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C5 Chemical Changes (Salts (Formulae of Salts (salt is an ionic compound…

- - - - reactions of acid and very reactive metals are too violent to carry out safely
        
        e.g. alkali metals
      - acid(aq) + metal(s) → a salt(aq) + hydrogen(g)
      - the more reactive metal displaces the hydrogen in the acid
    - - the metal atom loses electrons
        
        they are oxidised
      - the hydrogen ions gain electrons
        
        they are reduced
      - Example: magnesium + dilute sulfuric acid
        
        balanced symbol equation
        
        Mg + H₂SO₄ → MgSO₄ + H₂
        
        ionic equation
        
        Mg + 2H²⁺ → Mg²⁺ + H₂
        
        half equations
        
        Mg - 2e⁻ → Mg²⁺
        
        2H⁺ + 2e⁻ → H₂
  - - - hydrochloric acid (HCL) form chlorides (Cl⁻)
      - sulfuric acid (H₂SO₄) form sulfates (SO₄²⁻)
      - nitric acid (HNO₃) form nitrates (NO₃⁻)
  - - - +ve metal ions (or ammonium ions NH₄⁺)
        
        +ve ions can come from a metal, base or carbonate
      - -ve ion from acid
      - charges on the ions have to cancel out
        
        work out the ratio the ions must combine in so the +ve and -ve charges balance
    - - positive ions
        
        group 1 form 1+ ions
        
        group 2 form 2+ ions
        
        group 3 form 3+ ions
        
        ammonium is a 1+ ion (NH₄⁺)
        
        transition metals
        
        can form several different ions
        
        charge given by roman numeral
      - negative ions
        
        group 7 form 1- ions
        
        group 6 form 2- ions
        
        nitrate is a 1- ion (NO₃⁻)
        
        sulphate is a 2- ion (SO₄²⁻)
  - - - except group 1 metal carbonates
- - - - many of these metals have been oxidised
        
        had oxygen (in air) added
        
        to extract the metal, the metal oxide must be reduced
        
        have oxygen removed
        
        form oxides
      - an ore is a rock which contains enough metal or metal compound to make it economically worth extracting the metal
        
        ores may need to be concentrated (to remove the rock) before the metal is extracted
      - extracted by chemical reactions
        
        metals elements can be extracted by displacement using a more reactive element
        
        in practice carbon is used because it is very cheap
        
        oxides of metals below carbon in the reactivity series can be reduced by carbon
        
        metal oxide(s) + carbon(s) → metal(l) + carbon dioxide(g)
        
        the metal oxide is heated with carbon
        
        metals more reactive than carbon cannot be extracted using carbon
        
        they are extracted by electrolysis
        
        hydrogen is used to reduce tungsten from its oxide
        
        although more expensive than using carbon
        
        but carbon forms a compound with the metal
        
        tungsten carbide
        
        so using hydrogen as the reducing agent makes the tungsten more pure
        
        other more reactive metals are too expensive to use in displacement reactions (but they would also work)
    - - in their 'native state'
      - can be separated from rock using physical methods
    - - how easy to extract
      - how much metal ore contains
      - changing demands
  - - - Water
        
        group 1 metals react vigorously
        
        leaving an alkaline solution of metal hydroxide
        
        magnesium reacts slowly with cold water
        
        lead reacts slightly with steam
        
        copper does not react at all
      - Dilute Acid
        
        group 1 metals explode
        
        magnesium fizzes
        
        forms a salt
        
        lead reacts slowly with warm acid
        
        copper has no reaction
      - hydrogen gas produced
        
        gas "pops" with lighted splint
        
        the rate at which the metal gives off hydrogen can be used to judge relative reactivity
  - - - more reactive metals form +ve ions more easily
    - - Ionic equations
        
        *spectator ions that remain the same are not included
        
        show only the ions that change in the reaction
        
        no. of atoms of each element same on both sides
        
        total charge on each side are equal
      - Half equations
        
        shows what happens to each reactant in the ionic equation
        
        shows loss / gain of electrons
        
        electrons shown as e⁻
        
        no. of atoms of each element same on both sides
        
        total charge on each side are equal
      - OIL RIG
        
        Oxidation
        
        loss of electrons
        
        Reduction
        
        gain of electrons
        
        Oxidation is Loss. Reduction is Gain
    - - Hydrogen
        
        between copper and lead
        
        metal + acid reactions displace hydrogen
        
        copper can't displace hydrogen in acid but lead can
      - Carbon
        
        between aluminium and zinc
        
        used to extract metals from their oxides
        
        does not displace aluminium from its oxide but does displace lead
- - - - metal oxides and hydroxides are bases
      - alkalis are bases that are soluble in water
        
        soluble hydroxides
    - - Required Praticical
        
        pour a fixed volume of acid in the beaker
        
        add metal oxide/carbonate to the acid one spatula at a time whilst gently warming on a tripod and gauze and stirring
        
        filter the solution using filter paper and a funnel to remove excess metal oxide/carbonate
        
        pour the solution into an evaporating basin and heat using a bunsen burner over a beaker of water
        
        continue adding until the metal oxide/carbonate is in excess and all the acid has reacted
        
        stop heating at the point of crystallisation and leave the rest of the water to slowly evaporate to give you larger crystals
        
        decant the solution then gently pat dry the crystals using filter paper
        
        Safety: Wear safety goggles
      - acid(aq) + base(s) → a salt(aq) + water(l)
    - - acid(aq) + alkali(aq) → a salt(aq) + water(l)
      - represent neutralisation using an ionic equation
        
        H⁺ + OH⁻ → H₂O
      - ammonium salts as well as metal salts can be made
        
        ammonia reacts with water to form a weakly alkaline solution
        
        ammonia solution reacts with acid to form an ammonium salt and water
      - there is no visible change when acids react with alkalis so you can't tell when they've completely reacted
        
        you need to carry out a titration using an acid/base indicator to find the exact volumes that react together
        
        the two volumes of acid and alkali can be reacted again, this time without the indicator
        
        a pure, dry sample of the salt crystal can be prepared by crystallising the solution
      - when you dissolve a substance in water you make an aqueous solution
        
        acids release H⁺ ions when dissolved in water
        
        alkalis release OH⁻ ions when dissolved in water
  - - - acids have pH lower than 7
        
        0 is most acidic
        
        red/orange/yellow
      - alkalis have pH higher than 7
        
        14 is most alkaline
        
        blue/purple
      - pure water is neutral (pH 7)
        
        green
        
        neither acidic or alkaline
      - has values 0-14
      - pH curve
    - - universal indicator (UI)
        
        turns a range of colours as the pH changes
      - substances which change colour depending on the pH of the solution
      - litmus
        
        red in acid/ blue in alkali
      - acid/base indicator
        
        phenolphthalein
        
        clear in acid and neutral/ pink in alkali
    - - digital pH values
  - - - strong acids are diluted with water to make them less harmful
        
        less acid molecules for a given volume
      - the pH scale tells us about the concentration of H⁺ ions
        
        as the pH scale decreases by 1 unit, the concentration of H⁺ ions increases 10x (one order of magnitude)
        
        0.1 mol/dm³ = pH1 0.01 mol/dm³ = pH2 0.001 mol/dm³ = pH3
        
        in standard form, the negative exponent of the concentration of acid equals the pH value
        
        0.01 mol/dm³ = 1x10⁻² mol/dm³ exponent is -2 so =pH2
    - - acid molecules completely ionise in aqueous solutions
        
        split and to form H⁺ ions
      - hydrochloric acid sulphuric acid nitric acid
      - strong acids have a lower pH than weak acids for a given concentration
        
        stronger acids have higher concentrations of H⁺ ions
    - - partially ionise in aqueous solutions
        
        reversible reaction
        
        equilibrium is reached in which both whole molecules and their ions are present in the solution
        
        only some molecules split up to form H⁺ ions
        
        low concentration of H⁺ ions compared to acid molecules
      - carbonic acid ethanoic acid citric acid
        
        fizzy drinks/ rainwater vinegar
      - carboxylic acids