OCR Chemistry 21st Century (new) C5

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Chemicals in the atmosphere

Dry Air is a mixture of Gases
The Earth's atmosphere contains many gases. Some of these gases are elements, e.g. oxygen (O2), Nitrogen (M2) and Argon (Ar) - they contain only one type of atom. Other gases are compounds, e.g. carbon dioxide - they contain more than one type of atom bonded together. There are different amounts of each of these gases in the atmosphere - it's about 78% nitrogen, 21% oxygen, 1% argon and 0.04% carbon dioxide. Most of the gases in the atmosphere are molecular substances.

Molecule Substances have low melting and boiling points
Molecular substances usually exist as small molecules, like CO2 and H2O. The atoms within the molecules are held together by very strong covalent bonds. In contrast, the forces of attraction between these molecules are very weak. You only need a little bit of energy to overcome the weak forces between the molecules - so molecular substances have low melting and boiling points. This means they usually exist as liquids or gases at room temperature. They also don't conduct electricity if pure, simply because they're molecules aren't charged. Most non-metal elements and most compounds formed from non-metal elements are molecular.

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Covalent Bonding

Covalent Bonds- sharing electrons
Sometimes atoms make covalent bonds by sharing electrons with other atoms. This way both atoms feel that they have a full outer shell, and that makes them happy, Each covalent bond provides one extra shared electron for each atom. Each atom involved has to make enough covalent bonds to fill up it's outer shell. The atoms bond due to the electrostatic attraction between the positive nuclei and the negative electrons shared between them.

Hydrogen- H2
..just needs just one extra electron to fill its outer shell. So, two two hydorgen atoms share their outer electron so that they each have a full shell and a covalent bond is formed.

Carbon Dioxide - CO2
Carbon needs four more electrons to fill up. Oxygen needs two. So, two double covalent bonds are formed. A double covalent bond has two shared pairs of electrons. 

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Chemicals in the Hydrosphere

The Earth's hydrosphere is the Oceans

The Earth's hydrosphere consists of all the water in the oceans,seas lakes, e.t.c It also conatins any compounds that are dissolved in the water. Many of these compounds are ionic compounds called salts - that's why sea water is 'salty'. Examples of salts are sodium chloride (NaCl), Magnesium chloride (MgCl2).

Solid Ionic Compounds form crystals

Ionic compounds are made of charged particles called ions. Ions with opposite charges are strongly attracted to one anothe. You get a massive giant lattice of ions built up. There are very strong chemical bonds called ionic bonds between all the ions. A single crystal of salt is one giant ionic lattice, which is why salt crystals tend to be cuboid in shape.  

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Chemicals in the Hydrosphere (2)

Ionic Compounds have high melting and Boiling Points


The forces of attraction between the ions are very strong. It takes a lot of energy to overcome these forces and melt the compound, and even more energy to boil it. So ionic compounds have high melting and boiling points, which makes them solids at room temperature. 

They conduct electricity when dissolved or molten

When an ionic compound dissolves, the ions seperate and are all free to move in the solution. This means that they're able to carry an electric current. Similarly, when an ionic compound melts, the ions are free to move. So, they'll carry an electric current. When an ionic compound is a solid, the ions aren't free to move, and so an electric current can't pass through the substance.

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Identifying Positive Ions

Flame Tests - spot the colour
Compounds of metals give a characteristic colour when heated.
Sodium, Potassium, Calcium, Copper 

Add Sodium Hydroxide and looked for a coloured precipitate

A precipitate reaction is where two solutions react to form an insoluble solid compound called a precipitate.

  • Many metal hydroxides are insoluble and precipitate out of solution when you add an alkali. 
  • Some of these hydroxides have a charateristic colour.
  • If you get a coloured insoluble hydroxide you can then identify the metal ion that was in the compound.

Ionic Equations show just the useful bits of reactions

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Identifying Negative Ions

  • Hydrochloric Acid can Help Detect Carbonates

    If the limewater turns cloudy, there was carbon present in the original compound. 
  • Test for Sulfates with Hydrochloric Acid and Barium Chloride

    A white precipitate of barium sulfate means the original compound was a sulfate. 
  • Test for Halides (Cl-, Br-, I-) with Niric Acid and Silver NItrate

    Chloride --> white precipitate of silver chloride
    Bromide --> cream precipitate of silver bromide.
    Iodide ---> yellow precipitate of silver iodide. 
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Chemicals in the Lithosphere

The Earth's Lithosphere is made up of a mixture of minerals
The lithosphere is the earth's rigid outer layer - the crust and part of the mantle below it.  It's made up of a mixture of minerals, often containing silicon, oxygen and aluminium. Different types of rock contain different minerals and different elements. For example, limestone contains a lot of calcium, where as sandstone contains a lot of silicon.

Carbon forms giant covalent structures

Diamond

The carbon atoms in diamond each form four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance. All those strong covalent bonds give diamond a very high melting point. It doesn't conduct electricity because it has no free electrons - not even when molten. It's insoluble in water.

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Chemicals in the Lithosphere (2)

Graphite  

Graphite is also made from carbon but it has a different giant covalent structure. Each carbon atom only forms three covalent bonds, creating sheets of carbon atoms, which are free to slide over each other. This makes graphite slippery, so it's useful as a lubricant. The layers are held together so loosely that they can be rubbed off onto paper.Graphite also has a high melting point - the covalent bonds need lots of energy to break. Only three out of each carbon's four outer electrons are used in bonds, so there are lots of spare electrons. This means graphite conducts electricity - it's used for electrodes.

Silicon Dioxide is also a giant covalent structure

Most of the silicon and oxygen in the Earth's crust exists as the compound silicon dioxide. This is what sand is made of. each grain of sand is one giant structure of silicon and oxygen. Silicon dioxide has a similar structure to diamond so has similar properties too, e.g. high melting point and doesn't conduct electricity. 

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Metals from minerals

Ores contain enough metal to make extraction worthwhile

Rocks are made of minerals. MInerals are just solid elements and compounds. Metal ores are rocks that contain varying amounts of minerals from which metals can be extracted. In many cases the ore is an oxide of the metal For some metals, large amounts of ore need to be mined just to obtain small percentages of valuable minerals. A good example of this is copper mining - copper ores typically contain about 1% copper.

More Reactive metals are harder to get

A few unreactive metals like gold are found in the Earth as the metal itself, rather than as a compound. But most metals need to be extracted from their ores using a chemical reaction. More reactive metals, such as sodium, are harder to extract - that's why it took longer to extract them.

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Reduction using Carbon

Some metals can be extracted by reduction with carbon

A common way of extracting a metal from its ore is chemical reduction using carbon or carbon monoxide. When an ore is reduced, oxygen is removed from it. The reaction then produces carbon dioxide. When a metal oxide loses its oxygen it is reduced. The carbon gains the oxygen and is OXIDISED. How reactive the metal is compared to carbon determines whether it can be extracted by reduction with carbon or carbon monoxide.

  • metals that are less reactive than carbon can be extracted by reduction by heating with carbon.
  • This is because carbon can take the oxygen away from metals which are less reactive than itself. Metals that are more reactive than carbon can't be extracted by reduction - they have to use electrolysis.
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Electrolysis

Electrolysis means 'Splitting Up with Electricity'

Electrolysis is the decomposwition of a substance using electricity. It needs a liquid to conduct the electricity - called the elctrolyte. Electrolytes are usually free ions dissolved in water or molten ionic compounds. It's the free ions that conduct the electricity and allow the whole thing to work. For an electrical circiut to be complete, there's got to be a flow of electrons. In electrolysis, electrons are taken away from ions at the positive electrode and given to other ions at the negative electrode. As ions gain or lose electrons they become atoms or molecules.

Electrolysis removes aluminimum from its ore

The main ore of aluminium is bauxite which contains aluminium oxide. Molten Aluminium oxide contains free ions- so it'll conduct electricity. The positive Al 3+ ions are attracted to the negative electrode where they pick up 3 electrons and turn into neutral aluminium atoms. These then sink to the bottom. The negative O2- ions are attracted to the positive electrode where they each lose two electrons. The neutral oxygen atoms will then combine to form O2 molecules.

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Electrolysis (2)

At the negative electrode:
Al 3+   +  3e-  -----> Al

Reduction - a gain of electrons

At the positive elctrode:
2O2- ----> O2 + 4e-

oxidation -- a loss of electrons

Formula 

Aluminium oxide       ----->   Aluminium        +    Oxygen
2Al2O3 (l)                 ------>  4Al (l)                 +    3O2 (g) 

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Electrolysis (3)

Diagram


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Calculating Masses

Relative Atomic Mass -- Easy Peasy
This is just a way of saying how heavy different atoms are compared with the mass of an atom of carbon-12. 
You can work out an element's relative atomic mass by looking at the periodic table. The bigger number of the two is the relative atomic mass.

Relative Formula Mass
 If you have a compound like MgCl2, then it has a relative formula mass, which is just all the relative atomic masses added together. For MgCl2, it would be:

MgCl2
24 + (35.5 x 2) = 95

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Metals

Metal Properties are all due to the sea of free electrons

  • Metals consist of a giant structure
  • Metallic bonds involve the all-important free electrons, which produce all the properties of metals. 
  • These free electrons come from the outer shell of every metal atom in the structure.
  • The positively charged metal ions are held together in a crystal by a sea of free elctrons that can move.

They're good conductors of Heat and Electricity
..because of the free elctrons

Most Metals are Strong and Malleable

They generally have high melting and boiling points
..because the metallic bonds are very strong, so it takes a lot of energy to break them.  

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Environmental Impact (1)

Ores are Finite Resources

This means that there's a limited amount of them - eventually they'll run out. People have to balance the social, economic and environmental effects of minig the ores. So, mining metal ores is good because useful products can be made. It also provides local people with jobs and brings money to the area. This means services such as transport and health can be improved. But mining ores is bad for the environment as it uses loads of energy, scars the landscape and destroys habitats. Also, noise, dust and pollution are caused by an increasein traffic. Deep mine shafts can also be dangerous for a long time after the mine has been abandoned.

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Environmental Impact (2)

Recycling Metals is important 

Mining and extracting emtals takes lots of energy, most of which comes from buring fossil fuels. Fossil fuels are running out so it's important to conserve them,. Not only this, but buring them contributes to acid rain, global dimming and climate change. Recycling metals only uses a small fraction of the energy needed to mine and extract new metal. Energy doesn't come cheap, so recycling saves money. There's a finite amount of each metal in the earth so recycing helps conserve these resources. Recycling metal cuts down on the amount of rubbish in landfill which takes up space and pollutes the surroundings.

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Comments

Anokhi

Hi, These are great..... do you have any notes on C6?

Phoebe-rose Burris


Another great set of notes! I'm with Anokhi C6 revision notes would be great too!

Gingeh

These were really helpful thank you! Do you have any on C6?

navraj01

on the 2nd page, 2nd paragraph you spelled hydrogen wrong

navraj01

on the 2nd page, 2nd paragraph you spelled hydrogen wrong

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