Chemistry MYE 2015

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CHEMICAL BONDING

Atoms of elements gain/lose electrons to achieve stable octet structure

Chemical bond is a very strong force between atoms and depends on valency of atom

Valency of an atom is a numerical charge value (e.g. Al has valency of 3 as each atom loses 3 e-)

There are 3 types of bonding IONIC COVALENT METALLIC

IONIC

It is the transfer of electrons from one atom to another.

occurs between metals and non-metals

Metals lose e- and forms positively charge ions

NOn-metals gain e- to form negatively charged ions

IONIC BOND : ELECTROSTATIC FORCE BETWEEN OPPOSITELY CHARGED IONs

Ionic compounds have giant ionic lattice structures

eg. NaCl (Na+ and Cl-) ; have giant ionic lattice structures ; held together by strong electrostatic forces

PHYSICAL PROPERTIES :

High melting and boiling points : the electrostatic forces between positively charged and negatively charged ions is very strong, more energy is needed to overcome it.

Conduct electricity in molten or aqueous state: there are free moving ions to conduct a current. In solid state, they are held together at fixed positions.

IOnic compounds are generally soluble in water but not inn inorganic solvents.

The bigger the charges, the stronger the electrostatic forces hence the more energy required to overcome it.

Eg. MgO (Mg2+ and O2-) has a stronger electrostatic forces compared to NaCl (Na+ and Cl-)

'Dot and Cross" diagrams are used to demonstrate the electronic transfer between metals and non-metals.

COVALENT

Sharing of electrons equally between atoms of non-metals

Single bond: H - H

Double bond : O = O

Triple bond : N (3) N

PHYSICAL PROPERTIES:

Low melting and boiling points: The attractive forces between molecules is very weak hence lesser energy is required to overcome it.

Does not conduct electricity at any state: They do not have ions or delocalised electrons to conduct a current.

The simple covalent molecules are held together by weak intermolecular forces (van Der Waal's forces).

Within each covalent molecule, covalent bonds(intra molecular forces) are present. Between each molecule weak intermolecular force is present.

Simple covalent molecules are generally soluble in inorganic solvents but not in water.

'Dot and Cross' Diagrams are used to demonstrate the sharing of electrons between non-metals.

NOTE: The attractive forces is overcomed, not the covalent bonds

GIANT MOLECULAR STRUCTURE

Formed by large number of atoms covalently bonded to one another.

Structure consists of A NETWORK OF STRONG COVALENT BONDS.

Therefore, a giant molecular substance is significantly different from a simple covalent molecule.

CARBON : non-metallic element which is able to produce a giant molecular structure.

EG. DIAMOND AND GRAPHITE

DIAMOND

Each carbon atom is covalently bonded to four other atoms.

Due to network of covalent bonds, large amount of heat energy required, extremely high melting point.

Cannot conduct electricity : does not have delocalised electrons or ions

Strong and Hard : hence can be used s cutting tool.

Does not dissolve easily in any solvent

Another giant molecular structure similar to diamond is Silicon Dioxide (SiO2) a.k.a. Sand

GRAPHITE

Made of layers of planes which consists of carbon atoms joining together in a form of hexagonal rings

Each carbon atom is covalently bonded to three other atoms

One electron from each atom is not bonded hence it is known as delocalised electron ; the delocalised electrons are able to move freely, , hence able to conduct a current.

Each layer is held together by weak intermolecular forces, hence able to slide over one another easily, thus they are soft and slippery.

Suitable for : Lubricants and pencil leads

IN A NUTSHELL:

simple covalent bonds have relatively low melting and boiling points and density while giant molecular structures have relatively high melting and boiling points and density.

METALLIC

Atoms of metallic elements have the tendency to lose electrons, hence they become positive

Metallic lattice consists of positive ions surrounded by a 'sea of electrons'

Metallic bonding is a strong electrostatic force between the 'sea of electrons' and the positive ions.

There are free electrons/ delocalised electrons to move freely hence able to conduct a current

Requires a large amount of energy to overcome hence they have a high melting and boiling point generally.

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THE MOLE CONCEPT AND STOICHEMTRY

Relative atomic mass (Ar)

Ar = Average mass of one atom / 1/12 x mass of carbon-12 atom

Ar= sum of (% abundance x isotopic mass)

% mass of element in a compound = Ar x no. of atoms / Mr of compound x 100%

mass of element in compound = Ar x no. of atoms/ Mr of compound x mass of given sample

% mass of water in compound = Mr of water x number of moles/ Mr of compound x 100%

EMPIRICAL FORMULA

1. write down elements

2. % given

3. Mr

4. % / Mr

5. Divide by smallest denominator

6. write down empirical formula

n = Mr / sum of Ar

Mr . Mol . Ma .

24 . mol . Vol of gas .

V . M . Mass/Mol .

dm3 x 1000 = cm3

LIMITING AND EXCES REAGENTS

Limiting reagent: The reactant which is used completely

Excess reagent : The reactant which is left unreacted after the reaction

1. Find mole of each reactant

2. The one with more is excess

3. To determine volume, use limiting reactant's ratio.

EG. Mg : H

mole ratio 1 : 1

0.2 : 0.1

hence formula :MgH2

PERCENTAGE PURITY / PERCENTAGE YIELD

Percentage purity = (actual mass used / reacted substance) / total mass of sample x 100%

Percentage yield = (actual yield / theoretical yield) x 100%

1. Find mole

2. Find actual mass/ theoretical yield

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