Redox Processes. Eh-pH Relationships - Mozakkir Azad

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Jatiya Kabi Kazi Nazrul Islam University
Trishal, Mymensingh
Assignment on: “Redox Processes. Eh-pH Relationships”
Course Name: Environmental Geochemistry
Course Code: ESE 2203
DATE OF SUBMISSION: 27 March 2023
SUBMITTED BY:
Mozakkir Azad
Roll no: 20103429
Session: 2019-2020
2nd
Year 2nd
Semester
Department of Environmental Science and Engineering
SUBMITTED TO:
DR. ASHRAF ALI SEDDIQUE
Professor
Department of Environmental Science and Engineering.
Jatiya Kabi Kazi Nazrul Islam University
Trishal, Mymensingh

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Table of Contents
Introduction…………………………………………………………………………......03
Redox Reaction………………………………………………………………………….03
Types of Redox Reactions……………………………………………………………....03
Oxidation and Reduction Reaction……………………………………………………...04
● Reduction
● Oxidation
Oxidizing and Reducing Agents………………………………………………………..05
● Important Oxidizing Agents
● Important Reducing Agents
Applications Of Redox Reaction………………………………………………….....06
Eh-pH…………………………………………………………………….. 07
● Eh
● pH
Eh-pH Diagram………………………………………………………....….09
Eh-pH Relationships……………………………………………………….10
● Equilibrium Equations for Eh-pH
Conclusion………………………………………………………………....12
References………………………………………………………………….13

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Introduction:
Redox reactions are oxidation-reduction chemical reactions in which the reactants undergo a
change in their oxidation states. The term ‘redox’ is a short form of reduction-oxidation. All the
redox reactions can be broken down into two different processes – a reduction process and an
oxidation process.The oxidation and reduction reactions always occur simultaneously, such class
of chemical reactions was named as the redox reaction or Oxidation-Reduction Reaction. The
substance getting reduced in a chemical reaction is known as the oxidizing agent, while a substance
that is getting oxidized is known as the reducing agent.Eh - pH are also known as Pourbaix
Diagrams, after the author of the famous Pourbaix diagram handbook . The simplest type of these
diagrams is based on a chemical system consisting of one element and a water solution. The system
can contain several types of species, such as dissolved ions, condensed oxides, hydroxides, oxides,
etc. The Eh - pH diagram shows the stability areas of these species in the redox potential-pH
coordinates.
Redox Reaction:
A redox reaction can be defined as a chemical reaction in which electrons are transferred between
two reactants participating in it. This transfer of electrons can be identified by observing the
changes in the oxidation states of the reacting species.The loss of electrons and the corresponding
increase in the oxidation state of a given reactant is called oxidation. The gain of electrons and the
corresponding decrease in the oxidation state of a reactant is called reduction.
Electron-accepting species which tend to undergo a reduction in redox reactions are called
oxidizing agents. An electron-donating species which tends to hand over electrons can be referred
to as a reducing agent. These species tend to undergo oxidation. It can be noted that any redox
reaction can be broken down into two half-reactions, namely the oxidation half-reaction and the

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reduction half-reaction.
Types of Redox Reactions
The different types of redox reactions are:
● Decomposition Reaction
● Combination Reaction
● Displacement Reaction
● Disproportionation Reactions
Decomposition Reaction
This kind of reaction involves the breakdown of a compound into different compounds. Examples
of these types of reactions are:
2NaH → 2Na + H2
2H2O → 2H2 + O2
Na2CO3 → Na2O + CO2
All the above reactions result in the breakdown of smaller chemical compounds in the form of AB
→ A + B
But, there is a special case that confirms that all the decomposition reactions are not redox
reactions. For example CaCO3 → CaO + CO2
Combination Reaction
These reactions are the opposite of decomposition reactions and hence involve the combination of
two compounds to form a single compound in the form of A + B → AB. For example: H2 +
Cl2 → 2HClC+O2→CO2
4Fe+ 3O2→2Fe2O3
Displacement Reaction
In this kind of reaction, an atom or an ion in a compound is replaced by an atom or an ion of
another element. It can be represented in the form of X + YZ → XZ + Y. Further displacement
reaction can be categorized into
1. Metal displacement Reaction
2. Non-metal displacement Reaction

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3. Metal Displacement
In this type of reaction, a metal present in the compound is displaced by another metal. These types
of reactions find their application in metallurgical processes where pure metals are obtained from
their ores.
For example:CuSO4+Zn→Cu+ZnSO4
Non-Metal Displacement
In this type of reaction, we can find a hydrogen displacement and sometimes rarely occurring
reactions involving oxygen displacement.
Disproportionation Reactions
The reactions in which a single reactant is oxidized and reduced is known as Disproportionation
reactions.
For example: P4 + 3NaOH + 3H2O → 3NaH2PO2 + PH3
Oxidation and Reduction Reaction:
Oxidation Reaction
Oxidation may be defined as loss of electrons from a substance, the other definition of oxidation
reactions states that addition of oxygen or the more electronegative element or removal of
hydrogen or the more electropositive element from a substance is called an oxidation reaction.
examples:
2S(s) + O2 (g) → SO2 (g) CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
Reduction Reaction
Like oxidation reactions, reduction reactions are defined as the gain of electrons. Any substance
that gains electrons during a chemical reaction gets reduced.In other forms, the reduction reaction
is stated as the addition of hydrogen or more electropositive element or removal of a more
electronegative element or oxygen from a substance.
examples:
2CH2CH2 (g) + H2 (g) → CH3CH3 (g)
2FeCl3 (aq) + H2 (g) → 2FeCl2 (aq) + 2HCl (aq)
Now if we closely examine the above reaction we would find that all the reactions above have
both reduction and oxidation reactions.

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Oxidizing and Reducing Agents
The substance (atom, ion, and molecule) that gains electrons and is thereby reduced to a low
valency state is called Oxidising agent.The substance that loses electrons and is thereby oxidised
to a higher valency state is called a reducing agent.
Important Oxidizing Agents
Molecules made up of electronegative elements. Eg: O2, O3, and X2 (halogens) Compounds
containing an element which is in the higher oxidized state. Eg: KMnO4, K2Cv2O7, HNO3,
KClo3
Oxides of metals and non-metals. Eg: MgO, CuO, CrO3, P4O10
Fluorine is the strongest oxidizing agent.
Important Reducing Agents
All metals, Eg: Na, Zn, Fe, Al
A few non-metals Eg: C, Hydrogen, S, P
Hydracids, Eg: HCl, HBr, HI, H2S
Few compounds containing an element in the lower oxidation state, Eg: FeCl2, FeSo4, SnCl2,
Hg2Cl2
Metallic hydrides, Eg: NaH, LiH, CaH2, etc..
Organic compounds like HCOOH,
Lithium is the strongest reducing agent in the solution and Cesium is the strongest reducing agent
in the absence of water. The substances which act as oxidizing as well as reducing agents are H2O2,
SO2, H2SO3, HNO2, NaNO2
Applications Of Redox Reaction
Oxidation-reduction reactions have many diverse applications ranging from industries to our daily
lives. Some of the important applications of Redox Reaction are as follows: Redox Reaction in

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Electrochemistry. The batteries which are used for generating DC current use redox reaction to
produce electrical energy.
Batteries also called as electrochemical cells used in our day-to-day life are also based on redox
reactions. For example, storage cells which are used in vehicles to supply all the electrical needs of
the vehicles.
Redox Reaction in Combustion
Combustion involves oxidation-reduction reaction and hence it is a redox reaction. An explosion
is a swift form of combustion and hence explosion can be treated as a redox reaction. Even the
combustion in a space shuttle works on redox reactions. The combination of powdered aluminium
and ammonium perchlorate inside the rocket boosters gives rise to oxidation-reduction reaction.
Redox reaction in Photosynthesis
Water and carbon dioxide are converted by plants into carbohydrates and this process is defined as
photosynthesis. The reaction is given below:
From the reaction above, we can see that the carbon dioxide is reduced to carbohydrates while the
water gets oxidized to oxygen and hence it is a redox reaction. Sunlight provides energy for this

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reaction. This reaction is a source of food for animals and plants.
Extraction of metals
Metal oxides can be reduced to metals by using a suitable reducing agent. For example, ferrous
oxide is reduced to iron in the blast furnace using coke as the reducing agent. Production of
chemicals
Many chemicals which we use in our daily lives such as caustic soda, chlorine, fluorine, etc., are
produced by electrolysis which is based on redox reactions.
Quantitative analysis
Redox reactions are very useful in quantitative analysis by redox titrations. These titrations involve
the reactions between oxidising and reducing agents and help in estimating the amount of unknown
substances in solutions. These reactions are useful in pharmaceutical industries. Others Uses
application of Redox Reaction
● Electrolysis is used in the production of some important chemicals which in turn is
based on redox reactions. Many chemicals like caustic soda, chlorine, etc. are
produced using redox reactions.
● Oxidation-Reduction reactions also find their application in sanitizing water and
bleaching materials.
● The surfaces of many metals can be protected from corrosion by connecting them to
sacrificial anodes which undergo corrosion instead. A common example of this
technique is the galvanization of steel.
● Oxidation process is used in the industrial production of cleaning products. ● Nitric
acid which is a component in many fertilizers is produced from the oxidation reaction
of ammonia.
● Redox reactions are also used in the process of electroplating by applying a thin
coating of a material on an object. It is used in the production of gold-plated
jewellery.
● It is used for separating metals from their ores. One such example is the smelting of
metal sulphides in the presence of reducing agents.
Oxygen Is the main source of oxidation and therefore redox reaction or oxidation-reduction
reactions are responsible for food spoilage.
Eh-pH :

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pH:
pH is a measure of the acidity or alkalinity ty of a solution, where pH = -log a H+. pH is thus less
in more acidic (less basic) solutions and greater in less acidic (more basic) solutions. A change of
1 (i.e., from pH=3 t o pH=4) represents at en-fold change in the activity of H+. A 1N solution of
highly dissociated acid (e.g., 1M HCl) has a pH of 0 (because a H+ = 1 = 100). pH is commonly
said t o have a possible range from 1 t o 1 4, but highly acidic solutions can have pH = -1 (10N)
and even -2. At 2 5°C, pH + pOH =1 4.
Eh:
Eh is a measure of the redox (oxidation-reduction) state of a solution or, more exactly, its solutions.
Eh is a measurement of electrical potential and thus commonly expressed in volts. Values of Eh in
nature range from -0.6 t o +0.9V, with 0.0 characterizing a solution with no drive t o either oxidize
or reduce. Accurate measurement of Eh is much more difficult than that of pH, because
measurement of Eh requires equilibration with many solutes and even solids.
Unlike pH, Eh is not a measure of consent ratio of something, and use of the related parameter pE
= (-log ae-) requires recognition that pE is a completely theoretical expression, ignoring the reality
that the concentration of electrons in a solution is always zero. pE = Eh ÷ 0.05 916.
Eh-pH Diagram
The diagram at above shows data from a range of environments. The fields for rainwater, waters
of rivers and lakes, shallow groundwater, and deeper groundwater are highlighted and/or shaded
to draw attention to the evolution of water through the water cycle as it becomes less oxic and more
buffered with respect to pH.

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Eh-pH Relationship:
Equilibrium Equations for Eh-pH
The chemical equation between Species A and B in the water system, with or without electron
involvement, can be expressed as:
aA + cC ↔ bB + dD + hH+ + wH2O (+ne−) (1)
Species C and D are ligand and complexes produced with ligand. The stoichiometric coefficient
of a species is taken as positive if it is on the right-hand side of the equation, and vice versa. Species
H+, H2O and e- may not always be on the right-hand side of the equation. Because so many
equations and species are involved while performing equilibrium calculations for an Eh-pH
diagram, it is easier to use the free energy of formation of each involved species, ΔGi0, then to
calculate the free energy of reaction as,
(2)
where υi represents the stoichiometric coefficient of species i.
Depending on whether or not the reaction involves an electron and/or hydrogen ion, the equations
are:
The Nernst equation for oxidation-reduction reaction with or without acid-base:
where R is the universal gas constant, 8.314472(15) J/(K·mol); T is in
kelvins; F is the Faraday constant 96,485.3399(24) J/(V·equivalent); and {A} and the other species
are defined as the activities of Species A. The activities of solid and liquid are normally assumed
to be one; gas is taken as the atmosphere (atm). The activity of an aqueous solution is the
multiplication of the concentration in mol/L, symbolized as [A], with its activity coefficient. The
coefficient can be computed from one of the appropriate models. Without having the acid-base, the
“hpH” term in the equation will be dropped out.
The equilibrium equation for acid-base reaction without redox reaction:

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The equation for reaction involves neither an electron nor a hydrogen ion:
Species A will be favored if logQ − logK is positive, and vice versa.
As mentioned earlier, two different approaches may be used to construct an Eh-pH diagram. One
is to calculate equilibrium equations between pairs of species and to construct the diagram by
plotting the resulting equilibrium lines. The other is to perform equilibrium calculations from all
involved species at each point in a grid, then selecting the predominant species at each point.
Regardless of which method is used, these equilibrium equations have to be satisfied.

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Conclusion
Oxidation and reduction reactions always occur together, because the electrons that are donated
from one compound, must be received by another compound. This is why redox reactions are said
to be the product of two half reactions, the oxidation half reaction and the reduction half reaction.
Each half reaction has a measurable reduction potential E0, which is a measure in volts of how
easily the compound is reduced (how easily it gains electrons). Remember, the reduction potential
is how much a species "wants" to get reduced, and the higher the number, the greater the
potential.Redox reactions play an important part in our lives. Combustion reactions that generate
heat and electricity, such as the burning of natural gas, oil, gasoline or wood, are redox reactions,
and in our bodies, redox reactions are needed to generate ATP to power our metabolism and our
muscles.Eh- pH diagrams show the thermodynamic stability areas of different species in an
aqueous solution. Stability areas are presented as a function of pH and electrochemical potential
scales. Usually the upper and lower stability limits of water are also shown in the diagrams by
dotted lines.

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References
https://thefactfactor.com/facts/pure_science/chemistry/physical-chemistry/redox-reactions/11959/https://b
yjus.com/jee/redox-reactions/#oxidation-and-reduction-reaction
https://leverageedu.com/blog/class-11-redox-reactions/
https://www.w3spoint.com/applications-of-redox-reaction#:~:text=Redox%20reactions%
20are%20also%20used,the%20presence%20of%20reducing%20agents.
http://railsback.org/Fundamentals/8150RedoxPlot07LS.pdf
https://www.outotec.com/globalassets/products/digital-solutions/hsc/17-Eh---pH---
Diagrams.pdf