MU #1 An oxidation-reduction (redox) reaction involves the transfer of electrons (e-).

MU #2 Oxidation is the loss of electrons.

A loss of e- results in an increase in oxidation number. Chemists found that whenever an element combined with O it had a tendency of losing electrons and this tendency became associated with the element O.

Example

Na --> Na⁺ + e-     

Al  --> Al³⁺ + 3 e-

MU #3 Reduction is the gain of electrons.

A gain of e- results in a decrease in oxidation number.

Example

Cl  + e- --> Cl-

Si  + 4 e-  --> Si^4-

MU #4 Changes in oxidation numbers indicate that oxidation and reduction have occurred.

Redox reactions obey the law of conservation of mass and charge.

Example

2 Mg(s) + O₂(g) --> 2 MgO(s)

Mg⁰ changed to Mg²⁺ therefore is has been oxidized since it lost 2 e-

O⁰ changed to O^2- therefore it has been reduced since it gained 2 e-

MU #5 A half-reaction can be written to represent oxidation and reduction. In a redox reaction the number of electrons lost is equal to the number of electrons gained.

Example

2 Mg(s) + O₂(g) --> 2 MgO(s)

½ oxidation reaction is Mg --> Mg²⁺ + 2 e-             ½ oxidation reaction balanced is 2 Mg --> 2 Mg²⁺ + 4 e-

½ reduction reaction is O + 2 e- --> O^2-                    ½ reduction reaction balanced is O₂ + 4 e- --> 2 O^2-

Mg  +  Cl₂ --> MgCl₂

½ oxidation reaction is Mg --> Mg²⁺ + 2 e-             ½ oxidation reaction balanced is Mg --> Mg²⁺ + 2 e-    

½ reduction reaction is  Cl + e- --> Cl^-                       ½ reduction reaction balanced is Cl₂ + 2 e- --> 2 Cl^-

2 H₂ + O₂ --> 2 H₂O

½ oxidation reaction is H --> H⁺ +  e-                        ½ oxidation reaction balanced is  2 H₂ --> 4 H⁺ +  4 e-    

½ reduction reaction is  O + 2e- --> O^-2                     ½ reduction reaction balanced is O₂ + 4 e- --> 2 O^-2 

Problem

Write the 2 half-reactions for the following redox reactions.

    2 NH₃ --> N₂ + 3 H₂

   N₂ + 2 O₂ --> 2 NO₂

   4 Al + 3 O₂ --> 2 Al₂O₃

TEST YOUR UNDERSTANDING

1/07

20 In which reaction are electrons transferred from one reactant to another reactant?

(1) 2Ca(s) + O₂(g) ---> 2CaO(s)                                 (2) AgNO₃(aq) + KCl(aq) ---> AgCl(s) + KNO₃(aq)

(3) HCl(aq) + NaOH(aq) ---> NaCl(aq) + H₂O(l)     (4) H₃O⁺(aq) + OH^–(aq) ---> 2H₂O(l)

6/07

23 Which balanced equation represents an oxidation-reduction reaction?

(1) BaCl₂ + Na2SO₄ ---> BaSO₄ + 2NaCl                        (2) C+H₂O ---> CO + H₂

(3) CaCO₃ ---> CaO + CO₂                                                (4) Mg(OH)₂ + 2HNO₃ ---> Mg(NO₃)₂ + 2H₂O

47 Given the balanced ionic equation representing a reaction: 2Al³⁺(aq) + 3Mg(s)  ---> 3Mg²⁺(aq) + 2Al(s)

In this reaction, electrons are transferred from (1) Al to Mg²⁺ (2) Al³⁺ to Mg (3) Mg to Al³⁺ (4) Mg²⁺ to Al

8/07

43 Which balanced equation represents a redox reaction?

(1) CuCO₃(s) ---> CuO(s) + CO₂(g)                                  (2) 2KClO₃(s) ---> 2KCl(s) + 3O₂(g)

(3) AgNO₃(aq) + KCl(aq) ---> AgCl(s) + KNO₃(aq)      (4) H₂SO₄(aq) + 2KOH(aq) ---> K₂SO₄(aq) + 2H₂O(l)

44 Given the unbalanced ionic equation:

3Mg + ____ Fe³⁺ ---> 3Mg²⁺ + ____ Fe

When this equation is balanced, both Fe³⁺ and Fe have a coefficient of

(1) 1, because a total of 6 electrons is transferred (2) 2, because a total of 6 electrons is transferred

(3) 1, because a total of 3 electrons is transferred (4) 2, because a total of 3 electrons is transferred

6/02

1. Base your answers on the following redox reaction, which occurs spontaneously in an electrochemical cell.

                     Zn + Cr³⁺ --> Zn²⁺ + Cr

            a) Write the half-reaction for the reduction that occurs.

            b) Write the half-reaction for the oxidation that occurs.

            c) Balance the equation using the smallest whole-number coefficients. 

            d) Which species loses electrons and which species gains electrons?

            e) Which half-reaction occurs at the cathode?

            f) State what happens to the number of protons in a Zn atom when it changes to Zn²⁺ as the redox reaction occurs. 

2.  Given the reaction: Mg(s) + 2 H⁺(aq) + 2 Cl^-(aq) -->  Mg²⁺(aq) + 2 Cl^-(aq) + H₂(g) Which species undergoes oxidation?

            (1) Mg(s)           (2) H⁺(aq)      (3) Cl^-(aq)        (4) H₂(g)

3. Which particles are gained and lost during a redox reaction?  (1) electrons      (2) protons        (3) neutrons    (4) positions

8/02

1. As a Ca atom undergoes oxidation to Ca²⁺, the number of neutrons in its nucleus   (1) decreases    (2) increases    (3) remains the same

2.  Given the reaction: 4 Al(s) + 3 O₂(g) --> 2 Al₂O₃(s)

a) Write the balanced oxidation half-reaction for this oxidation-reduction reaction.

b) What is the oxidation number of oxygen in Al₂O₃?

3. In any redox reaction, the substance that undergoes reduction will

(1) lose electrons and have a decrease in oxidation number (3) gain electrons and have a decrease in oxidation number

(2) lose electrons and have an increase in oxidation number (4) gain electrons and have an increase in oxidation number

1/03

1. When a neutral atom undergoes oxidation, the atom's oxidation state

     (1) decreases as it gains electrons    (2) decreases as it loses electrons (3) increases as it gains electrons  (4) increases as it loses electrons

2. Given the equation: C(s) + H₂O(g) --->CO(g) + H₂(g) Which species undergoes reduction?  (1) C(s)     (2) H⁺      (3) C²⁺  (4) H₂(g)

6/03

28. Given the equation: 2 Al + 3 Cu²⁺ --> 2 Al³⁺ + 3 Cu The reduction half-reaction is   (1) Al --> A1³⁺ + 3 e-    (2) Cu²⁺ + 2 e- --> Cu   (3) Al + 3 e- --> A1³⁺   (4) Cu²⁺ --> Cu + 2e-

8/03

21 Which type of reaction occurs when nonmetal atoms become negative nonmetal ions?  (1) oxidation (3) substitution (2) reduction (4) condensation

27 Given the reaction: Zn(s) + 2 HCl(aq) --> ZnCl₂ (aq) + H₂ (g)

Which statement correctly describes what occurs when this reaction takes place in a closed system?

    (1) Atoms of Zn(s) lose electrons and are oxidized.     (2) Atoms of Zn(s) gain electrons and are reduced.
    (3) There is a net loss of mass.                                         (4) There is a net gain of mass.

53 Given the reaction: Cl₂ + 2 HBr --> Br₂ + 2 HCl Write a correctly balanced reduction half-reaction for this equation.

1/04

23 Given the reaction: 2 Al(s) + Fe₂O₃ (s) ---heat---> Al₂O₃ (s) + 2Fe(s)   Which species undergoes reduction? (1) Al (2) Fe (3) Al³⁺ (4) Fe³⁺

39 Which equation shows conservation of both mass and charge?
(1) Cl₂ + Br^- ---> Cl^- + Br₂   (2) Cu + 2Ag⁺ ---> Cu²⁺ + Ag   (3) Zn + Cr³⁺ ---> Zn²⁺ + Cr   (4) Ni + Pb²⁺ -->Ni²⁺ + Pb

6/04

26 Given the reaction that occurs in an chemical cell: Zn(s) + CuSO₄(aq) ---> ZnSO₄ (aq) + Cu(s) During this reaction, the oxidation number of Zn changes from

        (1) 0 to +2 (2) 0 to -2 (3) +2 to 0 (4) -2 to 0

48 Given the reaction for the corrosion of aluminum: 4 Al + 3O₂ ---> 2Al₂O₃ Which half-reaction correctly represents the oxidation that occurs?

(1) Al + 3e- ---> Al³⁺   (2) Al ---> Al³⁺ + 3e-   (3) O₂ + 4e- ---> 2O^2-   (4) O₂ ---> 2 O^2- + 4e-

Base your answers to questions 54 and 55 on the unbalanced redox reaction . Cu(s) + AgNO₃(aq) ---> Cu(NO₃)₂(aq) + Ag(s)

54 Write the reduction half-reaction.

55 Balance the redox equation using the smallest whole-number coefficients. 

8/04

24 Which half-reaction correctly represents reduction?     (1) Ag ---> Ag⁺+ e-    (2) F₂ ---> 2 F^– + 2e^–     (3) Au³⁺+ 3e- ---> Au   (4) Fe²⁺ + e^– ---> Fe³⁺

25 In a redox reaction, how does the total number of electrons lost by the oxidized substance compare to the total number of electrons gained by the reduced substance?

                (1) The number lost is always greater than the number gained. (2) The number lost is always equal to the number gained.

                (3) The number lost is sometimes equal to the number gained. (4) The number lost is sometimes less than the number gained.

26 Which reaction is an example of an oxidation-reduction reaction?

                (1) AgNO₃ + KI ---> AgI + KNO₃                      (2) Cu + 2 AgNO₃ ---> Cu(NO₃)₂ + 2 Ag

                (3) 2 KOH + H₂SO₄ ---> K₂SO₄ + 2 H₂O            (4) Ba(OH)₂ + 2 HCl ---> BaCl₂ + 2 H₂O

1/05

23 Which change in oxidation number indicates oxidation?  (1) –1 to +2 (2) –1 to –2 (3) +2 to –3 (4) +3 to +2

24 Given the redox reaction: Cr³⁺ + Al ---> Cr + Al³⁺ As the reaction takes place, there is a transfer of

                (1) electrons from Al to Cr ³⁺ (2) electrons from Cr³⁺ to Al (3) protons from Al to Cr ³⁺ (4) protons from Cr³⁺ to Al

6/05

49 Given the balanced ionic equation: Zn(s) + Cu2+(aq) <---> Zn2+(aq) + Cu(s) Which equation represents the oxidation half-reaction? 

                    (1) Zn(s) + 2e– ---> Zn2+(aq) (2) Zn(s) --->Zn2+(aq) + 2e– (3) Cu2+(aq) --->Cu(s) + 2e– (4) Cu2+(aq) + 2e– ---> Cu(s) 

1/06

6/06

8/06

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