Chemical changes

• Acids and alkalis:

  • acids produce H+ ions, and alkalis produce OH- ions in aqueous solutions
  • alkalis have a high conentration of OH- ions
  • acids have a high concentration of H+ ions: as the H+ concentration increases by 10, the pH decreases by 1
  • alkalis are soluble bases
• Indicators:

  • phenothalein is pink in alkali and colourless in acids
  • methyl orange is yellow in alkali and red in acid
  • litmus is blue in alkali and red in acid
• Core practical:

  • add dilute HCl(acid) to a beaker, record pH
  • add a mass of calcium hydroxide(alkali), record pH
  • repeat until no change in pH
• Describing acids & alkalis:

  • concentrated, larger amount of substance in a volume of solution
  • dilute, lesser amount of substance in a volume of solution
  • strong acid, fully breaks down to release ions in a solution
  • weak acid, partially breaks down in a solution
• General equations:

  • acid + base ---> salt + water
  • acid + metal ---> salt + hydrogen
  • acid + metal oxide/hydroxide ---> salt + water
  • acid + metal carbonate ---> salt + water + carbon dioxide
• Neutralisation:

  • H+ + OH- ---> H2O
• Soluble salts from an acid and an insoluble reactant:

  • excess of copper oxide(insoluble reactant) is added to ensure sulfuric acid reacts completely
  • excess copper oxide removed by filtration - salt and water remains
  • evaporate water, so that the copper sulphate crystals remains
• Soluble salts from an acid and a soluble reactant:

  • fill burette with 100cm3 acid, and fill a conical flask with 25cm3 alkali. Add indicator to the conical flask (phenolphthalein is pink in alkali).
  • add acid from burette to alkali until phenolphthalein is colourless
  • the titre, is the volume of alkali needed to neutralise the acid, is calculated and repeated
  • the salt solution is heated to evaporate the water, so that the crystals form
• Solubility:

  • soluble: sodium, potassium, ammonium, nitrates, chlorides, sulphates
  • insoluble: (silver and lead) chlorides, (lead, barium, calcium) sulphates, carbonates, hydroxides
• Insoluble salts:

  • mix two solutions needed to form the salt
  • filter the mixture, the insoluble salt will form a precipitate
  • wash salt with distilled water and leave to dry
• Electrolysis:

  • ionic compounds in a molten state or in a solution, ensure ions are free to move
  • electrolysis, when electrical energy decomposes electrolytes by breaking them down into their elements
  • in electrolysis, cations(+) move to the cathode(-), anions(-) move to the anode(+)
• Electrodes:

  • in an ionic solution, there are the ions making up the compound, but also OH- and H+ ions
  • at the cathode(-) H+ produced, unless less reactive than H+ (Cu, Au or Ag)
  • at the anode(+) OH- produced unless there is Cl, Br or I (halide ions)
• Half equations:

  • cathode(-), X+ + e- ->X, cations are reduced - they gain electrons
  • anode(+), X- -> e- + X, anions are oxidised - the lose electrons
  • Oxidation Is Loss, Reduction Is Gain
• Electrolysis of copper sulphate solution with copper electrodes:

  • anode is made of impure copper, cathode is pure copper
  • Cu2+ ions from the anode move to the solution
  • Cu2+ ions move to the cathode, where they gain electrons and are discharged as copper
  • impurities from the anode form a sludge at the bottom
• Electrolysis of copper sulphate solution with inert electrodes:

  • at the cathode Cu produced
  • at the anode, O2 produced - 4OH- —> O2 + H2O + 4e-
  • H+ and SO4 2- ions remain and react to form sulfuric acid
• REMEMBER the electrolysis of:

  • copper chloride solution
  • sodium chloride solution
  • sodium sulfate solution
  • water acidified with sulfuric acid
  • molten lead bromide