Additional Chemistry (C2)

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Stucture and Bonding

Ionic Bonding

Non-metal ions gain electrons.

Metal ions lose electrons.

Strong electrostatic forces of attraction between oppositely charged ions - This is called an IONIC BOND.

Covalent Bonding

Covalent Bond = Strong bond between two non-metal atoms.

Non-metal atoms share their electrons to form COVALENT BONDS.

When two atoms share a pair of electrons they get joined together.

The covalent bond that is formed is very strong and it takes a lot of energy to break it.

Compound = Atoms of different elements join together.

Covalent bonds join atoms together to form molecules.

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Stucture and Properties

Simple Molecules

These contain only a few atoms held together by STRONG COVALENT BONDS.

They have LOW MELTING and BOILING POINTS as the weak intermolecular forces break down easily.

They are NON-CONDUCTIVE - They don't have any free electrons or an overall charge.

Giant Covalent Structures - 'Macromolecules'

Contain a lot of NON-METAL ATOMS, joined by COVALENT BONDS.

The atoms are arranged into GIANT LATTICES, which are VERY STRONG structures because of the many bonds.

They have HIGH MELTING and BOILING POINTS because a lot of STRONG COVALENT BONDS must be broken.


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Polymers

Polymer = A large molecule formed from many identical monomers.

Thermosoftening: The forces between the chains are weak so it softens when heated and hardens when cooled.

Thermosetting: Strong covalent bonds form cross-links between the chains so they don't soften or melt when heated.

LDPE and HDPE

Low-Density Poly(Ethene) and High-Density Poly(Ethene) are made using DIFFERENT CATALYSTS and different REACTION CONDITIONS.

High-Density Poly(Ethene) = Higher softening temperature and is stronger.

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Ionic Compounds

Ionic Bonds form when a metal reacts with a non-metal.

Ionic compounds have giant structures in which many strong electrostatic forces of attraction hold the ions together.

They have HIGH MELTING and BOILING POINTS because of the strong bonds.

They CONDUCT ELECTRICITY when molten because the ions are free to move about and carry charge.

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Metals

Metals are MALLEABLE - They can be bent or shaped.

Metals consist of LAYERS of atoms. The layers can slide over one another.

Metals are GOOD CONDUCTORS of electricity and heat. This is because the free / delocalised electrons can move through the metal, carrying charge.

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Alloys

Alloy: A mixture of two or more elements where at least one element is a metal.

Many alloys are mixtures of two or more metals.

Alloys contain atoms of different sizes. This makes it harder for the layers to slide over each other.

Alloys are HARDER than pure metals.

Shape Memory Alloys can return to their original shape once heated.

Uses: Dental braces.

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Nanoscience

Nanoscience = The study of small particles that are between 1 and 100 nanometres in size

One nanometre = 10^-9m

Nanoparticles

Nanoparticles have a VERY LARGE SURFACE AREAS.

They can REACT VERY QUICKLY and so make USEFUL CATALYSTS.

Nanoparticles behave differently to normal particles.

Uses: Sun screen, cosmetics, deoderants, etc.

Disadvantages:

- Particle with a very large surface area can cause damage to the lungs and breathing.

- There may be dangerous side affects.

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Atomic structure

The mass of an atom is its total number of protons and neutrons.

Isotopes are atoms of an element with different numbers of neutrons.

Isotopes have the SAME CHEMICAL PROPERTIES but DIFFERENT PHYSICAL PROPERTIES.

A atom consits of;

-  a nucleus,

- protons,

- neutrons

- electons.

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Relative Formula Mass (Mr) & Percentage and Formul

Relative Formula Mass

We find the Relative Formula Mass of a substance by adding up the relative atomic masses (Ar) of all the atoms in its formula.

One mole of any substance is its relative formula mass in grams.

Percentage and Formulae

The Relative Atomic Masses (Ar) of the elements in a compound and its formula can be used to work out its percentage composition.

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Analysing Substances

Paper Chromotography is used to analyse colured substances.

Paper Chromatography can be used to analyse the artificial colours in food.

Method:

A spot of colour is put onto paper and solvent is allowed to move through the paper.

The colours move different distances depending on their solubility.

Insrumental Methods

Instrumental methods of analysis rely on machines.

There are several different types of instrumental analysis.

Modern instrumental techniques provide fast, accurate and sensitive ways of analysing chemical substances.

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Gas Chromatography

Compounds in a mixture can be separated using gas chromatography:

1) The sample is dissolved in solvent and injected into one end of a column.

2) An unreactive gas, usually Nitrogen, carries the sample through a column.

3) Different substances travel through the column at different speeds and so become separated from each other.

4) The separated substances leave the column one after the other. As they leave, they are detectd by a detector.

The time taken for a substance to travel through a column is called the RETENTION TIME.

A detector produces a graph where each substance is represented by a peak:

 - Number of peaks = Number of compounds present

 - Position of peak = Retention time for each compound

Once separated, compounds can be identified using a mass spectrometer.

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Quantitative Chemistry

In a reversible reaction, the products of the reaction can react to form the original reactants.

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Mass is NEVER LOST or GAINED in a chemical reaction. We say that mass is conserved.

Mass of products = Mass of reactants.

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The Yeild of a chemical reaction describes how much product is made.

The percentage yeild of a chemical reaction tells us how much product is made compared with the maximum amount that could be made.

It is not always possible to get the entire calculated amount of product;

 --> Reversible reactions ma not go to completion

 --> Some product may be lost

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Measuring Rates of Reaction

We can measure the changes in amounts of the chemicals taking part in the reaction by measuring their mass or volume.

For a liquid or a solid we measure the mass using a BALANCE.

For a gas we measure the volume with a GAS SYRINGE.

The rate of reaction = How quickly a reaction takes place.

There are two ways to measure the rate of a reaction:

 - The rate at which a reactant is used up

 - The rate at which a product is formed.

Rate of Reaction = Amount of reactant used / Amount of product formed                                     

                                                              Time Taken

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Factors Affecting the Rate

For a reaction to take place, particles must collide with enough energy.

This amount of energy is called the ACTIVATION ENERGY.

> Temperature: Increases the rate of reaction as the particles have more energy and collide more frequently and more energetically. At a higher temperature more particles have energy greater than the activation energy.

> Concentration: If one of the reactant chemicals is in solution the reaction rate will increase if the concentration is increased. There will be more of a chemical in a smaller volume so collisions are likely to take place more often.

> Preassure: Increasing the preassure means there are more reactant particles in a given volume. There will be more successful collisions as the particles are closer together.

> Surface Area: Increasing the S/A of a solid means that more particles are exposed to other reactants, therefore there is a greater chance of collision.

> Catalyst: Reduces the activation energy needed for a reaction to take place. (Different reactions require different catalysts.)

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Exothermic and Endothermic Reactions

Exothermic Reactions

 > Transfer energy to the surroundings - usually heat energy.

 > Temperature increase can be detected using a THERMOMETER.

 > Exothermic Reactions; Combustion, Oxidation reactions, Neutralisation reactions (acids and alkalis).

Endothermic Reactions

 > Surrounding get colder.

 > Examples; Electrolysis, Reactions between Ethanoic Acid and Sodium Carbonate, Thermal decomposition of Calcium Carbonate in a blast furnace.

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Reversible Reactions

In a reversible reaction, the reaction in one direction is Exothermic and in the other direction it's Endothermic.

In any reversible reaction, the amount of energy released when the reaction goes in one direction is exactly equal to the energy absorbed when the reaction goes in the opposite direction.

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Acids and Bases

We measure the strength of acids and bases by their pH value.

Acids have a pH of less than 7.

Bases have a pH of more than 7.

Acids turn blue litmus paper red. Strong acids turn universal indicator red too.

Bases are sunstances that neutralise acids. They're usually Metal Oxides/Hydroxides.

Not all bases dissolve in water.

Alkalis are bases that dissolve in water.

Alkalis turn red litmus paper blue.

Neutral chemicals have a pH value of 7. They turn litmus paper/universal indicator green.

When we dissolve a substance in water, we make an Aqueous Solution (aq)

* Acids = H+ , Alkalis = OH-

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Salts

A salt is created when an acid reacts with a base, carbonate or metal.

Acid + Base = Neutralisation - This produces salt and water.

Acid + Metal = Salt + Hydrogen

Carbon Dioxide causes bubbling during a reaction

Hydrogen can be detected with a lighted splint and burns with a squeaky pop.

Acids = HCl (aq) = H+ (aq) + Cl- (aq)

Alkalis = NaOH (aq) = Na+ (aq) + OH- (aq)

Neutralisation = H+ (aq) + OH- (aq) = H^2O (l)

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Naming Salts

To name a salt, we take the firstname of the metal in the base and the name of the acid used:

Hydrochloric Acid = Chloride

Nitric Acid = Nitrate

Sulfuric Acid = Sulfate

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Insoluble salts can be made by reacting two solutions to produce a percipitate.

Percipitaion is an important way of removing some metal waste ions from industrial waste water

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Making a salt from an acid and an insoluble base =

* Neutralisation --> Filtration --> Evaporation

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Electolysis

Electrolysis = The process by which ionic substances are broken down into simpler substances using electricity.

Ionic substances form when a metal reacts with a non-metal.

The metal atoms become positive ions and the non-metal atoms become negative ions.

Electrolysis splits up a substance and elements are produced.

The ions need to be free to move about, so the substance needs to be in molten liquid state or in solution dissolved in water.

The electrolysis liquid is called the electrolyte.

Electrodes = Cathode (negative) and Anode (positive)

Cathode - Positively charged ions move to the cathode. They RECIEVE electrons and so are REDUCED.

Anode - Negatively charged ions move to the anode. They LOSE electrons so are OXIDISED.

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Electrolysis - Part 2

The ions become atoms again.

The least reactive ions come out at the cathode.

If the metal we have is less reactive than Hydrogen, then it is the metal we get.

Least reactive = Product.

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The Chlor-Alkali Industry & The Extraction of Alum

The Chlor-Alkali Industry

When we electrolyse Brine, we get three products, Chlorine gas (positive electrode), Hydrogen gas (negative electrode) and Sodium hydrxide solution.

The products are important reactants used in industry:

> Hydrogen - Margarine, Hydrochloric Acid

> Sodium Hydroxide - Soap, Paper, Bleach

> Chlorine - Bleach, Kill bacteria, Disinfectants

The Extraction of Aluminium

Aluminium oxide is electrolysed to manufacture aluminium.

Molten cryolite is used to reduce the temperature needed for the process.

The positive Carbon electrodes are often replaced as they gradually burn away.

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