C2 Elements, compounds, and mixtures

C2 Elements, compounds, and mixtures

1. C2.1 Purity and separating mixtures

   • C2.1a explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’
     • A pure substance in science consists of just one element or compound
     • Purity is the measure of how pure a substance is.
     • 'Pure' in everyday life describes natural substances that have not been processed or changed.
   • C2.1b use melting point data to distinguish pure from impure substances
     • The melting point of a pure substance is a single temperature.
     • If a substance is impure, the melting point is less than that of a pure substance, and it often melts over a range of temperatures, not just one temperature
     • Usually, the greater the difference between the melting point is less than that of a pure substance and it's accepted melting point, the lower it's purity is likely to be
   • C2.1c calculate relative formula masses of species separately and in a balanced chemical equation
     • Relative atomic mass (Ar) - the mean mass of an atom of an element compared to 1/12 the mass of a 12/6C (Carbon- 12) atom
     • The relative atomic mass of 12/6C is defined as 12.0 exactly. This means that an atom has less mass than a carbon-12 atom if its Ar is below 12.0, and that an atom has more mass than a carbon- 12 atom if its Ar is above 12.0
     • Relative formula mass (Mr) - the mean mass of a unit of a substance compared to 1/12 the mass of a 12/6C atom
     • You work it out, by adding the Ar values for all the atoms in the formula for the substance. They have no units
     • 1) Write down the Ar values of the elements in the compound 2)Work out the number of atoms of each element 3)Multiply those values with the Ar values to calculate the relative formula mass
     • Relative molecular mass - relative formula mass applied to a molecular substance
   • C2.1d deduce the empirical formula of a compound from the relative numbers of atoms present or from a model or diagram and vice versa
     • Empirical formula - Formula showing the simplest whole-number ratio of the atoms of each element in a compound
     • 1) Divide mass or amount of the element in the formula by relative atomic mass, which will give you the mole 2) Divide all moles by the smallest mole
     • In a model, 1) Work out the number of atoms of each element in the molecule by counting how many times each chemical symbol is shown 2) Divide by the smallest number
   • C2.1e explain that many useful materials are formulations of mixtures
     • Many useful materials are mixtures of different substances, often deliberately chosen to produce the desired properties.
     • An alloy is a mixture of a metal with one or more other elements. e.g. pure gold is very soft, so copper is often added to it, to make it harder, it is often used for jewellery
   • C2.1f describe, explain and exemplify the processes of filtration, crystallisation, simple distillation, and fractional distillation
     • Filtration separates an insoluble solid from liquids. It works because the solids cannot fit through the holes in the filter paper, but liquids can. e.g. sand and water - afterwards the sand becomes the residue, and the water becomes the filtrate
     • Crystallisation separates dissolved solids in a liquid. You heat the solution gently until the solvent evaporates leaving the solute behind. After heating the solution for a while, it will become a saturated solution (no more solute can be dissolved at that temperature)
     • Simple distillation separates a solvent from a solution. The solvent has a much lower boiling point than the solute, so when the solution is heated, the solvent boils, however the solute does not. The solvent is evaporated and then condensed by a condenser.
     • Fractional distillation separates two or more miscible liquids from each other. It relies on the substances having different boiling points.
   • C2.1g describe the techniques of paper and thin layer chromatography
     • Thin-layer chromatography is used to separate its components. It works in the same way as paper chromatography, but the stationary phase is a thin layer of silica or alumina powder, spread over a plate of glass or plastic
     • Paper chromatography is used to separate the coloured substances in ink or sweets. You put the mixture on the baseline of a piece of paper, which is then put in water or propanone, however the mixture does not touch the water. The water will travel upwards through the paper and spread the mixture out. The stationary phase is the paper, and the mobile is the liquid
   • C2.1h recall that chromatography involves a stationary and a mobile phase and that separation depends on the distribution between the phases
     • A stationary phase does not move
     • A mobile phase does move
     • Separation depends on the distribution between the phases
   • C2.1i interpret chromatograms, including measuring Rf values
     • 1) Measure the distance travelled by the substance (the middle of the spot of colour) and the distance travelled by the solvent 2) Calculate the rf value by dividing the distance travelled by substance by the distance travelled by solvent
     • Rf values vary from 0-1 and have no units
   • C2.1j suggest suitable purification techniques given information about the substances involved
     • Insoluble and soluble substances - dissolving followed by filtration
     • A solute dissolved in a solvent (a solution) - crystallisation to obtain the solute, simple distillation to obtain the solvent
     • Two or more substances in a liquid state - fractional distillation
     • Coloured soluble substances - paper chromatography or TLC
   • C2.1k suggest chromatographic methods for distinguishing pure from impure substances
     • You can use paper chromatography or TLC to tell if a substance is pure, however TLC is quicker, more sensitive (so a smaller sample can be used), and there is a larger range of stationary phases and solvents to choose from.
     • You can also scrape an individual spot from a TLC for further analysis by gas chromatography
     • Gas chromatography separates many different kinds of substance

2. C2.2 Bonding

   • C2.2a describe metals and non-metals and explain the differences between them on the basis of their characteristic physical and chemical properties
     • Metals are usually shiny, have high melting and boiling points, solid at room temperature, malleable, ductile, and are good thermal and electrical conductivity
     • Non-metals are usually dull, have low melting and boiling point, brittle, non-ductile, and insulators - so they are poor conductors
     • Metals lose electrons to form positive ions, but non-metals gain electrons to from negative ions
     • Metals do not react with each other (they mix to form alloys, but non-metals react with each other to produce compounds that consists of molecules
     • Metals react with oxygen to produce metal oxides that produce alkaline solutions when dissolved in water
     • Non-metals react with oxygen to produce non-metal oxides that produce acidic solutions when dissolved in water
   • C2.2b explain how the atomic structure of metals and non-metals relates to their position in the periodic table
     • Metals are found on the left because they have extra electrons which can be given away in ionic bonding
     • However non-metals gain electrons in ionic bonding, meaning that before bonding they are missing a couple of electrons from a full outer shell.
   • C2.2c explain how the position of an element in the periodic table is related to the arrangement of electrons in its atoms and hence to its atomic number
     • Groups are vertical columns and represent how many electrons are in the outer shells (1-8/0)
     • Periods are horizontal rows and represent how many shells there are (1-7)
     • The periodic table is arranged by the proton number, and because p=e, it also means that electrons increase across the periodic table
     • Atomic number is the total amount of electrons in the element
   • C2.2d describe and compare the nature and arrangement of chemical bonds in: i. ionic compounds ii. simple molecules iii. giant covalent structures iv. polymers v. metals
     • Ionic compounds - metal reacting with non-metal, electrons are transferred from the metal atoms to the the non-metal atoms. The metal becomes positive ions, and the non-metal negative.
     • Simple molecules - It only contains a few atoms. The covalent bonds between atoms in a simple molecule are strong, but the intermolecular forces between the molecules are weak.
     • Giant covalent structures - many non-metal atoms joined by covalent bonds and arranged in a repeating regular pattern called a giant lattice.
     • Polymers - made from many molecules called monomers which join end to end in chemical reactions, producing longer polymer molecules
     • Metals - metal atoms are packed together in a regular way, forming a giant metallic lattice. Electrons leave the outer shells of the metal atoms, forming a 'sea' of electrons around positively charged metal ions. These electrons are free to move through the structure of the metal, so they are called delocalised electrons. Metallic bonds are the strong electrostatic forces of attraction between the delocalised electrons and the metal ions
   • C2.2e explain chemical bonding in terms of electrostatic forces and the transfer or sharing of electrons
     • Ionic compounds - in their solid state contain positive and negative ions arranged in a regular way, which is called a giant ionic lattice. The ions are held in place by ionic bonds, which act in all directions. Ionic bonds are strong electrostatic forces of attraction between oppositely charged ions.
     • Simple molecules- Covalent bonds involve electrostatic forces of attraction between the nucleus of each bonded atom and the shared atom and the shared electrons. The covalent bonds between atoms in a simple molecule are strong, but the intermolecular forces between the molecules are weak
     • Metallic bonds - strong electrostatic forces of attraction between the delocalised electrons and the closely packed, positively charged metal ions.
   • C2.2f construct dot and cross diagrams for simple covalent and binary ionic substances
     • Simple Covalent substances - the electrons from one of the bonded atoms are shown as dots, and the electrons from the other bonded atom are shown as crosses. Each pair of electrons in the shared area between the overlapping circles represents a covalent bond. Only the outer shells is usually shown
     • Binary ionic substances - You show the electrons from one atom as dots and the electrons from the other atom as crosses to model the ions.
   • C2.2g describe the limitations of particular representations and models to include dot and cross diagrams, ball and stick models and two- and three-dimensional representations
     • Dot and cross - 2D, not in correct size or proportion
     • Ball and stick - size of atoms and lengths of bonds are exaggerated, and it suggests that the electrons make the bonds do not move
     • 2D - It does not show the 3D shape of the molecule
     • 3D - it does not show the forces of attraction
   • C2.2h explain how the reactions of elements are related to the arrangement of electrons in their atoms and hence to their atomic number
     • In group 1, the elements get more reactive down the group, because as you go down the number of shells increases therefore the outermost electron gets further away from the nucleus meaning that the attraction between them gets less
     • In group 7, the elements get less reactive down the group, because the atoms get larger, meaning that the outer shell gets further away from the nucleus and electrons get weaker, so an electron is less easily gained
     • Group 1 - metal - very reactive - electronic structures end in 1 - +1 ions formed in reaction
     • Group 2 - metal - reactive- electronic structures end in 2 - +2 ions formed in reactions
     • Group 7 - non-metal - very reactive - electronic structures end in 7 -1 ions formed in reactions
     • Group 0 - non-metal - very unreactive - outer shells are full - do not react
     • Elements are arranged in order of increasing atomic number; The atomic number is the number of protons in an atom; The number of electrons in an atom is equal to the number of protons; Electronic structure is determined by the number of electrons; The electronic structure of an element determines its chemical properties
   • C2.2i explain in terms of atomic number how Mendeleev’s arrangement was refined into the modern periodic table
     • Mendeleev arranged all the elements known at the time in order of increasing atomic weight
     • He rotated the table so that groups were in columns and periods in rows, like the modern periodic table
     • Mendeleev developed his table without knowing about atomic structure, and the atomic number of an element was just its position in the Periodic Table. However, after his death, the proton was discovered, and Moseley discovered that an atom's atomic number was actually the number of protons in its nucleus

3. C2.3 Properties of materials

   • C2.3a recall that carbon can form four covalent bonds
   • C2.3b explain that the vast array of natural and synthetic organic compounds occur due to the ability of carbon to form families of similar compounds, chains and rings
     • It has four electrons in its outer shell, and can from up to four covalent bonds. It can join with other carbon atoms to form chains and rings, andcan also form bonds with other elements, producing many different compounds, and many different polymers
   • C2.3c explain the properties of diamond, graphite, fullerenes and graphene in terms of their structures and bonding
     • Diamond - exists as a giant covalent structure, each carbon atom is covalently bonded to four other carbon atoms. Covalent bonds are strong, and diamond has many of them, so it is very hard, and has a very high melting point. Because there re no spare electrons in the outer shell, it has no delocalised electrons meaning that it cannot conduct electricity.
     • These are all allotropes of carbon. Allotropes are different forms of an element in the same state but with different atomic arrangements.
     • Graphite - exists as a giant covalent structure, each carbon atom is covalently bonded to three other carbon atoms. This means that one electron in the outer shell of the atom is not involved in bonding and becomes delocalised, meaning that graphite can conduct electricity, even though it is a non-metal. Also, it has a very high melting point because of the strong covalent bonds Graphite has a layered structure the atoms in each layer form interlocking hexagons, however the intermolecular forces between each layer are weak, so layers can slide over each other easily, this is why graphite is slippery, and is used in pencils
     • Graphene - resembles a single layer of graphite. It is almost transparent, extremely strong and conducts electricity.
     • Fullerenes - form a large family of carbon allotropes in which the molecules are shaped like tubes of balls. A nanotube resembles a sheet of graphene rolled into a tube. They are strong and are used to reinforce sports equipment. A buckyball resembles a sheet of graphene closed to make a hollow ball, but the carbon atoms are in pentagons as well as hexagons. Their small size allows buckyballs to pass through cell membranes, and they might be able to deliver medical drugs directly to cells. They can also be used as lubricants with the molecules acting like tiny ball bearings.
   • C2.3d use ideas about energy transfers and the relative strength of chemical bonds and intermolecular forces to explain the different temperatures at which changes of state occur
     • When a substance melts or boils, forces of attraction between its particles are overcome. Some bonds break going from the solid to the liquid state. All remaining bonds break going from the liquid to the gas state.
     • The stronger the bonds, and the more of them there are, the more energy must be transferred from the surroundings to breaks them. Substances have high melting points if they have many strong bonds in the solid state, and high boiling points if they have many strong bonds in the liquid state.
     • Bonds form when a substance condenses or freezes. Some bonds form going from the gas to the liquid state. Many bonds form going from the liquid to the solid state. Stored chemical energy is transferred to the surroundings, usually by heating when chemical bonds form.
   • C2.3e use data to predict states of substances under given conditions
     • If the temperature is below the melting point then it is solid, if it is above the boiling point then it will be a gas, if neither, then it will be a liquid.
     • Metals, ionic compounds and giant covalent structures have strong bonds, so are usually solid at room temperature. Simple molecular substances are in the liquid or gas state at room temperature, or in the solid state but easily melted.
   • C2.3f explain how the bulk properties of materials (ionic compounds; simple molecules; giant covalent structures; polymers and metals) are related to the different types of bonds they contain, their bond strengths in relation to intermolecular forces and the ways in which their bonds are arranged
     • Metal (strong metallic bonds, e.g. iron mercury) Ionic compound (strong ionic bonds, e.g. sodium, chloride) Giant covalent ( strong covalent bonds, e.g. diamond silica) Simple molecules (weak intermolecular forces, e.g. oxygen, water wax)
     • Metals are malleable even though metallic bonds are strong, metal ions are held in a lattice by forces that attract them to a 'sea' of delocalised electrons. When a large enough force is applied, layers of ions slide over each other, however since the delocalised electrons are free to move, overall, no bonds are broken.
     • Giant covalent structures contain very many atoms held together in a giant lattice by strong covalent bonds. If a large enough force is applied many covalent bonds break at once, meaning the substance breaks, and also meaning that the substance is brittle.
     • In an ionic compound there are oppositely charges ions that are held together in a giant lattice by strong ionic bonds. If a large enough force is applied, many ionic bonds breaks at once, meaning that the substance breaks, also meaning that the substance is brittle.
     • Simple molecules and polymer molecules are attracted to each other by weak intermolecular forces which are easily broken. If these substances are arranged in a lattice when solid, they may be brittle. e.g. Iodine crystals and some polymers. If the molecules are not arranged in a lattice, the substance may be soft or flexible e.g. wax (particularly when it's warm)
     • A substance can conduct electricity if it has charged particles that are free to move. Metals conduct electricity when liquid or solid because their delocalised electrons are free to move through the lattice. Simple molecules, most polymers and substances with a giant covalent structure do not conduct electricity because they have no delocalised electrons.
     • The oppositely charged ions in ionic compounds are free to move when it is liquid, or dissolved in a solvent, such as water, but not when it is in the solid state. Therefore, ionic compounds conduct electricity when molten or in solution, but not when they are in the solid state.
   • C2.3j explain the possible risks associated with some nanoparticulate materials
     • Nanoparticles are so tiny that they may be breathed in, absorbed by the skin, or pass into cells. Nanoparticles may take a long time to break down once released into the environment, and toxic substances may stick to their surfaces. Some scientists are concerned that nanoparticles may be harmful to health and the environment in ways that are too difficult to predict, with risks that are too difficult to determine.
   • C2.3i describe how the properties of nanoparticulate materials are related to their uses
     • A material made from nanoparticles is described as nanoparticulate. Grains, lumps an sheets are made 'in bulk'. Nanoparticulate materials have different properties are due to the very small size of nanoparticles. Some of these properties are due to the very small size of nanoparticles. The small size of nanoparticles makes them useful for new paints, and new cosmetics, medicines, and sunscreens. e.g. titanium dioxide is white in bulk but transparent when it is nanoparticulate
   • C2.3h describe the surface area to volume relationship for different-sized particles and describe how this affects properties
     • Many properties of nanoparticles are due to their very large surface area to volume ratio compared with the same substance in bulk.Forces of attraction between surfaces can appear to be weak on a larger scale, but on a nanoscale they are strong.Atoms on the surface of a material are often more reactive than those in the centre, so a larger surface area means the material is more reactive. These properties are leading to exciting developments such as new catalysts to speed up industrial chemical reactions, and self-cleaning windows, ovens and clothes
   • C2.3g compare ‘nano’ dimensions to typical dimensions of atoms and molecules
     • A nanoparticle is a particle between 1nm and 100nm in diameter, and consists of just a few hundred atoms. Helium atom:0.062nm Methane Molecule:0.41nm Nanoparticle:1-100 Red Blood Cell:7000nm Human Hair:100 000nm

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