- Created by: isobel2001
- Created on: 28-04-15 17:26
Atoms, Elements and Compounds
All substances are made of atoms. There are over 100 different atoms, and they combine together to provide all the different substances we have on Earth. Elements contain only one type of atom. Compunds contain two or more which are chemically bonded.
Elements have different properties, and in the periodic table they are placed in columns with elements that have similar properties.
Atoms are made of a nucleus which contains protons and neutrons, and electrons which orbit the nucleus in shells. Protons have a +1 charge and a mass of 1, neutrons have a 0 charge and a mass of 1, and electrons have a -1 charge and a mass of almost zero, roughly 0.0005.
An atom doesn't have an overall electric charge because the numbers of protons and electrons are equal, so they cancel each other out. The proton or atomic number tells us how many protons are found in the atom. We can find the number of electrons as the number of protons and electrons is always equal. To calculate the number of neutrons, the number of protons needs to be subtracted from the atomic mass.
Electron Arrangement and Forming Bonds
The first shell orbiting a nucleus can carry a maximum of two electrons. The second, third, and subsequent shells can carry a maximum of eight electrons. Examples: Oxygen has eight electrons so its electron configuration is 2.6. Chlorine has 17 electrons so its electron configuration is 2.8.7. Each shell has a different energy level. The closer the electron shell to the nucleus, the lower its energy level. Elements with a full outer shell are very unreactive. This is because a full shell is the most stable electron configuration. Elements with partially full shells will react to try to get a full outer shell.
Sometimes atoms transfer electrons to form chemical bonds.This happens when a metal reacts with a non-metal. If both atoms which are reacting are non-metals they share electrons. When a metal bonds with a non-metal, the metal transfers its electrons to the non-metal atom. Both atoms become charged particles called ions. Metals form positively charged ions. Non-metals form negatively charged ions. Because opposite charges attract each other there is a strong attraction between the metal and non-metal ions and this forms an ionic bond. Non-metal atoms share electrons. They do this by overlapping, which means each pair of shared electrons forms a chemical bond between the atoms. These are called covalent bonds. (A set of atoms joined by covalent bonds is called a molecule).
Metallic bonds are bonds between metal atoms.
Symbol equations allow us to see how much of each reactant is reacting when a reaction takes place. In chemical reactions there is a conservation of mass. The total mass of products in the reaction is equal to the mass of reactants.
In symbol equations there is the same number of each type of atom on each side of the arrow.
Steps for balancing equations:
1.Write the word equation.
2.Write the correct formula underneath each name.
3.Use a trial and error approach to end up with the same number of each element on each side.
[You must not change any of the small numbers. You can only add big numbers in front of the formulae. Start with an element that is on its own. Look for common multiples].
4.Add in state symbols (if possible).
Limestone and Carbonates
Limestone is mainly made of calcium carbonate (CaCO3). It is mainly made of the remains of tiny plants and animals which lived in the sea. It is obtained by digging it out of the ground. Powdered limestone can be heated with powdered clay to form cement. The cement is mixed with sand, crushed rocks and water to produce concrete, a popular building material. When limestone is strongly heated, it produces carbon dioxide and calcium oxide. (CaCO3 ---> CaO + CO2). This process is called thermal decomposition.
If carbon dioxide is passed through limewater, the limewater turns cloudy. Limewater is a solution of calcium hydroxide (Ca[OH]2). It is alkaline. Carbon dioxide is weakly acidic so it reacts with the alkaline limewater. In the reaction, tiny solid particles of calcium carbonate are formed.
In the limestone cycle, calcium carbonate decomposes to form carbon dioxide and calcium oxide. If a small amount of water is added to calcium oxide, it forms calcium hydroxide. If lots of water is added, a solution of calcium hydroxide (limewater) is formed. If carbon dioxide is added to calcium hydroxide solution, calcium carbonate is formed.
Oxidation is when a substance reacts with oxygen. Combustion is when a substance burns. Displacement is when a more reactive element takes the place of a less reactive one. Decomposition is when a substance splits up, and neutralisation is when an acid and alkali react.
Cement and Concrete
The ancient Egyptians heated limestone and mixed it with water to produce a material that hardened with age. The Romans mixed calcium hydroxide with sand and water to produce mortar. Mortar is a paste that holds building materials together like blocks and bricks. Some people then found that reacting limestone with clay in a kiln produced cement which is a lot more useful.
Cement is a light grey powder made when limestone is heated with clay, then grounded. Cement is mixed with sand and aggregate (gravel) to make concrete. Reinforced concrete is made by pouring concrete around steel rods or bars. The difference between cement and concrete is that concrete is cheaper and stronger than pure cement. As concrete hardens, it binds carbon dioxide in carbonation.
Limestone is quarried from the ground, explosives are used to dislodge limestone from the rock face and this leaves scars on the landscape. After the quarrying has finished, it leaves a large crater in the ground.
Using lime mortar instead of cement would emit less carbon dioxide as it sets. Paper improves concrete by increasing its resistance to cracking, and may be a better option than using steel as it is cheaper.
Environmental factors involved with quarrying are that it will effect the landscape, impact wildlife, damgae the atmosphere e.t.c
Economic factors involved with quarrying are that there is a cost for extracting rock, has an effect on local businesses, an effect on tourism, price for the sold rock e.t.c
Social factors involved with quarrying are that there are jobs, there is disturbing noise, dirt is produced, extra traffic is generated e.t.c
Some metals are found in their native state like gold and silver. Others have to be extracted from their ores. Ores are rocks that contain enough metal compound to make it economic to extract. They can be reacted to remove the metal. Some metals such as potassium are found as compounds in the Earth's crust, because they are so reactive that they will react with oxygen in the air or with water and other chemicals.Iron oxide is a compound that is found in the ore haematite. Iron can be extracted from haematite by heating with carbon because it is lower in the reactivity series. Anything lower than carbon is extracted by reduction using carbon, and anything higher is extracted by electrolysis.
The iron that comes out of a blast furnace is very impure and has limited uses. It is mixed with other elements to form alloys which make it more useful. Carbon steels are simple steels. These are produced by removing most of the carbon from cast iron. There are cheap and used in products like cars, ships, knives.
High carbon steel-relatively high carbon content, very strong but brittle
Low carbon steel-soft and easy to shape, not very strong but less likely to shatter than high carbon steels.
Mild steel-Easy to press into shape, used in production of car bodies.
Alloy-A mixture made by combining two or more elements, one of which must be a metal. They are used for strength, protective layers, electrical plugs, hinges and wheels.
Malleable-Can be easily bent or shaped.
Ductile-Can be drawn out into wires.
Sonorous-Makles a ringing sound when hit.
Aluminium and titanium are metals which are very strong but have low densities. They are shiny, excellent conductors of heat and energy, malleable and ductile, not toxic, and resistant to corrosion.
Aluminium is extracted from its ore by electrolysis, and titanium is extracted by displacement from a more reacttive metal.
Crude oil is a mixture of hydrocarbons. Hydrocarbons are compounds that contain carbon and hydrogen only. Most of the hydrocarbons found in crude oil are alkanes. Alkanes are saturated hydrocarbons with the general formula of CnH2n+2.
Crude oil is separated into useful substances by Fractional distillation. Firstly, crude oil is heated in the furnace to about 370 C. Most of the oil turns into vapour but very high boiling point molecules remain as a liquid. The temperature decreases up the column. As the vapour rises it begins to cool and condense back into a liquid. Molecules with different boiling points condense at different levels in the column. These are called fractions. Molecules with low boiling parts do not condense. Each fraction contains a mix of hydrocarbons with similar properties. Fuel is a chemical which releases energy in a combustion reaction. This is when a substance reacts with oxygen.
Complete combustion occurs when there is lots of oxygen. Products are carbon dioxide and water.
Incomplete combustion occurs when there isn't enough oxygen for complete combustion. Products include carbon monoxide or carbon (soot/particulates) and water.
Other pollutants can be produced in the high temperatures inside combustion engines.
We can break up long alkane chains by a process called cracking. We do this because the smaller chain alkanes are more useful. When long chan alkanes undergo cracking a shorter chan alkane and an alkene are formed.
Alkenes are also hydrocarbons They have one double bond between two of the carbon atoms. Their general formula is CnH2n.
Alkenes are more reactive than alkanes. They undergo addition reactions (making bigger molecules). They can join together to form long molecules called polymers. A polymer is many monomers joined together, and a monomer is individual molecules. In a polymerisation reaction, molecules join together to form a long chain.