Wednesday 8 April 2009

iGCSE Chemistry - Principles of Chemistry

Atoms

 

  • Solids have fixed volume and shape.
  • Liquids have fixed volume but no definite shape.
  • Gases have no fixed volume or shape.
  • All matter is made from elements.
  • Elements cannot be broken down into simpler substances.
  • The number of atoms in a mole is called Avogadro’s constant.
  • Moles of atoms = mass/RAM
  • Avogadro’s number is 6 x 1023.

 

Atomic Structure

 

  • The smallest amount of an element that behaves like that element is an atom.
  • Atoms are made up of 3 subatomic particles: electrons, protons, and neutrons.

Subatomic particle

Relative mass

Relative Charge

Proton

1

+1

Neutron

1

0

Electron

1/2000

-1

  • Protons and neutrons are found in the centre of the atom in the nucleus.
  • Hydrogen is the only element that has no neutrons.
  • Atoms of the same element with the same number of protons and electrons but different numbers of neutrons are called isotopes.
  • Isotopes have the same chemical properties but different physical properties.
  • The relative atomic mass is the average mass of all isotopes.
  • Electrons are arranged in shells around the nucleus.

Electron Shell

Maximum number of electrons

1

2

2

8

3

18

  • The arrangement of electrons in an atom is called electronic configuration.
  • Number of electrons in outer shell is called periodicity.
  • Noble gases have filled outer shells and thus they are stable and unreactive.

 

Relative formula masses and molar volumes

 

  • The relative formula mass of a molecule can be calculated by adding up the relative atomic masses of the atoms in the molecule.
  • The relative atomic mass of an element tells you the mass of a mole of atoms of that element.
  • The relative formula mass tells you the mass of a mole of that substance.
  • The volume of one mole of gas contains Avogadro’s constant number of molecules of that gas.
  • One mole of any gas occupies the same volume under the same conditions of pressure.
  • One mole of any gas occupies 24,000cm3 (24dm3) at room temperature and pressure (rtp).
  • Moles = volume in cm3/24,000.

 

Chemical Formulae and Chemical Equations

 

  • To find the formula of magnesium oxide:
    1. Measure mass of magnesium.
    2. Heat magnesium so that it oxidises.
    3. Measure mass of magnesium oxide formed.
    4. Find difference in mass to ascertain how much oxygen has bonded with the magnesium.
    5. Convert mass change into moles.
    6. Convert original magnesium into moles.
    7. The moles come out to be equal, so formula is MgO.
  • To find the formula of water:
    1. Electrolyse water.
    2. Hydrogen is produced at the cathode and oxygen is produced at the anode.
    3. Observe that twice as much gas is produced at the cathode than at the anode.
    4. Thus, formula is H2O.
  • The formula of a compound can be calculated if the number of moles of the combining elements is known.
  • Empirical formulae show the simplest ratio of atoms present.
  • Molecular formulae show the actual number of atoms of each element in one molecule.
  • Percentage yield = actual yield/predicted yield x 100
  • Percentage purity = amount of pure substance/total amount of substance x 100
  • In chemical equations starting chemicals are reactants and finishing chemicals are products.
  • Diatomic elements, i.e. exist as molecules containing two atoms:

 

Element

Form in which it exists

Hydrogen

H2

Oxygen

O2

Nitrogen

N2

Chlorine

Cl2

Bromine

Br2

Iodine

I2

 

  • Balancing numbers can only be put in front of formulae.
  • States and their symbols:

 

State

Symbol

Solid

s

Liquid

l

Gas

g

Solution

aq

 

  • Ions and their charges:

 

Positive Ions (cations)

Negative ions (anions)

Ammonium

NH4+

Bromide

Br-

Hydrogen

H+

Chloride

Cl-

Copper

Cu+

Iodide

I-

Potassium

K+

Hydroxide

OH-

Sodium

Na+

Nitrate

NO3-

Silver

Ag+

Carbonate

CO32-

Calcium

Ca2+

Oxide

O2-

Magnesium

Mg2+

Sulphate

SO42-

Copper (II)

Cu2+

Sulphite

SO32-

Iron (II)

Fe2+

Sulphide

S2-

Zinc

Zn2+

Phosphate PO43-

Aluminium

Al3+

Iron (III)

Fe3+

 

  • Ionic equations show reactions involving ions.
  • In many ionic reactions some of the ions play no part in the reaction. These ions are called spectator ions.
  • A solution is made when a solute dissolves in a solvent.
  • Moles present in solution = volume (cm3) x mol cm3.

 

Ionic Compounds

 

  • Metals lose electrons from their outer shell to form positive ions.
  • Non-metals gain electrons in their outer shell to form negative ions.
  • Ionic bonding can be represented in dot and cross diagrams.

 

Properties of Ionic compounds

High melting + boiling points

Form giant lattices

Strong forces between ions

Electrolytes – conduct electricity when molten or dissolved in water

 

Covalent Substances

 

  • Covalent bonds involve electron sharing.
  • Covalent bonds only occur between non-metals.
  • The displayed formula of H2 is written as H—H.
  • Covalent bonds can be single, double, or triple. This means they each contribute one, two, or three (respectively) electrons to the molecule formed.
  • Shapes of molecules:

          Carbon dioxide is a linear molecule.

          Water molecules are v-shaped.

          Methane molecules are tetrahedral.

          Molecules of ammonia are pyramidal.

  • Covalent compounds can form simple molecular crystals.

 

Properties of covalent compounds

Low melting + boiling points

Very strong intramolecular forces

Weak forces between ions

Do not conduct electricity

 

  • Not all covalently bonded compounds exist as simple molecular structures: diamond exists as a giant structure.
  • Different forms of the same element are called allotropes.
  • Diamond ore is called kimberlite.

 

Electrolysis

 

  • Substances that can conduct electricity are called electrolytes.
  • For electrolysis to occur, the substance must be molten or dissolved in water, and it must contain ions.
  • Electrode connected to positive terminal is the anode.
  • Electrode connected to negative terminal is called cathode.
  • The negative ions are attracted to the anode and lose electrons (oxidised).
  • The positive ions are attracted to the cathode and gain electrons (reduced).
  • A Faraday is one mole of electrons.
  • Reactions at electrodes can be shown as ionic half equations.
  • e- stands for electron.
  • Electroplating is the process by which electrolysis is used to coat an object with a thin film of metal. The object is used as the cathode.
  • To purify metals, make the anode a large impure block and the cathode a thin pure block

 

Metallic Crystals

 

  • Metals are giant structures with high melting and boiling points.
  • Metal atoms give up electrons to form cations.
  • These electrons given off form a swarm around the metal ion, and thus are free to move (delocalised) meaning metals conduct electricity.

 

Properties of Metals

High melting + boiling points

Malleable (can be hammered into a sheet)

Ductile (can be drawn into a wire)

Conduct heat and electricity

6 comments:

  1. Is it me or is it there is something wrong with ur maximum no of electron in shell 3

    ReplyDelete
    Replies
    1. its just u.when full its normally with 8 electrons but it can expand to a total of 18.so,this guy who wrote these notes didnt make a mistake

      Delete
  2. God Bless you. This is really....awesome..

    ReplyDelete
  3. Formulas of all the Calculations pls

    ReplyDelete
    Replies
    1. This comment has been removed by the author.

      Delete
  4. what calculations?....like for empirical and molecular formula

    ReplyDelete