Unit 12: Redox Class Packet

Key Ideas

· An oxidation-reduction (redox) reaction involves the transfer of electrons (e-). (3.2d)

· Reduction is the gain of electrons. (3.2e)

· A half-reaction can be written to represent reduction. (3.2f)

· Oxidation is the loss of electrons. (3.2g)

· A half-reaction can be written to represent oxidation. (3.2h)

· In a redox reaction the number of electrons lost is equal to the number of electrons gained. (3.3b)

· Oxidation numbers (states) can be assigned to atoms and ions. Changes in oxidation numbers indicate that oxidation and reduction have occurred. (3.2i)

· An electrochemical cell can be either voltaic or electrolytic. In an electrochemical cell, oxidation occurs at the anode and reduction at the cathode. (3.2j)

· A voltaic cell spontaneously converts chemical energy to electrical energy. (3.2k)

· An electrolytic cell requires electrical energy to produce chemical change. This process is known as electrolysis. (3.2l)

Oxidation Numbers

Oxidation numbers are very important in this chapter “Redox Reactions.” Without the complete understanding of how to assign these numbers, we cannot move ahead with this chapter. They are much like ionic charges, except that every element will be assigned a number. The most important rules that cannot be broken are:

Unit 12: Redox Class Packet

· Free elements are zero.

· Group 1 is +1

· Group 2 is +2

· Fluorine is -1

Unit 12: Redox Class Packet

Assign oxidation numbers to each element in the following:

Unit 12: Redox Class Packet

(a) NaCl Na___ Cl___

(b) H2S H___ S___

(d) CO2 C ___ O___

(e) H2SO4 H ___ S___ O___

(f) FeCO3 Fe___ C___ O___

(g) AgI Ag___ I___

(h) H2 H___

(i) PbCl2 Pb___ Cl___

(j) BaCO3 Ba___ C ___ O___

(k) Fe2O3 Fe___ O___

(l) I2 I____

(m) BeO Be____ O____

(n) CaF2 Ca____ F ____

(o) FeCl3 Fe____ Cl____

(p) PF5 P____ F____

(q) H3PO4 H____ P____ O___

(r) KCl K ____ Cl____

(s) K2O K ____ O____

(t) O3 O ____

(u) LiH Li ____ H____

(v) HBr H ____ Br____

(w) Li+ Li____

(x) PO43- P____ O___

(y) CaH2 Ca___ H___

(z) Cr2O3 Cr___ O___

(aa) KClO K___ Cl___ O___

(bb) KClO2 K___ Cl___ O___

(cc) KClO3 K___ Cl___ O___

(dd) KClO4 K___ Cl___ O___

(ee) Na2SO4 Na___ S___ O___

(ff) Ca(OH)2 Ca___ O___ H___

(gg) Na2SO4 Na___ S___ O___

(hh) B2(Cr2O7)3 B___ Cr___ O___

(ii) Al2(SO4)3 Al___ S___ O___

(jj) Al(NO3)3 Al___ N___ O___

(kk) (NH4)3PO4 N___ H___ P___ O____

Unit 12: Redox Class Packet

REDOX Reactions

A redox reaction is a reaction in which electrons are transferred from one element to another. The reaction involves at least two elements, one that will give up an electron, and one that will receive that electron. The term redox comes from two words, “oxidation” and “reduction.” If something is oxidized, it “burns” in oxygen, as shown below:

Mg + O à Mg+2 + O-2

Take a moment to write out their Lewis dot structures. As you can see, Mg is being oxidized and it loses its two valence electrons, while oxygen gains them. Oxidation is defined as the loss of electrons.

Considering the name of this reaction, if something is being oxidized, the other element must be reducing. Neither reduction nor oxidation can happen alone. That means that oxygen, in this example, is being reduced because it gains the electrons. Reduction is defined as the gain of electrons.

A simple way to remember this is to remember that LEO the lion goes GER.

LEO = loss of electrons is oxidation.

GER = gain of electrons is reduction.

Also, Mg and O are “free” elements, which means their oxidation numbers are both 0. Now you can see that if and element’s oxidation number increases, that element is being oxidized. If an element’s oxidation number decreases it is being reduced (the number is reducing).

______

EXAMPLES: Indicate which element is being oxidized, and which is being reduced.

1. Cr3+ + Fe2+ à Cr2+ + Fe3+

2. F2 + O2- à F1- + O2

3. Sn + N5+ à Sn4+ + N4+

4. NaCl à Na+ + Cl-

5. Cu2O à Cu + O2

6. Cl2 + KBr à KCl + Br2

7. CH4 + O2 à CO2 + H2O

8. H3PO4 + Ca(OH) 2 à Ca3(PO4) 2 + H2O

RedOx Reactions

Redox reactions are usually synthesis reactions, decomposition, combustion or single replacement reactions. Double replacement and neutralization reactions are NOT redox reactions. Usually they are easy to spot because if an element goes from being “free” (with an oxidation number of 0) to being in a compound (with a new oxidation number) it shows there was an exchange of electrons. In the following examples, identify what type of reaction they are and then state if they are redox reactions.

1. N2 + O2 à 2NO ______

2. Cl2 + 2NaBr à NaCl + Br2 ______

3. 2NaOH + HCl à H2O + NaCl ______

Are these redox?

What type of reaction (S, D, C, SR, or DR) is NEVER redox? ______

Redox Half Reactions

A half reaction shows either the oxidation or reduction portion of a redox equation including if the electrons are gained or lost. A reduction half reaction shows an atom or ion gaining one of more e-: Fe3+ + 3e- à Fe Notice that the e- is on the left.

An oxidation half reaction shows an atom or ion losing one or more e-:

Mg à Mg2+ + 2 e- Notice that the e- is on the right.

In a half reaction, only one element is shown and the charges must be conserved. To write a half reaction, first assign all the oxidation numbers to all the elements. Second, cross out any elements that are spectators (they do not change oxidation number). Then, write a half reaction showing a change in oxidation state and label which element is being oxidized and which is being reduced. Last, add in the number of electrons needed to conserve the charge.

Directions: For the following examples, determine if they represent oxidation or reduction.

Unit 12: Redox Class Packet

1. e- + Cr+3 à Cr+2

2. 2e- + Mn+7 à Cr+5

3. Mg+2 + 2e- à Mg

4. K à e- + K+1

5. e- + Cu+2 à Cu+1

6. Alà Al+3 + 3e-

7. Li+ + e- à Li

8. S-2 à 2e- + S

Unit 12: Redox Class Packet

Directions: For the following examples, add in the e- to balance the charge.

Unit 12: Redox Class Packet

1. Cr+5 à Cr+2

2. Mn+4 à Mn+7

3. Ca+2 à Ca

4. Rb à Rb+1

5. Cu+1 à Cu+2

6. B+3 à B

7. N-3 à N+4

8. Te-2 à Te

Unit 12: Redox Class Packet

Directions: For the following examples, balance the elements first, then add in the e- to balance the charge.

Unit 12: Redox Class Packet

1. Cl- à Cl2

2. Br2 à Br-1

3. I2 à I-

4. H2 à H+1

5. F-1 à F2

6. N+3 à N2

7. S8 à S-2

8. O-2 à O2

Unit 12: Redox Class Packet

Directions: For each reaction, decide if it is redox. If it is redox, write the half reactions below. If they are not redox, leave the answers blank.

1. Cr3+ + Fe2+ à Cr2+ + Fe3+

2. F2 + O2- à F1- + O2

3. Sn + N5+ à Sn4+ + N4+

4. NaCl à Na+ + Cl-

5. Cu2O à Cu + O2

6. Cl2 + KBr à KCl + Br2

7. CH4 + O2 à CO2 + H2O

8. H3PO4 + Ca(OH) 2 à Ca3(PO4) 2 + H2O

Reduction Oxidation

1. ______

2. ______

3. ______

4. ______

5. ______

6. ______

7. ______

8. ______

Fill in the table below.

Reducing half Reaction / Oxidizing half reaction / How many e- are trans-ferred?
1 / H2 + O2 à H2O
2 / K + B2O3 à K2O + B
3 / C + S8 à CS2
4 / N2 + O2 à N2O5
5 / Na + O2 à Na2O
6 / Cs + N2 à Cs3N
7 / N2 + H2 à NH3
8 / Li + AlCl3 à LiCl + Al
9 / CH4 + O2à CO2 + H2O
10 / Li + H2O à LiOH + H2

Spontaneous Reactions and Activity Series

Use Table J to help you with the following questions.

1. Write the oxidation and reduction half reaction for:

Ca + Cu2+ à Ca2+ + Cu

2. According to Table J, the element higher on the list will oxidize. Which element is oxidizing here and does that mean that this reaction is spontaneous?

3. Write the oxidation and reduction half reaction for

Mg + Ca2+ à Mg2+ + Ca

4. Which element is oxidizing in number 3 and is the reaction spontaneous?

5. Which of the following ions is most easily oxidized?

a. F- b. Cl- c. Br- d. I-

6. Which element is more easily reduced?

a. Cu b. Mg c. Al d. Zn

7. Which element will reduce Mg2+ to Mg?

a. Fe b. Ba c. Pb d. Ag

8. Which ion will oxidize Fe?

a. Zn2+ b. Ca2+ c. Mg2+ d. Cu2+

9. Which metal will react spontaneously with Ag+ but not Zn2+?

a. Cu b. Au c. Al d. Mg

10. Which reaction will take place spontaneously?

a. Mg + Ca2+ à Mg2+ + Ca

b. Ba + 2Na+ à Ba2+ + 2Na

c. Cl2 + 2F-à 2Cl- +F2

d. I2 + 2Br- à 2I- + Br2

Unit 12: Redox Class Packet

RedOx Reaction Regents Review

Unit 12: Redox Class Packet

1. Which reaction is an example of an oxidation- reduction reaction?

(1) AgNO3 + KI → AgI + KNO3

(2) Cu + 2AgNO3 → Cu(NO3)2 + 2Ag

(3) 2KOH + H2SO4 → K2SO4 + 2H2O

(4) Ba(OH)2 + 2HCl → BaCl2 + 2H2O

2. In an oxidation-reduction reaction, reduction is defined as the
(1) loss of protons (3) loss of electrons
(2) gain of protons (4) gain of electrons

3. When a lithium atom forms a Li+ ion, the lithium atom

(1) gains a proton (3) loses a proton

(2) gains an electron (4) loses an electron

4. Which type of reaction occurs when nonmetal

atoms become negative nonmetal ions?

(1) oxidation (3) substitution

(2) reduction (4) condensation

5. 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

6. In a redox reaction, there is a conservation of

(1) mass, only

(2) both mass and charge

(3) neither mass nor charge

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

(1) lose e- & have a decrease in oxidation number

(2) lose e- & have an increase in oxidation number

(3) gain e- & have a decrease in oxidation number

(4) gain e- & have an increase in oxidation number

8. What occurs during the reaction below?

(1) The manganese is reduced and its oxidation number changes from +4 to +2.

(2) The manganese is oxidized and its oxidation number changes from +4 to +2.

(3) The manganese is reduced and its oxidation number changes from +2 to +4.

(4) The manganese is oxidized and its oxidation number changes from +2 to +4.

9. Given the balanced equation:

What is the total number of moles of

electrons lost by 2 moles of Al(s)?

(1) 1 mole (3) 3 moles

(2) 6 moles (4) 9 moles

10.Given the balanced equation:
Mg(s)+ Ni2+(aq)à Mg2+(aq)+ Ni(s)
What is the total number of moles of electrons lost by 2 moles of Mg(s)?

(1) 1.0 mol (3) 3.0 mol
(2) 2.0 mol (4) 4.0 mol

11. Given the equation representing a reaction:
Mg(s)+ Ni2+(aq)à Mg2+(aq)+ Ni(s)
What is the total number of moles of e- lost by Mg when 2.0 moles of e- are gained by Ni2+(aq)?
(1) 1.0 mol (3) 3.0 mol
(2) 2.0 mol (4) 4.0 mol

12. Given the reaction:

Which species undergoes oxidation?

(1) Mg(s) (3) Cl–(aq)

(2) H+(aq) (4) H2(g)

13. Given the redox reaction:

As the reaction takes place, there is a transfer of

(1) electrons from Al to Cr3+

(2) electrons from Cr3+ to Al

(3) protons from Al to Cr3+

(4) protons from Cr3+ to Al

14. Given the redox reaction:

2 Fe3+ + 3 Zn à 2 Fe + 3 Zn2+

As the reaction takes place, there is a transfer of electrons

(1) from Fe3+ to Zn

(2) from Zn to Fe3+

(3) from Zn2+ to Fe

(4) from Fe to Zn2+

Unit 12: Redox Class Packet

______

14. Circle the electrons in the half-reactions below and identify as oxidation or reduction.

15. Complete the half-reactions below by ADDING in electrons to the correct side in order to equalize charge (show conservation of charge).