Oxidation and Reduction Reactions

Oxidation and Reduction Reactions

Introduction
When some metallic objects are left outside in open moist air, they tend to lose their original colour to reddish brown. What do you think brings about such changes? Do you realise that there is an active part of air that interacts with particles in metallic objects to bring about such changes? The reactions responsible for such changes are known as oxidation and reduction. These reactions occur not only in metals, but also in other processes, like respiration and photosynthesis. In all these such processes, there is transfer of oxygen, hydrogen, or electrons from one substance to another. When oxidation and reduction reactions occur simultaneously, the entire process is termed as a redox reaction. “Red” is derived from reduction whereas “ox” is derived from oxidation, and the two make “redox”. In this chapter, you will learn more about oxidation, reduction and redox reactions in terms of gain or loss of oxygen and in terms of electron transfer. You will also appreciate that the two processes always occur at the time

1.1 Oxidation and Reduction Reactions
Oxidation Reactions
We say that a substance is oxidized if it gains oxygen, or loses hydrogen, or loses electrons. What happens to raw apple slices when they are left in an open place for some time?
Oxidation as gain of oxygen
In Activity 7.7(a), you will burn a piece of magnesium ribbon in oxygen, and then define the term oxidation” after making your observations.

Figure 1.1: A brown tint developing on apple slices

Activity 1.1 (a): Investigating the process of burning magnesium metal in oxygen
Work in groups.
What you need:

• magnesium ribbon
  . sandpaper
  . watch glass
  What to do
  pair of tongs
• spirit lamp or Bunsen burner
• safety goggles
  1.Obtain a 3 cm-long piece of magnesium ribbon.
  2.Using the sandpaper, clean the magnesium ribbon by scrubbing it.
  3.Hold the piece of the clean magnesium ribbon with a pair of tongs.
  4.Burn the magnesium ribbon in the flame, as shown in Figure 1.2. and observe
  what happens.
  5. Collect the product formed on the watch glass.

Observation and Analysis

1. How did the magnesium ribbon burn in air?
2. What was the color of the product formed when magnesium get burnt in air?
3. Why was it necessary to clean the magnesium ribbon with sandpaper?
4. Write an equation for the reaction that took place between maqnesium and oxyqen. Bunsen
   burner Figure

Conclusion

1. What name is given to the process by which magnesium ribbon is changed to the product you obtained?
2. How can you then explain oxidation in terms of oxygen?

Discussion question
Identify the substances which are oxidized in the reactions below.
a) copper + oxygen copper(ll) oxide
b) hydrogen + oxygen water
Oxidation as loss of hydrogen
Oxidation also occurs when a substance loses hydrogen.
Activity 1.1 (b): Investigating the process of passing dry ammonia gas over heated copper(ll) oxide
Work in groups.
What you need:

• rubber stoppers
• a source of dry ammonia gas
• combustion tube
• copper(ll) oxide
  Safety Precaution
  delivery tubes
  stands
  source of heat
• Smelling ammonia gas can cause irritation of the nose, throat, eyes and respiratory tract.
  Wear a gas mask and safety goggles when handling the gas.
  Put on gloves and use a spatula when handling copper(ll) oxide.

What to do
I. Set up the apparatus, as shown in Figure 7.3. Connect it to a source of dry ammonia gas.

2. Heat copper(ll) oxide for several minutes and pass dry ammonia gas over it.
3. Note the observations made as copper(ll) oxide reacts with ammonia.
4. Present your findings to the rest of the class.

Observation and Analysis

1. What did you observe in the combustion tube?
2. Write an equation for the reaction that took place.
   Conclusion
3. Identify the reactant substance that was:
   a) oxidised
   b) reduced.
4. Explain your answer in (7) above.

Reduction Reactions
What is reduction reaction?
A reduction reaction is a reverse process of an oxidation reaction. Reduction takes place when a substance loses oxygen, gains hydrogen, or gains electrons.
Reduction as loss of oxygen
When a mixture of zinc powder and copper(ll) oxide is heated, the following reaction occurs:

Reduction as gain of hydrogen

Importance of oxidation and reduction in everyday life

Many processes involve chemical reactions which are either oxidation or reduction reaction. Extraction of
some metals, like iron and zinc, involves reduction of their ores. Breakdown of food by cells to produce
energy is an oxidation reaction. Most of the electrochemical cells, such as car batteries and dry cells, use redox reactions to produce electrical energy. You will find out more about the importance
of oxidation and reduction reactions in chemical

industry in Activity 1. I(c).
Redox reactions are also used in electroplating, which involves coating a metal with a thin layer of
another metal. Electroplating is used to make gold-based jewellery, as shown in Figure 7.5.

For your Understanding
Electroplating is the application of electrolytic cells in which a thin layer of metal is deposited onto an electrically conductive surface. An electrolytic cell normally has an inert anode, but when used in electroplating, it is modified so that the anode is made of the metal whose ions are present
in the electrolyte and the cathode is the metal to be electroplated.
Assignment 1.1:

1. Research about how electroplating of iron with copper can be carried out.
2. Compile a reportofyour research findings and present it for appraisal.

Activity 1.1 (c): Exploring the importance of oxidation and reduction processes in chemical industry
Work in groups.
What you need:
Internet

• relevant Chemistry textbooks
  What to do

1. Collect and interpret data on the existence of various ores in Uganda and produce a chart. Copy and Complete Table 7.7.

2. Discuss the contribution of metal extraction to the economy of Uganda.
3. Explain the relevance of reduction / oxidation to metal extraction.
4. Discuss the importances of oxidation and reduction in chemical industry.
5. Share your findings with the rest of the class.

1.2 Redox Reactions in Terms of Electron Transfer
In Senior Three, you learnt about loss and gain of electrons by atoms of elements. In
this section, you may look at electrons as “burdens” and oxidation and reduction as
“relief” and “stress”, respectively. Metals lose electrons (burdens) and, in the process,
they are oxidized, while non-metals gain electrons (burdens) and get reduced.
Oxidation as loss of electrons
Oxidation reactions can take place even if there is no oxygen or hydrogen present.
When a substance loses electrons during a reaction, it is oxidized. Therefore, in terms of electron transfer, oxidation is defined as the loss of electrons from a substance.

Consider the reaction between magnesium and chlorine to form magnesium chloride

Discussion questions

1. What has been oxidised in the reaction between magnesium and chlorine?
2. What happens when copper is placed into silver nitrate solution?
   Activity 1.2(a): Investigating what happens when copper metal is placed in a solution of silver nitrate

Work in groups.
What you need:
silver nitrate solution

• strip of copper metal
  What to do
  1.mCoil the strip of copper metal.
  2.Place the coil of copper into a test tube containing silver nitrate solution
  as shown in Figure. 7.6
  3.Let the set-up stand for 3-4 minutes, observe and record what takes place.

Observation and Analysis

1. What happened to the aqueous solution of silver nitrate?
2. What happened to the copper wire?
3. Write an equation for the reaction that took place
   Conclusion
4. Which reactant loses electrons and which one gains electrons? Write the overall ionic equation for the reaction.
5. Give a reason why the reaction is redox.
   Reduction as gain of electrons
   In terms of electrons, reduction is gain of electrons by a substance. When hydrogen sulphide gas is passed through iron(lll) chloride solution, a green solution of iron(ll) chloride and a light yellow precipitate of sulphur are produced.

Redox Reactions
Oxidation and reduction always take place simultaneously. In other words, there can
be no oxidation without reduction, and vice versa. We call the combined process a redox reaction.
When steam is passed over heated magnesium, magnesium oxide and hydrogen gas are produced.

Has a redox reaction taken place?
Yes, a redox reaction has taken place. This is because in the reaction, magnesium has gained oxygen, and so has been oxidized to magnesium oxide, while water has lost oxygen and, thus, has been reduced to hydrogen.

What happens when a black and white film is exposed to sunlight? A black and white photographic film
contains tiny crystals of silver bromide suspended in gelatine. When such a photographic film is exposed
to sunlight, the following redox reaction

takes I

Activity 1.2(b): Exploring oxidation and reduction reactions
Work in groups.
What you need:
. Internet
What to do
in terms of electron transfer

• relevant Chemistry textbooks

1. Research about examples of redox reactions and identify the oxidized or reduced species in three (3) different reactions.
2. Record your findings in Table 7.2.
   Table 1.2: Oxidised and reduced species in redox reactions

Activity 1.2(c): Calculating oxidation numbers of atoms
Do this activity individually.
What you need:
. notebook
pen
What to do
I. Calculate the oxidation numbers of the following named atoms in the given
compounds:
a) sulphur in S03
b) carbon in C032-
c) lead in Pb02
d) magnesium in MgH2
e) nitrogen in NC13
f) sodium in Na202

2. Present your findings to the rest of the class.
   By finding the oxidation numbers of species in a reaction, you can tell what is being oxidized or reduced. The oxidation number describes the degree of oxidation of an atom in a chemical compound. Therefore, on the basis of oxidation number, oxidation may be defined as an increase in
   the oxidation number of a species in a reaction.

Oxidising and Reducing Agents You have learnt that oxidation and reduction reactions take place
simultaneously. In such reactions, a substance which oxidises another substance is called an oxidising agent. Therefore, an oxidising agent is a substance in a redox reaction that gains electrones, and so, gets reduced in the reaction. A reducing agent is defined similarly.

The two terms are illustrated by substances M and Y below.

Discussion question
1.Identify the oxidising and reducing agents in the above reaction.
2.Explain the reasons for your answers in (1).
3.What has happened to the oxidation numbers of zinc and copper in the reaction?
4.Write half ionic equations to illustrate your answers.

Activity 1.2(d): Investigating the effect of sulphur dioxide gas on acidified potassium dichromate solution and potassium permanganate solution
Work in groups.
What you need:

• iron(ll) sulphate
  boiling tubes
  rubber stopper
  5 cm3 of potassium dichromate solution
  5cm3 of potassium permanganate solution
  retort stand
  heat source
  3 cm3 of 2 M dilute sulphuric acid.

What to do

1. Transfer 5 cm3 of potassium dichromate solution into a boiling tube.
2. Add 3 cm3 of dilute sulphuric acid to potassium dichromate solution in a boiling tube.
3. Transfer 6 g of iron(ll) sulphate crystals into another boiling tube and set up the apparatus, as shown in Fiqure 7.8.

4.Hold the boiling tube containing acidified potassium dichromate solution in such a way that the delivery tube is dipped into the solution.
5.Heat the Iron(ll) sulphate crystals until there is no further change.
6.Repeat the experiment using potassium permanganate solution.
Observation
State the observations made using:
(i) acidified potassium dichromate solution.
(ii) acidified potassium permanganate solution.
Conclusion

1. How was sulphur dioxide obtained during the experiment?
2. What was the role of sulphur dioxide in the experiment?
   Activity 1.2(e): Exploring the oxidation of iodide ions to iodine
   Work in groups.
   What you need:
   10 cm3 of potassium iodide solution
   10 cm3 of 2 M dilute suphuric acid
   solution
   What to do
   heat source . test tube starch solution
3. Pour 4 cm3 of potassium iodide solution into a test tube.
   2. Add an equal volume of dilute suphuric acid.

3. Warm gently
4. Observe any colour changes.
5. Add a little starch solution.
   Observation
   L)
   What did you observe before and after adding starch solution?
   Conclusion
6. Identify the oxidising and reducing agents in the above reactions.
7. What was the purpose of adding starch solution?
8. Write an ionic half equation for the oxidation of iodide ions to iodine.
   Distinguishing between redox and non-redox reactions
   A reaction in which neither oxidation nor reduction takes place is called a non-redox reaction. Examples of such reactions include:

1.3 Electrolysis
In your communities, batteries are put to various uses. Disposable batteries power devices, like torches and radios, while rechargeable batteries, such as lithium-ion batteries, are used in digital cameras, mobile phones and many other devices. These and other batteries have two main components, that is, the electrolyte and The electrolytes, in form of aqueous salts, alkaline / acidic solutions or when in molten
state, conduct electricity, while electrodes are terminals where electric currents enter
(anode) and leave (cathode) the electrolyte.
The decomposition of an electrolyte by passage of an electric current is what is called electrolysis. Electrolysis takes place in an electrolytic cell. A liquid only shows conductivity of electricity with the presence of free moving charged particles called ions. What happens when electricity is passed through a
substance in molten or aqueous state?
In Activity 7.3(a), you will investigate what happens when electricity is passed through

Activity 1.3(a): Investigating the electrical conductivity of solid and molten substances

What to do
1.Fill two-thirds of the crucible with solid lead(ll) bromide.
2.Place the crucible and its contents on a pipe clay triangle on a tripod stand.
3.Dip the two carbon electrodes into the solid lead(ll) bromide and connect the batteries with the connecting wires, as shown in Figure 7.9.
4.Turn on the switch and check whether the bulb lights up. Record your observations in Table 7.5.
6.Now, heat the lead(ll) bromide in the crucible until it melts.
.Turn on the switch. Observe the changes that take place to the electrodes and bulb. Allow the current to pass through the melt for 20-30 minutes.
7.Repeat steps (7) to (6) by replacing solid lead(ll) bromide with naphthalene.

Analysis

1. How did you know whether a substance conducted electricity or not?
2. Explain the observed electrical conductivity in each of the following substances:
   a) solid lead(ll) bromide
   b) molten lead(ll) bromide
   c) solid sodium chloride
   d) Aqueous sodium chloride
   e) glucose solution

Meaning of electrolysis
Electrolysis refers to the chemical change produced by passing an electric current through a conducting solution or a molten salt. It involves passing a direct electric current through an electrolyte, producing chemical reactions at the electrodes and decomposition of the materials. During the process, electrical energy from the direct current is converted to chemical energy in an electrolysis cell.

Activity 1.3 (c): Analysing an electrolytic cell
What to do
Study Figure 1.17, showing the main components of electrolysis, and answer the
following questions:

1. Name the main components of an electrolytic cell.
2. Describe how the battery is connected in an electrolytic cell.
3. What kind of compounds conduct electricity in an electrolytic cell?
4. What happens to the compounds when they conduct electricity?

5. Describe the part of an electrolytic cell through which electrons enter leave the electrolyte.
6. Describe what happens during electrolysis.

Electrolysis of aqueous solutions
An aqueous solution of a salt is produced when the salt is dissolved in water. It will contain anions and cations of the salt, and hydrogen and hydroxide ions from the partial dissociation of water molecules:

For example, an aqueous solution of sodium chloride, NaCl, contains:
a) cations, which are sodium ions (Na+) and hydrogen ions (H+)
b) anions, which are chloride ions (CI-) and hydroxide ions (OH-)
During electrolysis of an aqueous solution, the cations move towards the cathode and the anions move towards the anode, as shown in Figure 7.77. More than one type of ions is attracted towards the cathode and anode. Thus, selective discharge takes place, because only one type of ion will be discharged.

For your Understanding
Ions which are high in the electrochemical series are not easily discharged because more energy is required to discharge them. Ions which are lower in the series are discharged with much ease

In Activities 7.3(d) and 7.3(e), you will demonstrate the electrolysis of some aqueous solutions.

Conclusion and Analysis
Explain the observed color change, if any.
1. State the electrode where:
a) oxidation occurred
b) reduction occurred
2.Write the overall equation for the reaction.
3.Why are carbon and platinum electrodes usually preferred during electrolysis?

Activity 1.3(e): Electrolysis of dilute sulphuric acid
Work in groups.
What you need:
dry cells
switch

• carbon electrodes with holders
• connecting wires
  trough
  What to do
  burning splint
  2 test tubes
  glowing splint
  0.5 M dilute sulphuric acid
  I. Fill the trough of the electrolytic cell, as set up in Figure 1.13, with 0.5 M dilute
  sulphuric acid until it is half-full.

Observation

1. What are the ions present in dilute sulphuric acid?
2. a) What did you observe at each of the electrodes?
   b) Write ionic equations for the reactions that occurred at each of the electrodes.
3. What happened to the colour of the solution at the end of the experiment?
   Conclusion and Application
4. What can you conclude about the electrolysis of dilute sulphuric acid using inert carbon electrodes?
5. State the application of this experiment.

Chapter Summary
In this chapter, you have learnt that:
a redox reaction is a reaction in which oxidation and reduction occur simultaneously.
a substance is oxidised if it gains oxygen, or loses hydrogen, loses electrons or if its oxidation number increases.
oxidation number is the total number of electrons that a species (atoms or ions) either gains or loses in order to form a chemical bond with another
species.
an oxidising agent is a substance in a redox reaction that gets reduced.
a reducing agent is a substance that gets oxidised in a redox reaction.
reduction is gaining of electrons, or loss of oxygen, or gaining of hydrogen.
electrolysis is a process in which a chemical change, especially decomposition,
is brought about by passing an electric current through an electrolytes, and
the electrolyte’s ions move towards the negative and positive electrodes, where they are selectively discharged.
a cathode is a negatively charged electrode by which electrons enter an electrical device.
an anode is a positively charged electrode by which the electrons leave an electrical device.
electroplating is the process of coating a layer of a metal onto another
metal by electrochemical means. It is most commonly used for decorative
purposes or to prevent corrosion of a metal.

a) Give the names of the electrodes A and B.
b) Which electrode is the oxidising electrode?

5. a) What is an electrolyte?
   b) Classify the following substances as strong electrolytes, or weak electrolytes, or non-electrolytes: acetic acid, ammonium hydroxide, ammonium chloride, carbon tetrachloride, dilute hydrochloric acid, sodium acetate, dilute sulphuric acid

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