[V] Chemistry - C5

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Molecules in the air

Air is a mixture of many different gases including water vapour. DRY AIR is air that has had the water vapour removed. The gases in dry air are the elements nitrogen, oxgen and argon and compounds such as carbon dioxide. The particles that make up these gases are atoms, such as argon atoms, and MOLECULES. Molecules are groups of atoms joined together. 

Oxygen and nitrogen form molecules containing two atoms of the element. The formulae of the molecules are written as O2, and N2. Carbon dioxide is a simple molecular compound. It's molecules are made up of two atoms of oxygen joined to an atom of carbon. The formula is CO2. 

Other non-metallic elements form molecules, such as Chlorine, Cl2-. Compounds between non-metallic elements are also molecular, for example water, H2O. 

Atoms in a molecule are joined together by one or more COVALENT BONDS. We cannot see molecules in a gas and we cannot see the bonds between the atoms. Nevertheless we can make models to show the arrangement of atoms in molecules. These models may be 3D, which you can hold and rotate to examine them, or they may be diagrams representing the molecules. One type of diagram is a 2D drawing showing the atoms as straight lines. Another type of diagram shows the 3D shape of the molecule. Arrows and dotted lines reveal whether atoms should be above or below the surface of the paper.

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Covalent bonds

A covalent bond is formed when atoms share a pair of electrons. Usually one electron comes from the outer shell of each atom. The atoms in the molecule have the same electron arrangement as unreactive gas atoms such as argon. This arrangemenet can be shown by "dot and cross" diagrams.

A bond is a force holding atoms together. In a covalent bond the force arises because of the positively charged nuclei of both atoms are attracted to the negatively charged shared pair of electrons. This force holds the atoms together strongly and rigidly. The atoms in a molecule cannot move freely, which explains why simple molecules have a fixed shape. 

Some properties of substances, such as melting and boiling points, can be measured. The data can be put as numbers into tables and chart. As these data show QUANTITES we refer to them as QUANTITATIVE data. 

We can also compare the QUALITIES of one substance with another, such as saying one is harder than another, or we can describe their behaviour. This is QUALITATIVE DATA.

Pure molecular substances with small molecules have similar properties. They are poor conductors of electricity. When they are solid (such as iodine) they are brittle and weak. 

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Forces between molecules

We can use ideas about forces between molecules to explain the properties of simple molecular substances. We can use our imagination to visualise what happens when the molecules of a substance are in the solid, liquid or gaseous states. The attractive forces between small molecules are very weak. Very little energy is needed for molecules to overcome these forces and move apart, so simple molecular substances have low melting and boiling points. This explains why the substances in the air are gases at room temperature.

Molecules of elements and compounds have no electrical charge, so pure molecular substances cannot conduct electricity. We can use these ideas to predict that other substances with low melting and boiling points, such as bromine, are also molecular substances. 

For small covalent molecules, the forces between molecules (known as INTERMOLECULAR forces) are eak, but the forces within molecules (known as INTRAMOLECULAR forces) are strong. This is because attractions between the nuclei and shared electrons form covalent bonds that hold atoms together inside molecules. These covalent bonds are strong.

Thus, when a molecular substance with small molecules is melted, boiled or stretched, the molecules are easily separated from one another but the molecules themselves are not broken up into separate atoms.

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Ionic crystals

The water, ice and snow on the Earth's surface and the water vapour in the atmosphere form the HYDROSPHERE. In the water, there are dissolved SALTS. When the water evaporates the salts are left as solid CRYSTALS with regular shapes and flat sides. Salts are IONIC COMPOUNDS made up of particles called ions which have a positive or negative electrical charge. The ions in the crystal are arranged in a regular 3D pattern called a LATTICE. The pattern is repeated over and over again in all directions, forming a giant ionic crystal lattice. 

Salts have high melting and boiling points and can conduct electricity when melted or dissolved in water. We can use data about salts and our imagination to explain why they have these properties. There is a strong force of attraction between positively charged and negatively charged ions which holds them together in the crystal lattice. We call this force an IONIC BOND. The size and shape of the ions determines the pattern they form in the lattice when the bonding force pulls them together. The ions need a lot of energy to break out of the lattice and become a liquid. This is why ionic compounds have a high melting and boiling point. 

Salts are hard and cannot be compressed because within the lattice the ions are packed closely together. This also means that a solid ionic compound cannot conduct electricity. The ions are not free to move. When the salt is melted, the ions become free to move so they can conduct electricity.

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Ionic compounds

When an ionic compound is dissolved in water, the water molecules surround the ions so the lattice breaks up. The dissolved ions are free to move and so can conduct electricity.

The attraction between ions acts in all directions so each ion is bonded to all its oppositely charged neighbours (6).

An ionic compound has no overall charge. This means that the number of positive charges must just balance and cancel out the negative charges.

In sodium chloride, the sodium ions have a 1+ charge and the chloride ions have a 1- charge. The charges are equal and opposite so sodium chloride is made up of equal numbers of sodium and chloride atoms. The formula is written as NaCl.

Magnesium ions have a 2+ charge and need two chlorine ions to balance them, so the formula of magnesium chloride s MgCl2-

The sulfate ion SO4,2- is called a MOLECULAR ION. It is formed from four atoms of oxygen and one of sulfur joined together by covalent bonds. Overall, it has two extra electrons so it has a charge of 2-. A sulfate ion needs two sodium ions to balance it, so the formula of sodium sulfate is Na2SO4.

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Testing metal ions

Ions have some properties which are the same whatever the compound they are part of, for instance compounds containing the copper ion are often blue. When an alkali such as sodium hydroxide solution is added to a solution containing copper ions, a blue solid is formed. The blue solid identifies the solution contains copper ions. The solid formed when two solutions mix is called a PRECIPITATE.

Other metal ions can be identifies from the solid formed when sodium hydroxide solution is added to solutions of their salts. 

Ion Observation

calcium, Ca2+ white precipitate (insoluble in excess)

copper, Cu2+ light blue precipitate (insoluble in excess)

iron (II), Fe2+ green precipitate (insoluble in excess)

iron (III), Fe3+ red-brown precipitate (insoluble in excess)

zinc, Zn2+ white precipitate (soluble in excess, giving a colourless solution)

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Solubility

In general, ionic compounds that are INSOLUBLE can be formed as a precipitate when a solution containing the positive ion of the compound is mixed with a solution containing the negative ion of the compound. This equation shows how to make a precipitate of copper hydroxide.

copper sulfate solution + sodium hydroxide solution ---> copper hydroxide precipitate Cu(OH)2     ---> sodium sulfate solution Na2SO4 

This is a PRECIPITATION REACTION.

Cu2+ (aq) + 2OH- (aq) ---> Cu(OH)2 (s)

This is the IONIC EQUATION for the precipitation of copper hydroxide. 

Since all metal nitrates and all sodium compounds are soluble, we can make an insoluble compound by mixing all the appropropriate solutions. For example, calcium carbonate is an insoluble compound. It can be precipitated from a solution containing calcium and carbonate ions.

Mixing calcium nitrate solution and sodium carbonate solution would give the precipitate that we want. Then we need to balance the equation to get a balanced equation for the precipitation reaction.

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Analysing salts

To test a chemical for the presence of negative ions, first add an acid. If the chemical contains the carbonate ion it will bubble and fizz. This is called EFFERVESCENE. 

Next, divide the solution of the sample into two parts. Add a few drops of silver nitrate to one part and a few drops of barium chloride or barium nitrate to the other part. A precipitate is a solid that makes the mixture look cloudy.

A compound containing the carbonate ion reacts with an acid to give off carbon dioxide gas. Nitric acid is used because it contains nitrate ions, which do not affect the other tests. Other salts will dissolve in nitric acid without effervescence. 

calcium carbonate + nitric acid ---> calcium nitrate + water + carbon dioxide gas

The tests for halide and sulfate ions make use of the fact that particular compounds containing these ions are insoluble. When barium ions in solution are added to a sample containing sulfate ions, for example, insoluble barium sulfate is formed. If chloride, bromide or iodine ions are in a sample, then a precipitate is formed with silver ions but not with barium ions. The colour of the precipitate is different for chloride, bromide and iodine. The silver chloride is white at first but in light it soon becomes pale blue-grey.

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Minerals and giant molecules

We find all the naturally occuring elements in the MINERALS that make up the LITHOSPHERE. The lithosphere is a layer of the Earth made up of the solid rocks of the crust and the top part of the mantle below. Minerals are solid substances found naturally in which the atoms or ions are arranged in a regular crystal lattice. Rocks are usually a mixture of minerals.

Diamond is a mineral; so is graphite. They are both crystalline forms of carbon.

Oxygen - 47% Silicon - 28% Aluminium - 8.1% Iron - 5%

Calcium - 3.6% Sodium - 2.8% Potassium - 2.6% Magnesium - 2.1%

Titanium - 0.4% Hydrogen - 0.1%

The information above shows some of the elements most commonly found in the Earth's crust. Silicon and oxygen are usually found joined together in compounds such as silicon dioxide. This is found in minerals such as quartz. 

Diamonds are formed in the top of the mantle, which is part of the Earth's lithosphere. 

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Giant covalent structures

Atoms of carbon can only form bonds with each other by sharing electrons in covalent bonds. Unlike simple molecules (such as methane), diamond and graphite are made up of GIANT COVALENT STRUCTURES in which a very large number of carbon atoms are linked together in a regular pattern. 

Covalent bonds are strong. In diamond the atoms are held together rigidly so diamond is very hard. It requires a lot of energy to allow the carbon atoms to move, so diamond has very high melting and boiling points and does not dissolve in water. There are no free charged particles in diamond so it does not conduct electricity when solid or when melted.

Silicon dioxide is also a giant covalent structure. The atoms are arranged in a similar pattern to the carbon atoms in diamond, so silicon dioxide is also hard, has high melting and boiling points, does not conduct electricity and does not dissolve in water.

Giant covalent and simple molecular substances both have strong covalent bonds holding atoms together.

Diamond and graphite are both forms of pure carbon with high melting points. They are both giant covalent structures but their different appearances and properties show that the arrangement of the atoms is different.

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Extracting metals

In diamond, each carbon atom is covalently bonded to four atoms in a tetrahedral 3D lattice.

In graphite each atom only has three covalent bonds and the atoms are arranged hexagonally in flat sheets. The sheets are strong but there is only a week force between the layers so they can slide over each other. This is what makes graphite a good lubricant and why graphite pencils can write on paper. Each carbon atom has one electron not used in the covalent bonding. These electrons can move easily between the layers, allowing graphite to conduct electricity. 

Different minerals contain different metals. Rocks that contain minerals from which metals can be extracted are called ORES. 

Some ores contain the metal oxide. The metal oxide can be heated with carbon to produce the metal. This method is used to obtain metals such as copper, iron and zinc from their ores.

Iron oxide and carbon are heated in a blast furnace. The carbon takes away the oxygen and REDUCES the iron oxide to iron. The iron runs out of the furnace as a liquid.

Iron oxide + carbon ---> iron + carbon dioxide

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Oxidation and reduction

Iron is one of the most abundant elements in the Earth's crust but it is very expensive to remove all the waste rock. It is not possible for the mining companies to make a profit if there is less than about 25% of the iron in the ore. Mineralogists look for ores with a very high percentage of minerals of iron, such as haematite and magnetite. These minerals are found in many parts of the world. Copper is a much more valuable metal and makes up a much smaller percentage of the Earth's crust. It is economic to extract copper from ores which have less than 1% of copper in them. This means that a huge amount of rock has to be mined to get at the copper.

The extraction of zinc uses carbon to reduce zinc oxide to zinc. This is the equation for the reaction:

2ZnO + C ---> 2Zn + CO2 zinc oxide + carbon ---> zinc + carbon dioxide

The zinc oxide loses oxygen. This process is called REDUCTION. The zinc oxide has been REDUCED. The carbon has gained oxygen. This process is called OXIDATION. The carbon has been OXIDISED. The reaction for extracting metals from their ore is a REDOX reaction. This means it involves both oxidation and reduction. OXIDATION IS THE GAIN OF OXYGEN BY A SUBSTANCE WHILE REDUCTION IS THE LOSS OF OXYGEN BY A SUBSTANCE.

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Chemistry in shorthand

Word equations show the chemicals that reacted (the reactants) in a chemical reaction and the chemicals that were made (the products). A reaction can be described in words, like this:

"Dilute nitric acid was added to solid sodium carbonate. There was effervescence as carbon dioxide was given off. Sodium nitrate was left in solution in water".

This can be summarised by the word equation:

nitric acid solution + sodium carbonate solid ---> sodium nitrate solution + carbon dioxide + water

Balanced symbol equations are the language of chemistry and are understood by chemists everywhere, regardless of their native language. Equations are also a short hand code for describing reactions. You need to recognise the symbols of the elements and understand how the symbols are put together to create formulae. The small letters in brackets after a formula show the state the substance is in. A balanced equation shows the reactants and products and also the number of atoms of each element and how many covalent molecules and ionic "formula units".

For example: 2Fe2O3 + 3C ---> 4Fe + 3CO2

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Balancing equations

To write a balanced equation you must follow these steps. 

1. Write a word equation for the reaction. 

2. Replace the words with the symbol of the elements that exist as atoms (metals and elements that have giant molecules such a carbon), the formulae of simple molecules and the formula units of ionic compounds.

3. Balance the equation to make the number of atoms of each element the same on both sides of the equation. Do this by placing numbers in front of each substance. 

2ZnO + C ---> 2Zn + CO2

Hydrogen, oxygen, nitrogen, fluorine, chlorine, bromine and iodine are all elements that exist naturally as simple diatomic molecules (eg: H2)

The RELATIVE ATOMIC MASS (RAM) of an atom is the mass of an atom compared to the mass of an atom of carbon, which is given the value 12. Magnesium atoms, which have twice the mass of carbon atoms, have an RAM of 24. You can find the RAM of an element in the Periodic Table. The RAM is the top number in the element's box.

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Formula mass

The RELATIVE FORMULA MASS (RFM) of a compound is the sum of the RAMs of all the atoms or ions shown in its formula. An ion has the same mass as the atom because the electrons that have been lost or gained have almost no mass. To find the RFM or water, H2O, we need the RAMs of hydrogen (1) and oxygen (16). The RFM of water is (2 x 1) + (16) = 18.

We can measure the number of grams of an element or compound represented by its RAM or RFM. This is the GRAM FORMULA MASS. For water it is 18g. 

What is the gram formula mass of sodium hydroxide NaOH? What mass of sodium is there in it? (RAM: Na = 23, O = 16, H = 1)

The RFM of sodium hydroxide is (23) + (16+1) = 40. 

The gram formula mass of sodium hydroxide is 40g, of which 23g is sodium. 

When calculating the relative formula mass, make sure you take account of the small numbers after the symbols in the formula. 

We can use gram formula mass and the formula to work out how much of a metal there is in any amount of a mineral.

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Calculating the mass of a metal

We can use gram formula mass and the formula to work out how much of a metal there is in any amount of a mineral.

Haematite is a common mineral of iron with formula Fe2O3 (iron oxide). What is the percentage of iron in haematite. RAM: Fe = 56, O = 16

The relative formula mass is (56 x 2) + (16 x 3) = 160

Percentage of iron in iron oxide is (56 x 2) = 112 / the relative formula mass of iron oxide = 160

112/160 x 100% = 70%

So, 70% of any mass of haematite can be extracted as iron. 

The equation for a reaction also represents the ratio of the gram formula masses that react. For example:

2Fe2O3 + 3C ---> 4Fe + 3CO2

This shows that 2 formula units of iron oxide give 4 atoms of iron. 

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Using electrolysis

ELECTROLYSIS means passing an electric current through a liquid ELECTROLYTE, which is broken down into its elements. Molten ionic compounds such as sodium chloride or aluminium oxide are electrolytes. When an ionic substance is heated until it melts the ions become free to move and travel with an electric current. 

Aluminium is extracted from aluminium oxide by electrolysis. An electric current is passed through molten aluminium oxide to DECOMPOSE it. Molten aluminium is formed, which is piped out of the cell. Oxygen is also formed. 

When an ionic compound is heated the ions become free to move and conduct electricity. The compound is an electrolyte. This process is called electrolysis. 

The oxides of reactive metals such as sodium and aluminium are not reduced by carbon, so this method cannot be used to extract these metals from their ores. Electrolysis is a way of obtaining reactive metals from their compounds. 

When molten ionic compounds are electrolysed the ions are attracted by, and move towards, the ELECTRODES which carry the electric current into the electrolyte. 

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Reactions at electrodes

Positive ions such as sodium (Na+) and aluminium (Al3+) move to the negatively charged electrode called the CATHODE. Metals, such as sodium and aluminium, form at the cathode. 

Negative ions such as chloride (Cl-) and oxide (O2-) move to the positively charged electrode called the ANODE. Non-metals, such as chlorine and oxygen, form at the anode. 

At the cathode, metal ions gain electrons and become neutral metal atoms. For example, in the electrolysis of molten aluminium oxide, aluminium is formed at the cathode.

At the anode, non-metal ions lose electrons and become neutral non-metal atoms. For example, oxygen is formed in the electrolysis of molten aluminium oxide. 

The oxygen atoms then combine to form oxygen molecules.

The electrons transferred to the anode flow in the circuit to the cathode, completing the electric circuit. 

Similar processes happen in the electrolysis of molten salts such as sodium chloride. Sodium and chloride ions carry the electric current through the electrolyte. At the cathode, sodium ions become neutral atoms. At the anode, chlorine ions become neutral atoms that combine to form chlorine gas.

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Using metals

Reactive metals are fored in electrolysis only when their molten ionic compounds are electrolysed and not when solutions oft their salts are electrolysed. 

We have replaced metals with plastics for some uses, such as pipes and buckets, but we use metals for many purposes. Metals are used because they have the particular properties that are needed. These properties include: 

- high melting point (this is needed for engines, furnaces and high-speed machines such as drills, which get very hot).

- strength (metals are strong when they are stretched, so can be used for structures like bridges and tall buildings that have to carry a large weight).

- MALLEABILITY (metals can be hammered, bent and pressed into complicated shapes such as car bodies).

- electrical conductivity (metals conduct electricity from power stations to homes and businesses and carry energy to lights, motors, computer chips and other components).

In metals, there is a force called the METALLIC BOND that attracts the atoms together.

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The structure of metals

In metals, there is a force called the METALLIC BOND that attracts the atoms together. The atoms are held together in a giant crystal lattice in which there is a repeated regular arrangement of the atoms.

The metallic bond is strong in most metals, so the atoms need a lot of energy to make them move out of their positions in the lattice. A high temperature is needed to melt most metals, and a lot of force is needed to bend or stretch them. Metal atoms are packed tightly together so that they are touching. Metals cannot be squashes into a smaller volume.

Metal atoms can lose their outer shell electrons, leaving positively charged ions. When metal atoms are packed together in a crystal, the outer shell electrons can move freely from one atom to another. They form a "sea of electrons" between the fixed positive ions. The positive ions are atracted to all the free electrons around them. This force keeps the ions in the lattice - it is what makes the metallic bond. The force is strong in most metals, giving them high strength and a high melting point. The free electrons can move through the metal structure, and this allows the metal to conduct electricity. In a pure metal all the atoms are identical. The atoms are all the same size so the layers in the giant lattice can roll across each other quite easily. The arrangement of the atoms is the same in the new position as before the movement. This means that metals can change their shape without affecting the strength of the bonds between the atoms. They are malleable.

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Metals in the environment

People have used some metals for centuries. There are disused quarries and mines in many places. Poisonous heavy metals, such as lead, mercury and cadmium have sometimes been left in the soil. People can be harmed by these metals, and it costs a lot of money to clean up the soil. Today we are using many more metals.

Mines are often huge and there is a lot of waste rock to dispose of. Mines destroy habitats and can damage water sources. Extracting metals uses energy and produces pollutants.

Some of the pollutants are gases that can contribute to acid rain. If metals are just dumped when they have stopped being useful, they can damage habitats.

Even if a metal is not toxic to plants and animals, dumping a lot of it in one place may still damage habitats.

Copper is one of the most important metals used today. Even the best ores contain about 1% of copper. This means that 99% of the ore is waste rock. Getting at the copper uses a lot of energy and often releases sulfer dioxide, which can cause acid rain. Copper is used for pipes and electric wiring. It can be recycled after use but about two thirds of waste copper is left to contaminate the environment. 

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Copper and lithium

There are similar environmental issues with many other materials used in todays "hi-tech" world. This poses ethical questions. We know that extracting materials damages the environment and may possibly harm the health of the people who live there. The materials extracted will benefit a large number of people. Does this justify any damage caused? Should we continue to simply throw away waste materials, or is this always wrong?

Since the 1990s, the lithium battery has become common in phones, games machines, music players and computers.

Lithium batteries are also being used increasingly in electric and hybrid cards. Vehicles need vastly more power than a mobile phone, so arrays of thousands of lithium batteries are needed. Although not a rare element, not much lithium has been used until recently. New sources will have to be found to meet the increase in demand.

Lithium batteries can be a fire hazard but they are not toxic. Dumping lithium batteries in landfill may be safe, but it is a waste of the lithium and other metals in them.

Electric cars use a lot of lithium in their batteries and also rare earth metals.  

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