Chemistry_Grade_10-12 (1).pdf

3 the standard hydrogen electrode 2 h aq 2 e h 2 g

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Figure 17.3: The standard hydrogen electrode 2 H + ( aq ) + 2 e H 2 ( g ) The position of this equilibrium can change if you change some of the conditions (e.g. concen- tration, temperature). It is therefore important that the conditions for the standard hydrogen electrode are standardised as follows: pressure = 100 kPa (1atm); temperature = 298 K (25 0 C) and concentration = 1 mol.dm 3 . In order to use the hydrogen electrode, it needs to be attached to the electrode system that you are investigating. For example, if you are trying to determine the electrode potential of copper, you will need to connect the copper half cell to the hydrogen electrode; if you are trying to determine the electrode potential of zinc, you will need to connect the zinc half cell to the hydrogen electrode and so on. Let’s look at the examples of zinc and copper in more detail. 1. Zinc Zinc has a greater tendency than hydrogen to form ions, so if the standard hydrogen electrode is connected to the zinc half cell, the zinc will be relatively more negative because the electrons that are released when zinc is oxidised will accumulate on the metal. The equilibria on the two electrodes are as follows: Zn 2+ ( aq ) + 2 e Zn ( s ) 2 H + ( aq ) + 2 e H 2 ( g ) In the zinc half-reaction, the equilibrium lies far to the left and in the hydrogen half- reaction, the equilibrium lies far to the right. A simplified representation of the cell is shown in figure 17.4. The voltmeter measures the potential difference between the charge on these electrodes. In this case, the voltmeter would read 0.76 and would show that Zn is the negative electrode (i.e. it has a relatively higher number of electrons). 2. Copper Copper has a lower tendency than hydrogen to form ions, so if the standard hydrogen electrode is connected to the copper half cell, the hydrogen will be relatively more negative. The equilibria on the two electrodes are as follows: 332
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CHAPTER 17. ELECTROCHEMICAL REACTIONS - GRADE 12 17.4 V - - - - - - - - - - H electrode (less negative) Zn electrode with electrons Figure 17.4: When zinc is connected to the standard hydrogen electrode, relatively few electrons build up on the platinum (hydrogen) electrode. There are lots of electrons on the zinc electrode. Cu 2+ ( aq ) + 2 e Cu ( s ) 2 H + ( aq ) + 2 e H 2 ( g ) In the copper half-reaction, the equilibrium lies far to the right and in the hydrogen half- reaction, the equilibrium lies far to the left. A simplified representation of the cell is shown in figure 17.5. V - - - - - - - - - - H electrode Cu electrode Figure 17.5: When copper is connected to the standard hydrogen electrode, relatively few elec- trons build up on the copper electrode. There are lots of electrons on the hydrogen electrode. The voltmeter measures the potential difference between the charge on these electrodes. In this case, the voltmeter would read 0.34 and would show that Cu is the positive electrode (i.e. it has a relatively lower number of electrons).
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