AS Chemistry Unit 1 Notes (edexcel)

Chapter 1.1 Formulae, equations and amounts of substances

THE FOUNDATIONS OF CHEMISTRY: What’s in an atom?  The nuclei of atoms has protons (+) and neutrons (0) [called nucleons]  Electrons (-) occupy empty space outside the nucleus  Element: substance that cannot be broken down chemically into simpler substances  Atomic number: number of protons of an atom of the element  Number of protons is equal to the number of electrons, so the atom is electrically neutral  Protons and neutrons have mass but electron’s mass is negligible  Mass number: number of protons + number of neutrons Isotopes:  Atoms with the same atomic number but a different numbers of neutrons are called isotopes  Some heavy isotopes are radioactive  They may have different physical properties but the same chemical properties as the number of electrons is the same Holding atoms together:  Bonding between metals and non-metals is ionic  An atom loses or gains an electron to have a complete outer shell, this makes the atom, an ion  Strong electrostatic forces hold the oppositely charged ions in a giant lattice  Covalent bonding: non-metals bond together by sharing pairs of electrons Ions in solutions:  Ions that are not involved in an equation are called spectator ions  Molecular equation shows complete formula of every substance Relative atomic mass:  The average mass of its isotopes compared with the mass of an atom of the carbon-12 isotope  It is an average of the differing isotopes Counting and Weighing atoms:  Mole: the amount of substance that contains as many particles (atoms, ions or molecules) as there are atoms in exactly 12g of carbon 12  Any mole of a substance contains 6.23x10^23 particles (Avogadro’s number)  Avogadro’s constant has the unit particles per mole (mol-1) and sometimes has the symbol L or NA

Relative formula mass and relative molecular mass:  The sum of the relative atomic masses of all the atoms within a chemical formula  Molar mass: the RMM or RFM in grams per mole; it tells you the number of grams of a substance that makes up one mole Molar Volume:  Molar volume (of the gas) [Vm]: one mole of any substance must occupy the same volume under the same conditions  STP: standard temperature and pressure [1 atmospheric pressure and 298K] is used to compare molar volumes  Under STP: 1 mole of any gas occupies 24dm3 The empirical formula:  Gives the ratio of the different atoms present; it is the simplest formula for a compound showing the whole number ratio of each atom of an element present  Does not say how many atoms are bound together in one molecule of a compound Moles in solution:  Solution: solute that has dissolved in a solvent  Molar solution: solution of concentration 1M  Concentration [molarity] is measured in mol dm-3 Other units of concentration Percentage by mass:  Percentage by mass = mass of solute/mass of solution x 100 Percentage by volume:  Percentage by volume = volume of one component/total volume x 100 Parts per million:  Concentration= mass of component/mass of solution x 1 000 000 ppm  Used for levels of pollution in air or water; used when percentages are not very useful The yield of a reaction:  Yields may not be 100% as: reactants may not continue until all reactants are used up (may be an equilibrium reaction); some product may be left on apparatus; volatile products may evaporate; human error Double salts:  Crystals that contain two different salts in a 1:1 ratio Atom economy:  Atom economy: mass of atoms in desired product/mass of atoms in reactants x 100%

Chapter Two: Energetics and Enthalpy Change Energy and Energetics:  Energetics: study of energy transfers between chemicals and their surroundings  Thermochemistry: study of these energy transfers Exothermic and Endothermic:  Endothermic reaction: reaction that needs energy from heating to take place  Exothermic reaction: reaction that releases energy  Bond breaking requires energy whilst bond making releases energy Energy changes in exothermic reaction:  Energy released by bond formation is greater than the energy needed to break the bonds in the reactant  Neutralisation reactions are exothermic Energy changes in endothermic reactions:  The energy required to break the bond is greater than the energy released when new bonds are formed in the product  Photosynthesis and thermal decomposition is endothermic Enthalpy changes:  An exothermic reaction releases energy to the surrounding in the form of heat (enthalpy change of reaction)  System: the reaction in which the changes are happening  Surrounding: everything outside the system  Boundary: separates system from the surroundings  Closed boundary: prevents particles leaving or entering the surroundings  Isolated boundary: prevents energy from leaving or entering the surroundings  Energy cannot be created or destroyed  Energy is transferred and spread through the surroundings (dissipated)  Principle of conservation of energy: total energy content of the universe is constant How much energy is transferred?  Enthalpy (H): energy content in a system held at a constant pressure  Enthalpy change (H): the amount of heat given off or absorbed in a reaction carried out at constant pressure  H = Hproducts – Hreactants Enthalpy level diagrams: In an exothermic reaction – the enthalpy change is negative as H[products]