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### Question 1

What is the oxidation state of potassium, K, in compounds?

Periodic Table and oxidation state

### Explanation

1. a number assigned to an element in chemical combination that represents the number of electrons lost (or gained, if the number is negative) by an atom of that element in the compound.

### Question 2

What is the expected oxidation state of nitrogen, N, in compounds based on the periodic table?
(O is entered as -2 not 2-.)

### Question 3

What is the expected oxidation state of Cl in compounds based on the periodic table?

### Question 4

What is the expected oxidation state of S in compounds based on the periodic table?

### Explanation

Rule 1: The oxidation number of an element in its free (uncombined) state is zero — for example, Al(s) or Zn(s). This is also true for elements found in nature as diatomic (two-atom) elements

and for sulfur, found as:

Rule 2: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion, for example:

Rule 3: The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic (many-atom) ion is equal to the charge on the ion. This rule often allows chemists to calculate the oxidation number of an atom that may have multiple oxidation states, if the other atoms in the ion have known oxidation numbers.

Rule 4: The oxidation number of an alkali metal (IA family) in a compound is +1; the oxidation number of an alkaline earth metal (IIA family) in a compound is +2.

Rule 5: The oxidation number of oxygen in a compound is usually –2. If, however, the oxygen is in a class of compounds called peroxides (for example, hydrogen peroxide), then the oxygen has an oxidation number of –1. If the oxygen is bonded to fluorine, the number is +1.

Rule 6: The oxidation state of hydrogen in a compound is usually +1. If the hydrogen is part of a binary metal hydride (compound of hydrogen and some metal), then the oxidation state of hydrogen is –1.

Rule 7: The oxidation number of fluorine is always –1. Chlorine, bromine, and iodine usually have an oxidation number of –1, unless they’re in combination with an oxygen or fluorine.

• Notice that the zinc metal (the reactant) has an oxidation number of zero (rule 1), and the zinc cation (the product) has an oxidation number of +2 (rule 2). In general, you can say that a substance is oxidized when there’s an increase in its oxidation number.

Reduction works the same way. Consider this reaction:

The copper is going from an oxidation number of +2 to zero. A substance is reduced if there’s a decrease in its oxidation number.

### Question 5

What is the expected oxidation state of gallium, Ga, in compounds based on the periodic table?

### Question 6

What is the composition of aluminum nitride?

Composition of Ionic Compounds

Select one:

### Question 7

What is the composition of calcium fluoride?

Select one:

### Question 8

What is the composition of sodium sulfide?

Select one:

### Question 9

What is the composition of aluminum sulfide?

Select one:

### Question 10

What is the oxidation state of P in PF5

### Question 11

What is the oxidation state of Cl in HClO4?

### Question 12

What is the oxidation state of P in PO43-?

### Question 13

What is the oxidation state of H in CaH2?

### Question 14

What is the oxidation state of O in OF2?

### Question 15

What is the oxidation state of I in IO3-?

### Question 16

What is the oxidation state of P in PBr3?

### Question 17

What is the oxidation state of C in CS2?

### Question 18

In the reaction given below, what is oxidized?

2Na(s) + Cl2(g) → 2NaCl (s) (In case image is missing)

Write: Na, Cl2 (for Cl2), or NaCl

Note, only reactants are oxidized and reduced.

### Explanation

Have you noticed that only reactants are oxidized or reduced?

### Question 19

In the above reaction, what is the reducing agent?

Write: Na, Cl2 (for Cl2), or NaCl

### Question 20

In the above reaction, what is reduced?

### Question 21

In the above reaction, what is the oxidizing agent?

### Question 22

In the reaction given below, what element is oxidized?

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) (In case image is missing)

Write: C1 (for the C in CH4), C2 (for the C in CO2), H (for the H in CH4),
O2 (for O2), Ox1 (for the O in CO2), or Ox2 (for the O in H2O)

### Question 23

In the above reaction, what element is the reducing agent?

### Question 24

In the above reaction, what element is reduced?

### Question 25

In the above reaction, what element is the oxidizing agent?

### Question 26

In the reaction given below, what is oxidized?

2K(s) + S(s) → K2S(s)

Write: K, S, or K2S (for K2S)

Remember, The oxidized substance is the same as the reducing agent--electrons are lost.

The reduced substance is the same as the oxidizing agent--electrons are gained.
Only reactants are reduced or oxidized.

### Question 27

In the above reaction, what is the reducing agent?

### Question 28

In the above reaction, what is reduced?

### Question 29

In the above reaction, what is the oxidizing agent?

### Question 30

In the reaction given below, what is oxidized?

8H+(aq) + MnO4-(aq) + 5Fe2+(aq) → Mn2+(aq) + 5Fe3+(aq) 4H2O(l)

Write: H+ (for H+), Mn (for Mn in MnO4)-, O (for O in MnO4)-, Fe2+ (for Fe2+), Mn2+ (for Mn2+), Fe3+ (for Fe3+)

### Question 31

In the above reaction, what is the reducing agent?

### Question 32

In the above reaction, what is reduced?

### Question 33

In the above reaction, what is the oxidizing agent?

### Question 34

Preparation for balancing complicated oxidation-reduction reactions.

Balance the following half-reaction: Fe(s) → Fe3+(aq) +xe- (electrons)

What is x, that is, how many electrons are needed?

Oxidation-reduction reactions are frequently split into a balanced oxidation half-reaction and a balanced reduction half-reaction.

### Explanation

Hint, the charge on the left is 0.  So, what is x such that the charge on the right is 0?

### Question 35

The above half-reaction is

Select one:

### Question 36

The above half-reaction is

Select one:

### Question 37

The above half-reaction is

Select one:

### Question 38

The last step is to combine the 2 half-reactions such that the electrons lost = electrons gains. For the above half-reactions this means

Select one or more:

### Question 39

Consequently, which of the oxidation-reduction reactions is balanced correctly?

In any reaction, including oxidation-reduction, both material and charge must be balanced.

Select one:

### Question 40

Preparation for balancing complicated oxidation-reduction reactions.

If necessary what is available to help balance a reaction in an acidic solution?

Select one or more:

### Question 41

Which of the following equations are correct for material and charge balance?

Select one or more:

### Question 42

The following reaction will be balanced in an acidic solution:

MnO4-(aq) + SO32- (aq) → Mn2+(aq) + SO42-( (aq)

First balance the half reaction with Mn. Follow the steps given in the Half Reaction Example.

Select one or more:

### Question 43

Continue to balance the following reaction in an acidic solution:

MnO4-(aq) + SO32- (aq) → Mn2+(aq) + SO42-( (aq)

This time, balance the half reaction with S. Using smallest integer coefficients, which of the following statements are correct about this balanced half-reaction?

Select one or more:

### Question 44

To balance the electron gain and loss, the Mn half-reaction is multiplied by

### Question 45

To balance the electron gain and loss, the S half-reaction is multiplied by

### Question 46

Using smallest integer coefficients, which of the following statements are correct about the balanced reaction?

Select one or more:

### Question 47

Convert the following balanced half-reaction in acid to a balanced half-reaction in base:

Cr2O72-(aq) + 14H+ + 6e- (aq) → 2Cr3+(aq) + 7H2O(l)

How many OH- are added to each side?

### Question 48

After adding the OH- and simplifying, how many water molecules are on the left?

### Question 49

Using smallest integer coefficients balance the half-reaction given below in acid. How many H+ are on the right?

As2S3(s) → AsO43-(aq) + SO42-(aq)

Hint given in general feedback.

### Explanation

Hint: (1) Balance As and S, (2) balance O with water, (3) balance H, (4) balance charge.

### Question 50

How many electrons are added to the right?

### Question 51

Convert the your balanced half-reaction to one balance in base. How many OH- are added to each side?

### Question 52

After simplifying, how many water molecules remain on the right side of the equation?