DEFINITION OF CHEMICAL COMBINATION

Chemical combination refers to the process by which two or more elements or compounds react to form new substances through the formation of chemical bonds.

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• LAWS OF CHEMICAL COMBINATION

There are four laws of chemical combination which describes the general features of a chemical change. They are:

• LAW OF CONSERVATION OF MATTER – Also known as Lavoisier’s Law. It states that matter can neither be created nor destroyed during a chemical reaction but changes from one form to another.
• LAW OF DEFINITE PROPORTION – Also known as Law of Constant Composition (proposed by Proust). It states that all pure samples of a particular chemical compound contain similar elements combined in the same proportion by mass.
• LAW OF MULTIPLE PROPORTION – It states that if two elements, A and B, combine to form more than one chemical compound, then the various masses of one element, A, which combines with the fixed mass of the other element, B, are in simple multiple ratio. Examples are:

S/N	ELEMENT A	ELEMENT B	COMPOUNDS FORMED
1	Copper	Oxygen	CuO and Cu2O
2	Iron	Oxygen	Fe2O3 and FeO
3	Iron	Chlorine	FeCl3 and FeCl2

• LAW OF RECIPROCAL PROPORTION – It states that the masses of several elements, A, B, C, which combine separately with a fixed mass of another element, D, are the same as, or simple multiples of, the masses in which A, B, C, themselves combine with one another.

• PARTICULATE NATURE OF MATTER

ATOMS – It is the smallest particle of an element which can take part in a chemical reaction.

MOLECULES – It is the smallest particle of a substance that can normally exist alone and still retain the chemical properties of that substance, be it an element or a compound.

IONS – An ion is any atom or group of atoms which possesses an electric charge.

• CHEMICAL SYMBOLS

Chemical symbols are used to represent elements. In 1814, Berzelius suggested a system of representing elements with symbols derived from:

• Latin names e.g Sodium, Na (Natrium); Iron, Fe (Ferrum); Copper, Cu (Cuprum); Gold, Au (Aurum) etc.
• First letter of the name of the element e.g Hydrogen (H), Oxygen (O), Sulphur (S), Fluorine (F), Carbon (C) etc.
• Initial letter and another letter from the name of the element e.g Ca for Calcium, Cl for Chlorine, Mg for Magnesium, Ne for Neon etc.

• CHEMICAL FORMULA

This is the representation of a compound or molecules of a substance by the symbols of its component elements.

Chemical formulae are majorly categorized as:

• Empirical formula – The simplest formula (shows the component elements and mole ratio).
• Molecular formula – Gives the exact number of moles of atoms of the component element in one mole of the compound.
• Structural formula – Describes the structure of a compound or molecule(s) of a substance.

Examples are O₂ for oxygen gas, H₂O for water, O₃ for ozone, HCl for hydrochloric acid, CaCO₃ for calcium trioxocarbonate (IV) etc.

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• CHEMICAL EQUATION

A chemical equation is the symbolic representation of a chemical reaction, in which the reactants and the products are expressed in terms of their respective symbols and chemical formulae. The reactants are written on the left-hand side (LHS), products on the right-hand side (RHS) and an arrow leads from the reactants to the products. Example:

S/N	INFORMATIONS PROVIDED BY A CHEMICAL EQUATION	INFORMATIONS NOT PROVIDED BY A CHEMICAL EQUATION
1	Reactants and products involved.	Speed of the reaction.
2	Individual elements and radicals involved.	Heat changes during reaction, (except when indicated).
3	Mental picture of the reaction.	Colours of substances involved.
4	Stoichiometry of the reaction.
5	Direction of the reaction.
6	State of matter of substances present.

NB: All chemical equations must be balanced to comply with the Law of Conservation of Matter.

RELATIVE ATOMIC MASS – It is the number of times the average mass of an element is heavier than one-twelfth the mass of one atom of carbon-12.

MOLE CONCEPT – A mole of a substance is defined as the amount containing as many elementary entities as the number of atoms in exactly 12 grams of carbon-12.

1 mole = 12 grams of carbon-12 = 6.02 x 10²³ atoms

Avogadro’s number = 6.02 x 10²³

Number of moles =

Number of moles =

STOICHIOMETRY OF REACTION

This is the quantitative relationships of the amounts of reactants to one another and to the products in a chemical reaction.

The mole ratio is the ratio in which reactants combine to form products in a chemical reaction, giving the stoichiometry of the reaction.

EXAMPLE 1 – Calculate the mass of solid product obtained when 16.8g of NaHCO₃ was heated strongly until there was no further change.

SOLUTION – The equation for the reaction is:

                                                            2NaHCO_3(s) → Na₂CO_3(s) + H₂­o_(g) + CO_2(g)

Molar mass of NaHCO₃ = 23 + 12 + (16 × 3)

Molar mass of Na₂CO₃ = (23 × 2) + 12 + (16 × 3)

From the equation:

            2 moles of NaHCO₃ produces 1 mole of Na₂CO₃

            2 × 84g of NaHCO₃ produces 106g of Na₂CO₃

Therefore:

            16.8g of NaHCO₃ will produce Xg of Na₂CO₃

            Xg of NaCO₃ = (106g × 16.8g) / (2 × 84g)

                                    = 1780.8g / 168g

                                    = 10.6g

Mass of solid product obtained = 10.6g

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EXAMPLE 2 – 20cm³ of CO was mixed and sparked with 200cm³ of air containing 21% of O₂. If all the volumes are measured at s.t.p., calculate the total volume of resulting gases.

SOLUTION – In 200cm³ of air,

Volume of O₂ = 21% of 200cm³

                        = (21 / 100) x 200cm³

                        = 42cm³

            Volume of N₂ and rare gases = 200cm³ – 42cm³

                                                            = 158cm³

The equation for the reaction is:

                                    2CO_(g) + O_2(g) → 2CO_2(g)

            Volume ratio               2          :           1          :           2

            Before sparking           20cm³  :           42cm³

            Reacting volume         20cm³  :           10cm³

            After sparking             32cm³  :           20cm³

Volume of resulting gases = 32cm³ of CO + 20cm³ + 158cm³ of N₂ and rare gases

                                                = 210cm³