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GCSE Chemistry - Introductory Unit

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Introductory Unit - Basics

(1) Definitions :

(2) Laboratory Equipment :

Below is a table with some of the common pieces of laboratory equipment used in GCSE Chemistry.

test tube
beaker
conical flask
Liebig condenser
measuring cylinder
filter funnel
Bunsen burner
burette
pipette
volumetric flask
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Introductory Unit - Laboratory Safety

(1) Hazard Symbols :

Here are the common chemical hazard labels to be found on bottles in a laboratory.

corrosive
highly flammable
toxic
harmful
irritant

(2) Equipment Hazards:

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Introductory Unit - The Periodic Table

The modern version of the Periodic table classifies elements into groups (columns) and periods (rows). Elements in the same group have a gradual change in physical properties such as melting point, boiling point and density as the group is descended and form compounds of similar chemical formulae. These properties allowed Mendeleev to propose the first version of the Periodic Table, though it has undergone changes since then. Below is a shortened version of the Periodic table, showing the common elements and where the groups encountered in GCSE can be found.

Note also that an imaginary line can be drawn in the table where elements to the left are metals and elements to the right are non-metals. Elements close to the line sometimes exhibit properties of metals and non-metals.

Here is a link to a simple interactive full Periodic table:

http://chemlab.pc.maricopa.edu/periodic/

There are also some very modern versions of the Periodic table, linking the elements more closely to the number and arrangement of electrons in their atoms. Here are two such links:

http://chemlab.pc.maricopa.edu/periodic/spiraltable.html

http://chemlab.pc.maricopa.edu/periodic/stowetable.html

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Introductory Unit - Chemical Formulae and Equations

(1) Chemical Formulae :

In order to write chemical formulae it is vital to know the charges of positive ions and negative ions and how to put them together. A positive ion is an atom (or group of atoms) that has lost electrons (see The Atom below) and a negative ion is an atom (or group of atoms) that has gained electrons.

In the tables below note the position in the Periodic table of elements with the same charge, e.g. the alkali metals all have +1 charge.

Positive ions -

Charge Element / Compound ion
1+ Na, K, Li, Ag, H, NH4, Cs, Rb
2+ Mg, Ca, Sr, Ba, Sn, Pb, Cu, Zn, Fe (iron(II) compounds)
3+ Al, Fe (iron(III) compounds)

Negative ions -

Charge Element / Compound ion
1- F, Cl, Br, I, OH, NO3, NO2, HCO3
2- O, S, SO4, SO3, CO3

Notes

1 Salts are composed of a positive ion and a negative ion. The overall charge on the compound formed must be zero.

2 If the positive ion and the negative ion both have the same charge, then the salt is simply a combination of the two ions.

e.g. MgSO4, NaOH, KCl, CaCO3, ZnO.

3 If the positive ion and the negative ion differ in their charges then more thought is required. The value of the positive ion becomes the number of negative ions required, and the value of the negative ion becomes the number of positive ions required. BUT note all "1's" are omitted from the final formulae.

e.g. Na(1+) and SO4(2-) = Na2SO4; K(1+) and O(2-) = K2O; Fe(3+) and O(2-) = Fe2O3; Al(3+) and SO4(2-) = Al2(SO4)3.

4 When compound ions such as NH4+, OH-, NO3-, SO42- and CO32- are required more than once in a salt, the compound ion has brackets around it.

e.g. (NH4)2SO4, Al2(SO4)3, Mg(OH)2.

Below are links to two pop-up formula quizzes that can test your ability to work out chemical formulae. You can try them with the charge tables visible on your screen or by themselves without any aid. The advanced quiz involves all the ions given in the table above and some that are not. The basic quiz uses the most common positive and negative ions that will be encountered at GCSE level.

Basic Formula Quiz Advanced Formula Quiz

(2) Chemical Equations :

There are two ways of writing the reactants and products involved in a chemical reaction - a word equation and a formula equation.

Word equations -

Word equations are the simplest way of conveying the information. The names of the reactant chemicals are written on the left , then an arrow is drawn, →, and the names of the products are put on the right of the arrow.

e.g. when a mixture of hydrogen and oxygen is ignited an explosion results in the production of water :

hydrogen + oxygen → water

Word equations do not refer to the number of molecules present in the reaction, nor do they show where particular atoms move during a reaction. For this a formula equation is required.

Formula equations -

The process of writing a formula equation involves three stages.

The first stage is to know the names of the reactants and products and possibly write a word equation. As your ability to write equations increases the need to write a word equation decreases; however, you still have to know what happens when particular chemicals react together.

e.g. hydrogen + oxygen → water

Then the formulae for the various chemicals are written out (under the respective chemicals if you have written out the word equation).

e.g.

hydrogen + oxygen → water

H2 + O2H2O

The final stage is to count up the number of each individual type of atom on the left hand side and right hand side of the arrow. For a proper formula equation these numbers must be balanced, i.e. they must be equal to each other.

This balancing is achieved by putting numbers in front of the chemicals as required, to multiply the number of them present in the equation.

e.g.

hydrogen + oxygen → water

2H2 + O22H2O

Before balancing there are 2 hydrogen atoms and 2 oxygen atoms on the left and 2 hydrogen atoms and 1 oxygen atom on the right.

So, to balance the equation the number of hydrogen molecules is doubled to give 4 hydrogen atoms in total on the left and the number of water molecules is doubled to give 4 hydrogen atoms and 2 oxygen atoms on the right.

This ability to write a balanced formula equation is one of the most important aspects of chemistry and requires a lot of practice. Here is another example drawn from the reaction of a metal with water (see alkali metals below),

sodium + water → sodium hydroxide + hydrogen gas

2Na + 2H2O → 2NaOH + H2


number of atoms on left number of atoms on right
Na O H Na O H
before balancing 1 1 2 1 1 3
after balancing 2 2 4 2 2 4

The finishing touch for a balanced formula equation is to put state symbols next to each chemical to denote what kind of matter it is, i.e. solid, liquid, gas or in an aqueous solution.

state symbol represents
(s) solid
(l) liquid
(g) gas
(aq) aqueous solution

e.g.

2H2(g) + O2(g) → 2H2O(g)

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Here are a couple of equations to try out for yourself. Enter the numbers, including any 1's in the boxes and press check to see if you are correct.

Mg(s) + HNO3(aq)Mg(NO3)2(aq) + H2(g)
 
Al(s) + H2SO4(aq)Al2(SO4)3(aq) + H2(g)
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Introductory Unit - The Atom

The word atom is derived from an ancient Greek word meaning the smallest particle, implying that an atom cannot be divided into anything smaller. This view held for thousands of years, until Niels Bohr in 1902 first proposed the idea of an atom having a central nucleus with electrons orbiting around this nucleus, as the moon orbits the Earth.

The nucleus is composed of two different types of particles - protons and neutrons. The number of protons that an element has is called the atomic number, and the sum of the protons and neutrons is called the atomic mass.

The elements in the Periodic table were originally placed in the order of their atomic masses, i.e. the number of protons and neutrons in the atom. However, in the modern version of the Periodic table they are placed in the order of the number of protons that the element's nucleus has.

In a neutral atom, i.e. one that is not positively or negatively charged, the number of protons equals the number of electrons. This is because the value of charge on a proton and an electron is the same. In a proton the charge is positive and in an electron the charge is negative. Neutrons have no charge.

An ion is formed when electrons are added to or removed from an atom. N.B.: the number of protons and/or neutrons is not affected when an ion is formed.

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Introductory Unit - Alkali Metals

(1) Introduction :

The elements in the left most column of the Periodic table are called Group I or the Alkali metals. Collectively they are all metallic-grey solids with a gradually decreasing melting point as the group is descended (shown in the table below).

Element Melting pt. (oC)
Lithium, Li 181
Sodium, Na 98
Potassium, K 63
Rubidium, Rb 40
Caesium, Cs 29

(2) Reaction with water :

The reactivity of the metals with water increases as the group is descended. Lithium is quite a slow reaction with only gentle bubbling showing any reaction at all; sodium fizzes more violently and the piece of metal will move around the surface of the liquid quickly; potassium is more violent still with the hydrogen gas produced igniting, giving a lilac flame, because of the exothermic nature of the reaction; rubidium and caesium react almost explosively when they are added to water.

The reactions all produce hydrogen gas, which can be tested for by placing a lit splint in the gas; if a "squeaky pop" is heard then the gas has exploded and this shows hydrogen is present.

General equations :

(alkali) metal + water → metal hydroxide + hydrogen gas

2M(s) + 2H2O(l) → 2MOH(aq) + H2(g)

Because of this potentially violent reaction with water, alkali metals must be stored under oil. This prevents even water vapour in air coming into contact with the metal and so stops any adverse reaction.

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Introductory Unit - Noble Gases

The elements in the right most column of the Periodic table are called Group 0 or the Inert or Noble Gases. They are all colourless gases at room temperature and pressure and exist naturally as individual atoms; they are monatomic. They are very unreactive, hence the names for the group, though xenon, krypton and argon can form compounds; however, they only form compounds with fluorine, an extremely reactive halogen.

These gases are found in the air and can be extracted by compressing air and collecting the liquefied gases as the pressure is lowered and the temperature drops.

They have limited and very specialized uses. Helium, as the gas with the lowest density, is used in balloons of all sizes; neon is used in lighting signs; argon provides an inert atmosphere in domestic light bulbs; and krypton is used in lasers.

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Introductory Unit - Transition Metals

This term does not apply to a group of metals as defined previously. It refers to the metals in between magnesium and aluminium in the Periodic table, i.e. the elements from scandium (Sc) to zinc (Zn).

Transition metals tend to have higher melting points than alkali metals, high densities, are hard and frequently form coloured compounds.

The main transition elements of concern at GCSE level are iron, nickel and copper. Iron, and as its alloy steel, is used as a construction material for buildings and ships and as a catalyst for making ammonia in the Haber process. Nickel is used as part of the alloy, along with zinc, in coinage and as a catalyst in the saturation of margarine. Copper is used to form electrical wiring and in household water and gas pipes.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

updated : Thursday, 12th July, 2012

mail to: chemistryrules

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