Topic 2 - Archived

• To tell if something is an acid, alkali or neutral we use universal indicator (UI) & the pH scale
• Acids contain H+ ions e.g. HCl or H2SO4
• Alkalis (or bases) contain OH- ions e.g. NaOH or Ca(OH)2
• When an acid is added to an alkali (or base) the H+ ions combine with the OH- ions forming water, neutralising the pH (7)
• A strong acid fully breaks apart into H+ ions
• A weak acid does not fully break apart into H+ ions
• Acids react with water when they are added to it, forming ions. The degree to which they do this is what determines whether they are strong or weak acids. Strong acids are essentially 100% ionised in solution. Weak acids ionise very little in solution.
• Concentration is distinct from strength. It refers to the amount of acid in a given solution. A concentrated acid contains a large amount of acid in a given volume; a dilute solution contains a small amount. The pH scale gauges the amount of hydrogen ions in solution.
• Strong and weak acids produce the same type of products but weak acids react more slowly.
• Ethanoic acid is a weak acid so reacts slowly
• Hydrochloric acid is a strong acid so reacts quickly
• When acid is neutralised a salt is formed and all products are neutral, so they are called neutralisation reactions
• Neutralisation reactions are exothermic. The highest temperature reached is the point of neutralisation (pH 7).
• Metal + Acid -> Salt + Hydrogen
• Metal Oxide (base) + Acid -> Salt + Water
• Metal hydroxide (alkali) + Acid -> Salt + Water
• Metal carbonate + Acid -> Salt + Water + Carbon Dioxide
• The reaction between a metal and an acid produces hydrogen gas which means you will see bubbles/fizzing. The more bubbles you see, the more reactive the metal is.
• Test for hydrogen - ignites with a squeaky pop
• Carbonate test

  When acid reacts with a carbonate fizzing is observed. Bubbles are seen, CO2 is a gas.
• Carbon dioxide test

  The carbon dioxide (CO2) causes bubbling/fizzing during the reaction between metal carbonate and acid. It can be detected using limewater, which turns a milky white when carbon dioxide is bubbled through it.
• Sulfate ions in solution, SO₄²⁻, are detected using barium chloride solution.

  The test solution is acidified using a few drops of dilute hydrochloric acid, and then a few drops of barium chloride solution are added.

  A white precipitate of barium sulfate forms if sulfate ions are present.
• Copper (II) oxide (black solid) + sulfuric acid -> Copper (II) sulfate (blue solution) + water (Part 1)

  1. Add copper (II) oxide in excess (until no more will disappear/react). This means all the sulfuric acid has reacted.
  2. There will be a colour change from colourless solution to a blue solution. Black solid (excess copper (II) oxide) will sink to the bottom.
• Copper (II) oxide (black solid) + sulfuric acid -> Copper (II) sulfate (blue solution) + water (Part 2)

  3. Filter to remove the excess copper oxide from the blue copper (II) sulfate solution. The excess copper oxide will be left in the filter paper and the copper (II) sulfate solution will drip through into the evaporating dish.

  4. Heat the copper (II) sulfate solution to evaporate some of the water, causing the volume to decrease by 1/3. This is then left to cool and the crystals will form.
• The same method for copper (II) oxide and sulfuric acid can also be done with copper (II) carbonate and sulfuric acid - the steps would be exactly the same

  Observation differences for copper carbonate:

  • Copper carbonate is a green solid instead of black (copper oxide)
  • Bubbling will be seen as carbon dioxide is produced
• Insoluble compounds may be made by precipitation reactions
• Three main stages of a precipitation reaction:

  1. Mixing the required reactant solutions
  2. Filtration to remove
  3. Washing and drying the residue
• Mixing (1/3)

  All nitrates are soluble, and all sodium salts are soluble. This means that, for example, if you want to make insoluble silver bromide you can mix together silver nitrate solution and sodium bromide solution

  
  

  silver nitrate + sodium bromide -> sodium nitrate + silver bromide

  AgNO₃ (aq) + NaBr (aq) -> NaNO₃ (aq) + AgBr (s)

  
  

  Ionic equation (replace H with the halide asked in the question):

  Ag⁺ (aq) + H⁻ (aq) -> AgH (s)
• Filtration (2/3)

  The insoluble precipitate must be separated from the soluble impurities using filtration. The precipitate stays behind in the filter paper as a residue, while the soluble impurities pass through in the filtrate
• Washing and drying (3/3)

  The precipitate can be washed while it is still in the filter funnel. Water cannot dissolve the precipitate, but it can wash off any remaining soluble impurities. The filter paper can then be removed and opened out flat. The precipitate is then dried in a warm oven.
• Titration method:

  1. Use the pipette and pipette filler to add 25cm³ of alkali to a clean conical flask
  2. Add a few drops of indicator and put the conical flask on a white tile
  3. Fill the burette with acid and note the starting volume
  4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix
  5. Stop adding the acid when the end-point is reached (the appropriate colour change in the indicator happens). Note the final volume reading
  6. Repat steps 1 to 5 until you get concordant readings
• The titration method also works for adding an alkali to an acid - just swap around the liquids that go into the conical flask and burette
• If the volume of the unknown is less than the known then the unknown must be more concentrated

  If the volume of the unknown is more than the known then the unknown must be less concentrated
• Moles = concentration x volume

  n = c × v
• Concentration = moles/volume

  c = n/v
• Volume = moles/concentration

  v = n/c
• If a volume is in cm³ then you need to divide by 1000 to convert into dm³
• Moles = mass/molar mass (Aᵣ or Mᵣ)

  n = m/M
• mass = moles × molar mass

  m = n × M
• Molar mass is calculated by adding up all the individual mass numbers of all the atoms (or particles) in a formula