Transcript File
C5 Revision Chemicals of the Natural Environment Objective •To revise the whole of C5 Three parts of the Earth •Core •Mantle •Crust The four spheres… • Lithosphere: crust and upper mantle • Hydrosphere: Oceans and rivers • Atmosphere: Layer of air around the Earth • Biosphere: sphere of life Chemicals of the atmosphere • • • • • 78% N2 21% O2 1% Ar 0.03% CO2 Small amounts water vapour • All chemicals are gases – low melting and boiling points Molecules • Most of the gases in the atmosphere consist of small molecules • Argon exists as single atom • Attractive forces between molecules weak – gases – moving too quickly Bonds • Small molecules such as O2 or H2 do not split up unless at extremely high temperatures • Very strong bonds between atoms Different bonds Single bonds Double bonds Covalent Bonding • Non-metal atoms combine to form molecules – share electrons in their outer shells. • Held by electrostatic attraction between two nuclei and shared pair electrons Chemicals of the Hydrosphere • Water has special properties: • Liquid at room temperature, even though has a smaller mass than the gases in atmosphere. • Melts at 0oC and boils at 100oC • Water molecules have a greater tendency to stick together. • Contracts until 0oC, then starts to expand – important in nature. Good Solvent • Good solvent for salts • Most common salts do not dissolve ions, but water does. • Does not conduct electricity, therefore does not contain charged particles free to move Water molecules • Three atoms appear at an angle • The hydrogen region has a slight positive charge • The oxygen region contains a slight negative charge Water Molecule • The angle is due to the fact that the electrons are not evenly shared. • Oxygen has a greater share which explains the charges on each side of the molecule. • Overall they are still electrically neutral. Small Charges • The small charges on opposite sides of the molecule cause slightly stronger attractive forces between them. • Also help water dissolve ionic compounds by attracting the ions out of their crystals. • Ice is less dense than water because of the molecules angular shape. They tend to line up in a very open structure. Why is the sea salty? • Soluble chemicals carried from rocks to sea in water cycle. • Main chemical in sea sodium chloride • Built up over millions of years. • Most of compounds are salts – positive metal ions and negative nonmetal ions. Lithosphere • Top part of lithosphere is crust • Rocks include boulders, stones and pebbles Rocks.. • Rocks are made of one or more minerals • Minerals are naturally occurring chemicals. • Can be elements such as gold and silver • More commonly compounds, such as silicon dioxide, calcium carbonate etc Sandstone • Made of mainly one mineral • Silicon dioxide Limestone • Made mainly of calcium carbonate Granite • Mixture of quartz, feldspar and mica Lithosphere • Two most common elements in lithosphere are oxygen and silicon. • These are abundant so form the major types of minerals, such as silicon dioxide. Evaporite minerals • Sea water contains lots of dissolved chemicals. • When evaporates ionic compounds crystallise Rock salt – Halite is one example. • Minerals formed this way are called evaporites – laid down millions of years ago Ionic Bonding • Ions held together strongly by the opposite charges of their ions • E.g. Cl- and Na+ • The structure is called a giant ionic structure. • Takes lots of energy to break down Silica • • • • Consists of silicon dioxide Commonest form is quartz One silicon to four oxygens Form a 3D giant covalent structure Strong and rigid Silicon dioxide • Very Hard – used as abrasive • High mp and bp – linings for furnaces and high temp glassware • Insoluble in water – building • Electrical Insulator – silica glass used as insulator Biosphere • • • • Biochemicals – Carbon, hydrogen, oxygen Carbon is base of all living things: Can form chains by joining to themselves Forms four strong covalent bonds so other atoms can join • Means can make a variety of compounds. • Most are polymers Proteins • • • • Hair skin and muscle. Enzymes Polymers made from amino acids 20 different amino acids Proteins from amino acids Carbohydrates • Carbon • Oxygen • Hydrogen Sugars such as glucose Photosynthesis produces glucose Glucose very soluble Nucleic Acids • DNA and RNA are nucleic acids • Molecules that carry the genetic code • Back bone of DNA is a polymer Natural Cycles • As living things grow die and decay, elements move between the spheres • Carbon cycle • Nitrogen cycle • Humans have an effect on these cycles Human Effects • Carbon Cycle: burning fossil fuels. • Raised CO2 from 277ppm to 360ppm • Nitrogen Cycle: Adding fertilisers to soil Nitrogen Cycle • • • • Essential for biochemicals, especially proteins N2 is a gas NO3- (nitrate) and NH4+ (ammonium) are ions. Attracted to water molecules so makes them dissolve Fixing Nitrogen • Action of micro organisms (bacteria or algae) • Chemical reaction in air during lightning strikes • Haber process Metals from Lithosphere • Rocks which contain useful minerals are ores. • Valuable minerals are often oxides or sulphides of metals. • Ore examples: bauxite (aluminium), haematite (iron), rock salt (sodium). • Gold occurs naturally so has been used for thousands years. Couldn’t use other metals until learned how to extract. Mineral Processing • Valuable mineral mixed with dirt and rock • Separating this off is called ‘concentrating the ore.’ • Issues: Reducing the ore; supply of ore; energy costs; impact on environment. Extracting metals • Reduction – removing oxygen (eg ZnO to Zn) • Reducing agent – something that removes oxygen (eg C) • In reducing something, the reducing agent itself becomes oxidised (CO) • Carbon is a good reducing agent as at high temperatures – it has a strong tendency to react with oxygen. Relative atomic masses Relative Formula Mass • If you know the formula of a compound, then you can work out its relative formula masses. • Eg Fe2O3 • Fe – relative atomic mass = 56 • O – relative atomic mass = 16 • Therefore relative formula mass of Fe2O3 = • 56+56+16+16+16 = 160 Electrolysis • Some reactive metals hold on to their oxygen too strongly so cannot use carbon as a reducing agent. • Use electrolysis • Aluminium extracted from bauxite. • Electrolysis turns ions back into atoms. Electrolysis – What happens? • The electrolyte (liquid for electrolysis) • Aluminium forms at negative electrode because is attracted because positive. • Oxygen is negative so forms at the positive electrode. Electrolysis• Electrolysis turns ions back into atoms by giving them electrons. • Metal ions are positive so attracted to negative electrode • The flow of electrons from the power supply into the electrode is what makes it negative • In the electrolysis of molten aluminium oxide, the aluminium ions turn into aluminium atoms. Electrolysis • Al3+ + 3eAluminium electrons supplied ion by the negative electrode Al Aluminium Electrolysis • Non-metal ions are negatively charged so attracted to positive electrode. Electrode is positive because electrons flow out of it to power supply. • Negative ions give up electrons to positive electrode and turn into atoms. • O2O + 2eion atom electrons removed by positive electrode Metal Properties • • • • Strong Can be bent and pressed into shape High melting points Conduct electricity Metallic Structures • Scientists imagine that the model for metallic structure uses: • Tiny spheres • Arranged in a regular pattern • Packed close together in a crystal as a giant structure. Metallic bonding • Metals have a special kind of bonding – not ionic, covalent. • Metallic • Strong but flexible – allow to move atoms to new positions Metallic Bonding • Metal ions tend to lose electrons in their outer shell easily. • In solid metal the atoms lose these electrons and become positive ions. • The electrons drift freely between the metal ions. • The attraction between them holds the structure together. Metallic Bonding • A metal crystal is not charged overall. • Total negative charge on electrons = total positive charge on ions. • Electrons can move freely between the ions – explains why metals conduct electricity well. Life Cycle of Metals • Mining – Large volumes waste rock • Large holes in ground • Explosives – noisy and produces dust • Open cast – large volumes red mud. Life Cycle of Metals • Processing Ores • Many ores high value but low grade • Large percentage waste rock Life Cycle of Metals • Metal Extraction • All stages of metal extraction and fabrication need energy • Use large volumes water • Give off air pollutants Life Cycle of Metals • Metals in Use • Lighter cars, trucks and trains mean less fuel consumption and emssions. • Less wear and tear on roads and tracks Life Cycles of Metals • Recycling • Scrap metal from all stages of production routinely recycled. • Metal recycled at the end of the usefulness of metal products. • Save ores and reducing agents. Save water.