CHEM 2060CHEMISTRY- AS AS-Level Chemistry (New Spec) Notes. AS-LEVEL CHEMISTRY NOTES

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SECTION 1: Atoms & Reactions 1 Atomic structure & Isotopes • Atoms are made up of protons, electrons and neutrons • Neutral atoms have the same number of protons and electrons • You can wor... k out numbers of protons, electrons and neutrons from the element symbol • To find the total number of electrons for positive ions take away the same number of electrons from the atom (e g 3 from a 3+ ion) and for negative ions add the same number of electrons Particle | Position | Relative Mass | Charge Proton Nucleus 1 0 +1 Neutron Nucleus 1 0 0 Electron Shells around nucleus 1/2000 -12 Isotopes • Isotopes are atoms of an element with the same number of protons, but different numbers of neutrons • They have the same numbers of electrons and identical chemical reactions as a result • The atomic number is often omitted because it stays the same and the element symbol indicates how many protons there are snaprevise.co.uk 8 3 Relative Masses • The relative isotopic mass is the mass of an atom of an isotope compared with the mass of 1/12th an atom of 12C • The relative atomic mass, A r, is the weighted mean mass of an atom compared with 1/12th of the mass of an atom of 12C 4 Mass spectrometry • Mass spectrometry allows the masses of individual molecules or isotopes to be determined by turning substances into positive ions and measuring their mass/charge (m/z) ratio • Mass spectra plot m/z against percentage abundance • Number of isotopes can be identified by number of peaks • Each peak represents an isotope • To find the relative atomic mass multiply each isotopic mass by percentage abundance, add the results together and divide by 100 5 Relative molecular mass • Relative molecular mass, M r, is the mass of all the atoms in a molecule of a simple molecular substance • It’s found by adding up the relative atomic masses of all atoms in a molecule • Relative formula mass is the mass of one formula unit of a compound that forms a giant structure e g NaCl • It’s found in exactly the same way as Mr snaprevise.co.uk 9 SECTION 2: Compounds, Formulae & Equations 1 Ionic formulae • The formulae of ionic compounds can be found by balancing the charges of the individual ions that make it up • You need to know the charges of the specific ions and groups of the periodic table shown below • Find the charges of the ions in a compound and multiply up the number of moles of positive and negative ions until they balance +1 | +2 | +3 | -3 | -2 | -1 Group 1 Silver Gold Hydrogen Ammonium (NH4+) Group 2 Zinc Tin Lead Iron (II) Copper (II) Group 3 Iron (III) Group 5 Phosphate (PO43-) Group 6 Sulfate (SO42-) Carbonate (CO32-) Group 7 Nitrate (NO3-) Hydroxide (OH-)EXAM TIP Brackets in a formula, like Ca(OH)2, mean the thing in brackets is one ion, and there may be more than one of them 2 Balancing equations • Match the number of moles of each element on each side • To write the ionic equation for reactions involving ions write out just the ions that react • Leave out all the spectator ions (the ones that don’t change oxidation number or state) • For ionic equations the charges also need to balance • Unbalanced, no state symbols: Cu2+ + OH- Cu(OH) • Balanced, state symbols: Cu2+(aq) + 2OH-(aq) Cu(OH)2(s) EXAM TIP It’s really easy to forget to put state symbols on during an exam, but you can lose marks for not including them so add them if you’re ever in doubt snaprevise.co.uk 10 SECTION 3: Amount of a Substance 1 Moles • A mole is a measurement of an amount of substance, with the symbol ‘mol’ • It is the amount of any substance containing as many particles as there are carbon atoms in 12g of carbon-12 • Avogadro’s constant, NA, is the number of atoms per mole of any substance and is equal to 6 02 x 1023 • The molar mass is the mass per mole of substance in g/mol • The molar volume is the volume per mole of gas At room temperature and pressure it is 24 0 dm3/mol EXAM TIP The molar volume and N A are in the data sheet, you don’t need to remember them 2 Empirical and molecular formulae • The empirical formula is the simplest whole number ratio of elements in a compound • The molecular formula is the formula that’s used and is the actual ratio of elements in a compound 3 Finding empirical and molecular formulae • You can find empirical and molecular formulae for a compound from experimental data • For the empirical formula, divide the mass of each element by the Ar to find the number of moles of each element present • Divide by the smallest number • Multiply up to get whole numbers for each element • To find the molecular formula, divide the molecular mass (Mr) by the mass of the empirical formula, and multiply the empirical formula by the result snaprevise.co.uk 11 4 Hydrated salts and water of crystallisation • A hydrated salt is one which contains water of crystallisation e g MgSO4·7H2O An anhydrous salt is one with no water of crystallisation e g MgSO4 • To work out the amount of water of crystallisation in a salt: ○ From the M r - Find the M r of the anhydrous salt and subtract it from the hydrated Mr then divide the remainder by the Mr of water ○ From experimental results — Find the moles of water lost when a hydrated salt is heated in a crucible and the number of moles of anhydrous salt left at the end 5 Mole calculations • You convert moles, mass and formula mass using • All gases take up 24 dm3/mol at RTP, so mol if V is in dm3 • Divide by 24000 instead if V is in cm3 • For solutions, EXAM TIP Sometimes mol/dm3 is abbreviated to ‘M’, they mean exactly the same thing 6 The ideal gas equation • Links pressure, P, volume, V, temperature, T, the gas constant, R, and the number of moles of gas present, n • pV = nRT • Use the molar volume to convert masses of gas to volumes EXAM TIP All the quantities in the ideal gas equation need to be in SI units, (p: Pa, V: m3, T:K) so you’ve got to remember to convert any data you’re given R is in the data book snaprevise.co.uk 12 7 Stoichiometric relationships • The ratios of moles in chemical equations shows how many moles of a product will give how many moles of a reactant and vice versa • Work out the number of moles of reactants • Use the ratio to find number of moles of products • Also works the other way round 8 Percentage yield and atom economy • Percentage yield = • Percentage yield may be low due to the reaction not going to completion, impurities or side reactions for example • Atom economy = • Atom economy will be low if more than one product is formed 9 Sustainability and atom economy • Processes with high atom economies are more sustainable • They can be productive without depleting resources, as they reduce the amount of raw materials required and reduce waste • Addition reaction have atom economies of 100% • Substitution and elimination reactions have lower atom economies [Show More]

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