Chapter 7 Redox reaction
7.1 What is redox reaction?
There are two ways of finding out whether or not a substance has been oxidised or reduced during a chemical reaction:
■■ electron transfer
■■ changes in oxidation number.
Oxidation is losing electrons. (OIL)
Reduction is gaining electrons .(RIG)
7.2 Redox and electron transfer
Half-equations
Example: 2Na(s) + Cl2(g) →2NaCl(s)
✓Na  → Na++ e–
Na – e– → Na+                  Oxidation is losing electrons. (OIL)
✓Cl2 + 2e–  → 2Cl–      Reduction is gaining electrons .(RIG)
Balancing half-equations
Example : 1  Construct the balanced ionic equation for the reaction between nickel and iron(III) ions, Fe3+, from the half-equations:
Ni(s) →Ni2+(aq) + 2e–
Fe3+(aq) + e–→ Fe2+(aq)
The balanced ionic equation is: ___________________________________________________
Example : 2 Construct the balanced ionic equation for the reaction of iodide ions (I–) with manganate(VII) ions (MnO4–) inthe presence of hydrogen ions (H+). Use the
following two half-equations to help you:
i 2I–(aq)  →I2(aq) + 2e–
ii MnO4–(aq) + 8H+(aq) + 5e–→ Mn2+(aq) + 4H2O(l)
The balanced ionic equation is: ___________________________________________________
Exercise : 2 a Write two half-equations for the following reactions. For each half-equation state
whether oxidation or reduction is occurring.
i Cl2 + 2I– → I2 + 2Cl–
ii 2Mg + O2→ 2MgO
iii 4Fe + 3O2 → 2Fe2O3
b Zin
c metal reacts with IO3– ions in acidic solution.Construct a balance
d ionic equation for this
reaction, using the two half-equations below:
2IO3– + 12H+ + 10e– → I2 + 6H2O
Zn → Zn2+ + 2e–
7.3  Oxidation numbers
What are oxidation numbers(oxidation states)?
Oxidising agent &reducing agent
Oxidation number rules
1.单质化合价均为0.
2.在化合物中，许多原子有固定化合价
--第一主族（_________________________________________）元素为+1价
--第二主族（_________________________________________）元素为+2价
--氟元素（F）总为-1价
ox
-
-氢元素（H）为+1价（除非为金属氢化物，如NaH, H为-1 ）
--氧元素（O）为-2 价（过氧化物peroxide中的氧为-1，F2O中氧为+2）
3. 化合物中各元素化合价之和为0.
4. 离子团的化合价之和等于离子的带电荷数。（如NO3- : N元素与O元素的化合价之和为-1）
5. 无论是离子团还是化合物中，电负性更强的元素带负电。
Exercise ：State the ox. no. of the bold atoms in these compounds or ions:
a P2O5
b S O42–
c H2S
d Al2Cl6
e N H3
f Cl O2–
g Ca C O3
7.4  Naming compound
Oxides of nitrogen
■■The ox. no. of N in N2O is +1. So this compound is nitrogen(I) oxide.
■■The ox. no. of N in NO is +2. So this compound is nitrogen(II) oxide.
■■The ox. no. of N in NO2 is +4. So this compound is nitrogen(IV) oxide.
Nitrate ions
■■ The ox. no. of N in the NO2– ion is +3. So NaNO2 is sodium nitrate(III).
■■ The ox. no. of N in the NO3– ion is +5. So NaNO3 is sodium nitrate(V).
7.5 From name to formula
Exercise : Give the formulae of:    a sodium chlorate(I)      b iron(III) oxide
c potassium nitrate(III)
d phosphorus(III) chloride.
7.6  Balancing chemical equations using oxidation numbers
Example: Copper(II) oxide (CuO) reacts with ammonia (NH3) to form copper, nitrogen (N2) and water.
Exercise: Use the oxidation number method to balance these equations.
a H2SO4 + HI → S + I2 + H2O
b HBr + H2SO4→ Br2 + SO2 + H2O
c V3+ + I2 + H2O → VO2+ + I– + H+
homework
Section A
Q2 A cheap carbon monoxide detector for a gas heater consists of a patch containing palladium chloride crystals. When carbon monoxide is present, the crystals turn from orange to black as the following reaction takes place.
CO(g) + PdCl2(s) + H2O(l) → CO2(g) + Pd(s) + 2HCl(aq)
orange                                      black
Which is the element whose oxidation number decreases in this reaction?
A carbon
B chlorine
C hydrogen
D palladium
Q4 The nickel-cadmium rechargeable battery is based upon the following overall reaction.
Cd + 2NiOOH + 4H2O → Cd(OH)2 + 2Ni(OH)2.H2O
What is the oxidation number of nickel at the beginning and at the end of the reaction?
Q5 In the treatment of domestic water supplies, chlorine is added to the water to form chloric(I) acid, HClO.
Cl2(aq) + H2O(I) → H+(aq) + Cl–(aq) + HClO(aq)
This reacts further to give the chlorate(I) ion.
HClO(aq) + H2O(I) → H3O+(aq) + ClO–(aq)
Both HClO and ClO– kill bacteria by oxidation.
What is the change in oxidation number of chlorine in forming the chlorate(I) ion from the aqueous chlo
rine?
A –1
B 0
C +1
D +2
Q6 Ammonium nitrate, NH4NO3, can decompose explosively when heated.
NH4NO3 → N2O + 2H2O
What are the changes in the oxidation numbers of the two nitrogen atoms in NH 4NO 3 when this reaction proceeds?
A –2, –4
B +2, +6
C +4, –6
D +4, –4  Q8 In which substance does nitrogen exhibit the highest oxidation state?
A NO
B N 2O
C N 2O 4
D NaNO 2  Q11 Chlorine dioxide is produced on a large scale as it is used for bleaching paper pulp. It is made by the following reaction.
2Cl O 3(aq) + SO 2(g) → 2Cl O 2(g) + SO 4 (aq)
How do the oxidation numbers of chlorine and sulphur change in this reaction?
Q12 In some early paintings, lead(II) carbonate was used as a white pigment. In the 19th century hydrogen sulphide from burning coal reacted with this pigment to form black lead(II) sulphide, PbS. The original colour of the painting may be restored by carefully treating the area with dilute hydrogen peroxide, producing lead(II) sulphate which is also white.  What is the role of the hydrogen peroxide?
A catalyst
B oxidising agent
C reducing agent
D solvent  Q13 In an experiment, 50.0 cm 3 of a 0.10 mol dm –3 solution of a metallic salt reacted exactly with 25.0 cm 3 of 0.10 mol dm –3 aqueous sodium sulphite.
The half-equation for oxidation of sulphite ion is shown below.
SO 3 (aq) +H 2O(I)→SO 4 (aq) +2H (aq) +2e If the original oxidation number of the metal in the salt was +3, what would be the new  oxidation number of the metal? A+1                    B+2                  C+4                    D+5  Q14 The amount of titanium dioxide in an ore can be determined by using the following reaction. 3
TiO2 + 4BrF3 → 3TiF4 + 2Br2 + 3O2
Which element increases in oxidation number in this reaction?
A bromine
B fluorine
C oxygen
D titanium
Q16 Sodium iodide reacts with concentrated sulfuric acid. The equation which represents one of the reactions that takes place is shown.
8NaI + 9H2SO4 → 8NaHSO4 + 4I2 + H2S + 4H2O
Which species has been oxidised in this reaction?
A H+
B I–
C Na+
D SO42–
Q17 In which reaction does an element undergo the largest change in oxidation state?
Q19 Which conversion involves a reduction of chromium?
Section B
Q20 Many crude oils contain sulphur as H2S. During refining, by the Claus process, the H2S is converted into solid sulphur, which is then removed.
reaction I        2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
reaction II      2H2S(g) + SO2(g) → 2H2O(l) + 3S(s)
Which statements about the Claus process are correct?
1 H2S is oxidised in reaction I.
2 SO2 oxidises H2S in reaction II.
3 SO2 behaves as a catalyst.
Q21 Which reactions are redox reactions?
1 CaBr
2 + 2H2SO4→ CaSO4 + Br2 + SO2 + 2H2O
2 CaBr2 + 2H3PO4 → Ca(H2PO4)2 + 2HBr
3 CaBr2 + 2AgNO3 → Ca(NO3)2 + 2AgBr
Q22 Zirconium, Zr, proton number 40, is a metal which is used in corrosion-resistant alloys. Zirconium metal is extracted from the oxide ZrO2 by the following sequence of reactions.