Chemistry Revision

The periodic table is a list of all the elements. There are around 100 elements and every substance is made of them.

An element is made of one type of atom. The type of atom is represented by chemical symbols such as Na for the atom of sodium.

Elements are arranged in columns called groups, according to their properties.

Atoms have a nucleus that is surrounded by electrons. 

Compounds are a combination of two or more elements that have joined together.

An atom has a nucleus made up of two types of particle called protons and neutrons. Protons have a positive charge and neutrons have no charge.

The nucleus is surrounded by electrons which are negatively charged.

An atom has no charge overall as there are the same number of electrons as there are protons so they are equal but opposite in charge.

The atomic number of an element is the number of protons it has (protons=electrons so is also number of electrons). 

The mass number is the total number of particles in the nucleus of the atom (protons and neutrons).

Each electron in an atom is in an energy level otherwise known as a shell. The first shell can hold two electrons. The second can hold eight. Electrons 'fill up' the first shell first before working their way up through the rest of the shells.

Elements in the same group on the periodic table have the same number of electrons in their highest level. Group 1 elements have one electron in their highest level.

Group 1(alkali) elements include lithium, sodium and potassium. They react quickly with water and oxygen.

Group 0 (noble gases) elements include helium. They are unreactive and have very stable arrangements of electrons.

When different elements combine they form a compound.

When a metal reacts with a non metal they form an ion. Metal atoms lose one or more electrons to form positively charged ions. Non metal atoms gain one or more electrons to form negatively charged ions. The oppositely charged ions attract each other strongly meaning the compound has ionic bonds.

The chemical formula of an ionic compound shows the simplest ratio of ions in the compound. NaCI shows that there is an equal number of sodium and chloride ions.

When non metals combine they form covalent bonds and molecules are formed.

H2O shows that a water molecule containing two hydrogen atoms and one oxygen atom.

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

In a chemical reaction atoms are not created or destroyed. The number and type of atoms stays the same before and after the reaction.

Mass of products=mass of reactants

Word equations give the name of the reactants and products. Symbol equations give the numbers and types of atom.

Symbol equations should always be balanced. The number of each type of atom should be the same on either side of the equation.

Limestone is quarried because it has many uses. It is used for building by mixing it with calcium oxide to make cement. The cement can then be mixed with sand, aggregate and water to make concrete.

Limestone is mainly calcium carbonate CaCO3.

When it is heated strongly it decomposes to make calcium oxide and carbon dioxide. This is done inside lime kilns and the type of reaction is called thermal decomposition.

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

Metal carbonates decompose to form metl oxide and carbon dioxide when heated strongly.

All carbonates react with acids to produce salt, water and carbon dioxide. Limestone is damaged by acid rain because the calcium carbonate reacts with the acid in the rain.

Limewater (calcium hydroxide solution) is used to test for carbon dioxide. The limewater turns cloudy because it reacts with carbon dioxide to form calcium carbonate.

1. When calcium carbonate is heated it produces carbon dioxide and calcium oxide.

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

3. When more water is added calcium hydroxide solution is produced.

4. The solution reacts with carbon dioxide to produce calcium carbonate.

Cement is made of limestone mixed with clay then heated in a kiln.

Mortar is made of cement, sand and water mixed together. It is used to hold bricks and blocks together in a building.

Concrete is made of cement, aggregate, sand and water mixed together.

We depend on limestone for building materials, and cement and concrete are also used for building.

Quarrying limestone can have negative impacts on the environment and economy.

Advantages:

More employement opportunities for local people, more customers and trade for local business and improved roads.

Disadvantages:

Dust and noise, more traffic and loss of habitats for wildlife.

An ore is a rock that contains enough of a metal or a metal compound to make it worth extracting the metal.

Many ores used to produce iron contain iron oxide. Using caron, iron oxide is reduced at high temperatures in a blast furnace. The cast iron produced contains 96% iron and is hard and brittle.

Removing all the impurities like carbon creates pure iron but it is too soft for many uses.

Iron is mostly used to make steels which are alloys of iron. They are a mixture of iron with carbon and other elements. Alloys can be made so that they have specific uses.

The amounts of carbon and other elements are adjusted when making steels. Low carbon steels are easily shaped and high carbon steels are hard.

Stainless steels contain larger amounts of other metals to resist corrosion.

Aluminium is resistant to corrosion and has a low density. It is more reactive than carbon so it cannot be reduced by carbon.

Aluminium oxide has to be extracted by electrolysis which requires large amounts of electricity and heat so is expensive to extract.

Pure aluminium is not very strong but aluminium alloys are stronger and harder.

Titanium is resistant to corrosion and has a low density. It is very strong and is not above carbon in the reactivity series.

Titanium oxide can be reduced by carbon but it makes it very brittle. Titanium is extracted as an ore which takes several stages and is very expensive.

Copper is extracted from copper rich ores by smelting, where it is heated strongly in a furnace.

Smelting produces impure copper which is then purified by electrolysis. These two processes require a lot of heat and electricity. Copper rich ores are a limited resource.

Phytomining uses plants to absorb copper compounds from the ground. The plants are then burned to produce ash from which the copper can be extracted.

Bioleaching uses bacteria to produce solution containing copper compounds.

The copper is either displaced by something such as scrap iron or is extracted by electrolysis from the compound.

Transition metals are in the centre block of the periodic table and are all metals. They are good conductors of both heat and electricity.

They are strong but can be bent into shape so are useful for vehicles, buildings, pipes and wires.

Copper is a good conductor of heat, doesn't react with water and can bend but is hard enough to keep it's shape. Therefore it is useful for pipes and wires.

Most of the metals we use are not pure elements because they are too soft and are easily bent.

Mining for metal ores produces large amounts of waste material and effects large areas of the environment.

Recycling metals saves energy and resources. Also less fossil fuel is needed to provide the energy to extract the metal from its ore.

Steel girders are used in many buildings for construction. Although this makes the building strong it also has drawbacks such as pollution.

Crude oil contains many different compounds. They boil at different temperatures and burn under different conditions. Crude oil has to be seperated by fractional distillation.

You can seperate a mixture of liquids using distillation and from crude oil it can produce liquids that boil within different temperature ranges. These liquids are called fractions.

Most compounds in crude oil are hydrocarbons, and many of them are alkanes. Alkanes contain as many hydrogen atoms as possible in each molecule and so are called saturated hydrocarbons.

The formula for alkanes is Cn H2n+2. Molecules are represented in different ways- a molecular formula shows the number of each type of atom in the molecule and displayed formula shows how the atoms are bonded together.

Crude oil is seperated into fractions using this process. The boiling point of the different hydrocarbons depends on the size of the molecule- the larger the molecule the higher the boiling point.

The oil is vaporised before it enters the fractional distillation column. Inside the column, the vapours more up, getting cooler as they get higher. The hydrocarbons condense when they reach the level that is their boiling point.

The hydrocarbons with the smallest molecules and lowest boiling points condense at the top.The hydrocarbons with the largest molecules and highest boiling points condense at the bottom.

Fractions with low boiling ranges have a low viscosity so are runny liquids and are very flammable. They burn with clean flames so don't produce much smoke so are useful for fuels.

When pure hydrocarbons burn completely they are oxidised to form carbon dioxide and water.

Incomplete combustion may produce carbon monoxide. Carbon and some unburnt hydrocarbons may also be produced, causing solid particles that contain soot and particulates,

Fossil fuels contain sulfur compounds, When the fuel burns sulfur dioxide is produced. This causes acid rain.

When fuels burn, high temperatures cause oxygen and nitrogen in the aire to combine creating nitrogen oxides that also cause acid rain.

Burning any fuel that contains carbon produces carbon dioxide. It is a greenhouse gas that causes global warming.

Incomplete combustion of fuels produces carbon monoxide. It can also cause tiny solid particulates that cause global dimming.

Burning fuels causes sulfur dioxide and nitrogen oxides. They produce acid rain.

Pollutants can be removed from waste gases after the fuel is burned.

Sulfur can be removed from fuels before they are sold and then burned so that less sulfur dioxide is produced.

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

Biodiesel can be made from vegetable oils extracted from plants:

It makes small carbon dioxide levels because the carbon dioxide given of when it burns was taken from the atmosphere by the plants as they grew. However the plants grown use large areas of farmland.

Ethanol is made from sufar cane or beet and is a biofuel. It is liquid and can be stored. It can be mixed with petrol.

Hydrogen can be used as a fuel and it only produces water when burned. However as it is a gas it takes up a large volume and so is difficult to store in the quantities needed for combustion in engines. It can be produced from water by electrolysis but this needs a large amount of energy.

Large hydrocarbon molecules can be broken down into smaller molecules by cracking.

You can either heat a mixture of hydrocarbon vapours and steam to a very high temperature or you can pass hydrocarbon vapours over a hot catalyst.

During cracking thermal decomposition reactions produce a mixture of smaller molecules. Some are alkanes with are saturated hydrocarbons. They are more useful as fuels because they are smaller molecules.

Some smaller hydrocarbon molecules are called alkenes. They are unsaturated hydrocarbons because they contain fewer hydrogen atoms in comparison to carbon atoms than alkanes. They have a double bond between two carbon atoms. They react with bromine water making it clear.

Plastics are made from large molecules called polymers. Polymers are made from lots of small molecules joined together called monomers. The reaction to make a polymer is called polymerisation.

For example, lots of ethene monomers can join to make polyethene. In the reaction the double bond in each monomer becomes a single bond.

Many plastics we use such as for bags are made from alkenes.

Polymers are designed for particular uses such as for packaging, clothing and medical applications.

Polymer dental fillings are now being used aswell as light sensitive polymers used in plasters to make them easy to remove.

Hydrogels are polymers that can trap water and are used for dressing wounds.

An example of a smart polymer is shape memory polymers that change back to their original shape when conditions are changed.

Fibres for clothing can be coated in polymers to make them waterproof and breathable.

Plastic used for drinks bottles can be recycled to make polyester fibre for clothing, pillows and duvets.

Many polymers are not biodegradable. Plastic rubbish gets everywhere and harms wildlife aswell as taking up space with landfill sites.

Biodegradable plastics are becoming more used. Microorganisms can break down biodegradable plastics.

Biodegradable plastics can be made from plant material for things such as food packaging.

Some plastics can be recycled but it is time consuming to sort the plastics.

Its formula is C2H6O. It can be produced by fermentation of sugar from plants using yeast. Enzymes in the yeast cause the sugar to convert to ethanol and carbon dioxide.

It can also be made by the hydration of ethene. Ethene is reacted with steam at a high temperature with a catalyst. The ethene is obtained from crude oil by cracking.

When produced by fermentation it uses renewable resources. However the ethanol produced must be seperated from the solution by fractional distillation.

When produced from ethene it uses a non renewable resource. However the reaction can run continuously and produces pure ethanol but requires high temperature.

Seeds, nuts and fruits are rich in vegetable oils which can be extracted by crushing and pressing then followed by removing water and impurities. Some oils are extracted by distilling the plant material mixed with water which produces a mixture of oil and water which are seperated.

Vegetable oils give us lots of energy and important nutrients. They can be used as fuels because of the amount of energy they release when burned. They are used to make biofuels such as diesel.

The molecules in veg oils have hydrocarbon chains. Those with double bonds are unsaturated. If there are several double bonds in each molecule they become polyunsaturated. They react with bromine water turning it colourless.

The boiling point of veg oils is higher than water so food is cooked at high temperatures in oil as it cooks faster. It also changes the colour and flavour of the food. Some oil is absorbed so the energy content of the food increases.

Unsaturated oils can react with hydrogen so the double bonds become single. This is called hydrogenation and is done at around 60 degrees with a nickel catalyst. The hydrogenated oils have higher melting points are they are saturated. The reaction is also called hardening as they are hard at room temperature so are used for spreads.

Oil and water don't mix but if we shake them together, tiny droplets form that can be slow to seperate. This mixture is called an emulsion.

They are opaque and thicker than oil and water. Their texture, appearance and ability to stick and coat solids is improved. They are used as milk, cream, salad dressings and ice cream.

Emulsifiers are substances that help stop the oil and water seperating. They keep the emulsion stable.

Emulsifier molecules have a hydrophilic head that loves water. They have a hydrophobic tail that hats water.

Vegetable oils are high in energy and contain important nutrients. Some unsaturated fats are supposed to be better than saturated,

Animal fats and hydrogenated oils contain saturated fats and have been linked to heart disease.

Emulsifiers stop oil and water seperating and makes food creamier. It also means food tastes better and is less obvious that it is high in fat so can be tempting to eat more.