SSLC Chemistry: Chapter 03 Reactivity Series and Electrochemistry - Questions and Answers


Study Notes for Class 10 Chemistry (English Medium) ക്രിയാശീല ശ്രേണിയും വൈദ്യുത രസതന്ത്രവും | Text Books Solution Chemistry (English Medium) Chapter 03 Reactivity Series and Electrochemistry. ഈ അദ്ധ്യായം Malayalam Medium Notes Click here
Reactivity Series and Electrochemistry - Questions and Answers & Model Questions
some metals engage in chemical reactions vigorously, certain others react sluggishly in the same reaction.
Let us analyse the reaction of some metals with water

The reaction of metals with water

1. Take three beakers having the same quantity of water. Take pieces of sodium, magnesium and copper of the same size and drop each one into each beaker. Observe the reactions.
Observation
The gas formed in the above reactions is hydrogen.

2. Based on your observation, arrange these metals in the decreasing order of their reactivity.
Answer: Sodium > Magnesium >Copper

Reactions of Metals with Air

3. Cut a piece of sodium using a knife. Observe the freshly cut portion. Don't you see that its shining fades after some time? Give reason:
Answer:  This is due to the conversion of sodium into its compounds by reacting with oxygen, moisture and carbon dioxide in the atmosphere

4. A fresh magnesium ribbon losing its lustre when kept exposed in the air for some days. Why?
Answer: This is also due to its reaction to atmospheric air. Magnesium reacts with oxygen in air forming a black layer of magnesium oxide over it.
2 Mg (s)+ O2(g) → 2MgO(s)
Aluminium vessels diminish as time passes by. In the case of copper vessels, it takes months for the loss of its lustre by the formation of verdigris. But the shining of gold fades even after a long time. This indicates that metals react with air at different rates.

5.  (a) Which metal among magnesium, copper, gold, sodium and aluminium, loses its lustre at a faster rate?
Answer: Sodium
     (b) List the above metals in the decreasing order of their reactivity with air and thereby losing lustre.
Answer:  Sodium > Magnesium > Aluminium> Copper > Gold

The reaction of Metals with Acids
The image given below shows the reaction of some metals with dilute HCl
This indicates that metals react with dilute HCl at different rates.

6. What is reactivity series?
The series obtained by arranging the metals in the decreasing order of their reactivity is known as the reactivity series. 
Note that hydrogen is also included in this series for the sake of comparison of chemical reactivity.

Reactivity series and displacement reactions
7. Prepare some CuSO₄ solution in a beaker and dip a Zn rod in it. Observe the changes after some time and write down the observations.
The blue colour of CuSO₄ solution is due to the presence of Cu²⁺ ions. When the Zn rod is dipped in CuSO₄ solution, the Cu²⁺ ions in the solution get deposited at the Zn rod as Cu atoms. The chemical reaction taking place here is given below.
Zn + CuSO₄ → ZnSO₄ + Cu

Zinc is more reactive than copper. Hence zinc will displace copper from the solution. As a result, ZnSO₄ and Copper are formed. The blue colour of the solution diminishes and disappears. The displaced copper gets deposited at the zinc rod. (The colour of the solution changes to the colour of the newly formed compound(solution).
Each Zinc Copper ion gains two electrons. That is, Copper ions undergo Reduction.
Since oxidation and redox reactions occur simultaneously, this is a redox reaction.
8. A copper plate is immersed in AgNO₃ solution,
(a) Identify and record the changes. 
Answer: Copper is more reactive than Silver. Hence copper will displace silver from silver nitrate solution. Silver gets deposited at the copper plate. Since copper nitrate solution is formed, the colour of the solution becomes blue.
Each Copper atom loses two electrons. That is, Copper undergoes Oxidation.
Each Ag⁺ ion gains one electron. Hence Silver ions undergo Reduction.
Since oxidation and redox reactions occur simultaneously, this is a redox reaction. 

Displacement reactions
Highly reactive metals can displace less reactive metals from their salt solutions. Such reactions are called displacement reactions. Displacement reactions are redox reactions.

9. Certain metals and the salt solutions in which they are dipped are given below.
Identify displacement reaction occurs.
10 . Arrange the above metals in the decreasing order of their reactivity.
Answer: Mg >  Al  >   Zn >  Fe >  Cu > Ag

Galvanic cell
We have learned that metals differ in their reactivity. A galvanic cell is an arrangement in which the difference in reactivity of metals is used to produce electricity. 
Arrange the apparatus as shown in the picture. Take two beakers, one containing 100mL ZnSO₄ solution and the second containing the same amount of CuSO₄ solution with the same concentration.
Connection details
Zn rod in ZnSO₄ solution, Cu rod in CuSO₄ solution.
the negative terminal of a voltmeter is connected to the Zn rod and the positive terminal to the Cu rod.
Two solutions in the beakers are connected using a salt bridge
(A long filter paper moistened with KCl solution can be used instead of the salt bridge).
Observation
The reading of the voltmeter changes. We can produce electricity using such arrangements.
Here electricity is produced due to chemical change.
A galvanic cell or voltaic cell is an arrangement in which chemical energy is converted into electrical energy by means of a redox reaction.

11. We have understood from the reactivity series than Zn has higher reactivity than Cu.
a. Which electrode has the ability to donate electrons in a cell constructed using these metals?
Answer: Zn
b. Which one can gain electrons?
Answer: Cu
c. Identify the chemical reaction that takes place at the Zn electrode.
concentration.
d. Which reaction takes place here? Oxidation/Reduction
Answer: Oxidation

That is, Zn loses two electrons and becomes Zn²⁺
An electrode at which oxidation occurs is called the anode. Anode has a negative charge in this case. The electrons liberated from the Zn rod reach the copper electrode through the external circuit. These electrons are received by copper ions in the solution changing them into copper.
a. Write the chemical equation for the reaction taking place at the Cu electrode.
Answer: Cu²⁺+ 2 ē  → Cu
b. Which reaction takes place here? Oxidation/Reduction
Answer: Reduction
That is, Cu gains two electrons and becomes Cu.
An electrode at which reduction occurs is called a cathode. Cathode has a positive charge in this case.
Normally highly reactive metals donate electrons
The electrode at which oxidation occurs is the anode and that at which reduction occurs is the cathode. Anode attains a negative charge and cathode gets a positive charge.
The combined form of these reactions can be written as
Since oxidation and reduction occur at the same time, it is a redox reaction.
The transfer of electrons produced by this redox reaction causes the flow of electric current in the cell. The direction of electron flow is from anode to cathode.
12. Construct a galvanic cell using silver and copper electrodes.
13. How many cells can be constructed using Zn, Cu and  Ag?
Find the cathode and anode of the cell.
Answer:

Electrolytic cells
Electrolysis of water is a chemical reaction employing electrical energy. We have learned about this chemical reaction in the lower classes. 
The process of chemical change taking place in an electrolyte by passing electricity is known as electrolysis.
Electrolytes are substances that conduct electricity in molten states or in aqueous solutions and undergo chemical change. 
Acids, alkalis and salts are electrolytes in their molten state or in an aqueous solution.
In a molten state or in an aqueous solution, ions of the electrolytes can move freely. These ions are responsible for the conduction of electricity by the electrolytes.
It was Michael Faraday who gave a scientific explanation for electrolysis for the first time. Electrodes are substances that pass electricity to the electrolytes. During electrolysis one electrode is connected to the positive terminal of a battery and the other to the negative terminal. The electrode which is connected to the positive terminal of the battery is the anode. The electrode which is connected to the negative terminal is the cathode.
The electrode at which oxidation takes place is an anode and the electrode at which reduction take place is a cathode. In an electrolytic cell, oxidation takes place at the positive electrode and reduction takes place at the negative electrode.

14. Compare and contrast electrochemical cells and electrolytic cells.
Answer:
15. a. To which electrodes are the positive ions attracted during electrolysis?
Answer:  During electrolysis, positive ions are attracted towards the negative electrode(Cathode)
b. To which electrodes are the negative ions attracted during electrolysis?
 Answer:  During electrolysis, negative ions attracted towards the positive electrode(Anode)
c. What changes happen to the ions which are attracted to the cathode?
Answer: Positive ions are attracted to cathode. They receive electrons to become atoms or molecules. (Positive ions get reduced at cathode)
d. What changes happen to the ions which are attracted to anode?
Answer: Negative ions are attracted to anode. They donate electrons to become atoms or molecules. (Negative ions get oxidised at anode).

The positive ions which are attracted towards the negative electrode are called cations and negative ions which move towards the anode are called anions.
1. Electrolysis of molten sodium chloride
Sodium chloride in a solid state is not an electrical conductor because its ions have no freedom of movement. But electricity flows through molten sodium chloride. When sodium chloride melts, the positively charged sodium ions (Na⁺ ) and the negatively charged chloride ions (Cl⁻) are free to move.
NaCl → Na⁺ + Cl⁻
● Which ion is attracted to the positive electrode (anode)?
Cl⁻
● What is the chemical reaction taking place there?
2Cl⁻ → Cl₂ + 2 ē
● Which is the gas liberated at the anode?
Cl₂ 
● Which is the ion attracted to the negative electrode (cathode)?
Na⁺
● Write the change happening to it?
Na⁺ + ē→ Na
● Which is the metal deposited at the cathode?
Na

2. Electrolysis of sodium chloride solution
As a result of the electrolysis of sodium chloride solution, chlorine is obtained at the anode, hydrogen is obtained at the cathode and NaOH is obtained in the solution.

The practical utility of electrolysis
1. Production of metals
Metals like Potassium, calcium, sodium and aluminium are produced by the electrolysis of their compounds.
2. Production of non-metals
Electrolysis can be utilised for the bulk production of non-metals.
Hydrogen, oxygen, chlorine etc. are some of the non-metals produced by this method.
3. Production of compounds
Electrolysis can be employed to produce compounds like sodium hydroxide, potassium hydroxide etc.
4. Refining of Metals
Metals such as copper, gold etc. are refined by electrolysis.

Electroplating
The process of obtaining a coating of one metal over another metal using electrolysis is known as electroplating. 
This thin coating is helpful for improving the appearance of the metal and also in preventing metallic corrosion. 
Clean the article to be coated and connect it to the negative terminal of the battery. 
The metal to be plated is connected to the positive terminal of the battery. 
A salt solution of the metal to be coated is taken as the electrolyte.
Copper plating on an iron bangle
When electricity is passed through the Cu²⁺ ions of the solution, they are attracted to the negative electrode (iron bangle) or to the cathode.
● What happens to Cu²⁺ ions at the cathode?
Cu²⁺ ions receive electrons to become Cu atoms(Reduction)
Cu²⁺ +2ē →Cu 
Cu²⁺ ions are deposited on the iron bangle as Cu atoms.
● Copper undergoes oxidation at the copper plate which is the positive electrode (anode).
Cu→ Cu²⁺+2ē 
Since both, the reactions are similar and opposite in direction, the concentration of ions in the solution remains constant during electrolysis.
Some of the examples for electroplating.
1. Gold plated jewellery.
2. Chromium-plated handlebars.
3. Silver plated utensils.
Let Us Access
1. The solution of ZnSO₄, FeSO4 and CuSO₄ are taken in three different test tubes. Suppose, an iron nail is kept immersed in each one.
a● In which test tube the iron nail undergoes a colour change?
b● What is the reaction taking place here?
c● Justify your answer.
Answers:
a. In CuSO4
b. Displacement
c.  Iron is more reactive than the metal in salt solution (Cu)

2. Compare the electrolysis of molten potassium chloride and solution of potassium chloride.
What are the processes taking place at the cathode and the anode?
Answer: Electrolysis Molten potassium chloride.
Potassium chloride in a solid state is not an electrical conductor because its ions have no freedom of movement. But electricity flows through molten potassium chloride. When potassium chloride melts, the positively charged potassium ions (K⁺) and the negatively charged chloride ions (Cl⁻) are free to move. KCl → K⁺+ Cl⁻
● Which ion is attracted to the positive electrode (anode)?
Cl⁻
● What is the chemical reaction taking place there?
2Cl⁻ → Cl₂ + 2ē 
● Which is the gas liberated at the anode?
Cl₂ 
● Which is the ion attracted to the negative electrode (cathode)?
K⁺
● Write the change happening to it?
K⁺ +  ē→ K
● Which is the metal deposited at the cathode?
K
Electrolysis of potassium chloride solution
3. You are given a solution of AgNO₃ a solution of MgSO₄ an Ag rod and an Mg ribbon.
How can you arrange a Galvanic cell using these? Write down the reactions taking place at the cathode and the anode.


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