- Created by: jennywoolcock
- Created on: 10-06-16 13:00
Dry air contains non-metal elements such as nitrogen, oxygen and argon. Air also contains small amounts of non-metal compounds such as carbon dioxide and water vapour. The gases are non-metal elements and non-metal compounds. They are molecular because they contain atoms joined together into small molecules.
Molecules can either be shown in 2D or 3D. The atoms in the molecules are held together by covalent bonds. A covalent bond forms when atoms share a pair of electrons. The atoms are held together because the positively charged nuclei of both atoms are attracted to the negatively charged shared pair of electrons.
Simple molecular substances, such as moleucles in the air, have very low melting points and boiling points. This is because the attractive forces between small molecules are very weak. Very little energy is needed for molecules to overcome these forces and move apart. Molecules of elements and componds have no electrical charge, so pure molecular substances cannot conduct electricity. For small covalent molecules, the forces between molecules are weak but the forces within the molecules (covalent bonds) are strong. When a molecular substance melts, the molecules are easily separated from one another but the molecules themselves are not broken up into separate atoms.
The earth’s hydrosphere is all the water on earth, including oceans, seas, lakes and rivers. The hydrosphere is mostly water, with some dissolved compounds called salts. When water evaporates, dissolved salts from solid crystals. Salts are ionic compounds. Ions have either a positive or a negative charge and are arranged in a giant 3D pattern called a lattice. The strong force of attraction between positively charged and negatively charged ions is called an ionic bond. Ionic compounds have high melting points and boiling points because a large amount of energy is needed to overcome the forces between ions in the lattice. Ionic compounds do not conduct electricity when solid because the ions are not free to move. When they are melted or dissolved in water, the ions can move and they conduct electricity. In the formula for an ionic compound, the number of positive charges just balance and cancel out the negative charges. Some ions such as the sulphate ion contain groups of atoms. This is called a molecular ion.
Testing for ions
Ions in a compound can be identified by their distinctive properties. For example, compounds containing the copper ion are often blue. Solutions of some ionic compounds make a precipitate (a substance to be deposited in solid form from a solution) of an insoluble compound when they mix. The colour of the precipitate can be used to identify the ions in the compound. Adding an alkali, such as dilute sodium hydroxide, to different positive metal ions gives different colours of precipitate.
Negative carbonate ions are identified by adding dilute acid and looking for fizzing. Negative chloride, bromide, iodide and sulphate ions are identified by adding dilute silver nitrate or dilute barium chloride and looking for precipitates.
Carbonates fizz when an acid is added because carbon dioxide gas is made in the reaction. Precipitates form when an insoluble solid is made in the reaction. For example, most metal hyroxides are insoluble, as is silver chloride from the reaction of silver ions and chloride ions.
Ionic equations with state symbols show what happens when precipitates form. For example:
- for positive ions reacting with hydroxide ions: Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s)
- for negative ions reacting with silver ions or barium ions: Ag+ (aq) + Cl- (aq) --> AgCl (s)
Metals, minerals and ores
The lithosphere is the rigid outer layer of the earth, made up of the crust and upper mantle. It contains rocks and minerals. Minerals are solids with atoms or ions arranged in a regular arrangement or lattice eg. carbon in the form of diamond or graphite. Silicon, oxygen and alumnium are very abundant elements in the earth's lithosophere. Most of the silicon and oxygen on earth are joined together in the earth's crust as silicon dioxide, for example, in the mineral quartz.
Some minerals contain metals. Rocks that contain metals are called ores. Some ores contain metal oxides. Copper, zinc and iron can be extracted from their ores by heating thier metal oxides with carbon. Carbon reduces the metal oxide by taking away oxygen. The amount of minerals in ores varies. For some metals such as copper, a huge amount of rock has to be mined to extract a small amount of metal.
Extracting metals by heating their oxides with carbon is a redox reaction, because both oxidation and reduction happen. Reduction happens because the metal oxide loses oxygen. Oxidation happens becase the carbon gains oxygen. For example, the extraction of zinc uses carbon to reduce zinc oxide to zinc: Zinc oxide + Carbon --> Zinc + Carbon dioxide. In this reaction the zinc has been reduced and the carbon has been oxidised.
Giant covalent structures: Graphite and Diamond
Diamond and graphite contain many carbon atoms covalently bonded together in a regular pattern. This is called a giant covalent structure. Covalent bonds form when atoms share electrons.
The bonds in diamond are very strong and need a large amount of energy to break them. This is why diamond has a very high melting point and boiling point and doesn't dissolve in water. There are no free charged particles in diamond so it does not conduct electricity when solid or when melted. Silicon dioxide also has a giant covalent structure, so it has similar properties to a diamond.
In diamond, each carbon atom is covalently bonded to four other atoms in a tetrahedral 3D lattice. In graphite meanwhile, each atom is strongly bonded to three others in sheets. The sheets are strong but there is only a weak force between the layers, so they can slide over each other. There are free moving electrons between the layers in graphite, so it conducts electricity.
Ionic compounds contain charged particles and so conduct electricity when they are molten. Covalent compounds have no charged particles and so cannot conduct electricity. Graphite is the only exception to this rule.
Giant covalent structures: Graphite and Diamond
Equations, atomic mass and formula mass
Equations show the chemicals that react together (the reactants) and the chamicals that are made (products). A balanced equation also shows the number of atoms of each element on each side of the equation. Equations are balanced because there is the same number of atoms of each element on both sides. State symbols show whether each chemical is solid (s), liquid (l), gas (g) or dissolved in water (aq)
The relative atomic mass of an atom is the mass of an atom compared to the mass of an atom of carbon, which is given the value 12.
The relative formula mass of a compound is the sum of the realative atomic masses of all the atoms or ions shown in its formula.
The gram formula mass of an element or compound is its relative atomic mass or relative formula mass in grams. For example, if the relative formula mass of water is 18, its garm formula mass is 18g.
Percentage of metal ina mineral = (total mass of metal atoms/ gram formual mass) X 100%
Electrolysis means passing an electric current through an ionic compound when it is either molten or dissolved in water.
The compound is called the electrolyte beacuse it conducts electricity. Electrolytes break down as the electricity passes through.
Aluminium is extracted from aluminium oxide by electrolysis. Aluminium and oxygen are made in the process.
Electrolysis is used to extract more reactive metals (aluminium) because their oxides cannot be reduced by carbon since they are more reactive than carbon. Metals form at the negative electrode (cathode) because positive metal ions are attracted to it. At the negative electrode, metal ions gain electrons and become neutral metal atoms.
Negative ions move to the positively charged electrode (anode). Non metals such as chlorine and oxygen, form at the anode. At the positive elctrode, non-metal ions lose electrons and become neutral non-metal atoms.
Metals are very useful because they are strong, malleable (can be hammered into shape), have high melting points and are good conductors of electricity. Atoms is metals are held together by metallic bonds. The atoms are arranged in a regular pattern in a giant lattice. Metallic bonds are stong and so a large amount of energy is needed to melt or reshape them.
Metal atoms lose their outer shell electrons to form positive ions. In solid metals, the outer-shell electrons form a 'sea of electrons', which can move freely. The attraction between the positive ions and the sea of electrons is very strong, so that metals have high melting points and high strength. Metals conduct electricity because the electrons are able to move. In pure metals, all the atoms are the same size and can roll over each other. This means that metals can be reshaped even though the bonds are strong.
Metals in the environment
Some metals are poisonous (eg. lead, mercury and cadmium). Waste poisonous metals from mines destroy habitats and damage soil and water sources. Extracting metals makes pollutant gases that cause acid rain.
Large amounts of waste rock need to be processed to produce very small amounts os some metals such as copper. Processing large amounts of rock uses a lot of energy. Some minerals contain compounds of metals with sulphur. During extraction of the metal, sulphur dioxide is made which forms acid rain and damages plants and fish. We need large amounts of copper for electrical wiring and circuits, pipes and building materials. Some waste copper is recycled.
Lead is a toxic metal that was used to make batteries for vehicles. Modern batteries are made using lithium. Lithium is not toxic but it is difficult to extract enough lithium to meet the demand for batteries.