Question

أثناء طلاء الحديد بالنحاس كهربيًا استخدم ساق من النحاس غير النقي، أي مما يلي يمثل التغير المحتمل في كتل هذه الأقطاب في نهاية عملية الطلاء ؟

• A. تقل كتلة الأنود g 5 ، وتزداد كتلة الكاثود g 5
• B. تقل كتلة الأنود g 5 ، وتزداد كتلة الكاثود g 4
• C. تزداد كتلة الأنود g 5 ، وتقل كتلة الكاثود g 4
• D. تزداد كتلة الأنود g 4 ، وتقل كتلة الكاثود g 5

Answer 1

Answer: (B)

② تقل كتلة الأنود g 5 ، وتزداد كتلة الكاثود g 4

Answer 2

The problem describes an electroplating process where an iron object is coated with copper using an impure copper rod. We need to determine the probable mass changes of the electrodes.

1. Identify the Electrodes and their Roles:

   • Anode: The impure copper rod. Oxidation occurs here.
   • Cathode: The iron object to be plated. Reduction occurs here.
   • Electrolyte: A solution containing copper ions (e.g., CuSO₄).

2. Reactions at the Electrodes:

   • At the Anode (Impure Copper):

     • Copper metal from the impure rod oxidizes and dissolves into the electrolyte: $\text{Cu(s)} \rightarrow \text{Cu}^{2+}\text{(aq)} + \text{2e}^-$

     • Any impurities in the copper rod that are more reactive than copper (e.g., zinc, iron) will also oxidize and dissolve into the electrolyte: $\text{M(s)} \rightarrow \text{M}^{\text{n}+}\text{(aq)} + \text{ne}^-$ (where M represents a more reactive impurity)

     • Impurities that are less reactive than copper (e.g., silver, gold, platinum) will not oxidize and will fall to the bottom as anode sludge.

     • Therefore, the mass of the anode will decrease as both copper and more reactive impurities dissolve.

   • At the Cathode (Iron Object):

     • Copper ions from the electrolyte are attracted to the cathode and get reduced, depositing as pure copper metal onto the iron object: $\text{Cu}^{2+}\text{(aq)} + \text{2e}^- \rightarrow \text{Cu(s)}$

     • The ions of more reactive impurities (like Zn²⁺, Fe²⁺) that dissolved from the anode will remain in the solution because their reduction potentials are lower than that of Cu²⁺, meaning they are less easily reduced and will not deposit on the cathode.

     • Therefore, the mass of the cathode will increase as pure copper deposits on it.

3. Comparing Mass Changes:

   • The anode loses mass due to the dissolution of copper and more reactive impurities.

   • The cathode gains mass only due to the deposition of pure copper.

   • Since the more reactive impurities dissolve from the anode but do not deposit on the cathode, the total mass lost by the anode will be greater than the mass gained by the cathode.

4. Evaluating the Options:

   • ① تقل كتلة الأنود g 5 ، وتزداد كتلة الكاثود g 5 (Anode mass decreases by 5g, Cathode mass increases by 5g): This implies equal mass change, which would only happen if the anode were pure copper or if impurities also deposited, which is not the case for more reactive impurities.

   • ② تقل كتلة الأنود g 5 ، وتزداد كتلة الكاثود g 4 (Anode mass decreases by 5g, Cathode mass increases by 4g): Here, the anode mass decrease (5g) is greater than the cathode mass increase (4g). This perfectly matches our analysis that the impure anode loses more mass than the pure copper deposited on the cathode.

   • ③ تزداد كتلة الأنود g 5 ، وتقل كتلة الكاثود g 4 (Anode mass increases by 5g, Cathode mass decreases by 4g): Incorrect direction of mass change for both electrodes.

   • ④ تزداد كتلة الأنود g 4 ، وتقل كتلة الكاثود g 5 (Anode mass increases by 4g, Cathode mass decreases by 5g): Incorrect direction of mass change for both electrodes.

Therefore, option ② is the correct representation of the mass changes.