Questions: Redox Reactions
Redox Reactions22 min readLesson 5 of 6
Q1. Define oxidation and reduction in terms of electron transfer.
Answer: Oxidation is the loss of electrons and reduction is the gain of electrons. The species that loses electrons is oxidised, and the species that gains them is reduced. The two processes always occur together, because the electrons lost by one species must be accepted by another. This definition applies to all redox reactions, including those in which oxygen and hydrogen are not involved.
Hinglish: Oxidation electron ka loss hai aur reduction electron ka gain. Jo electron kho deta hai woh oxidise hota hai aur jo leta hai woh reduce hota hai. Dono hamesha saath hote hain kyunki electrons kahin jaate hain. Yeh definition har redox reaction par lagti hai.
Q2. Identify the oxidising and reducing agents in the reaction of zinc with copper sulphate.
Answer: Zinc goes from oxidation number zero to plus two, so it loses electrons and is oxidised; zinc is the reducing agent. Copper goes from plus two to zero, so it gains electrons and is reduced; the copper ion is the oxidising agent. The overall reaction is zinc plus copper ion giving zinc ion plus copper, and the two electron changes match two electrons each.
Hinglish: Zinc ka oxidation number 0 se +2 ho jaata hai, matlab woh electron kho deta hai aur oxidise hota hai, isliye zinc reducing agent hai. Copper +2 se 0 ho jaata hai, matlab reduce hota hai, isliye copper ion oxidising agent hai. Dono mein do electrons ka change hai, isliye equation balanced hai.
Q3. State four rules for assigning oxidation numbers.
Answer: An element in its free state has oxidation number zero. A monoatomic ion has oxidation number equal to its charge. Oxygen usually has minus two and hydrogen usually plus one. The sum of the oxidation numbers in a neutral compound is zero, and in a polyatomic ion it equals the charge of the ion. Exceptions such as the minus one of oxygen in peroxides and the plus two of oxygen in fluorine compounds are decided by the more electronegative atom.
Hinglish: Free element ka number zero, monoatomic ion ka number uski charge, oxygen generally -2, hydrogen generally +1, aur compound mein numbers ka sum zero hota hai jabki ion mein sum charge ke barabar. Exceptions mein oxygen peroxide mein -1 aur OF2 mein +2 hota hai.
Q4. Find the oxidation number of manganese in potassium permanganate, KMnO4.
Answer: Potassium is plus one and each of the four oxygen atoms is minus two, so the oxygen contributes minus eight. Let the manganese be x. The sum for the neutral compound is plus one plus x minus eight, which must equal zero, so x is plus seven. The manganese in permanganate is therefore in the plus seven state, its highest oxidation state.
Hinglish: K +1, chaar O milakar -8, to +1 + x - 8 = 0, isliye x = +7. Permanganate mein manganese ka oxidation number +7 hai, jo uski highest state hai.
Q5. What is disproportionation? Give one example.
Answer: Disproportionation is a redox reaction in which the same element is simultaneously oxidised and reduced. In the decomposition of hydrogen peroxide, oxygen is minus one in the reactant and appears as minus two in water and zero in oxygen gas, so it is both reduced and oxidised. Another example is chlorine with cold dilute sodium hydroxide, where chlorine goes to minus one in chloride and plus one in hypochlorite.
Hinglish: Disproportionation mein same element ek hi reaction mein oxidise aur reduce dono hota hai. H2O2 ke decomposition mein oxygen -1 se -2 aur 0 dono ban jaata hai. Chlorine aur cold dilute NaOH mein chlorine -1 aur +1 dono state mein jaata hai.
Q6. Why is the zinc electrode the anode in the Daniell cell?
Answer: Zinc has a greater tendency to lose electrons than copper, because its standard reduction potential is more negative. It therefore undergoes oxidation and releases electrons, which makes it the anode, since oxidation always occurs at the anode. The electrons travel through the external circuit to the copper electrode, where copper ions are reduced. The copper electrode is the cathode.
Hinglish: Zinc ki reduction potential copper se zyada negative hai, isliye zinc electron kho deta hai aur oxidise hota hai. Oxidation anode par hota hai, isliye zinc anode hai. Electrons wire se copper tak jaate hain jahan copper ions reduce hote hain, isliye copper cathode hai.
Q7. What is the function of the salt bridge in a galvanic cell?
Answer: The salt bridge completes the electrical circuit by allowing ions to move between the two solutions without mixing them. It keeps the solutions electrically neutral by supplying ions to the half cell that is building up charge and taking ions from the other. Without it the charge would build up and the cell would stop working almost immediately.
Hinglish: Salt bridge circuit complete karta hai aur ions ko dono solutions ke beech jaane deta hai bina unhe mix kiye. Yeh dono half cells ko electrically neutral rakhta hai, warna charge build up hokar cell kaam karna band kar deta. Isliye iski zaroorat hoti hai.
Q8. Write the relation between the standard cell potential and the free energy change.
Answer: The standard free energy change equals minus n F E naught cell, where n is the number of electrons transferred, F is the Faraday constant and E naught cell is the standard cell potential. A positive cell potential makes the free energy change negative, which is the condition for a spontaneous reaction. A cell potential of zero corresponds to equilibrium, with a free energy change of zero.
Hinglish: Relation Delta G = -nFE_cell hai. Positive E_cell ka matlab Delta G negative, matlab reaction spontaneous. E_cell zero ho to system equilibrium mein hota hai aur Delta G bhi zero hota hai.
Q9. Given E naught for copper and silver couples, how do you decide whether copper can reduce silver ions?
Answer: Write the two half reactions and compute the cell potential as the silver reduction potential minus the copper reduction potential. If the result is positive, the reaction is feasible and copper can reduce silver ions. For these couples the silver value is more positive, so the cell potential is positive and the reaction proceeds: copper is oxidised and silver ions are reduced.
Hinglish: Do half reactions likho aur E_cell = E_Ag - E_Cu nikaalo. Agar positive aata hai to reaction feasible hai. Silver ka potential zyada positive hai, isliye E_cell positive aata hai aur copper silver ions ko reduce kar deta hai; copper oxidise hota hai.
Q10. Balance the half reaction for the reduction of permanganate in acidic medium.
Answer: Start with permanganate turning into manganese two plus. Balance the oxygen by adding four water molecules to the right, then balance the hydrogen by adding eight protons to the left, and finally balance the charge by adding five electrons to the left. The half reaction is permanganate plus eight protons plus five electrons giving manganese two plus plus four water. The charge on both sides is plus two.
Hinglish: MnO4^- se Mn^2+ banao. Oxygen ke liye right side par 4 H2O add karo, hydrogen ke liye left side par 8 H^+, aur charge ke liye left side par 5 electrons. Half reaction: MnO4^- + 8H^+ + 5e^- gives Mn^2+ + 4H2O. Dono side charge +2 hai.
Quiz below: five questions on oxidation numbers, agents, half reactions and electrode potentials. Attempt them before rereading the formula sheet.