Chemical Changes

• Ions in aqueous acids.


  Hydrogen ions (H+).
• What can be used to measure pH?

  Universal indicator

  pH probe
• Phenolphthalein.

  Acid - colourless

  Alkali - pink
• Methyl orange.

  Acid - Red

  Alkali - Yellow
• Blue litmus paper.

  Acid - Turns red

  Alkali - Stays blue
• Red litmus paper.

  Acid - stays red

  Alkali - turns blue
• Problem with universal indicator.

  pH chart used, subjective
• Neutralisation reactions.

  Acid and a base, H+ ions react with OH- ions and form water.
• Ionic equation for neutralisation reactions.


  H+(aq) + OH-(aq) - H20(l)
• Concentrated and dilute.

  More moles of acid per unit volume of water, not the same as strength which refers to if the acid completely disassociates in water or not.
• What is a base?

  Any substance that reacts with an acid to form salt and water only.
• What makes a base an alkali?

  If it is soluble.
• Acid + metal

  Salt + hydrogen
• Acid + metal oxide

  salt + water
• Acid + metal hydroxide

  Salt + water
• Acid + metal carbonate

  salt + water + carbon dioxide
• Why are metal oxides bases not alkalis?

  They are insoluble.
• Test for hydrogen.

  Insert a lit splint into a test tube of gas, ’squeaky pop’.
• Test for carbon dioxide.

  Bubbke the gas through limewater (calcium hydroxide), cloudy if present.
• Why is excess of an insoluble reactant added when reacting with acid to form a soluble salt?

  To ensure all the acid reacts.
• Titration.

  Method used to prepare a salt from an acid and a soluble reactant, since both reactants are soluble titration allows you to combine the reactions avoiding excess that would be hard to remove.
• 3 steps producing a pure dry salt from an acid and an alkali.

  Titration to find the volume of acid that reacts with a set volume of alkali, use results to mix correct proportions, evaporate the water from the solution.
• Titration method.

  Pipette to add 25cm3 of acid to the conical flask and add a few drops of phenolphthalein, place on a white tile.

  Fill burette with alkali, record volume.

  Add the alkali to the conical flask - when pink to colourless (swirling constantly) record how much added to achieve neutral solution.
• Which common chlorides are insoluble?

  Silver chloride and lead chloride are insoluble.
• Nitrates are soluble.


  All common sodium, potassium and ammonium salts are soluble.
• Which common sulfates aren’t soluble?

  Lead sulfate, calcium sulfate, barium sulfate
• Which common carbonates are soluble?

  Carbonates/hydroxides of sodium, potassium and ammonium.
• What is formed when a salt is insoluble?

  A precipitate.
• Electrolyte.

  Ionic compound in its molten or aqueous state.
• Why can an electrolyte carry charge?

  Molten or aqueous ionic compounds have mobile ions which can carry charge.
• Electrolysis.

  A process that used electrical energy (from a direct current supply) to decompose electrolytes.
• What happens at the anode?

  Oxidation, loss of electrons.
• What happens at the cathode?

  Reduction, gain of electrons.
• what is formed at each electrode?

  Anode - non metal

  Cathode - metal or hydrogen
• Rules for what is discharged at the cathode

  The least reactive, always hydogren unless copper is present
• Rules for what is discharged at the anode

  Always hydroxide unless a halide is present
• When is electrolysis used instead of reduction with carbon?

  When the metal is more reactive than carbon