San Francisco de Paula, Science Department.
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      • Unit 4 - Ecosystems & Biodiversity
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        • Unit 5a - The Geosphere and the Atmosphere
        • Unit 5b - The hydrosphere and the Biosphere
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        • Unit 6a: Internal Geodynamics
        • Unit 6b: External Geodynamics
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    • Contents >
      • Units 1, 2 and 3 - Measuring Matters >
        • Unit 1 Physical quantities and Scientific Activity >
          • Converting Units
        • Unit 2 Matter and its Properties
        • Unit 3 Pure Substances and Mixtures >
          • Concentration and units (g/L, % mass)
      • Units 4 and 5 - Chemical Cooking >
        • Models of the Atom
        • Electronic structure of matter.
        • Periodic table (distribution of elements).
        • Chemical formulation
        • Molecular mass
        • Moles
        • Types of reactions
        • Lavoisier's law: Balancing of equations
        • Reaction rates: Factors that affect the speed of a reaction
      • Units 6, 7 and 8 - Move It! >
        • Unit 6 - Motion (URM)
        • Unit 6 - Motion (UARM)
        • Unit 7 - Forces and Mass
        • Unit 8 - Energy and Transformations
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      • How to write a lab report
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    • Yr9 Syllabus & Assessment
    • Contents >
      • Unit 1 - General organization of the human body, health and illness >
        • Human cells: levels of organisation
        • Health and types of diseases
        • Preventing diseases
        • The Immune System
        • Transplants
        • Addictive substances and associated substances
        • Complete Unit 1
      • Unit 2 - Nutrition >
        • Nutrition and nutrients
        • The Mediterranean diet
        • The anatomy and physiology of the digestive system
        • The anatomy and physiology of the circulatory system >
          • Revision quizzes
        • The anatomy and physiology of the respiratory system
        • The anatomy and physiology of the excretory system
        • Nutrition Documentaries
      • Unit 3 - Interaction and Coordination >
        • The function and anatomy of the nervous system
        • The endocrine system
      • Unit 4 - The reproductive system >
        • Anatomy and physiology of the human reproductive system.
        • Gametogenesis
        • Menstrual cycle
        • Fertilization
        • Infertility, contraception and STDs
      • Unit 5 - Genetics >
        • DNA and genetic modification
        • Cells and Inheritance
        • MORE ON MITOSIS VS MEIOSIS
        • Mendelian genetics
      • Unit 6 - Evolution >
        • Evolutionary theories
        • Evolution Simulators
      • EXTRA Unit 7 - Ecology and the environment >
        • Ecology
    • Laboratory & Tasks
  • MYP Year 10
    • Yr10 Syllabus
    • Contents >
      • Unit 0 - Formulation
      • Unit 1 - Scientific Activity
      • Unit 2 - Kinetic theory
      • The Atmosphere
      • Unit 3 - Mixtures and pures substances: Colligative properties
      • Unit 4 - The structure of matter
      • Unit 5 - Stoichiometry
      • Unit 6 - Redox Reactions
      • Unit 7 - Movement
      • Unit 8 - Forces
      • Unit 9 - Electricity
      • Unit 10 - Energy
    • Revision
    • Assessments and Lab Sessionss >
      • Assessments >
        • Criterion D - Water crisis
      • Session 1 - Paper balls
      • Sessions 2 - Properties of substances
      • Session 3 - Preparing a Schlenk tube
      • Session 4 - Gas constant
      • Session 5 - Colligative properties
      • Session 6 - Job´s method
      • Session 7 - Redox titration
      • Session 8 - UARM
      • Session 9 - Hooke´s Law
    • PISA questions
  • DP Biology SL
    • Contents >
      • Topic 1: Cell biology >
        • 1.1 Introduction to cells
        • 1.2 Ultrastructure of cells
        • 1.3 Membrane structure
        • 1.4 Membrane transport
        • 1.5 The origin of cells
        • 1.6 Cell division
      • Topic 2: Molecular biology >
        • 2.1 Molecules to metabolism
        • 2.2 Water
        • 2.3 Carbohydrates and lipids
        • 2.4 Proteins
        • 2.5 Enzymes
        • 2.6 Structure of DNA and RNA
        • 2.7 DNA replication, transcription and translation
        • 2.8 Cell respiration
        • 2.9 Photosynthesis
      • Topic 3: Genetics >
        • 3.1 Genes
        • 3.2 Chromosomes
        • 3.3 Meiosis
        • 3.4 Inheritance
        • 3.5 Genetic modification and biotechnology
      • Topic 4: Ecology >
        • 4.1 Species, communities and ecosystems
        • 4.2 Energy flow
        • 4.3 Carbon cycling
        • 4.4 Climate change
      • C: Ecology and conservation >
        • C.1 Species and communities
        • C.2 Communities and ecosystems
        • C.3 Impacts of humans on ecosystems
        • C.4 Conservation of biodiversity
      • Topic 5: Evolution and biodiversity >
        • 5.1 Evidence for evolution
        • 5.2 Natural selection
        • 5.3 Classification of biodiversity
        • 5.4 Cladistics
      • Topic 6: Human physiology >
        • 6.1 Digestion and absorption
        • 6.2 The blood system
        • 6.3 Defence against infectious disease
        • 6.4 Gas exchange
        • 6.5 Neurons and synapses
        • 6.6 Hormones, homeostasis and reproduction
    • Labs >
      • Statistical analysis
    • Previous to 2015 >
      • Topic 1: Statistical analysis
      • Topic 2: Cells >
        • 2.1 Cell theory
        • 2.2 Prokaryotic cells
        • 2.3 Eukaryotic cells
        • 2.4 Membranes
        • 2.5 Cell division
      • Option F: Microbes and biotechnology >
        • F1 Diversity of microbes
        • F2 Microbes and the environment
        • F3 Microbes and biotechnology
        • F4 Microbes and food production
      • Topic 5: Ecology and evolution >
        • 5.1 Communities and ecosystems
        • 5.2 The greenhouse effect
        • 5.3 Populations
        • 5.4 Evolution
        • 5.5 Classification
      • Option D: Evolution >
        • D1 Origin of life on Earth
        • D2 Species and speciation
        • D3 Human evolution
      • Topic 3: The chemistry of life >
        • 3.1 Chemical elements and water
        • 3.2 Carbohydrates, lipids and proteins
        • 3.3 DNA structure
        • 3.4 DNA replication
        • 3.5 Transcription and translation
        • 3.6 Enzymes
        • 3.7 Cell respiration
        • 3.8 Photosynthesis
        • TO DELETE
      • Topic 4: Genetics >
        • 4.1 Chromosomes, genes, alleles and mutations
        • 4.2 Meiosis
        • 4.3 Theoretical genetics
        • 4.4 Genetic engineering and biotechnology
      • Topic 6: Human health and physiology >
        • 6.1 Digestion
        • 6.2 The transport system
        • 6.3 Defence against infectious disease
        • 6.4 Gas exchange
        • 6.5 Nerves, hormones and homeostasis
        • 6.6 Reproduction
    • Tasks >
      • Databases
    • Documents
    • DP Biology Resources
    • Interesting links
  • Biología 12º
    • Bloque 1. Bioquímica
    • Bloque 2. Biología celular
    • Bloque 3. Genética y evolución
    • Bloque 4. Microbiología y biotecnología
    • Bloque 5. Inmunología
  • IB Biología NS
  • IB Chemistry HL
    • Contents >
      • Year 1 >
        • Topic 0 - Formulation
        • Topic 1 - Stoichiometric relationships
        • Topic 2 and 12 - Atomic structure
        • Topic 3 and 13 - Periodicity
        • Topic 4 and 14 - Chemical structure and bonding
        • Topic 5 and 15 - Energetics
        • Topic 10 and 20 - Organic chemistry
      • Year 2 >
        • Topic 6 and 16 - Chemical kinetics
        • Topic 7 and 17 - Equilibrium
        • Topic 8 and 18 - Acids and bases
        • Topic 9 and 19 - Redox processes
        • Topic 11 and 21 - Measurement and data processing
        • Option D - Medicinal chemistry
    • Lab work >
      • SFP Campus lab reports
      • Internal assessment structure
      • Internal Assessment examples
    • Nature of science
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    • Selectividad
    • General information
    • Extended Essay
  • Other resources for students
    • Science Fair
    • Science essays >
      • How to Reference
    • Chemical formulation
    • Laboratory >
      • How to write a lab report
      • Microscopy
      • Excel for graphs and calculations >
        • Calculate the mean and SD
        • Drawing graphs
        • Add error bars to excel graphs
      • Lab videos!
    • Further Reading >
      • Women In Science
      • Infographics
    • Proyecto integrado
  • Department Home page
    • The International Baccalaureate
    • Conceptual learning
    • How will I be assessed?
    • Primary Science Club
    • Science Activities Noticeboard
  • MYP Year 6
    • Yr 6 Syllabus & Assessment
    • Contents >
      • Unit 1: Introduction to investigation >
        • How to write Lab Reports
      • Unit 2: Living things
      • Unit 3: Classification of living things
      • Unit 4: Fungi, Protists and Bacteria
      • Unit 5: Nutrition I
    • Laboratory and Tasks
  • MYP Year 7
    • Yr 7 Syllabus & Assessment
    • Contents >
      • 7º PAI en español
      • Unit 1 - What do scientists do? >
        • How to write a lab report
        • SI Units
      • Unit 2 - The Earth in the Universe
      • Unit 3: How can we study living things >
        • Unit 3a - Living things
        • Unit 3b - The Diversity of Life >
          • Monera
          • Protists
          • Fungi
          • The Plant Kingdom
          • The Animal Kingdom
      • Unit 4 - Ecosystems & Biodiversity
      • Unit 5: The systems of the Earth >
        • Unit 5a - The Geosphere and the Atmosphere
        • Unit 5b - The hydrosphere and the Biosphere
      • Unit 6: Our changing home >
        • Unit 6a: Internal Geodynamics
        • Unit 6b: External Geodynamics
    • Laboratory & Tasks
  • MYP Year 8
    • Yr8 Syllabus & Assessment
    • Contents >
      • Units 1, 2 and 3 - Measuring Matters >
        • Unit 1 Physical quantities and Scientific Activity >
          • Converting Units
        • Unit 2 Matter and its Properties
        • Unit 3 Pure Substances and Mixtures >
          • Concentration and units (g/L, % mass)
      • Units 4 and 5 - Chemical Cooking >
        • Models of the Atom
        • Electronic structure of matter.
        • Periodic table (distribution of elements).
        • Chemical formulation
        • Molecular mass
        • Moles
        • Types of reactions
        • Lavoisier's law: Balancing of equations
        • Reaction rates: Factors that affect the speed of a reaction
      • Units 6, 7 and 8 - Move It! >
        • Unit 6 - Motion (URM)
        • Unit 6 - Motion (UARM)
        • Unit 7 - Forces and Mass
        • Unit 8 - Energy and Transformations
    • Laboratory & Tasks >
      • How to write a lab report
  • MYP Year 9
    • Yr9 Syllabus & Assessment
    • Contents >
      • Unit 1 - General organization of the human body, health and illness >
        • Human cells: levels of organisation
        • Health and types of diseases
        • Preventing diseases
        • The Immune System
        • Transplants
        • Addictive substances and associated substances
        • Complete Unit 1
      • Unit 2 - Nutrition >
        • Nutrition and nutrients
        • The Mediterranean diet
        • The anatomy and physiology of the digestive system
        • The anatomy and physiology of the circulatory system >
          • Revision quizzes
        • The anatomy and physiology of the respiratory system
        • The anatomy and physiology of the excretory system
        • Nutrition Documentaries
      • Unit 3 - Interaction and Coordination >
        • The function and anatomy of the nervous system
        • The endocrine system
      • Unit 4 - The reproductive system >
        • Anatomy and physiology of the human reproductive system.
        • Gametogenesis
        • Menstrual cycle
        • Fertilization
        • Infertility, contraception and STDs
      • Unit 5 - Genetics >
        • DNA and genetic modification
        • Cells and Inheritance
        • MORE ON MITOSIS VS MEIOSIS
        • Mendelian genetics
      • Unit 6 - Evolution >
        • Evolutionary theories
        • Evolution Simulators
      • EXTRA Unit 7 - Ecology and the environment >
        • Ecology
    • Laboratory & Tasks
  • MYP Year 10
    • Yr10 Syllabus
    • Contents >
      • Unit 0 - Formulation
      • Unit 1 - Scientific Activity
      • Unit 2 - Kinetic theory
      • The Atmosphere
      • Unit 3 - Mixtures and pures substances: Colligative properties
      • Unit 4 - The structure of matter
      • Unit 5 - Stoichiometry
      • Unit 6 - Redox Reactions
      • Unit 7 - Movement
      • Unit 8 - Forces
      • Unit 9 - Electricity
      • Unit 10 - Energy
    • Revision
    • Assessments and Lab Sessionss >
      • Assessments >
        • Criterion D - Water crisis
      • Session 1 - Paper balls
      • Sessions 2 - Properties of substances
      • Session 3 - Preparing a Schlenk tube
      • Session 4 - Gas constant
      • Session 5 - Colligative properties
      • Session 6 - Job´s method
      • Session 7 - Redox titration
      • Session 8 - UARM
      • Session 9 - Hooke´s Law
    • PISA questions
  • DP Biology SL
    • Contents >
      • Topic 1: Cell biology >
        • 1.1 Introduction to cells
        • 1.2 Ultrastructure of cells
        • 1.3 Membrane structure
        • 1.4 Membrane transport
        • 1.5 The origin of cells
        • 1.6 Cell division
      • Topic 2: Molecular biology >
        • 2.1 Molecules to metabolism
        • 2.2 Water
        • 2.3 Carbohydrates and lipids
        • 2.4 Proteins
        • 2.5 Enzymes
        • 2.6 Structure of DNA and RNA
        • 2.7 DNA replication, transcription and translation
        • 2.8 Cell respiration
        • 2.9 Photosynthesis
      • Topic 3: Genetics >
        • 3.1 Genes
        • 3.2 Chromosomes
        • 3.3 Meiosis
        • 3.4 Inheritance
        • 3.5 Genetic modification and biotechnology
      • Topic 4: Ecology >
        • 4.1 Species, communities and ecosystems
        • 4.2 Energy flow
        • 4.3 Carbon cycling
        • 4.4 Climate change
      • C: Ecology and conservation >
        • C.1 Species and communities
        • C.2 Communities and ecosystems
        • C.3 Impacts of humans on ecosystems
        • C.4 Conservation of biodiversity
      • Topic 5: Evolution and biodiversity >
        • 5.1 Evidence for evolution
        • 5.2 Natural selection
        • 5.3 Classification of biodiversity
        • 5.4 Cladistics
      • Topic 6: Human physiology >
        • 6.1 Digestion and absorption
        • 6.2 The blood system
        • 6.3 Defence against infectious disease
        • 6.4 Gas exchange
        • 6.5 Neurons and synapses
        • 6.6 Hormones, homeostasis and reproduction
    • Labs >
      • Statistical analysis
    • Previous to 2015 >
      • Topic 1: Statistical analysis
      • Topic 2: Cells >
        • 2.1 Cell theory
        • 2.2 Prokaryotic cells
        • 2.3 Eukaryotic cells
        • 2.4 Membranes
        • 2.5 Cell division
      • Option F: Microbes and biotechnology >
        • F1 Diversity of microbes
        • F2 Microbes and the environment
        • F3 Microbes and biotechnology
        • F4 Microbes and food production
      • Topic 5: Ecology and evolution >
        • 5.1 Communities and ecosystems
        • 5.2 The greenhouse effect
        • 5.3 Populations
        • 5.4 Evolution
        • 5.5 Classification
      • Option D: Evolution >
        • D1 Origin of life on Earth
        • D2 Species and speciation
        • D3 Human evolution
      • Topic 3: The chemistry of life >
        • 3.1 Chemical elements and water
        • 3.2 Carbohydrates, lipids and proteins
        • 3.3 DNA structure
        • 3.4 DNA replication
        • 3.5 Transcription and translation
        • 3.6 Enzymes
        • 3.7 Cell respiration
        • 3.8 Photosynthesis
        • TO DELETE
      • Topic 4: Genetics >
        • 4.1 Chromosomes, genes, alleles and mutations
        • 4.2 Meiosis
        • 4.3 Theoretical genetics
        • 4.4 Genetic engineering and biotechnology
      • Topic 6: Human health and physiology >
        • 6.1 Digestion
        • 6.2 The transport system
        • 6.3 Defence against infectious disease
        • 6.4 Gas exchange
        • 6.5 Nerves, hormones and homeostasis
        • 6.6 Reproduction
    • Tasks >
      • Databases
    • Documents
    • DP Biology Resources
    • Interesting links
  • Biología 12º
    • Bloque 1. Bioquímica
    • Bloque 2. Biología celular
    • Bloque 3. Genética y evolución
    • Bloque 4. Microbiología y biotecnología
    • Bloque 5. Inmunología
  • IB Biología NS
  • IB Chemistry HL
    • Contents >
      • Year 1 >
        • Topic 0 - Formulation
        • Topic 1 - Stoichiometric relationships
        • Topic 2 and 12 - Atomic structure
        • Topic 3 and 13 - Periodicity
        • Topic 4 and 14 - Chemical structure and bonding
        • Topic 5 and 15 - Energetics
        • Topic 10 and 20 - Organic chemistry
      • Year 2 >
        • Topic 6 and 16 - Chemical kinetics
        • Topic 7 and 17 - Equilibrium
        • Topic 8 and 18 - Acids and bases
        • Topic 9 and 19 - Redox processes
        • Topic 11 and 21 - Measurement and data processing
        • Option D - Medicinal chemistry
    • Lab work >
      • SFP Campus lab reports
      • Internal assessment structure
      • Internal Assessment examples
    • Nature of science
    • General revision
    • Selectividad
    • General information
    • Extended Essay
  • Other resources for students
    • Science Fair
    • Science essays >
      • How to Reference
    • Chemical formulation
    • Laboratory >
      • How to write a lab report
      • Microscopy
      • Excel for graphs and calculations >
        • Calculate the mean and SD
        • Drawing graphs
        • Add error bars to excel graphs
      • Lab videos!
    • Further Reading >
      • Women In Science
      • Infographics
    • Proyecto integrado
San Francisco de Paula, Science Department.

Unit 5 - Redox reactions

Key concept - Change- How important is the rearrangement of matter in chemical reactions?

Related concepts - Consequences - How does science utilise the consequences of chemical reactions? Are the consequences always positive?

Global concept - Globalisation and sustainability - In what ways does improving knowledge of chemical reactions impact our goals of global sustainability?
Picture
Key words
  • ​Oxidation
  • Reduction
  • Oilrig
  • Oxidation state/valency
  • Electron loss/gain
  • Redox
  • Voltaic/Galvanic cell
  • Electrolysis
  • Faraday
  • Reactivity
  • Anode (oxidation)
  • Cathode (reduction)

Pre-knowledge - What are oxidation numbers?

Oxidation numbers are assigned to show (theoretically or actually) the number of electrons lost or gained by an atom or ion in a molecule or compound.

Example - Magnesium oxide, MgO (Ionic compound)
This ionic compound is composed of Mg 2+ ions and O 2- ions.

As magnesium has lost 2 electrons we assign a +2 oxidation state.

As oxygen has gained 2 electrons we assign a -2 oxidation state.

(Note: oxidation states must be given with the + or - before the number)
Imagen
Imagen
Example - Ammonia, NH3 (covalent compound)
As ammonia is a covalent molecule, we assign oxidation numbers by using the electronegativity values of each atom to predict where the electrons would go if it was to form an ionic compound.

As nitrogen is more electronegative than hydrogen, it would take an electron from a hydrogen. As it needs 3 electrons to complete its outer shell, it would need to take an electron from 3 hydrogens as shown in its formula - NH3.

As nitrogen has theoretically gained 3 electrons we assign a -3 oxidation state.

As each hydrogen has lost 1 electron we assign a +1 oxidation state.
You have already seen these oxidation states in chemical formulation. This is just the theory behind them.
Task 5a:
  1. Organise the following into ionic compounds (metal and non-metal ions) and covalent compounds (only non-metals): NaCl, H2O, O2, Mg(NO3)2, NH3, CH4, AgF
  2. Which of the following numbers could be oxidation states:     +1     -2     III     4+     +7      7+     -II      0
  3. Predict which oxidation states might be likely for the four new elements with atomic numbers 113, 115, 117 and 119.

5.1 Assigning oxidation states

The following rules can be used to help assign ox. states:
  1. Oxygen = -2                                                                       (except  in peroxide ion with -1)
  2. Hydrogen = +1                                                                     (except  in metal hydrides with -1)
  3. Group 1, 2 and 3 ions have an ox. state = Group number              (e.g. Mg2+  -->  +2)
  4. Group 17 (halides) = -1                                                          (e.g.  F-  -->  -1)
  5. Anything in its "elemental state" must = 0                               (e.g. Ne,  O2,  N2,  Mg,   Cl2)
Note: Something in its elemental state is the natural state of a pure element. For example, oxygen is found naturally as the molecule O2 and gold is found naturally as atomic gold, Au .
The sum of the ox. states in a compound/molecule will ALWAYS add up to ZERO.
The sum of ox. states in an ion will ALWAYS add up to the CHARGE on that ion.
Worked example: Assign oxidation numbers to the following equation:    Cl2 + SO2 + 2H2O --> H2SO4 + 2HCl

Cl2 --> In its elemental state so each Cl must be 0    (0 + 0 = 0 )

SO2 --> Oxygen is always -2 therefore sulfur must be +4      (+4 -2 -2 = 0 )

H2O --> Oxygen is always -2 and hydrogen is always +1     (+1 +1 -2 = 0 )

H2SO4 --> Oxygen is always -2 and hydrogen is always +1 so sulfur must be +6       (+1 +1 +6 -2 -2 -2 -2 = 0 )

HCl --> Hydrogen is always +1 and a chloride ion is always -1.    (+1 -1 = 0 )

0           +4  -2          +1   -2            +1   +6  -2          +1  -1      
Cl2  +  SO2  +  2H2O  -->  H2SO4  +  2HCl   
Note: Ox. states can also be represented as roman numerals to prevent confusion with any charges.

0           IV  -II          I   -II             I   VI  -II          I  -I      
Cl2  +  SO2  +  2H2O  -->  H2SO4  +  2HCl   
Task 5b: Quiz
Assigning oxidation numbers - ProProfs
redox_y10_theory_mcu.docx
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y10_redox_student_booklet_mcu.doc
File Size: 104 kb
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5.2 Oxidation and reduction

0           +4  -2          +1   -2            +1   +6  -2          +1  -1      
Cl2  +  SO2  +  2H2O  -->  H2SO4  +  2HCl   
As we can see in the equation above, some elements may experience a change in their oxidation state during the chemical reaction. This means that a transfer of electrons must have occurred.

          Element           Before           After
             Cl                   0                 -1
             S                   +4                +6
Imagen
A decrease in ox. state means electrons have been gained --> we call this REDUCTION
An increase in ox. state means electrons have been lost --> we call this OXIDATION

We can remember these processes using the acronym oil rig (above right): "oxidation is loss, reduction is gain"

We can also use the terms "oxidising agent" and "reducing agent​" to define these species:
Picture
Task 5c:
redox_questions.doc
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File Type: doc
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Task 5d:
ProProfs - Identifying redox processes » ProProfs Quiz Software
​Redox summary video:

5.3 Balancing redox equations

Balancing redox equations consists of being given an unbalanced equation in which redox reactions are occuring. You will have to use the following ion-electron method to carry this out.

5.3a Acidic conditions (using H+)

Example question - Balance the following equation using the ion-electron method (in acidic conditions).

                                                   HNO3 + Cu --> Cu(NO3)2 + NO2 + H2O
 
1.       If the equation is written in the molecular form we first have to change it to ionic. This means that:
  • For any ionic compounds - we write them down as seperate ions e.g. MgSO4 --> Mg2+  +  SO42-
  • For and acids - we also break them into sperate ions e.g. HCl --> H+  +  Cl-
  • For any covalent molecules we leave them as they are e.g. H2O and NO2
So for this question:
                                  +1         +5 -2           0             +2           +5 -2         +4 -2        +1 -2
                                  H+   +   NO3-   +   Cu   -->   Cu2+   +   NO3-   +   NO2   +   H2O
 

2.       Assign oxidation states to all elements in the ionic equation and identify which species has been oxidised and which has been reduced (note: we include the whole species that includes the oxidation state changes). Write a half equation for both processes.

                                                         NO3-  --> NO2                           REDUCTION

                                                           Cu --> Cu2+                               OXIDATION



3.       Atoms in each half reaction are balanced, according to this sequence:

a. Atoms other than O and H 
b. O atoms, adding the corresponding number of H2O molecules
c. H atoms, added as H+

4.       Balance the charges on opposite sides of each half reaction equation by adding electrons to the appropriate side.

                                           NO3- + 2 H+ + e- --> NO2 + H2O

                                                             Cu --> Cu2+ + 2 e-


5.       The number of electron lost in oxidation must equal the number of electrons gained in the reduction half-reaction. If necessary, multiply each half reaction by the adequate entire number (least common multiple, lcm)

                                           NO3- + 2 H+ + e- --> NO2 + H2O      (x2)

                                                             Cu --> Cu2+ + 2 e-
                                       -------------------------------------------
                                        2 NO3- + 4 H+ + Cu --> 2 NO2 + 2 H2O + Cu2+


 
6.       Half-reactions are added and common species that appear at both sides of the overall equation are cancelled.


7.       Check that mass and charge is balanced.


8.    Balanced Ionic Reaction

(Note: You will only need to know how to do this process in acidic conditions. If in basic conditions, we also add OH- ions to eliminate the H+ ions, since H+   +   OH- = H2O)

Add the necessary species to rebuild the initial reagents, and simplify if possible.

                                       2 NO3- + 4 H+ + Cu --> 2 NO2 + 2 H2O + Cu2+

                                       2 NO3-                                        2 NO3-

                                       -------------------------------------------
                                         4 HNO3 + Cu --> 2 NO2 + 2 H2O + Cu(NO3)2


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Example 1:
Balance this equation using the ion-electron method:

Li   +   CuSO4   -->   Li2SO4   +   Cu

Answer:    2Li   +   CuSO4   -->   Li2SO4   +   Cu
Example 2:
Balance this equation using the ion-electron method:

O2   +   H2S   -->   S   +   H2O


Answer:     O2   +   2H2S   -->   2H2O   +   2S





Balancing redox reaction with Stoichiometry calculation
  1. Given the chemical reaction: potassium dichromate + hydroiodic acid + perchloric acid, yielding to chromium(III) perchlorate + molecular iodine + potassium perchlorate + water: a) balance the reaction; b) determine which is the oxidant specie; c) calculate the number of grams of iodine obtained in the reaction if 200 cc of 2 M solution of the oxidant are consumed. Cr 52; K 39; I 127.
  2. Potassium permanganate react with hydrogen sulfide in presence of manganese(II) sulfate, sulfur, potassium sulfate and water: a) balance the corresponding redox equation; b) indicate which is the reductant specie and the anode; c) determine the volume of 0.5 M hydrogen sulfide solution needed if in the process 100 g of sulfur are obtained. S 32, K 39.
  3. Manganese dioxide and potassium iodide react in presence of sulfuric acid to give manganese(II) sulfate, iodine, potassium sulfate and water: a) balance the given reaction; b) determine the oxidant and the reductant species; c) calculate the maximum number of grams of iodine possible, if we begin with 1 kg de manganese dioxide 95.7 % in richness. Mn 55, K 39; I 127.
  4. Nitric acid attacks mercury in presence of hydrochloric acid to give nitrogen, mercury(II) chloride and water: a) balance the corresponding redox reaction; b) determine which is the reductant specie and the cathode; c) determine how many litres of water vapour will be given off at 127 °C and 1520 mmHg, it 500 cc of 2 M nitric acid solution are consumed in the process. N 14, Hg 200

5.3b Basic conditions (using OH-​)

Balancing redox reactions in basic conditions is almost exactly the same but because we are not in acidic conditions we cannot have H+. Therefore in Step 3 there is one extra thing we must do:

3.       
Atoms in each half reaction are balanced, according to this sequence:
a. Atoms other than O and H 
b. O atoms, adding the corresponding number of H2O molecules
c. H atoms, added as H+
d. Add an equal amount of OH- ions to each side to convert any H+ into H2O.     (H+   +   OH-   -->   H2O)
Task 5e: Complete the quiz below
Balancing redox equations

5.4 The reactivity series of metals

Imagen
A REACTIVE METAL LOSES ITS OUTER ELECTRONS MORE EASILY (is oxidised more easily)

Zinc is quite a reactive metal so is oxidised easily whe
n it reacts with an acid:
                                 0                                           +2
                               Zn   +   2HCl   --->     ZnCl2   +   H2
                               
Copper is very unreactive so rarely undergoes oxidation:
                                0                                           
                               Cu   +   2HCl   --->    No reaction
                             
Different metals have different levels of reactivity as shown in the "Reactivity Series".

5.5a Electrochemical cells - Galvanic cells

​These differences in reactivity actually allow use to produce ELECTRICITY (the flow of electrons) in something called a Galvanic cell. It uses a chemical reaction to produce electricity.

In the diagram below we see an example using a piece of zinc and a piece of copper.
Process:
  1. The zinc is more reactive so releases the electrons more easily (it is oxidised).
  2. Electrons flow from the piece of zinc to the copper which is reduced.
  3. To make sure the circuit is complete, we must have a salt bridge that allows charge to move around the whole circuit.
​
​
Key features:
  • Must have 2 metals with different reactivities
  • Must have a salt bridge
  • Anode = electrode where oxidation takes place
  • Cathode = electrode where reduction takes place
Picture


​Uses
: Batteries
Picture
Mr Canning video explanation - LINK
Simulation: LINK

5.5b Electrochemical cells - Electrolysis 

​The opposite of a Galvanic cell is called and electrolytic cell. This uses electricity to produce a chemical change.
Process:
  1. ​As the battery forces electrons to move in 1 direction, the electrode where they arrive will be negative and, therefore, will be where reduction takes place.
  2. This negative electrode will attract positive ions in the solution and so they will gain the electrons (reduction).
  3. The positive electrode will attract the negative ions and take their electrons (oxidation).
Picture
Uses: 
  • Can be used to collect pure metals from solutions. (click on diagram for video).
  • Can be use for electroplating (LINK)
Picture
Mr Canning video explanation - LINK
Task 5f: Copy and paste this document to your PCD and complete the questions once you have carried out the experiments.
​REDOX DEMONSTRATIONS

5.6a Faraday´s 1st Law 

Faraday came up with some very useful equations related to electrolysis. The first helps us calculate the mass of an element formed at one of the electrodes by calculating the number of electrons being passed around the circuit.
Picture
m = mass
mm = molecular mass (g/mol)
n = number of electrons needed to form metal
F = Faraday´s constant - 96500 C/mol
I = current (A)
t = time (s)

Example 1: Calculate the mass of aluminium produced when molten aluminium oxide is electrolysed using a current (I) of 20,000 A, for 5 hours, 21 min and 40s.    Data: F = 96500 C/mol;    Al - 27 g/mol
mm = 27
n = Al3+ requires 3 electrons to form Al
F = 96500
​I = 20000
​t = 19300
m = 36000 g 

5.6b Faraday´s 2nd Law

Faraday´s s 2nd Law simply describes how metals with ions with high oxidation states require more electricity than those with low oxidation states.

Example: If the same 12 moles of e- are passed through a Cu + and a Ni 3+ solution. Which will deposit the greatest number of moles of metal?

Answer: The 2 reduction half-equations for forming the metals are:
Cu +     +    e-     --->      Cu                                       Ni 3+    +    3e-     --->     Ni

So if we balance the stoichiometric coefficients with the 12 e- being produced:

12Cu+   +  12e-      --->  12Cu                                  4Ni 3+    +   12e-     --->  4Ni

We can see that 12 mol Cu and 4 mol Ni would be produced.



Revision

Use this LINK to practice balancing ionic charges, balancing equations, balancing redox equations.
 Redox equation questions

 1.       Calculate the oxidation state of each of the following chemical species:
a) NO2,     b) HIO3,     c) O3,     d) O2    , e) O22-
f) (NH4)2Cr2O7,     g) NO+,     h) PCl3,     i) C6H6,     j) CH2=CH2,
k) NaHCO3,     l) Fe3+,     m) Ne,     n) Sc2O3,     o) OsO4,
p) H2O2,     q) PdH4,     r) P4,     s) AsH3,     t) P2O54- 
u) NH4+,     v) SnI4,     w) ICl4-     x) K2O2.


2.       Which species are oxidising agents and which are reducing agents in the following balanced redox reactions?
a)                2Mg + O2 --> 2MgO
b)                CuSO4 + Zn --> ZnSO4 + Cu
c)                2Al + 3Cl2 --> 2AlCl3
d)                Cl2 + SO2 + 2H2O --> H2SO4 + 2HCl
e)                Zn + 2HCl --> ZnCl2 + H2
f)                MnO2 + 4HCl --> MnCl2 + Cl2 + 2H2O
g)                KMnO4 + H2O2 + H2SO4 --> MnSO4 + O2 + K2SO4 + H2O

3.       Balance the following redox reactions in acidic conditions: 
1)       HCl + KMnO4 --> Cl2 + KCl + MnCl2 + H2O
2)       Cu + HNO3 --> NO + Cu(NO3)2 + H2O
3)       Zn + HNO3 --> Zn(NO3)2 + (NH4)NO3 + H2O
4)       FeCl2 + H2O2 + HCl --> FeCl3 + H2O
5)       O2 + H2S --> S + H2O
6)        SO2 + HNO3 + H2O --> H2SO4 + NO
7)       NaClO + NaI + HCl --> I2 + NaCl
8)       HNO3 + H2S --> NO + S + H2O
9)       HNO3 + Fe 
--> Fe(NO3)3 + NO + H2O

4.       
Balance the following redox reactions in basic conditions: 

1)        KClO3 + KOH  + Cr(OH)3 --> KCl  + K2CrO4 + H2O 

ANSWERS
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Galvanic and electrolytic cell questions
1. Look a the following reactivity series of metals:
                               Most reactive     Mg > Al > Zn > Fe > Pb > Cu > Ag       Least reactive
Which is would be most easily oxidised and which least easily?

​2. An electrolysis has taken place for 48 minutes and 15 seconds using a 0.5 A current on a cobalt(III) chloride solution. How much (mass) cobalt will be deposited? (F = 96500 C/mol; Co = 59 g/mol)

3. ​a) How long will it take to deposit all the silver contained in a 100 mL beaker full of 0.1 M silver nitrate using a 0.1 A current? b) If the same time and current was used for a solution of gold(III) chloride. Would more or less gold be deposited? (Ag = 108 g/mol)

4. Without performing any calculation, answer these questions:
a) Which element will deposit a higher number of moles with the same current and time, copper from copper sulfate or aluminium from aluminium oxide?
b) In order to obtain the same number of moles of copper(II) and aluminium at the time, should I increase or reduce the current through the copper solution?


5. What electrical current should pass through an iron(III) nitrate solution if 28 g of iron are to be deposited in 6 hours? What if we had copper(II) instead of iron(III) nitrate?
Answers: 1. Mg most and Ag least.      2. 0.295 g (5x10`-3 mol)       3. a) 9650 s   b) less (one third less)       
4. a) Higher number for Cu than for Al,  b) reduce
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