Ocr 21St Century Science C5

Front 1

Chemicals in the atmosphere

Back 1

The earth's atmosphere is the sphere of gases surrounding the earth. It contains a mixture of gases, such as nitrogen, oxygen and argon. Some are compounds such as carbon dioxide and water vapour. Most of the gases in the atmosphere are molecular substances. It's 78% nitrogen, 21% oxygen, 1% argon and 0.04% carbon dioxide.

Front 2

Molecular substances

Back 2

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

Front 3

Interpreting data

Back 3

If a substance has a low boiling point in a table, it will either be a liquid or a gas at room temperature, which is around 25°C

Front 4

Covalent bonding

Back 4

Atoms make covalent bonds by sharing electrons with other atoms. This way, both atoms have a full outer shell so are happy. Each covalent bond provides one shared electron for each atoms outer shell. Each atom involved has to make enough covalent bonds to fill up its outer shell. The atoms bond due to electrostatic attraction between the positive nuclei and the negative electrons shared between them

Front 5

Examples of covalent bonding- hydrogen and carbon dioxide

Back 5

Hydrogen, H2 needs just one extra electron to fill it's outer shell, so two hydrogen atoms share their outer electron so that they have a full shell, and a covalent bond is formed.

Carbon needs two electrons to fill it's outer shell. Oxygen needs two. So two double covalent bonds are formed. A double covalent bond has two shared pairs of electrons.

Front 6

2D drawing of a molecule

Back 6

2D or 3D models are ways of representing molecules. For example, with O2 the 2D displayed formula is O=O, showing the atoms and covalent bonds so you can tell how they are joined. The 3D model shows the atoms, covalent bonds, and their arrangement in space next to eachother.

Front 7

The hydrosphere

Back 7

The earth's hydrosphere consists of all water in the seas, oceans, lakes, rivers, puddles and so on. It also contains any compounds that are dissolved in the water. Many of these compounds are ionic compounds called salts- which is why sea water is salty. Examples of salt include sodium chloride, magnesium chloride and potassium bromide.

Front 8

Solid ionic compounds

Back 8

Solid ionic compounds form crystals. Ionic compounds are made of charged ion particles. Ions with opposite charges are strongly attracted to eachother. You get a giant regular lattice of ions that builds up. There are strong ionic chemical bonds between ions. One crystal of salt is one giant ionic lattice, which is why salt crystals are usually cuboid in shape.

Front 9

Ionic compounds and melting points

Back 9

Ionic compounds have high melting and boiling points, as 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 them. When they are dissolved in water or molten, the ions separate and are free to move in the solution. This means that they are able to carry an electric current. When they are solid, the ions aren't free to move so an electric current can't pass through them.

Front 10

Identifying positive ions with flame tests

Back 10

The compounds of metals give a unique colour when heated. Sodium gives a yellow flame, potassium a lilac flame, calcium a brick red flame, and copper a green flame. You can use these colours to detect and identify different ions.

Front 11

Sodium hydroxide and a coloured precipitate

Back 11

A precipitation reaction is where the two solutions react to form an insoluble solid compound called a precipitate. Many metal hydroxides are insoluble and precipitate out of a solution when you add an alkali. Some of these hydroxides have a characteristic colour. To test for positive ions in a solution you can add sodium hydroxide, and form an insoluble hydroxide. The coloured insoluble hydroxide then tells you what metal ions are in the compound.

Front 12

Colours of precipitate

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Calcium= white precipitate

copper (II)= blue

iron (II)= sludgy gree

iron (III)= red brown

Zinc= white at first, but then rediscover in excess NaOH to form a colourless solution

Front 13

Ionic equations

Back 13

Ionic equations are "half" a full equation, they only show the bit you are interested in- nothing else. Eg, the formation of the solid calcium hydroxide from the calcium ions and the hydroxide ions in solution is shown by the ionic equation:

Ca2+ (aq) + 2OH- (aq) = Ca(OH)2 (s). The full equation of this reaction is:

CaCl2 (aq) + 2 NaOH (aq) = Ca (OH)2 (s) + 2 NaCl (aq)

Front 14

Identifying negative ions

Back 14

With dilute hydrochloric acid, carbonates (CO3 2-) will fizz because they give off carbon dioxide. You can test for carbon dioxide using lime water. Carbon dioxide turns lime water cloudy. If it goes cloudy you've identified a carbonate ion.

Front 15

Testing for sulphates

Back 15

You can test for sulphates with Hydrogen chloride and barium chloride. To identify a sulphate ion (SO4 2-) add dilute hydrogen chloride, followed by barium chloride solution. A white precipitation of barium sulphate means the original compound was a sulphate. The hydrochloric acid is added to get rid of any traces of carbonate ions before you do the test. These would also produce a precipitate and confuse the results.

Front 16

Test for halides

Back 16

To identify a halide ion, add dilute nitric acid followed by silver Nitrate solution. The acid is added to get rid of any carbonate ions before the test, however here you use nitric acid instead of hydrogen chloride.

A chloride gives a white precipitation of silver chloride

A bromide gives a cream precipitate of silver bromide

An iodide gives a yellow precipitate of silver iodide.

Front 17

Chemicals in the lithosphere

Back 17

The lithosphere is the earth's rigid outer layer- the crust and part of the mantle below it. It's made of a different 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, whereas sandstone contains a lot of silicon.

Front 18

Diamond

Back 18

Carbon can form 2 types of giant covalent structures- diamond and graphite. Both diamond and graphite are minerals which are found in the earth's crust.

Carbon atoms in diamond each have four covalent bonds in a very rigid giant covalent structure. This structure makes diamond the hardest natural substance, an ideal cutting tool. All those strong covalent bonds give diamond a high melting point. It doesn't conduct electricity because it has no free electrons- not even when molten. It is insoluble in water.

Front 19

Graphite

Back 19

Graphite is also made of carbon but it has a different giant covalent structure. Each carbon atoms only forms three covalent bonds, creating sheets of carbon atoms which are free to slide over each other. This makes graphite slippery. The layers are held together so loosely that they can be rubbed off onto paper to leave a mark- that's how pencils work. Graphite also has a high melting point as the covalent bonds need a lot of energy to break. Only three out of carbons four outer electrons are used in bonds, so there are lots of spare electrons meaning that graphite can carry an electric current.

Front 20

Silicon dioxide

Back 20

Silicon dioxide is also a giant covalent structure, most of the silicon and oxygen in the earth's crust exists as this compound. It is also called silica, and 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 and so has similar properties, such as high melting point and doesn't conduct electricity.

Front 21

Metals from minerals

Back 21

Minerals are solid elements and compounds, rocks are made of these. 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, such as iron ore (called haematite) which is iron oxide, and copper ore (called chalcopyrite) which is copper iron sulphide. For some metals, large amounts of ore need to be mined to obtain a small percentage of valuable minerals. Copper ores contain only around 1% copper.

Front 22

Extraction of more reactive metals

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A few unreactive metals such as gold are found in the earth's crust as the metal itself, rather than as a compound. Most metals need to be extracted from their ores using a chemical reaction. More reactive metals like sodium, are harder to extract- which is why they took longer to discover.

Front 23

Extraction from reduction with carbon

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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. When a metal loses oxygen it is REDUCED. THE carbon gains 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. Less reactive metals can be extracted by heating with carbon. This is because carbon takes away the oxygen from metals less reactive than itself. Metals that are more reactive than carbon can't be extracted by reduction.

Front 24

Electrolysis definition

Back 24

Electrolysis means the decomposition of a substance using electricity. It needs a liquid to conduct the electricity- called the electrolyte. Electrolytes are usually free ions dissolved in water (eg dissolved salts) or molten ionic compounds. It's the free ions that conduct electricity and allow the process to work. For an electrical circuit 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.

Front 25

Electrolysis of aluminium

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The main ore of aluminium is bauxite. Molten aluminium contains free ions so it'll conduct electricity. The positive aluminium ions are attracted to the negative electrode where they gain 3 electrons per ion. This turns them into neutral aluminium atoms, which sink to the bottom. The negative oxygen ions are attracted to the positive electrodes where they each lose two electrons. The neutral oxygen atoms then combine to form O2 molecules.

Front 26

Negative and positive electrodes

Back 26

Metals form positive ions so are attracted to the negative electrode. Non metals form negative ions so are attracted to the positive electrode. Aluminium is produced at the negative electrode, oxygen at the positive electrode. Oxidation is a loss of electrons, reduction a gain of electrons- OIL RIG.

Front 27

Calculating masses

Back 27

Relative atomic mass- Ar, is a way of saying how heavy different atoms are compared to the mass of carbon-12. Carbon-12 has an atomic mass of exactly 12. You can see an elements atomic mass by looking at the periodic table- it is the larger number. To work out the relative formula mass of a compound, add the relative atomic masses together. If there is a small number next to an element, you multiply it's relative atomic mass by this.

Front 28

Calculating masses of compounds with brackets

Back 28

The small number after the brackets means that there's that number of everything inside the bracket. You add together the masses of the elements inside the brackets and then multiply these by the number outside the bracket.

The gram formula mass is the same as the relative atomic mass but has the units in grams. Eg the relative of formula mass of MgCl is 95, and the gram formula mass is 95g.

Front 29

Calculating how much metal can be extracted

Back 29

You can work out how much metal can be extracted from an ore by using the relative atomic mass of the metal and the relative formula mass of the ore. To figure this out, use the equation:

RAM of element x number of atoms÷ RFM of ore

And then multiply your answer by the mass of the ore used.

Front 30

Metals

Back 30

Metals consist of a giant structure. Metallic bonds involve the free electrons which produce all of the properties of metals. The free electrons are found in the outer shell of every metal atom. The positive metal ions are held together in a crystal by the sea of free electrons that can move.

Front 31

Properties of metals- conductors

Back 31

Metals conduct electricity because their free electrons carry an electrical current. They also conduct heat.

Front 32

Properties of metals- malleable

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Metallic bonds mean that metals have a high tensile strength- they're strong and hard to break. The layers of atoms in a metal can slide over each other, meaning that the metals can be hammered, rolled into flat sheets or bent.

Front 33

Properties of metals- high melting and boiling points

Back 33

Metallic bonds are strong so it takes a lot of energy to break them. This makes them useful for making cooking utensils.

Front 34

Ores are finite resources

Back 34

There is a limited amount of ores in the lithosphere- eventually they will run out. People need to balance the social, economic and environmental effects of mining ores. It's good for producing useful products and provides people with jobs, bringing money to the area. Mining ores is bad for the environment as it uses energy and can cause subsidence, destroying habitats. Deep mine shafts can also be dangerous

Front 35

Recycling materials

Back 35

Mining and extracting metals uses a lot of energy, most of which comes from burning fossil fuels. Fossil fuels are running out, and burning them causes the greenhouse effect. Recycling old materials only uses a small fraction of the energy needed to mine and extract a new metal. Recycling saves money and energy, and also conserves the resources for mining metals and also cuts down on the amount of rubbish that gets sent to landfill.