O level Chemistry Notes

O level Chemistry Notes (Page 1)

O level Chemistry Notes (Page 1)

O level Chemistry Notes covering Matter, Atomic Structure, Elements, Mixtures and Compounds. These notes only show the key points in O’level chemistry curriculum.

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States of matter

SOLIDS:

  • particles do not move but only vibrate about fixed position
  • strong forces of attraction hold the particles in fixed positions
  • have definite shape and volume
  • solids are incompressible (i.e. cannot be squashed up into a smaller volume)
  • the particles of a solid have very low kinetic energy.

LIQUIDS:

  • particles can only move by sliding over one another
  • liquids have a definite volume
  • liquids take the shape of a container
  • liquids are incompressible
  • the particles of a liquid have moderate kinetic energy.

GASES:

  • particles widely separated
  • little or no attraction between the particles
  • particles move freely and randomly in all directions
  • gases fill whatever container they are placed in
  • diffuse into all the space that is available
  • are compressible (i.e. can be squashed up into a smaller volume)
  • the particles of a gas have very high kinetic energy.

Change of State

When substances change state, energy is either absorbed or given out.

Melting

Solid to Liquid. Heat is taken in because energy is needed to break the bonds between the particles in the solid.

Vaporisation

Liquid to Gas. Heat is taken in because energy needed to break the forces between the particles in the liquid.

Solidifying

Liquid to Solid. Heat is given out because energy is given out as particles bond together.

Condensation

Gas to Liquid. Heat given out because energy is given out as particles bond together

Sublimation

Gas to Solid directcly. Heat is given out because energy is given out as particles bond together.

Solid to Gas directly. Heat is taken in because energy needed to break the forces between the particles in the solid.

Kinetic Theory

The properties of solids, liquids and gases can be explained by kinetic theory.

Kinetic theory states that matter is made of tiny particles that move all the time.

The main points of the theory are;

• All matter is made of tiny, invisible, moving particles.

• The particles move all the time. The higher the temperature the faster they move.

• Heavier particles move more slowly than light ones at a given temperature.

Diffusion

Diffusion is the mixing of atoms or molecules due to their continuous and random motion. e.g. Mixing of bromine vapour and air.

Experiment to show that Gases diffuse at different rates.

Diffusion

One piece of cotton wool soaked in concentrated ammonia
solution and another soaked in concentrated hydrochloric acid are put at opposite ends of a dry glass tube. After a few minutes
a white cloud of ammonium chloride appears which shows the position at which the two gases meet and react.

The white cloud forms in the position close to the cotton wool soaked in hydrochloric acid as shown above. That is because the ammonia particles are lighter and have a smaller relative molecular mass than the hydrogen chloride particles (released from the hydrochloric acid) and so the ammonia particles move faster.

Brownian Movement

Brownian movement is the constant random movement of tiny particles (e.g. smoke particles, or pollen on a drop of water) caused by collision with (invisible) air or water molecules, which are themselves in continuous and random motion.

Heating Curve

Heating curve

  • At O – In a solid the particles are held in fixed positions by strong forces of attraction.
  • From O to A – Heat is applied and the particles start to faster until at A they start to break the bonds holding them together. The temperature increases propotionally.
  • From A to B – The applied heat is used to break the bonds holding the particles together so they is no net increase in temperature until the solid has fully melted at B.
  • From B to C – The substance is now in liquid form and the temperature increases propotionally to the applied heat energy.
  • From C to D – The applied heat is used to break the bonds holding the particles together so they is no net increase in temperature until the liquid has fully vaporised.

The main points to note from the heating curve are:

  • at every change of state there is no temperature rise.
  • the energy absorbed during change of state is called latent heat energy (hidden energy). Latent heat of vaporisation during vaporisation and latent heat of fusion during melting.

Cooling Curve

Cooling curve

  • At O – The substance is in gaseous state.
  • From O to A – Heat is lost to the surroundings and the particles start to slow down and bond together. There is no loss in temperature because the heat lost to the surroundings is balanced to the heat released by bond formation. That is until the gas is fully condensed at A.
  • From A to B – The substance is now in liquid form and the temperature decreases propotionally to the heat energy being lost to the surroundings.
  • From B to C – There is no net decrease in temperature because the heat lost to the surroundings is balanced to the heat released by bond formation as the liquid turns to solid.
  • After C – the substance is now a solid and the temperature is dropping as the heat is lost to the surroundings.

The main points to note from the cooling curve are:

  • at every change of state there is no temperature rise.
  • the energy released during change of state is called latent heat energy (hidden energy). Latent heat of vaporisation during vaporisation and latent heat of fusion during melting.

Atoms

The atomic theory assumes that all elements are made up of "atoms".

An atom is the smallest particle of an element that can exist or take part in a chemical change.

Molecules

A molecule is a simplest group of atoms joined together by a chemical bond.

All elements are made up of atoms. Some gaseous elements (e.g. noble gases) are composed of single atoms moving around freely. But in other gaseous elements (e.g halogens) single atoms cannot exist on their own at ordinary temperatures. In these elements the free-moving particles consist of pairs of atoms. The two atoms forming a pair ( a molecule ) are joined together by a chemical bond.

This is the case with hydrogen (H2), oxygen (O2 and nitrogen (N2). Such substances are said to be diatomic.

Elements, Mixtures and Compounds

  • An element is a pure substance made up of only one type of atoms.
  • A compound is a substance which contains two or more elements, chemically bonded together in a fixed proportion.
  • A mixture is a group of substances that are physically bonded together.
Compounds Mixtures
Proportions of elements are fixed.Proportions may vary.
Properties different from those of the mixed elements.Properties are simply those of mixed the elements.
Cannot be separated into elements by physical means.Can be separated by physical means.
There is usually an energy change when a compound is made.Usually no energy change when the elements are mixed to form a mixture.

Separation Techniques

A mixture can be seperated by the following physical means.

Solid and Liquid

  • If the solid has not dissolved in the liquid (i.e. is suspended in the liquid), the two substances can be separated by filtration.nThe liquid filtrate passes through the filter paper and the undissolved solid residue stays on the paper.
  • If the solid has dissolved in the liquid, forming a solution and if only the solid is required, it is obtained by evaporation else if the liquid is required, it is obtained by distillation.

During distillation, the solution to be separted is placed in a flask and heated. The liquid evasporates, and its vapour passes into a condenser, where it cools and turns back to liquid. The solid remains behind in the flask as a residue.

Distillation

  • X is the solution to be separated.
  • Y is the level where the thermometer bulb should be, if you decide to add a thermometer.
  • Z is the condenser.

Liquid and Liquid

  • If the liquids are immiscible seperate by using a seperating funnel. When the two liquids do not mix (e.g. paraffin and water), they can be separated simply by running the denser liquid from a separating funnel by opening the tap.
  • If the liquids are miscible they can be separated by fractional distillation. Both liquids are evaporated, and their vapours pass into a fractionating column, where they are condensed and re-boiled many times. The vapour of the liquid which has the lower boiling point. emerges from the top of the column first, and passes into the condenser. When all of this liquid has distilled, it is followed by the liquid having the higher boiling point.

Fractional Distillation

Solid and Solid

  • By dissolving. Use a solvent that will dissolve one solid but not the other. Then filter off the residue on the filter-paper and dry it. To obtain the second solid (now in solution, in the filtrate), evaporate the filtrate and crystallise the solid produced.
  • By chromatography. This is a method for separating two solids that are both soluble in the same solvent. Make a solution of the mixture and place a drop of the solution on a piece of filter-paper. Allow a suitable liquid to spread gradually across the paper. The solid that is more soluble in the liquid will move through the greatest distance, and the solid that is least soluble will move least. Thus the dissolved solids are separated on the paper.

Atomic Structure

Atoms are built up from three basic subatomic particles.

ParticleRelative MassRelative charge
Proton1+1
Neutron10
Electron1/1860–1

Each atom consists of a very small, very dense nucleus, which contains nucleons (protons and neutrons), surrounded by orbiting electrons. Atoms are represented as. All atoms of a particular element have the same no. of protons. e.g. all atoms with 11 protons are Sodium atoms.

  • The number of protons in an atom is called its proton number or atomic number.
  • In any atom, there is no overall electrical charge so because the number of electrons = the number of protons in the nucleus.
  • The total number of protons and neutrons in an atom is called its mass number or nucleon number or atomic mass.
  • Number of neutrons = Mass number – Atomic number.
  • Isotopes are atoms of the same element, with the same number of protons and electrons, but different numbers of neutrons in the nucleus.

For example, natural chlorine consists of two types of atom:

  • Chlorine-35 containing 17 protons and 18 neutrons.
  • Chlorine-37 containing 17 protons and 20 neutrons.

Relative Atomic Mass (Ar) of an element

The Relative Atomic Mass (Ar) of an element is the weighted average of the masses of all of the isotopes of that element relative to the mass of pure Carbon-12.

For example:

Natural chlorine has two isotopes, chlorine-35 which has a relative abundance of 75%, and chlorine-37 has a relative abundance of 25%. The Ar of chlorine is therefore the weighted mean of the isotopic masses:

Ar = (75/100 × 35) + (25/100 × 37) = 35.5

Electronic Configuration

Electrons are arranged in shells around the nucleus. The first shell can hold up to two electrons, the second up to eight, and the third up to eight.

Thus an atom of Li (with three electrons) will have two in the first shell, and one in the second shell.

We write this arrangement 2.1

An atom of Mg (12 electrons) is 2.8.2.

The chemical properties of elements depend on the number of electrons in the outer shell. On a periodic table, elements are arranged vertically in columns called groups based on the number of electrons in their outer shells. For example, Group 1 contains Li (2.1), Na (2.8.1), K (2.8.8.1), which all have one electron in their outer shells and have the same chemical properties.

By the same token, elements in Group 3 all have three electrons in their outer shells and elements in Group 7 always have seven electrons in their outer shells.

The elements on the right of the periodic table, Group 0 or Group 8 are inert (unreactive) because they have full outer shells, normally with eight electrons in them (Ne is 2.8, Ar is 2.8.8 etc), expert for He which has one shell that is full with 2 electrons.

Atoms with 1, 2 or 3 electrons in their outer shells are metals (apart from hydrogen, helium and boron). This means that elements in groups 1, 2 and 3 (except Boron) are all metals.

Atoms with 4, 5, 6 or 7 electrons in their outer shells are non-metals (i.e. groups 4, 5, 6 and 7).

Note: This is not always true for the lower members after element 20.

Atoms with full outer shells are noble gases. Although these are also non-metals, they fall into a special category because they are unreactive.