# Chemistry Revision

## Periodic Table and the elements

Periodic Table

• All things are made up of elements
• The first modern periodic table was created by Mandeleev,he left gaps for elements that were not yet discovered

The Atom

• Elements are made up from atoms. An atom has:
• - a nucleus containing protons and neutrons
• -electrons surronding the nucleus

Atomic particle                Mass                   Charge

• Proton                          1                            +1
• Neutron                        1                             0
• Electron                  Negligiable                  -1

Atomic number - Found at bottom of the symbol (number of protons) Relative atomic mass - found at the top of the symbol (number of protons and neutrons)

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## Isotopes and percentage composition

Isotopes are atoms of the same element that have different numbers of neutrons. Isotopes have the same atomic number but different mass number.

Mass Number - Number of protons and neutrons

Example: Chlorine has two isotopes

Relative atomic mass - Value of the element

e.g. - Carbon 12

Percentage composition:

K            N             O3

Ar         39           14           (3 x 16) = 48

Mr         39     +     14     +        48            = 101

Percentage of oxygen = (48/100) x 100

= 47.5%

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## Relative formula mass (Mr) and Emprical Formula

Relative formula (molecular) mass:

Water = H20

• 2 x hydrogen
• 1 x oxygen

Calculation = (1x16) + (2 x 1) = 18

Empirical formula

Symbol for element                                                    Ca                            Cl

• Mass in g                                                             10.0                          17.8
• Relative atomic mass (ram)                                  40.0                          35.5
• Divide mass of each element by ram                  10/40 = 0.25            17.8/35.5 = 0.5
• Divide answers by smallest number                   0.25/0.25                   0.5/0.25

= CaCl2

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## Theoretical, Actual and Percentage Yield

Theoretical Yield = How much of a product you should have

Actual Yield = This is the amount that you do get, this may be less

Percetage Yield:

• Actual Yield / Theoretical Yield x 100
• Value will always be less than 100%

- What mass of co2 is formed when 6g of carbon is burned in air:

• Carbon + oxygen - Carbon Dioxide
• C + O2 - CO2
• 12 + 32 - 12 + (16 x 2) = 44

-12g of carbon produces 44g of co2

• But we dont have 12g of carbon we have 6g
• If we half the amount of carbon, we will get half the amount of co2 out - so 44 / 2 = 22g
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## Bonding Types and Ionic Bonding

There are 3 types of bonding:

Ionic - Formed between a metal and non-metal

Covalent - Formed between two non-metals

Metallic - Between metal atoms

Ionic Bonding: - This involves a transfer of electrons from one atom to another. Ionic bonding results in the formation of electrically charged ions. Each ion has 8 electrons in the outer shell which can be: Positively charged (cations) or negatively charged (anions)

Ionic compounds.....

• high melting and boiling points due to string electrostatic forces of attraction
• Conduct electricity when in a solution because the charged ions are free to move about

Examples of ionic compounds - Sodium chloride (NaCl), Magnesium oxide (MgO)

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## Covalent Bonding and Simple molecular structures

Covalent bonds occur when electrons are shared between non-metal atoms to attain a full outer shell. Covalent bonds result in the formation on molecules.

A single covalent bond is formed when two atoms share one pair of electrons e.g. Chlorine

A double covalent bond is fomed when two pairs of electrons are shared e.g. Oxygen

Simple molecules

• simple molecular covalent structures have relatively few atoms in their molecules.
• There are strong forces between the atoms in the molecules but there are weak inter-molecular forces this means that:
• -have low melting and boiling point
• -can't conduct electricity
• -are often gases at room temperature
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## Giant molecules and Bonding in metals

Giant molecules

These are structures made from millions of atoms joined with covalent bonds. Examples of these are diamond and graphite.

Diamond

• each carbon bond forms 4 covalent bonds with other atoms
• high melting and boiling points
• very hard
• unable to conduct electricity

Graphite

• eacg carbon atom forms three covalent bonds with other carbon atoms in a layered structure
• high melting and boiling points
• soft and slippery
• conduct heat and electricity
• weak forces of attraction between layers
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## Metallic Bonding

Metals form giant structures in which electrons in the outer shells of the metal atoms are free to move. The metallic bond is the force of attraction between these free electrons and metal ions.

Metallic bonds are strong so metals can maintain a regular structure and usually high melting and boiling points.

Metals are good conductors of heat and electricity because the free electrons carry a charge of heat or energy through the metal.

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## Electrolysis

Electrolysis is the breaking down of a compound, using a direct current

When a direct current is passed through a liquid containing ioons, the ions move to the electrode of the opposite charge:

• positively charged ions - Negative Cathode
• negatively charged ions - Positive Anode

P                    A                    N                   I                   C

Positive           Anode          Negative            Is               Cathode

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## Pure Metals and Alloys

Pure Metals - Only contain atoms of that element. Pure metals have the following properties:

• good conductors of heat and electricity
• stong and hard
• dense
• malleable
• solids at room temperature
• high melting point
• Shiny when polished

Alloys - Many metals are better when they are not pure. Alloys are formed by mixing metals together. This gives the metal a greater range of uses than the main metal.

An alloy can have:

• lower melting points
• increased corrosion resistance
• increased chemical resistance
• increased strength and hardness
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## Electronic Configuration

Electronic configuration tells us how the electrons are arranged in an atom, in shells:

• first shell - max. 2
• shells after this - max. 8

Electronic configuration is written as a series of numbers e.g. Potassium, K - 2, 8, 8, 1

• elements in group 1 have 1 electron in the outer shell
• elements in group 2 have 2 electrons in the outer shell etc.
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## Bonding in Metals

• Metal atoms form giant crystalline structures
• atoms are packed tightly together so the outer electrons get seperated from atom
• The result is a lattice structure of positive ions in a sea of free electrons

Metals are good conductors of electricity because their electrons can move freely within the structure, carrying the electric charge.

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## Alkali metals and Noble Gases

Alkali Metals

• Found in group 1
• All have 1 electron in outer shell
• Low melting and boiling points
• low densities
• very soft metals

The alkali metals are more reactive because as the atomic number increase, the futher away the outer electron becomes from the nucleus, this means an electron is more easily lost

Noble Gases

• Found in group 0/8
• All have 8 electrons in outer shell
• low boiling point and density
• very unreactive
• non-metals

The electrons are arranged in such a way as to discourage any bonding with other elements.

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## Halogens

Halogens

• Found in group 7
• All have 7 electrons in outer shell
• Low melting and boiling points
• State at room temperature

They become less reactive as the atomic number increases because the outer electron becomes futher away from the nucleus and so an extra electron is less easily attracted.

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## Alkanes and Alkenes

Alkanes

A hydrocarbon is an organic compund containing hydrogen and carbon only. The spine of a hydrocarbon is made up of carbon atoms (chains)

• they are saturated hydrocarbons
• single covalent bond C - C

Alkenes

• unsaturated hydrocarbons
• contain one double bond C=C
• Have carbon atoms that are bonded to less than four other atoms

Testing Alkanes and Alkenes:

• Alkene will dicolour bromine water
• Alkane has no effct
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## Exothermic

Exothermic

• Transfer energy to surrondings (usually as heat energy) Some examples of reaction are:
• - Burning
• - Explosion
• - Reaction between water + calcium oxide
• - Energy released is more than energy absorebed
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## Endothermic

• Take in energy from surrondings (usually as heat energy) - colder. Some examples of endothermic:
• - electrolysis
• - photosynthesis
• - thermal decomposition of calcium carbonate
• - bond (breaking is endothermic process)
• energy absorbed is more than energy released
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## Rates of Reaction

The factors that affect the rate if reaction are:

• Temperature
• Surface area
• Concentration
• Pressure of reacting gases
• Catalyst

Chemical reactions usually occur when particles collide with each other with sufficient energy, this is called the collision theory.

Temperature:- Higher the temperature the faster the reaction rate, as there are more successful collisions.

Surface Area:- Larger the surface area the faster the reaction rate this is because there are more particles available to react with a larger surface area.

Concentration:- Higher concentration means there are more particles in a given volume, this means they are more likely to collide.

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## Rates of Reaction

The factors that affect the rate if reaction are:

• Temperature
• Surface area
• Concentration
• Pressure of reacting gases
• Catalyst

Chemical reactions usually occur when particles collide with each other with sufficient energy, this is called the collision theory.

Temperature:- Higher the temperature the faster the reaction rate, as there are more successful collisions.

Surface Area:- Larger the surface area the faster the reaction rate this is because there are more particles available to react with a larger surface area.

Concentration:- Higher concentration means there are more particles in a given volume, this means they are more likely to collide.

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## Catalysts

A catalyst is a substance which increase the rate of reaction without being used up.

• Catalysts work by reducing the activation energy needed for a reaction to happen. This means that more collisions will be successful and the reaction will be faster.
• Only a small amount of the catalyst is needed to increase reaction
• Catalysts are used in industrial processes to speed up reactions and make production more economical.

Catalytic Converters:

• Cars have this to help reduce amount of toxic waste (gases)
• High surface area
• Designed to work at high temperature
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## Enzymes and Making and Breaking Bonds

Enzymes - Biological catalysts that control the rate of chemical reactions in living organisms. If these were not used, the rate of reactions would be so slow that life wouldnt exist.

Making and Breaking Bonds

• In a chemical reaction, the bonds in the reactants must be broken and new bonds made to form the products.

Chemical reactions that require more energy to break bonds than is released when bonds are formed are endothermic reactions.

-

Chemical reactions that release more energy when bonds are formed than is needed to break them are exothermic reactions.

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## Extra Notes on C2

Immisable - Doesn't dissolve (separating funnel)       Missable - Dissolves (fractional distilation)

Fractional distilation of air:

• water vapour condenses
• carbon dioxide freezes at -79 degrees
• oxygen liquifies at -183 degrees
• nitrogen liquifies -196 degrees

Chromotography - It is used to separates mixtures of colour compounds. Different componants of a mixture move at different rates, this separates the mixture. RF = distance moved by compound / distance moved by solvent.

Displacement reactions - A more reactive halogen replaces a less reactive halogen from a solution of one of its salts.

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