Chemistry KeyPoints

1. INTRODUCTION TO CHEMISTRY

Chemistry is an experimental science which can be studied through experiments. Scientists apply scientific methods like observation, hypothesis, experiments before a particular law can be established. Chemistry being a subject that is fundamental to all science, makes every substance indeed a chemical substance.

The products of some of the chemical processes are polythene bags from plastic industry, creams and perfumes from pharmaceutical industry etc. chemistry is used in so many industries like fertilizer, insecticides, cement, paints, etc.

2. NATURE OF MATTER

Two types of change in chemistry are physical and chemical changes. The physical change is easily reversible e.g. the dissolution of sodium chloride (common salt) in water. Chemical change is not reversible and a new substance is formed e.g. burning of magnesium ribbon in a Bunsen burner.

States of matter: matter exists in three states: solid, liquid and gas. Matter changes from one state to another. Water when heated or cooled undergoes the following changes:

[Diagram]

ELEMENTS, MIXTURES AND COMPOUNDS

Element: an element is a substance which cannot be split up into simpler substances by any chemical means. Examples are sodium, aluminium, copper, silver and gold.

Compound: a compound is a substance formed by two or more different elements which and chemically joined together e.g. calcium trioxocarbonate (iv) called marble (CaCO3), salt (sodium chloride NACI), and glucose (C6H12O6).

Mixture: a mixture is a substance formed when two or more substances are physically joined together. Examples are air, blood, sea water, brass, etc.

3. SEPARATION TECHNIQUES

The method to be used to separate a mixture involves only physical means and this depends on the physical properties of the components of that mixture. The following method can be used to separate mixtures.

Filtration: is a method of separating an insoluble solid from a liquid by the use of porous materials like filter paper or pig bladder. Example is the filtration of water containing sand.

Evaporation: is a method of obtaining a soluble solid from solution by heating. Example is common salt and water.

[diagram]

Distillation: solvents are obtained from solution by heating in order that the solvent should vaporize and condense later. Example is two miscible liquids e.g. ethanol and water with different boiling points. Ordinary water can be distilled to make it pure.

Fractional Distillation: this involves the separation of two or more miscible liquids with close boiling points e.g. fractional distillation of crude oil into paraffin petrol, diesel, cooking gas, etc.

 Separating Funnel: immiscble liquids such as water and oil can be separated by using separating funnel as illustrated below. The liquids must have different densities.

[diagram]

Sublimation: solid substances change straight to gaseous state when heated. Examples are iodine and ammonium chloride.

[diagram]

Magnetization: is applied to separate magnetic substances e.g. iron filings from a non-magnetic substances e.g sulphur powder in a mixture.

Decantation: is used to separate a mixture of liquid and a denser substance e.g. a mixture of sand and water.

[diagram]

Chromatography: is a method used to separate mixtures by taking advantage of their different rate of movement in a solvent over an adsorbent material  e.g. paper. It is used to separate the dyes in black ink.

[diagram]

Precipitation: this is carried out by mixing two different solutions which react to produce one soluble compound and insoluble substance called precipitate e.g. copper (ii) tetraoxosulphate (vi) and an insoluble copper (ii) hydroxide which is the precipitate.

CuSO4+2NaOHàCu(OH)2+Na2SO4

Centrifugation: the spinning of materials in a centrifuge, tends to make the heavier solids to settle first, leaving the lighter ones on top. Example is found in blood where red and white blood cells settle down leaving plasma as supernatant.

Test for purity: the purity of a substance is determined by its melting and boiling points. The temperature is affected if the substance contains impurities. Capillary tube can be used to determine the melting point of a solid.

[diagram]

4. FORMULAE AND VALENCY

Valency: is the combining power of any element. It serves as the key point or the most important factor required for writing the correct formula of a compound e.g. calcium chloride.

[diagram]

Radicals: a radical is a group of atoms which behave as one unit. Radicals are divide into two parts namely cations and anions. The cations are the positive or metallic radicals e.g. Ca2+, Na+, while anions are negative or non-metallic radicals e.g. CINO3, SO42-.

5. PARTICULATE NATURE OF MATTER

CONCEPT OF ATOMS AND MOLECULES

Atom: An atom is the smallest amount of an element which can take part in chemical reaction. It is made up of tiny particles called protons, neutrons and electrons.

Molecule: A molecule is the smallest particle of a substance, either are element or compound which can exist in a free state.

Ions: An ion is any atom or group of atoms which possesses an electric charge. There are two types, the positively charged ions known as cations and the negatively charged ions known as anions.

ATOMIC STRUCTURE

An atom contains three types of particles, protons, neutrons and electrons. It consists of centrally placed nucleus, which is made of protons and neutrons. The nucleus is surrounded by a cloud of electrons.

[DIAGRAM]

Atomic Mass: The mass number A, is the sum of protons and neutrons in the nucleus of the element, e.g. sodium has mass number 23; protons 11 and neutrons 12.

Atomic Number: Atomic number Z is the number of protons in an atom of the element.

ORBITALS

Each shell or orbit that surrounds the nucleus is subdivided into orbitals which are called s, p, d, f orbitals.

Each orbital has a maximum number of electrons it can hold.

S = 2 electrons

P = 6 electrons

D = 10 electrons

R = 14 electrons

Orbital types in a shell

[DIAGRAM]

The sequence of filling up the orbitals with electrons is as follows:

1s 2s 3s 3p 4s 3d

4s is filled before 3d because 4s energy level is lower than 3d energy level.

e.g. calcium = 1s2, 2s2, 2p6, 3s2, 3p6, 4s2

ISOTOPY

Isotopy is a phenomenon whereby atoms of an element exhibit different mass numbers but the same atomic number e.g. chlorine atom has

[DIAGRAM]

RELATIVE MOLECULAR MASS

The relative molecular mass of a compound is the mass of the correctly written formula or one molecule of that compound, obtained by adding together the appropriate relative atomic masses of all the atoms of the element present in the molecule.

The molecular mass and relative molecular mass are the same except that relative molecular mass has no unit

Relative molecular mass of sodium trioxocarbonate (iv) Na2CO3 is

2 x Ar of Na + 1 x Ar of C + 3 x Ar of O

(2 x 23) + 1 x 12 + 3 x 16

     46     +    12    +    48

        Rmm = 106

EMPIRICAL FORMULA AND MOLECULAR FORMULA

Empirical formula of a compound is the simplest formula of that compound, which shows the simplest ratio of the number of atoms present in it.

Molecular formula: The molecular formula of a compound shows the actual number of atoms of the different elements in one molecule of it.

Compound    Empirical Formula       Molecular Formula

Ethyne            CH                                   (CH)n = C2H2

Benzene         CH                                   (CH)n = C6H6

6. CHEMICAL LAWS
Law of conservation of mass: The law of conservation of mass states that matter can neither be created nor destroyed in the course of chemical reaction.

Law of multiple proportion: The law of multiple proportion states that if two elements A and B combine together to form more than one compound then the several masses of A which separately combine with a fixed mass of B are in a simple ratio.

Law of definite proportion: The law of definite proportion states that all samples of the same chemical compound contain the same elements combined in the same proportion by mass.

7. CHEMICAL COMBINATION

Two main types of chemical combinations are:

(i) Electrovalent Combination

(ii) Covalent Combination

Electrovalent Combination: This involves a transfer of electrons from one atom to another giving rise to electrovalent bond between ions.

[diagram]

Covalent Combination: This involves the sharing of electrons between atoms such that each atom in the molecule has the electron arrangement of a noble gas. Consider two chlorine atoms each having electronic structure 2, 8, 7.

[diagram]

CHARACTERISTIC PROPERTIES OF ELECTROVALENT BOND

(i) Compounds form ions in water

(ii) ions conduct electricity

(iii) high melting and boiling points

(iv) occurs in inorganic compounds                              

COVALENT BOND

(i) Compounds consist of molecules

(ii) do not conduct electricity

(iii) occurs mainly in organic compounds

(iv) low melting and boiling points.

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