UNIT 17:REDUCTION AND OXIDATION REACTION
UNIT17: REDUCTION AND OXIDATION REACTION
Key unit competency: Explain the concept of reduction and oxidation and balance
equations for redox reactions
Learning Objectives
By the end of this unit, students should be able to
• Explain the redox reactions in terms of electron transfer and changes in
oxidation state (number).
• Explain the concept of disproportionation
• Differentiate the reducing agent from the oxidizing agent in a redox
reaction.
• Work out the oxidation numbers of elements in the compounds.
• Perform simple displacement reactions to order elements in terms of
oxidizing or reducing ability.
• Apply half-reaction method to balance redox reactions.• Deduce balanced equations for redox reactions from relevant half equations.
17.1. Definition of electrochemistry and its relationship with
redox reactions.
Activity 17.1.
1. Use examples to differentiate redox reactions from other chemical reactions
2. Explain this statement: “Electrochemistry is a chapter of chemistry that studies
the chemical reactions that produce electricity”
Electrochemistry is defined as the study of the interchange of chemical and Electrical
energy. It is primarily concerned with two processes that involve oxidation–reduction
reactions: the generation of an electric current from a spontaneous chemicalreaction
and, the opposite process, the use of a current to produce chemical change.
Electrochemistry is important in other less obvious ways. For example, the corrosion
of iron, which has tremendous economic implications, is an electrochemical process.
In addition, many important industrial materials such as aluminum, chlorine, and
sodium hydroxide are prepared by electrolytic processes.
Hence a redox reaction is a combination of two half-reactions: an oxidation halfreaction and a reduction half-reaction. Nevertheless, one half-reaction cannot exist
without the other, because electrons lost in the oxidation process must be captured
in the reduction process, this explains why we talk of oxidation-reduction or redox
reaction.
The characteristic of a redox reaction is that there is exchange or transfer of electrons
between chemical species participating in the reaction.
We can compare this to the emigration-immigration movement: when a person
leaves a country, emigration for that country, he/she must enter another country,
immigration for that country and this constitutes an emigration-immigration
movement.We notice that any chemical species whose oxidation state increases is oxidized:
1. Aqueous copper (II) ion reacts with aqueous iodide ion to yield solid copper (I)
iodide and aqueous iodine.
a. Write the net ionic equation,
b. Assign oxidation numbers to all species present, andc. Identify the oxidizing and reducing agents.
The oxidation number of an atom is the apparent or real charge that the atom has
when all bonds between atoms of different elements are assumed to be ionic. By
comparing the oxidation number of an element or chemical species before and
after reaction, we can tell whether the atom has gained or lost electrons. Note that
oxidation numbers don’t necessarily imply ionic charges; they are just a convenientdevice to help keep track of electrons during redox reactions.
17.5. Balancing of redox equations
3. To the second portion add a few drops of hydrogen peroxide followed by one
or two drops of dilute surphuric acid and warm gently. Allow the solution to
cool (or cool it under running tap water). To the cold solution add drop wise 2M
NaOH until there is no further change. Record your observations. Add dilute
sulphuric acid to the resultant product and note down your observations. Rinse
the test tube thoroughly
4. To the third portion, add about 1 cm3
of dilute hydrogen peroxide solution followed by one or two drops of dilute sulphuric acid. Warm gently and test the
gas produced with a glowing splint. Allow the solution to cool (or cool it using
running tap water).To the cold solution add ammonia solution drop wise until
no further change. Compare the product formed when ammonia solution tothat obtained when sodium hydroxide was used.
Study Questions
1. Name the products formed when dilute sulphuric acid reacts with iron powder.
Write a balanced formula equation for the reaction
2. When dilute sulphuric acid reacts with iron powder, iron atoms are oxidized and
hydrogen ions are reduced. Write a balanced
a) oxidation half-equation
b) reduction half-equation and
c) overall redox equations for the reaction between iron and sulphuric acid
3. What is the effect of adding a hydrogen peroxide in step 4?
4. What will be the effect of adding concentrated nitric acid to any iron salt?
Explain why concentrated nitric acid does not react with pure iron metal
17.5.1. Rules for balancing redox reactions
The Half-Reaction Method for Balancing Oxidation–Reduction Reactions in
Aqueous Solutions
For oxidation–reduction reactions that occur in aqueous solution, it is useful to
separate the reaction into two half-reactions: one involving oxidation reaction and
the other involving reduction reaction. Then after balancing those half reactions,
find the overall oxidation-reduction (redox) reaction by combining the two halfreactions.
For example, consider the unbalanced equation for the oxidation– reduction
reaction between cerium(IV) ion and tin(II) ion:
2. Place the test tubes in a 400 mL beaker that is about 1/3 full of boiling water.
After
a few minutes, look for evidence of reaction. Note any changes. Did some metals
that didn’t react with cold water, react with hot water?
3. Place a small sample of each metal in test tubes containing 5 mL of 1.0 mol/L
hydrochloric acid, HCl. Watch for evidence of reaction. Note any changes
4. Place a small sample of magnesium ribbon in test tube containing 5 mL of 1M
copper (II) sulphate. Watch for evidence of reaction and note any changes
Study questions
1) Considering sodium, magnesium, zinc, and copper:
Arrange the metals in order of increasing reactivity (from least reactive to most reactive)
2) Which of the four metals are reacting with cold water? For those metals that did
react, write a balanced symbolic equation.
3) Which of the four metals are reacting with hot water? For those metals that did
react, write a balanced symbolic equation.
4) Which of the four metals are reacting with the hydrochloric acid? For those
metals that did react, write a balanced symbolic equation.
5) Which metal did not react with either water or hydrochloric acid?
6) Which of the four metals would be suitable for making saucepans? Explain why
the others are not.
7. Describe what you would see if you dropped a piece of magnesium ribbon into
some copper (II) sulphate solution in a test tube. Write a chemical equation for
the reaction.
The reactivity series is a series of metals, in order of reactivity, as reducing agents,
from highest to lowest reducing agent. It is used to determine the products of single
displacement reactions, whereby metal A will displace another metal B in a solution
if A is higher in the series. Although hydrogen is not a metal, it is included in the
reactivity series for comparison (Table 17.2).
When a metal is placed in a solution of another metal salt, and if the metal is more
active than the metal in the salt, the more active metal displaces the other metal
17.7. End unit assessment
b) Write balanced equations for each reaction that took place.
18. Sulfur dioxide reacts with water to form sulfite ion. Is this a redox reaction?
Justify your answer.
19. In each of the following balanced redox equations, identify:
i) the species oxidized and their new oxidation numbers
(ii) the species reduced and their new oxidation numbers.
(iii) the reducing agent(iv) the oxidizing agent
i) copper + chromium sulfate
ii) magnesium + chromium sulfate
iii) chromium + copper sulfated. Compare the reactivity of chromium with those of iron and zinc