(N06) Redox




Things to note:
  • Word equations consist of chemical names only. Do not include the physical state of substances present, such as aqueous, solid, liquid or gas, in the equation.
  • Chemical equations consist of chemical formula only. Always ensure that you have balanced the equation. Never leave any chemical equation unbalanced.
  • Half-equations are equations involving the transfer of electrons.
  • Ionic compounds consists of positive and negative ions.
  • When drawing the ions, always ensure that the brackets and the charge of the ion is present. The charge should be written as '3+', '3-', etc. The quantity of the charge should come first before the + or - sign.
  • Covalent molecules consist of atoms.
  • When determining the chemical formula of any covalent compounds, use the valency of the elements present to decide. 
  • The valency of the element will also tell us how many electrons it needs to share in order to form a stable electronic configuration.
  • For structural representation or structural formula, a short straight line (horizontal/ vertical/ diagonal, doesn't matter as long as it is straight) will indicate a pair of electrons (2 electrons) shared between 2 elements. 
  • 2 shared electrons will be represented by a straight line, known as single bond.
  • 4 shared electrons will be represented by a double line, known as double bond.
  • Example: structural formula of water, H-O-H. 
  • Metals are made up of positive ion and sea of delocalised electrons.
  • When drawing dot-cross diagrams, different elements should adopt different symbol as its electrons.
Things to note for (N03) The Periodic Table:

1. General points about the Periodic Table
  • The elements in the Periodic Table are arranged in increasing order of proton number.
  • The Group number indicates the number of valence electrons of an atom of the element.
  • The Period number indicates the number of occupied shells of an atom of the element. 
  • Elements are placed in different groups because of the number of valence electrons they possess.
  • Elements in the same group will have the same chemical properties due to the same number of valence electrons they posses. Hence they react in the same way.
  • No. of valence electrons = No. of electrons in the outermost shell
  • Valency of an element = No. of electrons involve in a reaction regardless whether the electrons are gained or lost
  • Charge is written as, for example '3+', '2-', with the quantity first, followed by the +/- sign.
  • Reactivity of an element is related to how readily it forms an ion. Higher the tendency to form a stable ion, more reactive will be the element. The reaction of reactive elements will also appear to be more vigorous or violent. 
  • Tendency to form a stable ion, depends on the distance between the nucleus and the valence electron, which will in turn affect the forces of attraction between the nucleus and the valence electrons. 
  • (By convention) When naming or writing the chemical formula the compounds consisting of a metal and non-metal, the name of the metal will come first, followed by the non-metal. The name of the metal remains the same, while that of the non-metal will change.
Example: Sodium reacts with chlorine, the product is known as sodium chloride, NaCl, instead of sodium chlorine.

2. Group I elements (Alkali metals)

  • The elements consist of only 1 valence electron.
  • Melting and boiling point decreases down the group.
  • Density increases down the group.
  • In order to form a stable ion, the elements tend to lose 1 electron to form a positively charged ion. Therefore, valency will be 1.
  • Reactivity increases down the group. (Fr is the most reactive metal.)
  • Group I metal reacts chlorine to produce metal halide.
  • Group I metal reacts with water to produce metal hydroxide and hydrogen
Note: In general, when a reactive metal reacts with water, metal oxide and hydrogen gas will be obtained. BUT for Group I elements, they form compounds which are soluble in water, the metal oxide which is formed will further react with water to produce metal hydroxide. (see example below.)

         Example:  When sodium reacts with water, the reaction should be: 
                            sodium + water → sodium oxide + hydrogen 
   BUT because sodium oxide is soluble in water, the reaction will then be:
           sodium + water → sodium hydroxide + hydrogen

  • Most Group I metal compounds are white solids and when dissolved in water they form colourless solution. THERE IS NO WHITE SOLUTION. 
Example: Sodium chloride, the common table salt which we consume, is made by reacting sodium and chlorine. However, it will not take the colours of the elements which make it up. Being a Group I metal compound, it will be a white solid and when dissolved in water, a colourless solution is obtained. 

3. Group VII elements (Halogens)
  • The elements consist of only 7 valence electrons.
  • They exist as diatomic molecules, 2 atoms per molecule. 
  • Melting and boiling point increases down the group.
  • Density increases down the group.
  • Colour of the element darkens down the group.
  • In order to form a stable ion, the elements tend to gain 1 electron to form a negatively charged ion. Therefore, valency will be 1.
  • Reactivity decreases down the group. (F is the most reactive non-metal.)
  • Group VII elements can undergo displacement reactions, whereby the more reactive element displaces the less reactive element from its salt solution.

4. Group 0 elements (Noble gases)
  • The elements consist of a stable electronic configuration also known noble gas configuration.
  • They exist as monoatomic atoms, individual.
  • Melting and boiling point increases down the group.
  • They are unreactive due to their stable electronic configuration.

some of the above points came from questions or comments contributed by students, from 3E4 and 3E5, who came for consultation on Saturday (27 March '10).

------------------------------ Part 1: Elements and their reactions --------------------------------

Video 1: (Group I elements) Alkali metals and their reactions with water

Video 2: Alkali metals and their reaction with water (with Universal Indicator). 
Video 3: (Group VII elements) Halogens and their reactions

* For more videos related to reactions of the elements in the Periodic Table:
        http://www.periodicvideos.com/ (by the University of Norttingham)

In the presence of Universal Indicator, water will appear to be green in colour as it is neutral. However, when a Group I metal is placed in it, the Group I metal hydroxide will be produced. Thus, changing the colour from green to blue/purple. This is because the metal hydroxide which is formed is alkaline in nature. 

For Universal Indicator:
pH 1 (Acidic): Red
pH 7 (Neutral): Green
pH 14 (Alkaline): Purple
Things to note for (N02) Atomic Structure:

1. Sub-atomic particles: Proton, Neutron, Electron
The symbol for electron is 'e-', as each electron has a relative charge of 1-. 

(i) Proton number/ Atomic number
It is the number of protons present in an atom. It determines the identity of the element.
(ii) Nucleon number/Atomic mass number/ Mass number:
It is the total number of protons and neutrons present in an atom.
(iii) Atomic mass:
It is the mass of the atom. Calculation: Sum up the relative mass of all the protons, neutrons and electrons present in the atom.
(iv) Relative atomic mass:
Calculation: Average mass of all the isotopes of the element, the relative abundance of the isotopes will be required for the calculation.

2. Drawing of the atomic structure of an atom or ion: 
If you are asked to draw 
(i) all sub-atomic particles
draw all electrons, label to no. of protons and no. of neutrons in the nucleus.
(ii) all the electrons
draw only the electrons, replace the nucleus with the symbol of the element. 
(iii) only the valence electron
draw only the electrons in the outermost/valence shell, do not reflect any of the inner shells in your diagram, the nucleus will be replaced with the symbol of the element.

For all diagrams, please use either dot or cross as the symbol of electrons. A legend must be provided. If you are drawing a negatively charged ion, the electrons which the atom gained should be indicated with a different symbol. If you are drawing a positively charged ion, the electrons which are given away should no longer be shown in the diagram, as they have been given away. 

3. Symbol, Chemical formula:
(i) Symbol:
It is the only the symbol as shown in the Periodic Table.
(ii) Chemical formula:
It depends if the elements exist as individual particles or as diatomic molecules. Most of the elements in the Periodic Table exist as individual particle, while all Group VII, Nitrogen, Oxygen and Hydrogen exist as diatomic molecules.
(iii) Symbol/notation showing the number of sub-atomic particles present:
It will include the symbol of the element and on the top left position, it will be the nucleon number, the bottom left position will be the proton number. 
(No examples are provided, see A02.2 Q6 answer (d) for the correct notation.)

Example 1: Fluorine
(i) Symbol of fluorine will be F.
(ii) Chemical formula will be F2. 
(the number '2' should be at the bottom right position, subscript position.since fluorine exists as a diatomic molecule.

Example 2: Sodium
(i) Symbol of sodium will be Na.
(ii) Chemical formula will be Na.
since sodium exists as an individual particle.

Other forms of representation also include, for example, C-12, which represents the element Carbon with total number of protons and neutrons (nucleon number) being 12. 

More examples:
C-13: Carbon with nucleon number 13;
O-18: Oxygen with nucleon number 18;
N-15: Nitrogen with nucleon number 15 and so on.

4. Word, Chemical, Half- equations:
All equations must include an ARROW and at least a '+' sign.

(i) Word equations: 
They consist of only the chemical names or words in them.
(ii) Chemical equations: 
They consist of only the chemical formula in them.
(iii) Half equations/ Chemical equation for the formation of an atom or ion: 
They consist of only the chemical formula and electron(s) in them.

Example 1: Formation of sodium ion.
(i) Word equation: sodium 
→ sodium ion + electron
(ii) Chemical equation/Half-equation: Na → Na+ + e-           
(the '+' change should be at the top right position, superscript position)

5. Formation of ions: 
Atoms either gain or lose electron(s) in order to obtain a stable electronic configuration.

Rule for formation of an ion:
(i) Less than 4 valence: LOSE all the valence electrons.
(ii) More than 4 valence electrons: GAIN electrons to make it stable.

Example 1: Formation of the ion of aluminium.
Aluminium atom has 3 valence electrons, thus it will lose all 3 valence electrons to become a stable ion. 

Half-equation: Al → Al3+ + 3e-   
(the '3+' charge should be at the top right position, superscript position)

Example 2: Formation of the ion of bromine.
Bromine atom, a Group VII element, has 7 valence electrons, thus it will gain 1 electron to become a stable ion. The atom must accept the electron before forming the ion.

Half-equation: Br + e- → Br-     
(the '-' charge should be at the top right position, superscript position)

Note: If the electron is taken in order to form the ion, it will appear before the arrow. However, if it is lost in order to form the ion, it will appear after the arrow as it is given away by the atom. The symbol for electron should always be written as e-. It should never be written as e+ to represent electron gained. 

6. Isotopes: 

The are atoms with the same number of protons but different number of neutrons.

most of the above points came from questions or comments contributed by students, from 3E4 and 3E5, who came for consultation on Saturday (27 March '10).

------------------------------ Part 1: HISTORY OF AN ATOM ---------------------------------------

Video 1: Our Friend the Atom 1/5 (8:10 ~ End; Democritus + Aristotle)
Video 2: Our Friend the Atom 2/5 (3:10 ~ 4:50 min; Dalton)
Video 3: Our Friend the Atom 3/5 (3:50 ~ 7:40 min; Rutherford)

* To view more Science related videos from Disney, click here: Banyt's Channel

--------------------------- Part 2: STRUCTURE OF AN ATOM: -------------------------------------

Video 1: A is for Atom Part 1 (start ~ 5:43 min; structure of atom and isotopes)
Video 2: A is for Atom Part 3 (2:22 ~ 3.25 min; uses of radioactive isotopes)

* Part 2 of the video and start of Part 3 are on radioactivity. 
   To view, click here: Part 2

Things to note for (N01) Experimental Chemistry:

Test tube & Boiling tube:
  1. Vol. of a test tube = 20 ml (approx.)
  2. Vol. of a boiling tube = 50 ml (approx.)
  3. Test tube and boiling tube should not be used for measuring volumes of liquids. They can be used to contain liquids. However, do take note of their capacity to avoid transferring too much liquid into them. 
  4. When heating substances in a test tube/ boiling tube, avoid filling it to more than half of its height, unless otherwise stated. 

Measuring cylinder:
  1. Accuracy: +/- 1 ml (Example: 11 ml, 12 ml, 13 ml)
  2. Refer to (P01A), when finding average volume of test tube and boiling tube: If the average reading is 22.3 ml, round it down to 22 ml. If the average reading is 55.6 ml, round it up to 56 ml. This is because we are using a measuring cylinder to measure the volume of the test tube and boiling tube. And the accuracy of the measuring cylinder is only +/- 1 ml.

Burette and Pipette:
  1. Accuracy of burette: +/- 0.1 ml (Example: 0.0 ml, 11.1 ml, 12.4 ml, 13.9 ml)
  2. Accuracy of pipette: 1 d.p (Example 20.0 ml, 15.0 ml, 10.0 ml)

  1. Record to the nearest second.

Electronic weighing balance:
  1. Accuracy of burette: 2 d.p (Example 7.67 g, 5.00 g)

Recording observations:
  1. Always state the physical state of the substance (solid/liquid/gas).
  2. Always state the colour of the substance.
  3. When identifying the colour of a very pale solution, focus on the meniscus. The colour at the meniscus will indicate the colour of the solution.  

------------------------  Self-check: Can you perform the following? -----------------------

(P01A) Laboratory Apparatus Part I
  1. Reading the volume of the liquid in a measuring cylinder?
  2. Transferring of liquids and solids safely?
  3. Reading the temperature shown on the thermometer?
  4. Recording time taken?
  5. Recording observations?

(P01B) Laboratory Apparatus Part II
  1. Filling a burette with the use of a filter funnel?
  2. Filling a pipette using a pipette filler?
  3. Identifying the solution to be added to the burette and pipette?
  4. Identifying the end-point of the reaction?
  5. Recording of liquid level as shown in the burette?

(P01C) Crystallisation
  1. Folding a filter paper?
  2. Carry out filtration of a suspension?
  3. Evaporating a solution?
  4. Lighting a Bunsen burner?
  5. Recording observation? 
  6. Identifying the filtrate and residue?

(P01D) Chromatography
  1. Transferring liquids safely?
  2. Identifying the solvent to be used based on the procedure?
  3. Calculating Retention factor for the different components?
-------------------------------------------- VIDEOS ------------------------------------------------

Video 1: Separating iron and sand mixture using a MAGNET.