AS revision, atomic structure, bonding and periodicity

A level as notes: atomic structure, bonding and periodicity

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Periodicity -Acid Base Character

Acid-base character of oxides
Base behaviour : Na2O, MgO
Na2O(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l)
MgO(s) + 2HCl(aq)→MgCl2(aq) + H2O(l)

Amphoteric behaviour Al2O3
Al2O3(s) + 6HCl(aq) → 2AlCl3 + 3H2O(l)
Al2O3(s) + 2NaOH(aq) +3H2O(l) → 2NaAl(OH)4(aq) sodium aluminate

Acid behaviour : SiO2, P4O10, P4O6
SiO2 + 2NaOH(aq) → Na2SiO3(aq) + H2O
P4O10 + 12NaOH → 4Na3PO4 + 6H2O
P4O6 + 12NaOH → 4Na3PO3 + 6H2O

Formulae of chlorides and reactions with water

NaCl + aq →Na+(aq) + Cl-(aq) dissolves to form neutral solution , pH = 7

MgCl2 + aq → Mg2+(aq) + 2Cl-(aq) dissolves with slight hydrolysis to form slightly acidic solution , pH = 6.5

AlCl3 + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl-(aq) dissolves with hydrolysis to form acidic solution
[Al(H2O)6]3+(aq) →[Al(OH)(H2O)5]2+(aq) + H+(aq) , pH = 3

SiCl4 + 2H2O→SiO2 + 4HCl , pH = 2

PCl3 + 3H2O → H3PO3 + 3HCl, pH = 2

PCl5 + H2O → POCl3 + 2HCl, pH =2

Chemical bonding

Types of bonding

Ionic bond (between ions)
Ionic bond is the electrostatic force of attraction between the positive and negative ions as a result of transfer of electrons from an atom to another. (eg NaCl, MgCO3 )

Covalent bond (between atoms)
Covalent bond is the electrostatic force of attraction resulted by sharing of two electrons by two atoms.
Covalent bond can be found in simple molecular structure ( discrete molecules eg O2, CO2 ) and giant covalent structure (with a network of covalent bonds eg C(graphite), C(diamond), SiO2,,Si ]

Metallic bond ( between positive ions surrounded by sea of delocalised electrons)
Metallic bond is the electrostatic attractions between the cations and the delocalised electrons surrounding the cations. (eg Mg, Al , Na, Ag, Pb, Sn)

3 types of intermolecular forces ( weak forces between molecules)

London or dispersion forces occurs are weak forces occur between non- polar molecules , eg: CH4, Cl2, CO2

Dipole-dipole forces are forces occur between polar molecules, eg HCl , H2S, SO2

Hydrogen bonding is the electrostatic attraction forces between H atom bonded to a very electronegative atom and the lone pair of electrons from a very electronegative atom such as. F,O or N atom (eg HF, NH3, H2O ,amines and alcohols) Hydrogen bonding causes higher boiling points of hydrides, solubility of carboxylic acids, alcohols and amines, dimerisation of carboxylic acids and low density of ice.

Change of ph to f

CIE has changed the following terms:
New term for sulphate is sulfate
thiosulphate is thiosulfate
sulphite is sulfite
sulphur is sulfur
disulphide to disulfide

Important AS equations for Inorganic Chemistry

Practical P31/32 (Part 2)

Maximum error

Maximum error is usually marked on the glassware.

Burette
• A pipette has a maximum error of 0.05 cm3 in each measurement.

Calculation of percentage error

Percentage error = (maximum error ÷ quantity measured ) x 100%

Reading burettes
A burette is graduated in divisions every 0.1 cm3.
Using the half-division rule, the estimation is 0.05 cm3.
Burette is recorded to two decimal places with the last figure either ‘0’ or ‘5’.
The maximum error in each measurement = 0.05 cm3.
The overall maximum error in any volume measured always comes from two measurements, so
the overall maximum error = 2 x 0.05 cm3 = 0.1 cm3.
In a titration, a burette will typically deliver about 25 cm3 so the percentage error is small.

Percentage error = (2 × 0.05 ÷25.00) x 100% = 0.4%

The percentage error becomes more significant when burette is used to deliver small volume
For delivery of 2.50 cm3,

Percentage error = (2 × 0.05 ÷ 2.50)× 100% = 4%

Practical 31/32 Learn More (Part 1)

1. Titres should be within 0.20 cm3.
2. Candidates were expected to produce a single table.
3. The table should include correct headings and units.
4. Burette readings and the volumes added recorded to the nearest 0.05 cm3.
5. Select appropriate scales when plotting a graph
6. In plotting graph, a line should be best-fit straight line or a smooth curve.
7. A graph should contain at least five ponts.
8. Answer for calculation should be given to at least 3 significant figures
9. Thermometer readings and temperature differences expressed to the nearest 0.5 °C,
10. All weighings and the masses of the solid used should be recorded consistently to 1 decimal place or 2 decimal place depending on the sensitivity of the electronic balance. The balance in our lab can measure up to 2 decimal points.
11. To calculate mcT equation, the m is the mass of solution not the mass of solid.
12. +ve sign or –ve sign should be written on the H value.
13. Common error in thermochemistry experiments:
· heat loss/gain with the surroundings
· acid spray from the vigorous reaction
· the use of a thermometer calibrated by 1 °C
14. Suggested improvement for thermochemistry experiments :
· Suitable additional insulation or use of a lid
· Use of a thermometer with closer graduation or use of a digital thermometer

Important AS Definitions Part II

Activation energy is the minimum energy required by colliding particles to start off a reaction.


A catalyst is a substance which changes the rate of a chemical reaction without itself undergoing any chemical change at the end of the reaction.

Homogenous catalyst is a catalyst in which the physical state is in the same phase as the reactants.

Heterogenous catalyst is a catalyst in which the physical state is in the different phase as the reactants.

Increase the concentration of reactants will increase the rate since there will be more molecules per unit volume resulting in an increase in the collision frequency between the reactants. This leads to higher number of effective collisions.

Increase in the temperature will increase the rate since at higher temperature the particles gain more kinetic energy resulting an increase in collision frequency at higher temperature. In addition, more molecules have kinetic energy greater than or equal to the activation energy. Consequently, the number of effective collisions increases.

A catalyst operates by providing an alternative route of lower activation energy for the reaction.

Geometric isomers are compounds with different spatial arrangement due to the restriction in the rotation about the double bond in an alkene. In geometric isomers, two different groups at each end of the double bond can either be on the same side (cis) or on opposite sides (trans)

Optical isomers are mirror images that have the same geometric and structural formula, but cannot be superimposed onto one another.

Cracking is a process to break long alkane into a shorter alkane and an alkene. Hydrogen may also be produced. Thermal cracking requires temperature between 400°C to 900°C and high pressure whereas catalytic cracking requires a zeolite catalyst and a temperature of about 450°C. The purpose of cracking is to form extra gasoline and to produce alkenes with higher commercial value

Homolytic fission is the breaking a covalent bond in which one electron goes to each atom to form free radicals.

Heterolytic fission is the breaking a covalent bond in which one atom takes both electrons forming negative and positive ions.

A free radical is a highly reactive atom or molecule with an unpaired electron.

An electrophile is an electron-deficient species that can form a new covalent bond by accepting an electron pair provided by the carbon compound. Example : positive ion such as hydrogen ion or nitronium ion and a polar molecule with a d+ charge

A nucleophile is an electron-rich species with a lone pair of electrons that can form a new covalent bond by donating a pair of electrons to the electron deficient carbon atom. Example : negative ions with at least a lone pair.

Organic AS Reactions (Spider Diagram)

Spider diagram is one of the methods used in mind mapping. Using spider diagram to study the organic reactions is an effective method to remember reactions. You can also see the links between the functional groups. You can magnify it by clicking the diagram.

My dear students, don't forget to wear red or pink this coming Friday 23/1/09 in conjunction with the Chinese New Year festival. Hopefully, it will bring us luck, success and happiness.

Important AS definitions (Part I)

Atomic number or proton number is the total number of protons in the nucleus of an atom.

Mass Number or nucleon number is the total number of protons and neutrons in the nucleus of an atom.

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

The relative isotopic mass is the mass of one isotope of an element compared to 1/12 the mass of the 12C isotope.

The relative atomic mass is the average mass of one atom compared to 1/12 the mass of the 12C isotope.

First Ionisation Energy is the energy required to remove one mole of electrons from 1 mole of gaseous atoms to produce 1 mole of gaseous unipositive ions.

First Electron Affinity is the energy liberated when one electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous ions with a uninegative charge.

Electronegativity is the power of an atom to attract bonding electrons.

Oxidation is loss of electrons. Remember OIL RIG
Reduction is gain of electrons.

Oxidising agent is a substance which removes electrons from another substance and itself is reduced.

Reducing agent is a substance which gives electrons to another substance and itself is oxidised.

Disproportionation is simultaneous oxidation and reduction of one species in a chemical reaction.

Metallic bond is the electrostatic attractions between the positive ions and a 'sea' of delocalised valence electrons surrouding the positive ions.

Ionic bond is the attraction between the oppositely charged ions formed by transfer of one or more electrons from one atom to another.

Covalent bond is the attraction of the nuclei for the shared pair of electrons between two atoms.

Sigma bond is the covalent bond formed by ‘head on’ overlap of atomic orbitals.

Pi Bond is the covalent bond formed by sideways overlap of atomic orbitals.

Valence Shell Electron Pair Repulsion (VSEPR) Theory states that the shape of a molecule is determined by the repulsion between the electron pairs in the valence shell. According to the VSEPR , the electron pairs will arrange themselves so as to be as far apart as possible.

Polarising power is the ability of a positive ion to attract electrons away from a negative ion.

Dipole-Dipole Forces are intermolecular attractions between molecules which are permanently polarised.

Hydrogen Bond is the electrostatic attraction between a hydrogen atom which is bonded to a very electronegative atom and an electronegative atom of a neighbouring molecule.

Van Der Waals Forces are intermolecular forces of attraction which arise from temporary
dipoles in molecules.

Standard Enthalpy Of Formation is the enthalpy change when 1 mole of the substances is formed from its elements at their natural states at 298K and 1 atm pressure.

Standard Enthalpy Of Combustion is the enthalpy change when 1 mole of an element or compound is completely burned in oxygen at 298K and 1 atm pressure.

Standard Enthalpy Of Neutralisation is the enthalpy change when 1 mole of water is formed from the reaction between an acid and a base at 298K and 1 atm pressure.

Hess’s Law states that the total energy change resulting from a chemical reaction is dependant only on the initial and final states of the reactants and is independent of the reaction route.

Bond energy is the energy required to break 1 mole of bonds from gaseous molecules into gaseous atoms.

Reference : Stephen Oakes