2. Give short answer.
(i) What is oxidation state?
Answer: The oxidation state (or oxidation number) is a measure of the degree of oxidation of an atom in a chemical compound. It is the hypothetical charge that an atom would have if all bonds to atoms of different elements were treated as completely ionic. It is a bookkeeping device used to track electron transfer in redox reactions. Oxidation states can be positive, negative, or zero.
(ii) What is the oxidation number of Cr in chromic acid (H₂CrO₄)?
Answer: Let x be the oxidation state of Cr. H is +1, O is -2. The compound is neutral: 2(+1) + x + 4(-2) = 0 [source symbol unavailable] 2 + x - 8 = 0 [source symbol unavailable] x = +6.
(iii) Identify reducing agent in the following reaction: CuO + H₂ [source symbol unavailable] Cu + H₂O
Answer: The reducing agent is H₂ (hydrogen gas). Hydrogen is oxidized (oxidation state goes from 0 to +1) and causes the reduction of CuO (Cu goes from +2 to 0). The species that is oxidized is the reducing agent.
(iv) Why tin plated steel is used to make food cans?
Answer: Tin-plated steel is used to make food cans because tin is non-toxic, resistant to corrosion, and does not react with food acids. The tin coating protects the steel from rusting and prevents the food from coming into contact with the iron, which could cause discoloration and off-flavors. Tin also provides a shiny, attractive appearance.
(v) Explain one example from daily life which involves oxidation-reduction reaction?
Answer: Rusting of iron is a common example of an oxidation-reduction reaction. Iron (Fe) is oxidized by oxygen (O₂) in the presence of moisture to form hydrated iron(III) oxide (rust). Iron loses electrons (oxidation), and oxygen gains electrons (reduction). This reaction is undesirable as it weakens iron structures.
3. Compare and contrast oxidation and reduction.
Answer:
| Feature | Oxidation | Reduction |
| Definition | Loss of electrons | Gain of electrons |
| Oxidation State | Increases | Decreases |
| Location | Occurs at anode | Occurs at cathode |
| | |
| Example | Zn [source symbol unavailable] Zn²⁺ + 2e⁻ | Cu²⁺ + 2e⁻ [source symbol unavailable] Cu |
| Relationship | Always accompanied by reduction | Always accompanied by oxidation |
4. Define oxidation and reduction in terms of loss or gain of electrons.
Answer:
Oxidation: Oxidation is the loss of electrons by a chemical species. When a substance is oxidized, its oxidation state increases. The electrons lost are gained by another species. Reduction: Reduction is the gain of electrons by a chemical species. When a substance is reduced, its oxidation state decreases. The electrons gained are lost by another species. Oxidation and reduction always occur simultaneously in a redox reaction.
5. Explain how food and beverage industries deal with corrosion.
Answer: Food and beverage industries deal with corrosion by:
Using Corrosion-Resistant Materials: Stainless steel, aluminum, glass, and plastics are used for equipment and packaging. Coatings: Tin plating, lacquers, and polymer coatings are applied to metal cans to prevent contact with food. Cathodic Protection: Connecting metal equipment to a more active metal (e.g., magnesium) to prevent corrosion. pH Control: Adjusting the pH of food products to minimize corrosive effects. Inhibitors: Adding food-safe corrosion inhibitors.
6. State the substances which are oxidized or reduced. Give reason for your answer.
(a) N₂ + 3H₂ [source symbol unavailable] 2NH₃
Answer: N₂ is reduced (0 to -3). H₂ is oxidized (0 to +1).
(b) CO₂ + 2Mg [source symbol unavailable] 2MgO + C
Answer: CO₂ is reduced (C goes from +4 to 0). Mg is oxidized (0 to +2).
(c) Mg + H₂O [source symbol unavailable] MgO + H₂
Answer: Mg is oxidized (0 to +2). H₂O is reduced (H goes from +1 to 0).
(d) H₂S + Cl₂ [source symbol unavailable] 2HCl + S
Answer: H₂S is oxidized (S goes from -2 to 0). Cl₂ is reduced (0 to -1). (e) 2NH₃ + 3CuO [source symbol unavailable] 3Cu + N₂ + 3H₂O
Answer: NH₃ is oxidized (N goes from -3 to 0). CuO is reduced (Cu goes from +2 to 0).
7. (a) Define oxidation number or oxidation state.
Answer: The oxidation number is the charge that an atom would have if the compound was composed of ions. It is a measure of the degree of oxidation of an atom in a chemical compound.
(b) Find the oxidation state of nitrogen in the following compounds.
(i) HNO₃
Answer: +5
(ii) N₂O
Answer: +1
(iii) NO₂
Answer: +4
(iv) NH₃
Answer: -3
8. Find the oxidation state of S in the following compounds.
(a) H₂S
Answer: -2
(b) H₂SO₄
Answer: +6
(c) Na₂S₂O₃
Answer: +2 (Average oxidation state)
9. (a) Define oxidizing and reducing agents.
Answer:
Oxidizing Agent: A substance that accepts electrons and causes another substance to be oxidized. It is itself reduced. Reducing Agent: A substance that donates electrons and causes another substance to be reduced. It is itself oxidized.
(b) Identify the oxidizing agents and reducing agents in the following reactions:
(i) H₂S + Cl₂ [source symbol unavailable] 2HCl + S
Answer: Oxidizing agent: Cl₂. Reducing agent: H₂S.
(ii) 2FeCl₂ + Cl₂ [source symbol unavailable] 2FeCl₃
Answer: Oxidizing agent: Cl₂. Reducing agent: FeCl₂.
(iii) 2KI + Cl₂ [source symbol unavailable] 2KCl + I₂
Answer: Oxidizing agent: Cl₂. Reducing agent: KI.
(iv) Mg + 2HCl [source symbol unavailable] MgCl₂ + H₂
Answer: Oxidizing agent: HCl. Reducing agent: Mg.
10. Hydrogen peroxide reacts with silver oxide and lead(II) sulphide according to the following equations. Is hydrogen peroxide an oxidizing or reducing agent in these reactions. Argue.
(i) H₂O₂ + Ag₂O [source symbol unavailable] 2Ag + H₂O + O₂
Answer: In this reaction, H₂O₂ acts as a reducing agent. It reduces Ag₂O to Ag (Ag goes from +1 to 0) while itself being oxidized (O goes from -1 to 0 in O₂).
(ii) 4H₂O₂ + PbS [source symbol unavailable] PbSO₄ + 4H₂O
Answer: In this reaction, H₂O₂ acts as an oxidizing agent. It oxidizes PbS to PbSO₄ (S goes from -2 to +6) while itself being reduced (O goes from -1 to -2 in H₂O).
