CHEM 120
Quiz 5 Study Guide
1. We have the following aqueous reaction: Ba(NO3)2 + Na2SO4 ==> BaSO4(s) + 2NaNO3. Which statement
below is true?
a) Barium is oxidized. b) Sodium is reduced. C) Nitrate ion is the reducing agent.
d) Sulfate ion is the oxidizing agent.E) This reaction is not an oxidation-reduction reaction.
Let’s look at the oxidation states for each species.
2+ 1-
In the compound Ba(NO3)2, we have a Ba paired with two (NO3) . On the product side, Ba remains in a 2+
2-
oxidation state when it pairs with one (SO4 ) anion. So, we can say that barium has neither been oxidized
(moving to a higher oxidation number) nor reduced (gaining electrons, giving lower oxidation numbers).
4 + 2-
For the compound Na2SO , we have two Na combining with one (SO4) . On the product side, the sulfate
1-
ion remains the same, as does the Na+ when it combines with a (NO 3) . Therefore, no oxidation nor
reduction occur here. It is just as important to recognize when we have oxid-reduc and when we don’t.
2. We have a reaction N2 + 3H2 ==> 2NH3. Which statement is true?
a) Nitrogen is oxidized. b) Hydrogen is reduced. C) Nitrogen is the reducing agent.
d) The reaction is not an oxidation-reduction reaction. e) Hydrogen is the reducing agent.
Let’s look at each species…Both hydrogen and nitrogen start out in their native states. Any element in its
native form (e.g., O2, Na(s), Br2, etc.), the oxidation number is zero. So both H and N have an oxidation number
of zero.
+ 3-
What about NH3? We have (3) H to balance out the single N .
+
H goes from an oxidation state of 0 ==> 1 . An increase in oxidation number is OXIDATION.
3-
N goes from an oxidation state of 0 ==> . A decrease in oxidation number is REDUCTION.
The other thing to know is that the species that is reduced is the oxidating agent – causes the other species to
give up electrons, as it were. Similarly, the species that is oxidized is the reducing agent – the one transferring
electrons to the other species. Here, since hydrogen is oxidized by nitrogen, hydrogen reduces nitrogen by
transferring (3) electrons. So we could write two half reactions:
+ - - 3-
3H2 ==> 6H + 6 e and N2 + 6 e 2 N
3. Which of the following statements are true (check all that apply)
a) In an oxidation-reduction (redox) reaction, one species transfers electrons to another.
b) The species receiving the electrons is oxidized.
c) The species receiving the electrons is reduced.
d) The species losing electrons is being oxidized.
e) During oxidation, the oxidation number decrease (becomes less positive).
f) During reduction, the oxidation number decreases (becomes less positive)
Quiz 5 Study Guide
1. We have the following aqueous reaction: Ba(NO3)2 + Na2SO4 ==> BaSO4(s) + 2NaNO3. Which statement
below is true?
a) Barium is oxidized. b) Sodium is reduced. C) Nitrate ion is the reducing agent.
d) Sulfate ion is the oxidizing agent.E) This reaction is not an oxidation-reduction reaction.
Let’s look at the oxidation states for each species.
2+ 1-
In the compound Ba(NO3)2, we have a Ba paired with two (NO3) . On the product side, Ba remains in a 2+
2-
oxidation state when it pairs with one (SO4 ) anion. So, we can say that barium has neither been oxidized
(moving to a higher oxidation number) nor reduced (gaining electrons, giving lower oxidation numbers).
4 + 2-
For the compound Na2SO , we have two Na combining with one (SO4) . On the product side, the sulfate
1-
ion remains the same, as does the Na+ when it combines with a (NO 3) . Therefore, no oxidation nor
reduction occur here. It is just as important to recognize when we have oxid-reduc and when we don’t.
2. We have a reaction N2 + 3H2 ==> 2NH3. Which statement is true?
a) Nitrogen is oxidized. b) Hydrogen is reduced. C) Nitrogen is the reducing agent.
d) The reaction is not an oxidation-reduction reaction. e) Hydrogen is the reducing agent.
Let’s look at each species…Both hydrogen and nitrogen start out in their native states. Any element in its
native form (e.g., O2, Na(s), Br2, etc.), the oxidation number is zero. So both H and N have an oxidation number
of zero.
+ 3-
What about NH3? We have (3) H to balance out the single N .
+
H goes from an oxidation state of 0 ==> 1 . An increase in oxidation number is OXIDATION.
3-
N goes from an oxidation state of 0 ==> . A decrease in oxidation number is REDUCTION.
The other thing to know is that the species that is reduced is the oxidating agent – causes the other species to
give up electrons, as it were. Similarly, the species that is oxidized is the reducing agent – the one transferring
electrons to the other species. Here, since hydrogen is oxidized by nitrogen, hydrogen reduces nitrogen by
transferring (3) electrons. So we could write two half reactions:
+ - - 3-
3H2 ==> 6H + 6 e and N2 + 6 e 2 N
3. Which of the following statements are true (check all that apply)
a) In an oxidation-reduction (redox) reaction, one species transfers electrons to another.
b) The species receiving the electrons is oxidized.
c) The species receiving the electrons is reduced.
d) The species losing electrons is being oxidized.
e) During oxidation, the oxidation number decrease (becomes less positive).
f) During reduction, the oxidation number decreases (becomes less positive)
