Atoms with a full outer shell of electrons are stable. An atom will react chemically with another atom that either gives, takes or shares electrons so that both atoms have a full outer shell.
We use atoms and symbols to represent and explain what happens to substances in chemical reactions.
When atoms of two (or more) different elements react chemically, they form a compound.
- In water, H2O, each hydrogen atom shairs a pair of electrons with the oxygen atom. Each shared pair of electrons is a covalent bond.
- A magnesium atom joins an oxygen atom to form magnesium oxide, MgO. Magnesium gives two electrons, and oxygen takes them. Both then have a charge- they are ions. Their opposite charges attract, forming an ionic bond.
Two atoms of the same element can join to form molecules: in chlorine, Cl2, the atoms share electrons in a covalent bond.
Extraction of limestone
Limestone is calcium carbonate, CaCO3. It is the main compound in chalk and marble which, like limestone, are mined from quarries.Limestone is a building material. It is also used to make cement, concrete and glass and extract iron from it's ore.
A limestone quarry is likely to supply jobs for local people, have a good road system and local income to pay for health care, recreational and other facilities.
It may also scar the landscape and eliminate natural habitats, cause dust pollution and heavy traffic that damages buildings and roads, and produce noise from quarry blasting.
Limestones rocks were formed at the bottom of warm seas, millions of years ago.
Thermal decompositon of limestone
Limestone is mainly calcium carbonate.Its formula CaCO3 tells you the type and ratio of atoms joined together. The ratio is: 1 calcium atom: 1 calcium atom: 3 oxygen atoms.
When strongly heated, limestone undergoes a thermal decompostion reaction, decomposing into similar compounds. The word and symbol equations are:
Calcium Carbonate ---> Calcium Oxide + Carbon Dioxide
CaCO3 ---> CaO + CO2
The equation is balanced, with the same number of each type of atom on either side. In this and all reactions, no atoms are lost or made, and: mass of products = mass of reactants.
Calcium Oxide is quicklime. It reacts with water to produce Calcium Hydroxide, Ca(OH)2, slaked lime:
Calcium Oxide + water ---> Calcium Hydroxide
Adding water to quicklime forms solid slake lime, calcium hydroxide, Ca(OH)2, adding more water dissolving it, giving calcium hydroxide solution - limewater.
Uses of Limestone
Cement- is formed when crushed limestone and clay are strongly heated. Cement reacts with water: it is "hydrated".
Cement Mortar- is a mixture of cement, sand and water used to join bricks. Cement Mortar has largely replaced lime mortar: It sets faster and stronger, so lasts longer.
Concrete- is cement with sand and rock chippings added. The mixture reacts with water and sets to make a much stronger material than cement mortar.
To make glass, sand and sodium carbonate are heated with limestone. A very high temp melts the sand and breaks down the sodium carbonate and limestone to sodium oxide and calcium oxide.
When cooled slowly, pure molten silicon dioxide gives glass with a regular crystalline lattice structure. It shatters easily when hit. When cooled rapidly, the glass mixture fails to form a lattice structure. As ions, the sodium and calcium makes the structure more irregular, increases toughness of glass.
The raw materials for making iron are abundant and economical to extract and use. Haematite, iron ore is mainly iron(iii) oxide, Fe2O3. Coke is purified coal, C (carbon). Limestone is calcium carbonate, CaCO3. Air contains oxygen, O2. Iron ore is reduced to iron in a blast furnace.
Carbon reacts with oxygen, forming carbon dioxide. More carbon converts the carbon dioxide to carbon monoxide.
Metals that are less reactive than carbon can be extracted from their ocides by reduction with carbon. This applies to iron (ii) oxide: carbon in carbon monoxide reduces it to iron (carbon monoxide is oxidised to carbon dioxide): 3CO+Fe2O3 --> 3CO2 + 2Fe.
Limestone helps remove impurities that form ****. Molten iron runs off at the bottom. Oxidisation (addition of oxygen) and reduction (removal of oxygen) always occur together in a reaction.
Different types of iron
Iron from the blast furnace is 96% iron. It is remelted and mouled to form cast iron objects which are hard and strong. Impurities that make cast iron brittle are removed to give wrought iron (pure iron) that is softer and easily shaped. Carbon and other metals are added in different quantities to pure iron, to make different types of steel.
The atoms in pure iron are bonded together in a 3-D regular, closely oacked lattice. Layers of atoms can slide over eachother, so wrought iron is relatively soft and easily bent and shaped.
In steels, the different sized atoms of added carbon and other metals distort the lattice. Without regular layer of atoms to slide over one another, steels are much harder and more rigid than pure iron.
Steel is an alloy if iron. Types of steel contain other metals and carbon in varying amounts. Compared with pure iron, steel is harder, stronger, resists corrosion and is not brittle.
The different sized atoms in steels make alloys hard and rigid:
- Low carbon steels (0.4% carbon or less) are soft and easy to shape and are used in car bodies.
-Medium carbon steels (about 0.8% carbon) are harder, more rigid and stronger, suitable for hand tools.
-High carbon steeks (1.0-1.5% carbon) are very hard, ideal for knives but may snap if bent
- Stainless steel is 70% iron, 20% chromium, 10% nickel. It is very hard and resistant to corrosion (rust). Stainless steel is generally more costly to produce than other steels.