Chemistry_Grade_10-12 (1).pdf

Mg s 2 h aq mg 2 aq h 2 g answer step 1 write

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Mg ( s ) + 2 H + ( aq ) Mg 2+( aq ) + H 2 ( g ) Answer Step 1 : Write equations for the two half reactions involved Mg 2+ + 2 e Mg (E o V = -2.37) 2 H + + 2 e H 2 (E o V = 0.00) Step 2 : Determine which reaction takes place at the cathode and which is the anode reaction From the overall equation, it is clear that magnesium is oxidised and hydrogen ions are reduced in this reaction. Magnesium is therefore the anode reaction and hydrogen is the cathode reaction. Step 3 : Represent the cell using standard notation Mg | Mg 2+ || H + | H 2 Step 4 : Calculate the cell potential E 0 ( cell ) = E 0 (cathode) - E 0 (anode) = 0.00 - (-2.37) = +2.37 V 339
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17.4 CHAPTER 17. ELECTROCHEMICAL REACTIONS - GRADE 12 Predicting whether a reaction will take place spontaneously Look at the following example to help you to understand how to predict whether a reaction will take place spontaneously or not. In the reaction, Pb 2+ ( aq ) + 2 Br ( aq ) Br 2 ( l ) + Pb ( s ) the two half reactions are as follows: Pb 2+ + 2 e Pb (-0.13 V) Br 2 + 2 e 2 Br (+1.06 V) Important: Half cell reactions You will see that the half reactions are written as they appear in the table of standard electrode potentials. It may be useful to highlight the reacting substance in each half reaction. In this case, the reactants are Pb 2+ and Br ions. Look at the electrode potential for the first half reaction. The negative value shows that lead loses electrons easily, in other words it is easily oxidised. The reaction would normally proceed from right to left (i.e. the equilibrium lies to the left), but in the original equation, the opposite is happening. It is the Pb 2+ ions that are being reduced to lead. This part of the reaction is therefore not spontaneous. The positive electrode potential value for the bromine half-reaction shows that bromine is more easily reduced, in other words the equilibrium lies to the right. The spontaneous reaction proceeds from left to right. This is not what is happening in the original equation and therefore this is also not spontaneous. Overall it is clear then that the reaction will not proceed spontaneously. Worked Example 88: Predicting whether a reaction is spontaneous Question: Will copper react with dilute sulfuric acid (H 2 SO 4 )? You are given the following half reactions: Cu 2+ ( aq ) + 2 e Cu ( s ) (E 0 = +0.34 V) 2 H + ( aq ) + 2 e H 2 ( g ) (E 0 = 0 V) Answer Step 5 : For each reaction, look at the electrode potentials and decide in which direction the equilibrium lies In the first half reaction, the positive electrode potential means that copper does not lose electrons easily, in other words it is more easily reduced and the equilibrium position lies to the right. Another way of saying this is that the spontaneous reaction is the one that proceeds from left to right, when copper ions are reduced to copper metal. In the second half reaction, the spontaneous reaction is from right to left.
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