Chemistry C2

Ionic bonding- when a chemical bond is formed when one atom gives up one or more electrons to another atom

Ionic compounds always have giant ionic lattices. They have very strong electrostatic forces of attraction between oppositely charged ions, in all directions. For example, sodium chloride (salt)

Ionic compounds have high melting points and high boiling points. This is because of the strong attraction between the ions. This takes a large amount of energy to overcome. When ionic compounds melt, the ions are free to move and carry electric current. They also dissolve easily.

Mass number- total number of protons and neutrons

Atomic number- number of protons

Compound- when atoms of two or more elements are chemically bonded

Isotope- different atomic forms of the same element, which have the same number of protons but a different number of neutrons

Covalent Bonding- the sharing of electrons, only in the outer shell, having a full outer shell gives them the electronic structure of a noble gas.

Covalent Bond- a shared pair of electrons.

Examples:

• hydrogen- H2
• chlorine- Cl2
• methane- CH4
• hydrogen chlorine- HCl
• ammonia- NH3
• water- H2O
• oxygen- O2

Simple Molecular Substances: form very strong covalent bonds to form small molecules of several atoms, very weak intermolecular forces, low melting and boiling points, when the simple molecular forces are melted the intermolecular forces break, NOT the stronger covalent bonds, does not conduct electricity  cecause there are no ions to carry the electric charge

Giant Covalent Structures: no charged ions, all atoms are bonded by strong covalent bonds, high melting and boiling points, do no conduct electricity. Examples include:

• Diamond- 4 covalent bonds, rigid structure, hardest substance
• Graphite- 3 covalent bonds, creates layers which can slide over each other making it soft and slippery, weak intermolecular forces, good conductor of heat and electricity because of the delocalised electrons that can carry electric charge.

Metal Properties: giant, regular structure, delocalised electrons- free to move through the whole structure- good conductor of heat and electricity, strong forces of electrostatic attraction between the positive metal ions and the negative electrons

Alloys: different elements have different sized atoms- distort layers of metal atoms- difficult to slide over each other, alloys are harder

Shape Memory Alloy: e.g. nitinol- metal alloy, used for glasses frames and dental braces. When cool it can be bent but if you bend it too far it stays. If you heat it above a certain temperature it goes back to a "remembered shape"

Nanoparticles: very very tiny particles, nanotubes conduct electricity

uses include:

• new industrial catalysts
• sensors
• stronger, lighter building materials
• new cosmetics
• nanomedicine

Weak Forces: individual tangled chains of polymers held together by weak intermolecular forces- free to slide over each other.

Thermosoftening polymers- no cross links, forces weak forces between chains,  easy to melt, when cools polymer can be remoulded into a new shape.

Strong forces: stronger intermolecular forces betwenn polymer chains, contain cross links, which hold the chains firmly together

Thermosetting polymers- have cross links, hold chains in a solid structure, polymer does not soften when heated, they are hard, strong and rigid

What affects the properties of a polymer?starting materials and reaction condition

LD polythene- 200 degrees under high pressure- bags and bottles

HD polythene- low temperauture and pressure - water tanks and drainpipes

Formula mass - (top number) e.g Helium- 4

Relative formula mass of a substance in grams is one mole of that substance

Number of moles= mass in grams/Mr

Percentage mass of an element in a compound= (Ar x no. of atoms of that element /Mr of whole compound) x 100

Empirical Formula- 1. list elements 2. experimental masses 3. divide by Ar 4. simplest formula

Percentage yield= (actual yield/predicted yield) x 100

YIELDS ARE ALWAYS LESS THAN 100%

Chromotography- separate and identify substances

Advantages of using machines:

• very sensitive
• very fast
• very accurate

Gas chromotography- can separate out a mixture of compound and help identify the substances present

Depends on:

• temperatue
• catalyst
• concentration
• surface area

Rate of reaction=amount of reactant used or amount of product formed/time

Rate of reaction can be measured by

• precipitate
• change in mass
• volume of gas given off

More collisions increases the rate of reaction

1) higher temperature

2) higher concentration

3) larger surface area

Increasing the temperature causes faster collisions

CATALYST: a catalyst is a substance which speeds up a reaction without being changed or used up in the reaction

Exothermic reactions-  a reaction which transfers energy to the surroundings usually in the form of heat and usually shown by a rise in temperature

Endothermic reactions- a reaction which takes in energy from the surroundings usually in the form of heat and usually shown bya fall in temperature

Reversible reactions can be endothermic and exothermic.

Acids- have a pH of less than 7

Bases- have a pH og more than 7, substances which react with acids and neutralise them to make salt and water

Alkalis- are bases which dissolve in water

Acids and Bases neutralise each other

ACID + BASE --> SALT + WATER

H+  +  OH-  --> H20

ACID + METAL --> SALT + HYDROGEN

Hydrochloric acid will always produce chloride salts

Sulfuric acid will always produce sulfate salts

Metal oxides and metal hydroxides are bases

ACID + METAL OXIDE --> SALT + WATER

ACID + METAL HYDROXIDE --> SALT + WATER

The combination of metal and acid decides the salt....

• hydrochloric acid + copper oxide --> copper chloride + water
• sulfuric acid + calcium hnydroxide --> calcium sulfate + water
• nitric acid + magnesium oxide --> magnesium nitrate + water

Ammonia can be neutralised with HNO3 to make fertiliser, ammonia dissolves in water to make an alkaline solution, when it reacts with nitric acid you get a neutral salt- ammonium nitrate (NO WATER PRODUCED)

• ammonia + nitric acid --> ammonium nitrate
• NH3 + HNO3 --> NH4NO3 

You can make a salt by:

• using a metal or an insoluable base
  • add the metal, metal oxide ot hyrdoxide to the acid
  • filter out the excess
  • (evaporate) crystalisation
• using an alkali
  • use an indicator to find out how much alkali you need to neturalise the acid
  • evaporate
  • crystalise
• precipitation reactions
  • mix two solutions containing the ions you need
  • filter
  • wash
  • dry
    • can be used to remove poisionous ions from drinking water

Electrolysis- "splitting up using electricity"

• pass an electric current through an ionic molten substance or solution- separate the elements it's made of
• requires a liquid to conduct the electricity called an electrolyte
• electrolytes contain free ions- which conduct electricity
• e- are taken away from ions at the positive electrode and given to other ions at the negative electrode

OILRIG- oxidation is loss, reduction is gain

lead bromide electrolysis:

• -ve electrode: Pb2+ + 2e- --> Pb
• +ve electrode: 2Br --> Br2 + 2e-

At the -ve electrode, if a metal and H+ ions are present, the metal ions stay in the solution if the metal is more reactive than hydrogen, hydrogen will be producedthe more reactive an element, the keener to stay in the solution

At the +ve electrode if OH- ions and halide ions (Cl-, Br-, I-) then the molecules of chlorine, bromine or iodine will be formed,if no halide is present, pxygen will be formed

Electrolysis of sodium chloride solution

• -ve electrode: 2H+ +2e- --> H
• +ve electrode: 2Cl- --> Cl2 + 2e-

useful products, chlorine: bleach/plastics, sodium hydroxide: soap

electrolysis is used to extract aluminium from its ore, aluminium's main ore is bauxite, aluminium can be extracted from aluminium oxide (Al2O3) using electrolysis

cryolite is used to reduce the temperature and cost of extracting aluminium by electrolysis

aluminium forms at the -ve electrode and oxygen forms at the +ve electrode, the oxygen reacts with the carbon to produce carbon dioxide!

Electroplating:  uses electrolysis to coat the surface of one metal with another metal

-ve electrode: the metal that you want to plate

+ve electrode: the metal that you wnat to cover it with

Uses: silver- decoration - cheaper    copper- conduction- electronic circuits and computers