Chemistry 30
Redox Reactions and Electrochemistry		60	

I.	Multiple Choice	20

1.	The sum of the oxidation number in K2Cr2O7 is:

	A.	-2			C.	+2
	B.	0			D.	+11

2.	Which answer correctly indicates the change in oxidation number for Fe in the reaction
			2 Fe2O3  4 Fe + 3 O2
	
	A.	0 to + 3		C.	+3 to 0
	B.	+2 to +4	D.	+3 to –2

3.	The sum of the oxidation numbers in CO32- is:

	A.	0				C.	-1
	B.	+2				D.	-2

4.	Oxidation is best defined as

	A.	gaining electrons	C.	gaining protons
	B.	losing electrons	D.	losing protons

5.	Which of the following is not a redox reaction? 

	A.	Sn4+ + Fe2+  Sn2+ + Fe3+
	B.	SO3 + H2O  H2SO4
	C.	Ba + S  BaS
	D.	Cl2 + 2 Br-  Cl- + Br2

6.	An element that changes in oxidation number in a chemical reaction from –2 to 0 is said to be:
	A.	oxidized	
	B.	reduced
	C.	both oxidized and reduced
	D.	neither oxidized or reduced.

7.	Monatomic ions (ions consisting of a single atom) are all assigned oxidation numbers equal to:
	A.	+1			C.	their charge
	B.	zero		D.	-1

8.	The oxidation number of hydrogen in most compounds is:
	A.	-1			C.	-2
	B.	zero		D.	+1

9.	The oxidation number of Mn in MnO2 is:
	A.	0			C.	+2
	B.	+4			D.	+8

10.	The substance that is oxidized in a chemical reaction is also the:
	A.	oxidizing agent	C.	reducing agent
	B.	dissolving agent	D.	buffering agent

11.	In the reaction
Cu(s) + 2 Ag+(aq)  Cu2+(aq) + 2 Ag(s)
	the oxidizing agent is
	A.	Cu(s)		C.	Ag+(aq)
	B.	Cu2+(aq)	D.	Ag(s)

12.	In the reaction
4NH3 + 5O2  6H2O + 4NO
	the oxidation number of nitrogen changes from
	A.	+3 to +1
	B.	+3 to +2
	C.	-3 to +2
	D.	-3 to -2

13.	The half-reaction
ClO3- + 6H+ + 5e-  ½Cl2 + 3H2O
	 is best described as the:
	A.	oxidation of chlorine
	B.	oxidation of H+
	C.	reduction of chlorine
	D.	reduction of H+
14.	An electrochemical cell involves:
	A.	the conversion of chemical energy into electrical energy
	B.	the conversion of electrical energy into chemical energy
	C.	either A or B
	D.	neither A or B

15.	In the electroplating of silver, Ag, on an iron metal spoon, the reaction taking place at the spoon is:
	A.	Ag  Ag+ + e-	C.	Ag+ + e-  Ag
	B.	Fe  Fe2+ + 2e-	D.	Fe2+ + 2e-  Fe
16. In an electrochemical cell, electrons travel through the external circuit:
	A.	from the diode to the cathode
	B.	from the cathode to the diode
	C.	from the cathode to the anode
	D.	from the anode to the cathode

17.	The primary difference between electrolysis and an electrochemical cell is that in the electrochemical cell the reaction is:

	A.	reduction only	C.	non-spontaneous
	B.	oxidation only	D.	spontaneous

18.	A metal can be protected from corrosion by making it:
	A.	either electrode in an electrochemical cell.
	B.	the electrolyte in an electrochemical cell.
	C.	the anode in an electrochemical cell.
	D.	the cathode in an electrochemical cell.

19.	The electrode at which reduction occurs is the:
	A.	neutrode.	C.	diode.
	B.	cathode.	D.	anode.

20.	Consider the following half-reactions; electrode potentials are given:
		Zn   Zn2+ + 2e-	E = -0.76 V
		Cu Cu2+ + 2e-	E = +0.34 V

	If an electrochemical cell were constructed using Zn|Zn2+ and Cu|Cu2+, what would be the voltage of the cell?
	A.	+0.42V		C.	+1.10 V
	B.	-0.42 V		D.	+0.55 V

II.	Short Answer			40  marks

1.	Determine the oxidation number of each element in the following:		4 marks

	a.	MgCl2		Mg = ____________	Cl = _______________

	b.	H2SO4		H = ____________	S = ________________	O = ____________

	c.	PO43-		P = ____________	O = ________________

	d.	NH4+		N = ____________	H = _________________

2.	a.	Show, by the change in oxidation numbers, that the following equation represents a redox reaction.
	b.	Identify the elements that undergo oxidation and reduction.
	c.	Also identify the oxidizing agent and the reducing agent.		3 marks			4 marks

			I2 + SO32- + 2 OH-  H2O + SO42- + 2I-

		Element oxidized:  _____________________		Element reduced: ____________________

		Oxidizing agent:	____________________		Reducing agent:   _____________________

3.	Balance the following redox reactions, using either the oxidation number method or the half-reaction method.

							6 marks
	a.	    HBr  +     H2SO4         SO2   +      Br2   +     H2O

	b.	     MnO4-   +      H2S   +      H+        S   +      Mn2+   +    H2O

	c.	   NF3   +      AlCl3         N2   +      Cl2   +      AlF3

4.	Will a reaction occur if a copper(II) sulfate solution is stored in an aluminum container?  Explain and support your answer (a “yes” or “no” alone is not worth any marks) and provide a balanced equation.	3 marks

5.	Write the two balanced half-reaction equations for the following reaction, and identify each half-reaction as oxidation or reduction.			4 marks

		Ca(s) + F2(g)  CaF2(s)

6.	Use a table of standard reduction potentials to determine the voltage of the following electrode pairs.  
							4 marks
	a.	Co|Co2+   and   Mg|Mg2+

	b.	Cl-|Cl2   and   Ni|Ni2+

7.	Use a table of standard reduction potentials or activity series to determine whether or not the following reactions will occur spontaneously. If it does, write a balanced equation for the reaction. Write N.R. if a reaction does not occur.			4 marks

	a	Cr3+ + K(s) 

	b.	Mn2+ + Zn(s) 

8.	Refer to a table of standard reduction potentials to complete the following diagram and questions concerning the electrochemical cell created using copper and silver half-cells:	8 marks

	a.	Write the equation for the oxidation half-reaction:	(1 mark)

	b.	Write the equation for the reduction half-reaction:	(1 mark)

	c.	What is the voltage produced by this cell:	(2 marks)

	d.	Label the following items on the diagram:	(4 marks)
•	anode
•	cathode
•	positive post
•	negative post
•	direction of flow of electrons
•	direction of flow of cations and anions from the salt bridge (show on the diagram or describe below)

9.	In the electrolytic cell shown here, an iron nail is being plated with copper.	4 marks

	a.	Which object, the iron nail or the copper bar, should be connected to the negative post of the battery?

	b.	Write the equation for the half-reaction that occurs at the cathode of the electrolytic cell.

	c.	Write the equation for the half-reaction that occurs at the anode of the electrolytic cell.

	d.	Show on the diagram the flow of electrons both entering and leaving the battery.