Unit 4: chemical changes

• pH scale

  Measure of how acidic or alkaline a solution is, ranging from 0 to 14 with low numbers being most acidic and high numbers being most alkaline
• Neutral substances like pure water have a pH of 7, which is neither acidic nor alkaline
• Acidic substances

  • Stomach acid (pH 2)
  • Acid rain (pH 4)
• Alkaline substances

  • Washing up liquid (pH 9)
  • Bleach (pH 12)
• Measuring pH

  1. Using an indicator (chemical dyes that change color)

  2. Using a pH probe connected to a pH meter
• Indicator

  A group of chemical dyes that change color depending on the pH, some are wide range indicators like universal indicator
• pH probe and meter

  Can provide more accurate and precise pH measurements than indicators
• Acid

  Any substance that forms aqueous solutions with a pH less than 7, releasing hydrogen ions in water
• Base

  Any substance with a pH greater than 7
• Alkali

  A subgroup of bases that are soluble in water, forming hydroxide ions
• Neutralization reaction

  Reaction between an acid and a base, producing a salt and water
• Common acids

  • Hydrochloric acid
  • Sulfuric acid
  • Nitric acid
• Common bases

  • Sodium hydroxide
  • Calcium carbonate
• Strong acid


  Acid that ionizes completely, all acid particles dissociate to release hydrogen ions
• Weak acid


  Acid that doesn't fully ionize, only a small portion of acid particles dissociate to release hydrogen ions
• Dissociation of ethanoic acid

  1. Reactants form products

  2. Products can react to reform reactants (reversible reaction)
• Equilibrium of weak acid dissociation lies to the left, more undissociated acid molecules than dissociated
• Acid strength


  Proportion of acid molecules that ionize in water
• Concentration


  Amount of acid per unit volume
• Strong and weak acids can both be at high or low concentrations
• pH


  Measure of hydrogen ion concentration in solution
• As hydrogen ion concentration increases

  pH decreases
• Decrease of 1 pH unit


  Hydrogen ion concentration increases by a factor of 10
• High concentration of strong acid


  Low pH
• High concentration of weak acid


  Low pH
• At any given concentration, a strong acid will have a lower pH than a weak acid
• Electrolysis


  The process of breaking down an ionic substance into its elements by passing an electric current through it
• The process of electrolysis


  1. Ions are free to move in a molten or dissolved ionic substance

  2. Passing a current through the substance breaks it down into elements

  3. Positively charged ions move to the negative electrode (cathode)

  4. Negatively charged ions move to the positive electrode (anode)

  5. Ions are discharged at the electrodes producing elements
• Electrolysis of molten ionic compounds


  • The metal is produced at the cathode
  • The non-metal is produced at the anode
• Metals extracted by electrolysis


  Metals more reactive than carbon, which are too reactive to be extracted by reduction with carbon
• Extracting metals by electrolysis


  1. Large amounts of energy are used to melt the compounds and produce the electrical current

  2. Aluminium is manufactured by electrolysis of a molten mixture of aluminium oxide and cryolite using carbon as the positive electrode

  3. The positive electrodes need to be continually replaced as oxygen reacts with the carbon, forming carbon dioxide
• Electrolysis of aqueous solutions


  • The ions discharged depend on the relative reactivity of the elements
  • At the cathode, hydrogen is produced unless the metal is less reactive than hydrogen
  • At the anode, if OH- and halide ions are present, one of the halide ions will be produced, otherwise oxygen is formed

  
• Half equations


  • Represent the reactions at the electrodes
  • The small number is always the same as the 2 larger numbers within the equation
  • Electrons are represented by the symbol 'e-'
• Writing half equations for the reactions at each electrode

  1. Negative electrode: X+ + e- -> X, positive ions are reduced

  2. Positive electrode: X- -> e- + X, negative ions are oxidised
• Procedure for carrying out a titration


  1. Transfer 25 cm³ of sodium hydroxide solution into a conical flask

  2. Add 5 drops of indicator (e.g. methyl orange) to the alkali

  3. Place conical flask on a white tile

  4. Fill a burette with sulfuric acid

  5. Add acid to the alkali until the solution is neutral (colour change from yellow to permanently red)

  6. Read the volume of acid added from the burette (at the bottom of the meniscus)

  7. Repeat the titration until two readings are within 0.1 cm³, then take the mean
• Titration


  A procedure to determine the concentration of an acid or alkali by neutralization
• Neutralization reaction


  Reaction between an acid and an alkali to form a salt and water
• Indicator


  A substance that changes colour at a specific pH, used to determine when neutralization is complete
• Meniscus


  The curved upper surface of a liquid in a container, caused by the surface tension of the liquid
• It is important to read the burette at the bottom of the meniscus to ensure an accurate volume measurement