Balance the following redox reactions by ion – electron method :
(a) MnO4 – (aq) + I – (aq) → MnO2 (s) + I2(s) (in basic medium)
(b) MnO4 – (aq) + SO2 (g) → Mn2+ (aq) + HSO4– (aq) (in acidic solution)
(c) H2O2 (aq) + Fe 2+ (aq) → Fe3+ (aq) + H2O (l) (in acidic solution)
(d) Cr2O7 2– + SO2(g) → Cr3+ (aq) + SO42– (aq) (in acidic solution)
Step 1:
The two half reactions involved in the given reaction are:
-1 0
Oxidation half reaction: l (aq) → l2(s)
+7 +4
Reduction half reaction: Mn O-4(aq) → MnO2(aq)
Step 2:
Balancing I in the oxidation half reaction, we have:
2l-(aq) → l2(s)
Now, to balance the charge, we add 2 e- to the RHS of the reaction.
2l-(aq) → l2(s) + 2e-
Step 3 :
In the reduction half reaction, the oxidation state of Mn has reduced from +7 to +4. Thus, 3 electrons are added to the LHS of the reaction.
MnO-4(aq) + 3e- →MnO2(aq)
Now, to balance the charge, we add 4 OH- ions to the RHS of the reaction as the reaction is taking place in a basic medium.
MnO-4(aq) + 3e- →MnO2(aq) + 4OH-
Step 4:
In this equation, there are 6 O atoms on the RHS and 4 O atoms on the LHS. Therefore, two water molecules are added to the LHS.
MnO-4(aq) + 2H2O + 3e- →MnO2(aq) + 4OH-
Step 5:
Equalising the number of electrons by multiplying the oxidation half reaction by 3 and the reduction half reaction by 2, we have:
6l-(aq) → 3l2(s) + 2e-
2MnO-4(aq) + 4H2O + 6e- → 2MnO2(s) + 8OH-(aq)
Step 6:
Adding the two half reactions, we have the net balanced redox reaction as:
6l-(aq) + 2MnO-4(aq) + 4H2O(l) → 3l2(s) + 2MnO2(s) + 8OH-(aq)
(b) Following the steps as in part (a), we have the oxidation half reaction as:
SO2(g) + 2H2O(l) → HSO-4(aq) + 3H+(aq) + 2e-(aq)
And the reduction half reaction as:
MnO-4(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l)
Multiplying the oxidation half reaction by 5 and the reduction half reaction by 2, and then by adding them, we have the net balanced redox reaction as:
2MnO-4(aq) + 5SO2(g) + 2H2O(l) + H+(aq) → Mn2+(aq) + HSO-4(aq)
(c) Following the steps as in part (a), we have the oxidation half reaction as:
Fe2+(aq) → Fe3+(aq) + e-
And the reduction half reaction as:
H2O2(aq) + 2H+(aq) + 2e- → 2H2O(l)
Multiplying the oxidation half reaction by 2 and then adding it to the reduction half reaction, we have the net balanced redox reaction as:
H2O2(aq) + 2Fe2+(aq) + 2H+(aq) → 2Fe3+(aq) + 2H2O(l)
(d) Following the steps as in part (a), we have the oxidation half reaction as:
SO2(g) + 2H2O(l) → SO2-4(aq) + 4H+ (aq) + 2e-
And the reduction half reaction as:
Cr2O2-7(aq) + 14H+(aq) + 6e- → 2Cr3+(aq) + 3SO2-4(aq) + H2O(l)
Multiplying the oxidation half reaction by 3 and then adding it to the reduction half reaction, we have the net balanced redox reaction as:
Cr2O2-7(aq) + 3SO2(g) + 2H+(aq) → 2Cr3+(aq) + 3SO2-4(aq) + H2O(l)
The mass of an electron is 9.1 × 10–31 kg. If its K.E. is 3.0 × 10–25 J, calculate its wavelength.
Calculate the amount of carbon dioxide that could be produced when
(i) 1 mole of carbon is burnt in air.
(ii) 1 mole of carbon is burnt in 16 g of dioxygen.
(iii) 2 moles of carbon are burnt in 16 g of dioxygen.
Calculate the wavelength of an electron moving with a velocity of 2.05 × 107 ms–1.
In a process, 701 J of heat is absorbed by a system and 394 J ofwork is done by the system. What is the change in internal energy for the process?
What will be the minimum pressure required to compress 500 dm3 of air at 1 bar to 200 dm3 at 30°C?
In a reaction A + B2 → AB2 Identify the limiting reagent, if any, in the following reaction mixtures.
(i) 300 atoms of A + 200 molecules of B
(ii) 2 mol A + 3 mol B
(iii) 100 atoms of A + 100 molecules of B
(iv) 5 mol A + 2.5 mol B
(v) 2.5 mol A + 5 mol B
At 0°C, the density of a certain oxide of a gas at 2 bar is same as that of dinitrogen at 5 bar. What is the molecular mass of the oxide?
Which one of the following will have largest number of atoms?
(i) 1 g Au (s)
(ii) 1 g Na (s)
(iii) 1 g Li (s)
(iv) 1 g of Cl2(g)
Density of a gas is found to be 5.46 g/dm3 at 27 °C at 2 bar pressure. What will be its density at STP?
What is meant by the term bond order? Calculate the bond order of: N2, O2,O2+,and O2-.
What is the relationship between the members of following pairs of structures? Are they structural or geometrical isomers or resonance contributors?
(a)
(b)
(c)
Arrange the following
(i) CaH2, BeH2 and TiH2 in order of increasing electrical conductance.
(ii) LiH, NaH and CsH in order of increasing ionic character.
(iii) H-H, D-D and F-F in order of increasing bond dissociation enthalpy.
(iv) NaH, MgH2 and H2O in order of increasing reducing property.
Explain, why an organic liquid vaporises at a temperature below its boiling point in its steam distillation?
What is meant by the conjugate acid-base pair? Find the conjugate acid/base for the following species:
HNO2, CN– , HClO4, F –, OH–, CO2–3 and S-
An electron is in one of the 3d orbitals. Give the possible values of n, l and ml for this electron.
Why does boron trifluoride behave as a Lewis acid?
Why does the following reaction occur ?
XeO4– 6(aq) + 2F – (aq) + 6H + (aq) → XeO3(g) + F2(g) + 3H2O(l)
What conclusion about the compound Na4XeO6 (of which XeO4– 6 is a part) can be drawn from the reaction.
An organic compound contains 69% carbon and 4.8% hydrogen, the remainder being oxygen. Calculate the masses of carbon dioxide and water produced when 0.20 g of this substance is subjected to complete combustion.
The electron energy in hydrogen atom is given by En = (–2.18 × 10–18)/n2 J. Calculate the energy required to remove an electron completely from the n = 2 orbit. What is the longest wavelength of light in cm that can be used to cause this transition?
Explain why is sodium less reactive than potassium?
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