z in descending order of electropositivity of metal.

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   ____________________________________________________________________________________

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  • Metals X and Z are used as electrodes in the diagram. State which metal act as positive terminal

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   ____________________________________________________________________________________

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  • Give reason for your answer in 2(a)

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   ____________________________________________________________________________________

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  • Predict the voltage of the cell in 2(a)

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   ____________________________________________________________________________________

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