Matric Part-1 Chemistry Notes Chapter number 7 Electrochemistry short questions, and long questions.
Q.1) Define oxidation in terms of electrons. Give an example.
Answer:
Oxidation is the loss of electron(s) by an atom or an ion.
Examples:
Lithium loses an electron to form a positive charge.
Li → Li+1 + e–
Ferrous loses an electron to form ferric
Fe+2 → Fe+3 + e–
Q.2) Define reduction in terms of loss or gain of oxygen or hydrogen. Give an example.
Answer:
Reduction is the addition of hydrogen or loss of oxygen.
Examples:
N2 molecule gains hydrogen and forms NH3.
N2 + 3H2 → 2NH3
CuO loses oxygen and forms Cu.
Zn + CuO → Zn + CuO
Q.3) What is the difference between valency and oxidation state?
Answer:
| Valency | Oxidation state |
| Valency is the measure of an element’s ability to combine with other elements. It is determined by the number of electrons in the outermost shell of each atom of an element. | Oxidation state is a number assigned to an element in a chemical combination. It represents the number of electrons that an atom can gain, lose, or share. |
| The number precedes the sign while writing the valency. | The sign precedes the number while writing the oxidation state. |
| Example:Valency of nitrogen = N3+ | Example:Oxidation state of nitrogen in HNO3 = N+5 |
Q.4) Differentiate between oxidizing and reducing agents
Answer:
Q.5) Differentiate between strong and weak electrolytes.
Answer:
| Strong electrolytes | Weak electrolytes |
| Strong electrolytes almost completely dissociate in their aqueous solutions and produce more ions. | Weak electrolytes only partially dissociate in solution and produce relatively few ions. |
| Examples:Aqueous solutions of NaCl, NaOH, HBr, H2SO4 etcNaCl → Na+ + Cl– | Examples:CH3COOH, NH3, HF dissolved in water.CH3COOH + H2O → CH3COO– + H3O+ |
Q.6) How is the electroplating of tin on steel carried out?
Answer:
A clean sheet of steel is dipped into a container of a solution of tin salt. The steel acts as the cathode and another electrode made of tin acts as anode. An electric current is passed through the circuit and tin metal ions present in the solution deposit on steel.
Q.7) Why is steel plated with nickel before the electroplating of chromium?
Answer:
Steel is plated with nickel before the electroplating of chromium because it does not adhere well on the surface of steel. Chromium sits best on nickel which itself provides adhesion. Thus, chromium deposited over the layer of nickel lasts longer. Nickel is the source of shine, and resistance to corrosion in the chromium plating.
Q.8) How can you explain that the following reaction is oxidation in terms of increase of oxidation number?
AI° →Al3+ + 3e–
Answer:
Oxidation is loss of electrons. Oxidation state increases during oxidation. Aluminum loses three electrons and acquires +3 oxidation state. The oxidation state has increased from 0 to +3. Thus the reaction is an oxidation reaction.
Q.9) How can you prove with an example that conversion of an ion to an atom is an oxidation process?
Answer:
Oxidation is the loss of electrons. Anions undergo oxidation to convert to an atom. For example, chlorine ion (anion) loses electrons and converts into a chlorine molecule.
2Cl– → Cl2 + 2e–
Q.10) Why does the anode carry a negative charge in galvanic cells but a positive charge in electrolytic cells? Justify with comments.
Answer:
The anode is defined as the electrode where oxidation takes place. In a galvanic cell Zn is anode, Zn atoms lose electrons and give Zn+2 ions to solution. Since electrons are always present on the Zn plate, it is negative.
In an electrolytic cell, anions migrate from the solution towards the anode. And anions are negatively charged, so anode is positive.
Q.11) Where do the electrons flow from the Zn electrode in Daniel’s cell?
Answer:
In Daniel’s cell electrons flow from the Zn electrode (cathode) to the Cu electrode (anode) through an external circuit. Zn metal loses electrons which flow to Cu electrode and are gained by the copper ions of the solution and copper atoms deposit at the electrode.
Q.12) Why do electrodes get their names ‘anode’ and’ cathode ‘in galvanic cells?
Answer:
The two electrodes in a galvanic cell are Zn electrode and Cu electrode. Zn metal loses electrons i.e. oxidation takes place at Zn electrode. Therefore, it is named anode.
Zn → Zn2+ + 2e–
The electrons lost by Zn metal are gained by copper ions i.e. Cu+2 reduces and Cu atoms are deposited at the electrode. Therefore, it is named cathode.
Cu2+ + 2e– → Cu
Q.13) What happens at the cathode in a galvanic cell?
Answer:
Reduction takes place at cathode in a galvanic cell.
Cu2+ + 2e– → Cu
The electrons lost by Zn metal are gained by copper ions of the solution i.e. Cu+2 reduces and Cu atoms are deposited at the electrode.
Q.14) Which solution is used as an electrolyte in Nelson’s cell?
Answer:
Concentrated solution of sodium chloride called brine is used as an electrolyte in Nelson’s cell.
Q.15) Name the by-products produced in Nelson’s cell?
Answer:
Nelson’s cell is used for manufacturing of caustic soda (NaOH) from brine solution on industrial scale. Hydrogen gas (H2) and chlorine gas (Cl2) are the by-products produced.
2NaCl + 2H2O → H2 + Cl2 + 2NaOH
Q.16) Why is galvanizing done?
Answer:
Galvanizing is done to protect iron (Fe) against corrosion. It is done by coating a thin layer of zinc (Zn) on iron.
Q.17) Why is an iron grill painted frequently?
Answer:
Action of moist air with the iron causes it to rust. Iron grill is painted frequently to prevent it from rusting, weathering and other atmospheric effects.
Q.18) Why is O2 necessary for rusting?
Answer:
Stains and dents on the iron surface provide the site for rusting. This region is called anodic region. Iron dissolves in water to give Fe+2 and release electrons as follows:
Fe → Fe+2 + 2e–
The electrons liberated from the anode portion of the iron flow to the cathode. Cathode is the region with relatively high concentration of O2 near the surface surrounded by water. The free oxygen and dissolved iron bond into the salt Fe2O3.nH2O, which is called rust.
2Fe+2 + ½ O2 + (2+n)H2O → Fe2O3.nH2O + 4H+
This shows that O2 is necessary for rusting.
Q.19) In electroplating of chromium, which salt is used as an electrolyte?
Answer:
An aqueous solution of chromium sulfate containing a little sulphuric acid acts as an electrolyte in electroplating of chromium.
Q.20) Write the redox reaction taking place during the electroplating of chromium?
Answer:
An aqueous solution of chromium sulfate containing a little sulphuric acid acts as an electrolyte in electroplating of chromium.
Electrolyte ionizes as follows:
Cr3+ ions migrate towards the cathode. At cathode they discharge and deposit on the object to be electroplated.
Reaction at anode:
4OH- → 2H2O + O2 + 4e–
At cathode:
Cr3+ + 3e– → Cr
Q.21) In electroplating of silver, from where Ag+ ions come and where do they deposit?
Answer:
In electroplating of silver, a pure piece of silver strip acts as anode and a metallic object to be coated such as a spoon acts as cathode. These electrodes are dipped in silver nitrate solution. On passing the electric current, anode dissolves to produce Ag+ ions. These ions migrate towards the cathode, where they are discharged and deposited on the object.
At anode:
Ag → Ag+ + e–
At cathode:
Ag+ + e– → Ag
Q.22) What is the nature of electrodes used in electroplating of chromium?
Answer:
In electroplating of chromium, anode is made of antimonial-lead and the cathode is made of the object to be electroplated.
Long Questions
Q.1) Describe the rules for assigning the oxidation state.
Answer:
Rules for assigning oxidation state
i) The oxidation number of all elements in their free (uncombined) state is zero.
ii) The oxidation number of a monatomic ion is the same as the charge on the ion.
iii) The oxidation number of: Group 1 element is +1, Group 2 element is +2, Group 13 element is +3. Group 15, Group 16 and Group 17 elements have oxidation states of -3, -2 and -1 respectively.
iv) Hydrogen (H) has two possible oxidation numbers. +1 when it is bonded to a nonmetal and -1 when it is bonded to a metal.
v) The oxidation number of oxygen in a compound is usually –2. But it is -1 in peroxides and +2 in OF2.
vi) The sum of the oxidation numbers in a neutral compound is equal to 0.
vii) The more electronegative atom in a substance has the negative oxidation number.
viii) The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
Q.2) Find out the oxidation numbers of the underlined elements in the following compounds.
a) Na2SO4
b) AgNO3
c) KMnO4
d) K2Cr2O7
e) HNO2
Answer:
a) Na2SO4
The sum of the oxidation number is zero.
2 (Oxidation no. of Na) + Oxidation no. of S + 4 (Oxidation no. of O) = 0
2 (+1) + Oxidation no. of S + 4 (-2) = 0
+2 + Oxidation no. of S -8 = 0
Oxidation no. of S -6 = 0
Oxidation no. of S = +6
b) AgNO3
The sum of the oxidation number is zero.
Oxidation no. of Ag + Oxidation no. of N + 3 (Oxidation no. of O) = 0
+1 + Oxidation no. of N + 3 (-2) = 0
+1 + Oxidation no. of N -6 = 0
Oxidation no. of N -5 = 0
Oxidation no. of N = +5
c) KMnO4
The sum of the oxidation number is zero.
Oxidation no. of K + Oxidation no. of Mn + 4 (Oxidation no. of O) = 0
+1 + Oxidation no. of Mn + 4 (-2) = 0
+1 + Oxidation no. of Mn -8 = 0
Oxidation no. of Mn -7 = 0
Oxidation no. of Mn = +7
d) K2Cr2O7
The sum of the oxidation number is zero.
2 (Oxidation no. of K) + 2 (Oxidation no. of Cr) + 7 (Oxidation no. of O) = 0
2 (+1) + 2 (Oxidation no. of Cr) + 7 (-2) = 0
+2 + 2 (Oxidation no. of Cr) -14 = 0
2 (Oxidation no. of Cr) -12 = 0
2 (Oxidation no. of Cr) = +12
Oxidation state of Cr = +12/2 = +6
e) HNO2
The sum of the oxidation number is zero.
Oxidation no. of H + Oxidation no. of N + 2 (Oxidation no. of O) = 0
+1 + Oxidation no. of N + 2 (-2) = 0
+1 + Oxidation no. of N -4 = 0
Oxidation no. of N -3 = 0
Oxidation no. of N = +3
Q.3) How can a non-spontaneous reaction be carried out in an electrolytic cell? Discuss in detail.
Answer:
An electrolytic cell is a type of electrochemical cell that undergoes a non-spontaneous chemical reaction when electrical energy is applied. It is often used to decompose chemical compounds by the process called electrolysis.
An electrolytic cell consists of two electrodes (cathode and anode) dipped in an electrolytic solution. A source of direct current is connected to the pair of inert electrodes. The electrode connected to the negative terminal is called cathode while the electrode connected to the positive terminal is called anode.
When electric current is applied, an electrolytic reaction takes place. The negative ions migrate towards the anode and release electrons as they oxidize. When positive ions collide with the cathode, these ions pick up electrons and as a result reduction takes place.
For example when fused salt of NaCl is electrolyzed, the Cl– ions move towards the anode and release electrons and form chlorine atoms. The chlorine atoms combine together to form chlorine gas which bubbles away.
At anode
2Cl– → Cl2 + 2e–
Na+ ions move towards the cathode and pick up electrons to form sodium metal.
At cathode
2Na+ + 2e– → 2Na
Overall reaction
2Na+ + 2Cl– → 2Na + Cl2
Q.4) Discuss the electrolysis of water.
Answer:
Electrolysis of water
Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) by passing electric current through water.
Apparatus for electrolysis of water consists of two inert electrodes that are sealed in opposite ends of a container. The electrodes are then connected to a source of electric current.
Water by itself is a very poor conductor of electricity. It ionizes to a very small extent. Hydrogen ions (H+) and hydroxyl ions (OH–) are at concentrations of 10-7 mol dm-3. Therefore, few drops of an acid are added to water to provide ions that can flow through the solution. The added acid must be soluble in water and must contain ions that are harder to reduce or oxidize than water.
When electric current is passed through the acidified water, the OH– ions move towards anode, and discharge O2 gas. H+ ions move towards the cathode and discharge H2 gas. The redox reaction taking place is as follows:
At anode (oxidation)
4OH → 2H2O + O2 + 4e–
At cathode (reduction)
4H+ + 4e– → 4H
4H → 2H2
Overall reaction
2H2O → 2H2 + O2
Q.5) Discuss the construction and working of a cell in which electricity is produced.
Answer:
A galvanic cell (also known as a voltaic cell) is an electrochemical cell that uses spontaneous redox reactions to generate electricity.
Construction of galvanic cell
It consists of two separate half-cells. Each half-cell is composed of an electrode dipped in 1 M solution of its own salt and connected to an external circuit through a wire. The two half cells are linked together by a wire running from one electrode to the other. A salt bridge also connects to the half cells.
One half-cell consists of zinc metal electrodes dipped in 1 M solution of zinc sulfate. The other half-cell consists of copper electrodes dipped in 1 M solution of copper sulfate. The salt bridge maintains the charge neutrality of each half-cell by allowing the ions to flow with minimal mixing of the half-cell solutions.
Working of the cell
Oxidation occurs at the Zn electrode. Zn metal loses electrons which flow through external wire to the copper electrode. Copper ions of the solution pick these electrons to form copper atoms. These atoms deposit at the electrode. The redox reaction taking place is as follows:
Half-cell reaction at anode (oxidation)
Zn → Zn2+ + 2e–
Half-cell reaction at cathode (reduction)
Cu2+ + 2e– → Cu
Overall reaction
Zn + Cu2+ → Zn2+ + Cu
Because of this redox reaction, electric current is produced.
Q.6) How can we prepare NaOH on a commercial scale? Discuss its chemistry along with the diagram.
Answer:
Manufacture of NaOH
NaOH (caustic soda) is produced on a commercial scale by the electrolysis of aqueous concentrated solution of NaCl called brine. For this purpose a special type of cell called Nelson’s cell is used.
The cell consists of a U-shaped perforated iron vessel, which acts as a cathode. The cathode is coated inside with a diaphragm of asbestos. A carbon rod is suspended inside the U-shaped vessel which acts as anode.
Brine solution is poured into the cell and electric current is passed through it. Aqueous solution of sodium chloride consists of Na+, Cl–, H+ and OH– ions.
Cl– ions move towards anode where they lose electrons and chlorine gas (Cl2) is discharged. Na+ ions move towards the cathode where water splits into hydrogen gas (H2) and OH– ions.
The Na+ ions combine with OH– ions to form NaOH (sodium hydroxide) which is collected in the basin.
At anode (oxidation)
2Cl– → Cl2 + 2e–
At cathode (reduction)
2H+ + 2e– → 2H
2Na+ + 2OH- → 2NaOH
Overall reaction
2NaCl + 2H2O → H2 + Cl2 + 2NaOH
Q.7) Discuss the redox reaction taking place in the rusting of iron in detail.
Answer:
Rust is the name for a very common compound, iron oxide. Stain and dents on the surface of iron provide the site for the rusting process to occur. It is the anodic region and oxidation takes place there.
2Fe → Fe+2 + 4e–
The electrons released move through the iron and reach a point where O2 concentration is relatively high near the surface surrounded by a water layer. This region is a cathodic region and reduction takes place there. Oxygen molecules are reduced by the electrons in the presence of H+ ions. The sources of H+ ions are atmospheric moisture and carbon dioxide. Water combined with carbon dioxide to form carbonic acid which gives H+ ions.
O2 + 4H+ + 4e– → 2H2O
The overall redox reaction does not form rust.
2Fe + O2 + 4H+ → 2Fe2+ + 2H2O
The free oxygen and dissolved iron bond into the salt Fe2O3.nH2O, which is called rust.
2Fe+2 + ½ O2 + (2+n)H2O → Fe2O3.nH2O + 4H+
Q.8) Discuss why galvanizing is considered better than that of tin plating.
Answer:
The protective coating of zinc works in a different way than tin plating. Zinc is a more active metal than iron. This means when both of them are exposed, zinc reacts. So the zinc dissolves being sacrificed in the protection of the iron. Therefore, zinc protects the iron against corrosion even after the coating is damaged.
The thin tin coating on the iron is used in the beverage and food cans. The can is unreactive to foods in it, while the more active iron would dissolve in acidic foods. When the less active tin coat is scratched, the large area of the tin encourages corrosion of the iron. So the tin protects the iron from corrosion only as long as they are intact. Thus, galvanizing is considered better than tin plating.
Q.9) What is electroplating? Write down the procedure of electroplating.
Answer:
Electroplating
Electroplating is the process of coating one metal onto another by electrolysis. This process is used most commonly for decorative purposes or to prevent corrosion of a metal.
There are also specific types of electroplating such as copper plating, silver plating, and chromium plating. The object to be plated is made cathode (negative electrode) and the metal to be deposited is made anode (positive electrode). The electrolysis is carried out in aqueous solution of a salt of the metal being deposited.
Procedure for electroplating
Generally, one of the electrodes is made from the metal to be plated and the electrolyte is a solution of a salt of the same metal. This electrode is anode.
The electrode on which metal is to deposit is generally made from a cheaper metal or a nonmetal coated with a conducting material such as graphite. This electrode is cathode and is first cleaned with sand, washed with caustic soda solution and finally washed with water. These electrodes are connected with a battery.
When electric current is passed, positive ions (for example Zn ions in the above figure) are attracted to the cathode and slowly deposit on it. As a result, a thin layer of metal deposits on the object, which afterward is pulled out and cleaned.
Q.10) What is the principle of electroplating? How is the electroplating of chromium carried out?
Answer:
Principle of electroplating
The principle of electroplating is to establish an electrolytic cell. The object to be plated is made cathode (negative electrode) and the metal to be deposited is made anode (positive electrode). The electrolysis is carried out in an aqueous solution of a salt of the metal being deposited.
Electroplating of chromium
The object to be electroplated is dipped in an aqueous solution of chromium sulfate. This solution contains sulphuric acid acting as an electrolyte. The object to be plated is made the cathode and the antimonial-lead is made anode. On passing the electric current the electrolyte ionizes and forms Cr3+ ions. These ions move towards the cathode and deposit there.
Ionization of electrolyte
At anode
4OH– → 2H2O + O2 + 4e–
At cathode
Cr3+ + 3e– → Cr
The steel first undergoes a copper, then nickel plating prior to the chromium. This is done because chromium does not adhere well to the surface of the steel. Chromium sits best on nickel which itself provides adhesion. Thus chromium deposited lasts longer. Nickel is the source of shine and resistance to corrosion in the chromium plating.
