Chemistry

HideShow resource information

Atoms and Elements

Atoms have a small nucleus surrounded by electrons. The nucleus is in the middle of the atom and it contains protons and neutrons. Protons have a positive charge and neutrons have no charge therefore, the nucleaus has a positive charge.

The electrons move around the atom and are negatively charged. They occupy shells around the nucleus. 

Atoms have no charge overall, they're neutral. The number of protons is always the same as the number of electrons. If some electrons are added or removed, the atom becomes charged and is then an ion

Elements consist of one type of atom only. Atoms have different numbers of protons, neutrons and electrons. It is the number of protons in the nucleus that decides what type of atom. There are around 100 different elements. 

1 of 16

The Periodic Table

Atoms can be represented by symbols. For example: C = Carbon, O = Oxygen etc. 

The periodic table puts elements with similar properties together. The periodic table is laid out so that elements with similar properties form columns. These vertical columns are called groups and Roman numerals are often used for them. All of the elements in a group have the same number of electrons in their outer shell. This is why they have similar properties. For example, Group 1 elements are all metals and they react the same way. The elements in the final column (Group 0) are called the Noble Gases. They have 8 electrons in their outer shell, arpart from Helium (which has 2). This means that they're stable and unreactive

2 of 16

Electron Shells

Electron shell rules:

1) Electrons must always occupy shells.

2) The lowest energy levels are always filled first - these are the ones closest to the nucleus.

3) Only a certain number of electrons are allowed in each shell:

1st Shell - 2  2nd Shel - 8  3rd shell - 8  etc.

4) Atoms are much happier when they have full electron shells - like the noble gases in Group 0.

5) In most atoms the outer shell is not full and this makes the atom want to react to fill it.

3 of 16

Compounds

When different elements react, atoms form chemical bonds with other stoms to form compounds. It's usually difficult to separate the 2 original elements out again. Making bonds involves atoms giving away, taking or sharing electrons. Only the electrons are involved - it's nothing to do with the nuclei of the atoms at all.

A compound which is formed from a metal and a non-metal consists of ions. The metal atoms llse electrons to form positive ions and the non-metal atoms gain electrons to form negative ions. The opposite charges of the ions mean that they're strongly attracted to each other. This is called IONIC bonding. 

A compound formed from non-metals consists of molecules. Each atom shares an electron with another atom - this is called a COVALENT bond. Eahc atom has to make enough covalent bonds to fill up its outer shell. 

The properties of a compound are totally different from the properties of the original elements. For example, if iron and sulfur react, the compound formed is a dull-grey solid lump,  and doesn't behave anything like either iron or sulphur. 

Compounds can be small molecules like water or huge lattices like sodium chloride. 

4 of 16

Balancing Equations

Atoms aren't lost or made during chemical reactions. You always have the same amount of atoms at the end of a chemical reaction as you did at the start. They're just arranged in different ways. Balanced symbol equations show the atoms at the start and the atoms at the end and how they're arranged. For example:

2Mg  +  O2  -->  2MgO

There must always be the same number of each element on both sides - they can't just diappear. You balance the equation by putting number in front of the formulas where needed. Balance just ONE atom at a time. 

1) Find an element that doesn't balance and pencil in a number to try and sort it out.

2) See where it gets you. It may create another imbalance - if so, just pencil in a new number and see where it gets you. 

3) Carry on chasing unbalanced elements and it'll sort itself out pretty quickly.

5 of 16

Using Limestone 1

Limestone is mainly calcium carbonate.

Limestone is quarried out of the ground - it's great for making into blocks for building with. Fine old buildings like cathedrals are often made purely from limestone blocks. It is sturdy but reacts a lot. 

When it is heated it thermally decomposes to make calcium oxide and carbon dioxide:          CaCO --> CaO + CO2

  • When magnesium, copper, zinc and sodium carbonates are heated, they decompose in the same way e.g. magnesium carbonate --> magnesium oxide + carbon dioxide                (MgCO3 -> MgO + CO2)

Carbon carbonate also reacts with acid to make a calcium salt, carbon dioxide and water e.g.

Calcium carbonate + Sulphuric acid --> Calcium sulphate + carbon dioxide + water

The type of salt produced depends on the type of acid. For example, a reaction with hydrochloric acid would make a chloride. 

6 of 16

Using Limestone 2

Calcium oxide reacts with water to produce calcium hydroxide.

Calcium hydroxide is an alkali which can be used to neutralise acidic soil in fields. Powdered limestone can be used for this too, but the advantage of calcium hydroxide is that it works much faster.

Calcium hydroxide can also be used in a test for carbon dioxide. If you make a solution of calcium hydroxide in water (called limewater) and bubble gas through it, the solution will turn cloudy if there's carbon dioxide in the gas. The cloudiness is caused by the formation of calcium carbonate.

Limestone is used to make other useful things too. Powered limestone is heated in a kiln with powdered clay to make cement.

Cement can be mixed with sand and water to make mortar. Mortar is the stuff you stick bricks together with. You can also add calcium hydroxide to mortar.

Or you can mix cement with sand and aggregate (water and gravel) to make concrete

7 of 16

Using Limestone 3

Quarrying Limestone

Digging limestone out of the ground can cause environmental problems. 

Disadvantages:  For a start, it makes huge ugly holes which permanently change the landscape. -- Quarrying processes, like blasting rockes apart with explosives, make lots of noise and dust in quiet, scenic areas. - Quarrying destroys the habitats of animals and birds. The limestone needs to be transported away - usually in lorries. This causes more noise and pollution. - Waste materials produce unsightly tips

Making stuff from limestone causes pollution too. Cement factories make a lot of dust, which can cause breathing problems for some. Energy is needed to produce cement and quicklime. The energy is likely to come from burning fossil fuels

Advantages: Limestone provides things that people want. - Limestone products are used to neutralise acidic soil. - Limestone is also used in power station chimneys to neutralise sulphur dioxide which is a cause of acid rain. - The quarry provides jobs for people. - Once quarrying is complete, landscaping and restoration of the area is normally required. 

8 of 16

Using limestone 4

Limestone products have advantages and disadvantages. 

  • Limestone is widely available and is cheaper than granite or marble.
  • It is easy to cut.
  • Some limestone is more hard-wearing than marbke but it still looks attractive.
  • Concrete can be poured into moulds to make blocks or panels that can be joined together. It is a very quick and cheap way of constructing buildings.
  • Limestone, concrete and cement do not rot.
  • Concrete does not corrode. 
9 of 16

Getting metals from rocks 1

A metal ore is a rock which contains enough metal to make it worthwhile extracting the metal from it. In many cases the ore is an oxide of the metal. For example, the main aluminium ore is called bauxite - it is aluminium oxide. 

Most metals need to be extracted from their ores using a chemical reaction.

The economics of metal extraction can change over time. 

Metals are extracted from their ores chemically

A metal can be extracted from its ore chemically - by reduction or by electrolysis. Some ores may have to be concentrated before the metal is extracted - this just involves getting rid of the unwanted rocky material. Electrolysis can also be used to purify the extracted metal. 

Some metals can be extracted by reduction with carbon

A metal can be extracted from its ore chemically by reduction using carbon. When an ore is reduced, oxygen is removed from it. The position of the metal in the reactivity series determines whether it can be extracted by reduction with carbon. Metals higher in the series must be removed by electrolysis but anything below carbon can be extracted by reduction using carbon

10 of 16

Getting metals from rocks 2

Some metals have to be extracted by electrolysis

Metals that are more reactive than carbon have to be extracted using electrolysis of molten compounds. An example of a metal that has to be extracted this way is aluminium. However, this process is much more expensive than reduction by carbon because it uses a lot of energy.

Copper is purified by electrolysis

Copper can be easily extracted by reduction with carbon. The ore is heated in a furnace - this is called smelting. However, the copper produced this way is impure - and impure doesn't conduct electricity very well. This isn't very useful because a lot of copper is used to make electrical wiring. So electrolysis is also used to purify it, even though it's quite expensive. This produces very pure copper which is a better conductor. 

11 of 16

Getting metals from rocks 3

Electrolysis means 'splitting up with electricity'

Electrolysis is the breaking down of a substance using electricity. It requires a liquid to conduct the electricity, called the electrlyte. Electrlytes are often metal salt solutions made from the ore (e.g. copper sulphate) or molten metal oxides. The electrolyte has free ions - these conduct the electricity and allow the whole thing to work. Electrons are taken away by the (positive) anode and given away by the (negative) cathode. As ions gain or lose electrons they become atoms or molecules and are released.

You can extract copper from a solution using a displacement reaction

More reactive metals react more vigorously than less reactive metals. If you put a reactive metal into a solution of a dissolved metal compound, the reactive metal will replace the less reactive metal. This is because the more reactive metal bonds more stongly to the non-metal bit of the compound and pushes out the less reactive metal. For example, scrap iron can be used to displace copper from solution: copper sulphate + iron --> iron sulphate + copper

12 of 16

Getting metals from rocks 4

Copper-rich ores are in short supply

The supply of copper-rich ores is limited, so it's important to recycle as much copper as possible. The demand for copper is growing and this may lead to shortages in the future. Scientists are looking into new ways of extracting copper from low-grade ores (ores that only contain small amounts of copper) or from the waste that is currently produced when copper is extracted. Examples of new methods to extract copper are bioleaching and phytomining:

Bioleaching:

This uses bacteria to separate copper from copper sulphide. The bacteria get energy from the bond between copper and sulphur, separating out the copper from the ore in the process. The leachate (the solution produced by the process) contains copper, which can be extracted.

Phytomining:

This involves growing plants in soil that contains copper. The plants can't use or get rid of the copper so it gradually builds up in the leaves. The plants can be harvested, dried and burned in a furnace. The copper can be collected from the ash left in the furnace.

13 of 16

Impacts of extracting metals

Metal extraction can be bad for the environment

People have to balance the social, economical and environmental effects of mining the ores. Most of the issues are exactly the same as those who do with quarrying limestone.

Advantages: Mining metal ores is good because it means that useful products can be made. It also provides local people with jobs and brings money into the area.

Disadvantages: Mining ores is bad for the environment as it causes noise, scarring of the landscape and loss of habitats. Deep mine shafts can also be dangerous for a long time after the mine has been abandoned. 

Recycling metals is important

Mining and extracting metals takes lots of energy, most of which comes from burning fossil fuels. Fossil fuels are running out so it's important to conserve them. Not only this but burning the contributes to acid rain 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 too. Also, there's a finite amount of each metal in the earth, recycling conserves these resources. Recycling metal cuts down on the amount of rubbish that goes to landfill. 

14 of 16

Properties of metals

Metals are strong and bendy and they're great conductors

Most of the elements are metals - so they cover most of the periodic table. All metals have some fairly similar basic properties: - metals are strong but can be bent or hammered into different shapes - they're great for conducting heat - they conduct electricity well. Metals have lots of everyday uses because of these properties..

their strength and bendability makes them handy for making into things like bridges and car bodies, metals are ideal for things like saucepan bases (things that heat must travel through), their conductivity makes them great for making things like electrical wiring

A metal's exact properties decide how it's best used

Copper is a good conductor of electricity, so it's ideal for drawing out into electrical wires. It's hard and strong but can be bent. It doesn't react with water. Used in plumbing. 

Aluminium is corrosion-resistant and has a low density. Pure aluminium isn't very strong but it forms hard, strong alloys. Used in airplanes.

Titanium also has a low density. It is very strong unlike aluminium. It's also corrosion-resistant. Used in hip replacements etc.

15 of 16

Alloys

Pure iron tends to be a bit too bendy

Iron straight from the blast furnace is only 96% iron. The other 4% is impurities such as carbon. The impure iron is used as cast iron - handy to make ornamental railings but it is brittle. All the impurities are removed from most of the blast furnace iron. This pure iron has a regular arrangement of identical atoms. The layers can slide over each other which makes it soft and easily shaped

Most iron is converted into steel - an alloy

Most of the pure iron is changed into alloys called steels. Steels are formed by adding small amounts of carbon and sometimes other metals to the iron

Alloys are harder than pure metals

Different elements have different sized atoms. So when an element such as carbon is added to pure iron, the smaller carbon atom will upset the layers of pure iron atoms, making it more difficult for them to slide over. Alloys are harder

Bronze = Copper + Tin     Cupronickel = Copper + Nickel   Gold alloys are used in jewellery and aluminium alloys are used in aircraft.

16 of 16

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Atoms resources »