Laws of Chemical Change

• By Antoine Lavoisier- the Father of Modern Chemistry• It states that matter is neither created nor

destroyed during an ordinary chemical change.• The total quantity of matter and energy

available in the universe is a fixed amount and never more or less. • In solving: Proper measurement

is significant. All components of a reaction must be accounted for.

Laws of conservation of mass

examples

If 12 grams of Carbon is burned completely and combines with 32 grams of Oxygen from air, the product is 44 grams of Carbon dioxide.

Law of defi nite composition

• It states that elements combine to form a compound in ratios of simple whole numbers.

• A compound always contains two or more elements combined in a definite proportion by mass.

• Also known as Law of Definite Proportion• Stated by French Chemist, Joseph Proust 

FORMULA

examplesWashing soda,  Na2CO3,is used in the manufacture of glass, pulp, and

paper. What is its percentage composition?

Given:  Na2CO3

Find: %Na, % C, % OSolution:Step 1. Calculate the mass of eachelement and the total mass of the compound.

Mass Na = 2 x 22.99g = 45.98gMass C = 1 x 12.01g = 12.01gMass O = 3 x 16.00g = 48.00g

Total mass = 105.99g

Step 2. Calculate the percentage composition by mass .

Step 3.Check your answer by adding the three percentages. (the answer must be 100 or approximately 100).

Law of multiple proportions

•The law of multiple proportions states that if two elements form more than one compound between them, the masses of one element combined with a fixed mass of the second element form in ratios of small integers. •Stated by John Dalton

examples

Using methane and ethylene, both compounds of carbon and hydrogen, illustrate the law.

• 12g of C combines with 4g of H in methane CH4 ,

a major component of natural gas.• 24g of C combines with 4g of H in ethylene C2H4,

a hydrocarbon that plays an important role in the ripening of fruits.

• Taking the ratio of the masses of C in both compound that combine with 4g of H:

• This small whole number ratio exist because there also exist the same ratio between the number of atoms of C which combineswith the same number of atoms of H.

DALTON’S ATOMIC THEORY

DALTON'S ATOMIC THEORY MAKES THE FOLLOWING ASSUMPTIONS:

• All matter consists of tiny particles. The existence of atoms was first suggested more that 2000 years before Dalton's birth. Atoms remained pure speculation through most of this time, although Newton used arguments based on atoms to explain the gas laws in 1687. (Newton's speculations about atoms in the Principia were carefully copied by hand into Dalton's notebooks.)

• Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, broken into smaller parts or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass and on centuries of experimental evidence.

With the discovery of subatomic particles after Dalton's time, it became apparent that atoms could be broken into smaller parts. The discovery of nuclear processes showed that it was even possible to transform atoms from one element into atoms of another. But we don't consider processes that affect the nucleus to be chemical processes. The postulate is still useful in explaining the law of conservation of mass in chemistry. A slightly more restrictive wording is "Atoms cannot be created, destroyed, or transformed into other atoms in a chemical change"

• Elements are characterized by the mass of their atoms. All atoms of the same element have identical weights, Dalton asserted. Atoms of different elements have different weights. (Dalton used the word "weight" rather than mass, and chemists have called atomic masses "atomic weights" ever since).

We now know that atoms of the same element sometimes have slightly different masses, but always have identical nuclear charge. In modern atomic theory, the postulate has been amended to read: "Elements are characterized by the nuclear charge of their atoms".

•When elements react, their atoms combine in simple, whole-number ratios. This postulate suggested a practical strategy for determining relative atomic weights from elemental percentages in compounds. Experimental atomic weights could then be used to explain the fixed mass percentages of elements in all compounds of those elements!By suggesting that compounds contained

characteristic atom-to-atom ratios, Dalton effectively explained the law of definite proportions

•When elements react, their atoms sometimes combine in more than one simple, whole-number ratio. Dalton used this postulate to explain why the weight ratios of nitrogen to oxygen in various nitrogen oxides were themselves simple multiples of each other. Even Dalton's critics were impressed by the power and simplicity of his explanation, and it persuaded many of them that his atomic theory was worthy of further investigation.

MOLECULAR THEORY

The experimental observations about the behavior of gases discussed so far can be explained with a simple theoretical model known as the kinetic molecular theory. This theory is based on the following postulates, or assumptions:

•Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion.

•These particles move in a straight line until they collide with another particle or the walls of the container.

•These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space.

•There is no force of attraction between gas particles or between the particles and the walls of the container.

•Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container.

•The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else.

CHEMICAL BONDS

Chemical Bonding is the way in which atoms join together with each other.

Sodium metal Chlorine Table salt

Chemical Bonding

During a chemical reaction, atoms of the elements joined together to form ionic compound or covalent compound.

ionic compounds covalent compounds

Two types of chemical bonding

Ionic bonding - between metals and non- metals

Covalent bonding - between non-metals

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What is ionic bonding?

Ionic bonding involves transferring of electrons from metal to non-metal.

Ionic bonding

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FORMATION OF A SODIUM ION

To attain an octet configuration, a sodium atom (Na) loses 1 valence electron.

It forms a sodium ion (Na+) and has a noble gas structure.

sodium atom, Na

Lose 1 electron

sodium ion, Na+

+

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11 p11 e12 n

sodium atom, Na

Lose one electron

sodium ion, Na+

+

11 p10 e12 n

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During bonding Non-metallic atoms take in (accept) electrons and change into negative ions

or anions.

Cl - Cl + e-

Non-metallic atomAnion

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FORMATION OF CHLORINE ION

A chlorine atom (Cl) gains an electron to form a chloride ion (Cl-).

The chloride ion has an octet configuration.

gains one

electron

Chlorine ion, Cl-

-

Chlorine atom, Cl

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gains one

electron

Chlorine ion, Cl-

-

Chlorine atom, Cl

17 p17 e18 n

17 p18 e18 n

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By taking in and giving out electrons, both the metallic and non-metallic atoms achieve a completely filled outermost shell.Þ Indicates they attain a stable noble gas structure.

-+

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The electrostatic force of attraction between the positive and negative ions is called an ionic bond.

Ionic bond

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HOW DO WE SHOW IONIC BONDING?

We show it through the ‘dot and cross’ diagram.

The diagram here shows the formation of an ionic bond in sodium chloride.

ClNa

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EXAMPLES

Magnesium oxideCalcium fluorideLithium oxideSodium chlorideFerrous oxide

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STRUCTURE OF IONIC COMPOUNDS

• All ionic compounds are solids with giant lattice structure.

• They consist of positive ions of metals and negative ions of non-metals.

E.g. sodium chloride

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In a crystal of sodium chloride, Na+ and Cl- are held in fixed position by strong electrostatic force of attractions (ionic bonds)

EXAMPLE: SODIUM CHLORIDE

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1. High melting and boiling point

Ions are held firmly to their position by strong electrostatic force of attraction.

A great amount of energy is needed to break these forces for the ions to move out of its fixed position to become a liquid.

Characteristics of ionic compounds

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2. Solubility

Soluble in water

Insoluble in organic solvent (e.g. oil, ethanol, petrol)

Ethanol

(Insoluble)

Salt

Water(soluble)

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3. Conducts electricity when molten (melted) or aqueous (dissolved in water) but not in solid state.

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ELECTRICAL CONDUCTIVITY – IN AQUEOUS SODIUM CHLORIDE

Bulb lights up.

In molten or aqueous, ions are free to move about, thus carry charges to conduct electricity.

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What is covalent bonding?

Covalent bonding involves sharing of electrons between non-metallic atoms.

HH

Sharing of electrons

Transferring of electrons

Covalent bonding

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During bonding:

The non-metallic atoms share electrons to achieve noble gas configuration.

H2 molecule

H H

2H atoms

Rule 1: Each atom must contribute an equal number of electrons for sharing.

Y Y

Rule 2:

A shared pair of electrons forms a single covalent bond.

1 pair of electrons a single bond is formed.

2 pairs of electrons a double bond is formed.

3 pairs of electrons a triple bond is formed.

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Y Y

Structural formula

‘Dot and cross’ diagram

(Electronic Structure)

Y Y

A shared pair of electrons forms a single covalent bond.

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Y Y

Structural formula

‘Dot and cross’ diagram

(Electronic Structure)

Y Y

Two shared pair of electrons forms a double covalent bond.

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How do we show covalent bonding?

Through ‘Dot and Cross’ Diagram or Structural formula

Structural formula

‘Dot and cross’ diagram

(Electronic Structure)

Y YY Y

Examples:

o Hydrogen molecule

o Oxygen molecule

o Nitrogen molecule

o Methane (CH4)

o Carbon dioxide

o Water

o Ammonia (NH3)

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1. Low melting and boiling point

( high volatility)

Molecules are held together by very weak intermolecular forces.

Little energy is required to overcome the forces of attraction.

Characteristics of Covalent Compounds

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2. Solubility

Soluble in organic solvent

Insoluble in water.

3. Does not conduct electricity in any state.

• Molecules in covalent compounds do not carry charges.

• No ions to carry electric current.

Other types of chemical bonds

Hydrogen Bonds

Metallic Bonds

• A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule.

• Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge.

Hydrogen Bonds

EXAMPLE OF HYDROGEN BOND

Each hydrogen atom is covalently bonded to the oxygen via a

shared pair of electrons. Oxygen also has two unshared pairs of

electrons. Thus there are 4 pairs of electrons surrounding the

oxygen atom, two pairs involved in covalent bonds with hydrogen,

and two unshared pairs on the opposite side of the oxygen atom.

Oxygen is an "electronegative“ atom compared with hydrogen.

Metallic Bonds

• Metallic bonding is the type of bonding found in metallic elements. This is the electrostatic force of attraction between positively charged ions and delocalized outer electrons.

• Metallic bonding refers to the interaction between the delocalized electrons and the metal nuclei.

EXAMPLE OF METALLIC BOND

As the metal cations and the electrons are oppositely charged,

they will be attracted to each other, and also to other metal

cations. These electrostatic forces are called metallic bonds, and

these are what hold the particles together in metals.