Chemistry - Unit 1

• Atoms, Elements and Compounds

There are about 100 different elements from which all substances are made. The periodic table is a list of elements.

Each element is made of one type of atom.

Atoms are represented by chemical symbols.

The elements are organised into columns called groups whereby those in a group have similar properties.

Atoms have a tiny nucleus surrounded by electrons.

When elements react they join with atoms from other elements. Compounds are formed when two or more elements combine together.

• Atomic Structure

The nucleus contains protons and neutrons. The protons are positively charged whilst the neutrons have no charge.

Electrons are tiny negatively charged particles that move around the nucleus.

The atom has no overall charge because number of protons is equal to the number of electrons, with opposite charges they cancel eachother out.

All atoms within the element have the same number of protons this is the atomic number. The atomic number is also the number of electrons surrounding the nucleus.

The mass number is teh total number of protons and neutrons.

• The Arrangement of Electrons in Atoms

Each electron in an atom is in an energy level. These are represented as shells.

The shells can hold a certain amount of electrons, the first 2, the second 8, the third 8 and the fourth 16. The electron structure is the amount of electrons an atom has organised into shells.

Elements in the same group of the periodic table have the same amount of electrons in their outer shell.

The atoms of un-reactive noble gases all have very stable arrangements of electrons.

• Forming Bonds

When different elements combine they form compounds.

When a metal reacts with a non-metal ions are formed. Metal atoms lose electrons to form positively charged ions, whilst non-metals gain electrons to form negatively charged ions.

The oppositely charged ions attract eachother ad therefore the compound has ionic bonds.

The chemical formula of an ionic compound tells us the simplest ratio of ions in the compound.

When non-metals combine their atoms share electrons to form covalent bonds and molecules are formed.

The chemical formula of a molecule tells us the number of atoms that have bonded together in the molecule.

• Chemical Equations

In chemical reactions the atoms in the reactants re-arrange themselves to from new substances the products.

Atoms can not be destroyed or created therefoe the same amount have to be on both sides of the equation.

When symbol equations are written they should always be balanced.

This means that the number of each type of atoms should be the same on both sides.

Symbol equations are balanced by changing the large numbers in front of the formulae of the reactants and the products.

• Limestone and Its Uses

We quarry large amounts of limestone rock becasue it has many uses.

Limestone is used to make calcium oxide and cement.

We make concrete by adding sand, aggregate and water to cement.

Limestone is mainly calcium carbonate CaCO3.

When heated strongly it decomposes through thermal decomposition to make calcium oxide adn carbon dioxide.

• Reactions of Carbonates

All metal carbonates react in similar ways when heated or reacted with acids.

Metal carbonates decompose to metal oxide and carbon dioxide when they are heated strongly.

A bunsen burner cannot decompose sodium carbonate or potassium carbonate.

All carbonates react with acid to produce a salt, water and carbon dioxide.

Limestone is damaged by acid rain because the calcium carbonate reacts with acids in the rain.

Calcium hydroxide solution is called limewater. It is used to test for carbon dioxide because it turns cloudy when it reacts with carbon dioxide to produce insoluble calcium carbonate.

• The Limestone Cycle

When heated calcium carbonate decomposes to produce calcium oxide and carbon dioxide.

When water is added to calcium oxide, calcium hydroxide is produced.

Calcium hydroxide is an alkali so it can neutralise acids.

Calcium hydroxide is not very soluble in water but dissolves slightly in limewater.

Calcium hydroxide reacts with carbon dioxide to produce calcium carbonate.

• Cement and Concrete

To make cement, limestone is mixed with clay and heated. The product is then ground up into a powder.

Cement is mixed with sand and water to make mortar, which is used to hold bricks together in buildings.

Concrete is made by mixing aggregate with cement, sand and water. 

The mixture can be poured into moulds before it sets solid.

• Limestone Issues

Cement and concrete are needed in most buildings however quarrying limestone can have negative effects. Such as:

• Air pollution
• Noise pollution
• Destroying habitats
• Expensive
• Waste rock

• Extracting Metals

Rock that contains enough of a metal or metal compound to make it worth extracting is called an ore.

Mining ores often requires digging up large amounts of rock. Also the ore may need to be concentrated before the metal is extracted. These processes can produce large amounts of waste and have an effect on the environment.

There are a few metals low in the reactivity series that can just be dug up as pure metal. However most metals are found as compounds so these have to be extracted by chemical reactions.

Metals can be extracted by displacement where are a more reactive element is used. For example metals lower in the reactivity series than carbon can be displaced by it. As the carbon removes the oxygen from the oxide and produces the metal.

• Iron and Steels

Iron oxide is reduced at high temperatures in a blast furnace using carbon. The iron produced contains about 90% iron, making it hard and brittle giving it only a few uses whilst it is cast iron.

Removing all the carbon and impurities makes pure iron but this is too soft for many uses.

Most iron is used to make steels.

Steel is an alloy of iron because it is a mixture of iron and other elements. 

Alloys can be manufactured so they have specific purposes.

The amounts are carefully adjusted as low-carbon steels are easily shaped whilst high-carbon steels are hard.

Some steels such as stainless steel contain larger quantities of other metals and they resist corrosion.

• Aluminium and Titanium

Aluminium

• Low denisty
• Resisitant to corrosion
• Cannot be reduced by carbon
• Extracted by electrolysis
• Expensive to extract
• Alumimnium alloys are strong and hard.

Titanium

• Low density
• Resistant to corrosion
• Can be reduced by carbon
• Several stage extaction
• Expensive to extract
• Strong

• Extracting Copper

Copper can be extracted from copper rich ore by smelting - heating it in a furnace.

Smelting produces impure copper which can be purified by electrolysis.

Smelting and purifying require large amounts of heat and electricity.

Copper rich ores are a limited resource so new ways of extraction are being developed.

Phytoming uses plants to absorb the copper coumpounds from the ground and then the plants are burned so the copper can be extracted from the ash.

Bioleaching uses bacteria to produce solutions containing copper compounds.

The solutions can be reacted with a more reactive metal such as scrap iron to displace copper.

Copper can aslo be extracted from solutions using electrolysis.

• Useful Metals

Elements located in the central block of the periodic table are called the transition metals.

They are all metals and haev similar properties, such as: good conductors of heat and electricty.

Many of them are strong but can be bent or hammered into shape, these make them useful materials for buildings, vehicles, containers, pipes and wires.

Copper is a good conductor of heat and does not react with water and it can be bent but is still strong, making it useful for pipes and tanks in water and heating systems. It is also used for electrical wiring.

Most of the metals we use are not pure elements.

Pure iron, copper, gold and aluminium are soft and so they are usually mixed with other elements to make alloys. iron - Steel, Gold - Jewellry and Aluminium - Aircraft. All of these have been alloyed.

• Metallic Issues

Mining for metal ores involves digging up large amounts of rock and processing it. This can create masses of waste material and take up large amounts of space.

Recycling saves the energy needed to extract the metal also less ore needs to be mined. 

Furthermore less fossil fuel is used in the process of extracting the metal from its ore.

There are a variety of advantages and disadvantages with using metals.

Advantages

• They are strong and good conductors.
• They can be bent into shape and be made into flexible wires.

Disadvantages

• Obtaining metals from ores causes pollution
• More expensive

• Fuels from Crude Oil

Crude oil contains a variety of different compounds that boil at different temperatures. These burn under different conditions and so it needs to be separated in order to produce useful fuels.

Distilation of crude oil separates it into liquids called fractions.

Most of the compounds in crude oil are hydrocarbons, many of these are alkanes whith the formula CnH2n+2.

Alkanes have no double bonds therefore they are called saturated hydrocarbons.

These can be represented in a displayed formula which shows how the atoms are bonded together.

• Fractional Distillation

Crude Oil is separated using fractional distillation where the crude oil is boiled and separated depending on the length of the hydrocarbon.

The Crude Oil is vapourised and fed into the fractioning column this is hotter at the bottom and cooler at the top.

Inside the column the vapours rise cooling as they go. They then condense at their boiling point and escape out the side of the column where it collected.

Hydrocarbons with the smallest molecules are collected at the top of the column, and those with the largest molecules are collected at the bottom.

Fractions with low boiling points have low viscosity and are very flammable burning with clean flames. Therefore they are very useful fuels.

• Burning Fuels

When pure hydrocarbons burn completely they are oxidised to carbon dioxide and water. However they may not always be burned completely.

In a limited supply of air incomplete combustion can produce carbon monoxide and carbon may also be produced. The solid particles produced contain soot and these are called particulates.

Most fossil fuels conatin sulphur compounds and when it burns sulphur dioxide is produced which causes acid rain.

At high temperatures when oxygen and nitrogen combine nitrogen oxides may be produced which also cause acid rain.

• Cleaner Fuels

We burn large amounts of fuels and substances which are spread throughout the atmosphere and damage the environment.

Burning any fuel with carbon in it creates carbon dioxide which causes global warming.

Incomplete comubustion creates carbon monoxide which stops the red blood cells from carrying oxygen.

Solid particles of carbon cause global dimming.

Burning fuels also creates sulphur dioxide and nitrogen oxide which produces acid rain.

Harmful substances can be removed from waste gases before they get into the air.

• Alternative Fuels

Biofuels are made from plant or animal products and are renewable. 

Biodiesel can be made from vegetable oils extracted from plants.

Biodiesel makes little contribution to carbon dioxide because all the carbon dioxide given off it taken in by the plants as they grow, therefore it is good for the environment.

However growing plants takes up large amounts of farmland.

Ethanol made from sugar cane or beet is a biofuel and can be stored and distributed like other fuels. 

• Cracking Hydrocarbons

Large hydrocarbons can be broken into smaller molecules by a process called cracking.

Cracking can be done by heating a mixture of hydrocarbon vapours and steam to a very high temperature and by passing hydrocarbon vapours over a hot catalyst.

During cracking hydrocarbons thermal decompostion produces smaller molecules called alkenes with the formula Cn2n. These are unsaturated because they contain a double bond.

Alkenes react with bromine water turning it from orange to colourless.

• Making Polymers from Alkenes

Plastics are made from very large molecules called polymers. These are made from various small molecules joined together which are called monomers. This reaction is called polymerisation.

Lots of ethene molecules can be combined to produce polyethene in which the double bond breaks to become a single bond and therefore thousands of molecules can now join together.

When drawing the diagram always extend the bonds outside of the brackets and add an N.

Monly plastics we use as bags, bottles and containers are made from alkenes.

• New and Useful Polymers

Scientists can now design polymers with special properties for particular uses. Many of which are used in packaging, clothing and medicine.

New dental fillings only set under UV light.

Hydrogels trap water and can be used when dressing wounds.

Shape-memory polymers change back to their original shape when the temperature or other conditions are changed.

Fibres that make clothing can be coated with polymers to make them waterproof and breathable.

The plastic in drinks bottles can be recycled to make polyester fibres for clothes.

• Plastic Waste

Many plastics are not biodegradable therefore waste builds up and is unslightly and harmful to the environment. Even in landfill sites it takes up space.

Microorganisms can break down biodegradable plastics and they start to break down when in contact with soil.

Plastics made from non-biodegradable polymers can be mixed with cornstarch so the microorgansims can break the cornstarch down and leave the plastic in tiny pieces to be mixed with the soil.

Biodegradable plastics can be made from plant material.

Some plastics can be recycled but there are various types and sorting is difficult.

• Ethanol

Ethanol has the formula C2H6OH, the OH shows it is an alcohol.

It can be produced by the fermentation of sugar from plants using yeast.

Enzymes in the yeast cause the sugar to convert to ethanol and carbon dioxide.

Ethanol can also be produced by the hydration of ethene.

Ethene is reacted with steam at a high temperature in the presence of a catalyst.

Ethanol by fermentation is renewable and it is done at room temperature, however it produces a dilute solution of ethanol which has to be separated by fractional distillation.

Ethanol by ethene is non-renewable and needs to be done at high temperatures, however it can be done continously and produces pure ethanol.

• Extracting Vegetable Oil

Some seeds, nuts and fruits are rich in vegetable oils. These can either be extracted via crushing adn pressing or distillation.

When we eat vegetbale oils thye provide us with a lot of energy and nutrients.

When they burn in air they release a lot of energy therefore they can be used as fuels.

The molecules in vegetable oils have hydrocarbon chains.

Those that have double bonded chains are unsaturated and if it has multiple of these molecules it is polyunsaturated.

Unsaturated oils react with bromine water turning it from orange to colourless.

• Cooking with Vegetable Oils

The boiling point of vegetable oils is much higher than that of water, so food is cooked at higher temperatures in vegetable oils.

It also changes the flavour, colour and texture of the food.

Some of the oil is bsorbed during cooking so the energy content of the food increases.

Unsaturated oils ca be reacted with hydrogen making some of the double bonds become single bonds, this is called hydrogenation and is done at about 60 degrees celsius, using a nickel catalyst.

The hydrogentated oils have a higher melting point because they are more saturated.

The reaction is also called hardening because they are solid at room temperature.

• Everyday Emulsions

Oli and water do not mix and therefore they will ususally seperate. If we stir, shake or beat the liquids together the seperation is slowed down and this type of mixture is called an emulsion.

Emulsions are thicker and opaque in comparison with the oil and water that they are made from. 

This improves their texture, appearance and their ability to coat and stick to solids.

Milk, cream, salad dressings and ice cream are all examples of emulsions.

Emulsifiers are substances that stop the water and oil from sperating.

Emulsifier molecules have a hyrdophillic head and a lypophillic tail, the head is attracted to the water and the tail is attracted to the oil. The molecules surround the oil droplet and suspend it within the water stopping it from separating.

• Food Issues

Vegetable oils are high in energy and contain important nutrients, they contain unsaturated fats and are said to be better for you than saturated fats.

Animal fats and hydrogenated vegetable oils contain saturated fats and are used in a variety of fats.

Saturated fats have been linked to heart disease.

When emulsifiers are added to fats they taste better and therefore people may not release how much fat is in the product.

• Structure of the Planet

The Earth is almost spherical with a diameter of about 12800km.

At the surface there is a layer about 5km to 70km which is called the crust, this varies in thickness all over the Earth.

The mantle under the crust is about 3000km thick it goes about half way to the centre of the Earth. The majority of the mantle is solid however some of it can flow slowly.

The core is about half the diameter of the Earth, it has a high proportion of magnetic metals such as iron and nickel. It has a liquid outer part and a solid inner part.

The atmosphere surrounds the Earth, most of the air is 10km from the surface and most of the atmosphere is 100km from the surface.

All the resources that we depend on come from the crust, the oceans and the atmosphere. This means resources are limited.

• The Restless Earth

Scientists believe the Earth's crust and upper part of mantle is cracked into tectonic plates. Tectonic plates move due to convection currents in the mantle. 

Convection currents are caused by energy released in the mantle from decaying radioactive elements.

Where the plates move, huge forces build up and eventually rocks change shape or move suddenly causing earthquakes, volcanoes or mountains to form.

However scientists can not predict when this will happen.

Alfred Wegner put forward the idea of Continental Drift in 1915, however he could not explain why the continents moved and therefore some scientists did not accept he ideas.

They believed the Earth was shrinking as it cooled.

In the 1960's scientists found new evidence for the idea of plate tectonics.

• The Earth's Atmosphere in the Past

Scientists believe the Earth was formed 4.5 billion years ago, in the first years it was very volcanic and they released carbon dioxide, water vapour and nitrogen.

As the Earth cooled the water vapour condensed into the oceans so the atmosphere was mainly carbon dioxide and nitrogen. Some scientsists believe there are also methane and ammonia.

In the next two billion years bacteria, algae and plants evolved. These took in the carbon dioxide during respiration and released oxygen.

As the number of plants increased the amount of carbon dioxide reduced and the amount of oxygen increased.

• Life on Earth

The plants that produced the oxygen on Earth were probably formed from simple organisms such as plankton and algae.

But we do not know how living things were formed and so many scientsists created theories.

In 1952 the Miller-Urey experiment was created where they used a mixture of gases they believed to be in the early atmosphere and a high voltage spark to simulate lightening. 

After a week amino acids had been produced which are the building blocks for proteins.

Over time a variety of theories have been created but there is no evidence to prove any theory.

• Gases in the Atmosphere

Plants took up the majority of the carbon dioxide in the atmosphere through respiration, therefore when animals ate the plants the carbon ended up as sedimentary rocks and fossil fuels. 

Limestone was formed from the shells and skeletons of marine organisms.

Carbon dioxide dissolves in the oceans and some insoluble compounds formed sedimentary rock.

By 200 million years ago the levels of gases had balanced and are pretty much the same now.

The atmosphere is almost 4/5 nitrogen and 1/5 oxygen. Other gases such as carbon dioxide, water vapour and noble gases make up 1% of the atmosphere.

Gases in the air can be separated by fractional distillation which can produce pure versions of the gases.

• Carbon Dioxide in the Atmosphere

For about 200 million the same amount of carbon dioxide has remained in the atmosphere.

This because natural processes balance out the levels of carbon dioxide.

This is blanced by the carbon cycle whereby things take in and release carbon dioxide to keep the cycle moving.

In the recent years the amount of carbon dioxide has increased massively.

This is due to the large amounts of fossil fuels we burn.