2.2 Chemistry- Acids bases and salts

• pH scale

  Measures the acidity or alkalinity of a solution, can be measured using universal indicator or pH probe
• pH 7

  • Neutral
• pH less than 7

  • Acidic, the lower the pH the stronger the acid
• pH more than 7

  • Alkaline/basic, the higher the pH the stronger the alkali
• As the pH decreases by one unit, the H+ concentration of the solution increases by a factor of 10
• Alkali

  A base that dissolves in water
• Acids

  Release hydrogen ions, H+, in solution
• Strong acid

  • Completely dissociates to release H+ ions in aqueous solution
• Weak acid

  • Partially dissociates to release H+ ions in aqueous solution
• Strong base

  • Fully dissociates to release OH- ions in aqueous solution
• Weak base

  • Partially dissociates to release OH- ions in aqueous solution
• Dilute/concentrated

  Refers to the amount of substance present - the number of moles of that acid/base in solution
• Weak/strong

  Refers to the degree of ionisation of the acid or base - how readily the acid releases H+ ions or how readily the base releases OH- ions
• Reaction of dilute acid with metals

  Acid + metal → salt + hydrogen
• Neutralisation of dilute acids

  1. Acid + alkali → salt + water

  2. Acid + base → salt + water

  3. Acid + metal carbonate → salt + water + carbon dioxide
• Neutralisation involves the reaction of hydrogen ions with hydroxide ions: H+ (aq) + OH- (aq) → H2O (l)
• Salts

  The salt produced depends on the acid used and the positive ions in the base, alkali or carbonate
• Preparing soluble salts

  1. Measure a set volume of acid

  2. Heat the acid gently

  3. Add the chosen base in excess

  4. Filter the excess base

  5. Heat the salt solution to evaporate the water

  6. Leave the rest to evaporate slowly so crystals of the salt form
• Preparing insoluble salts

  1. React 2 solutions that contain the ions of your desired salt

  2. A solid precipitate of your salt is produced

  3. Filter the salt out using filter paper and a funnel

  4. Wash with distilled water and leave the salt to dry on the filter paper
• Titration

  1. Add acid to burette using a funnel, record the start volume

  2. Add a known volume of alkali to a conical flask and add a few drops of indicator

  3. Place the conical flask on a white tile

  4. Slowly add acid to alkali until you reach the neutralisation point when the indicator changes colour

  5. Calculate the volume of acid, this is called the titre

  6. Repeat until you get concordant titres
• Once you have carried out a titration and obtained concordant results, use all concordant results to calculate the mean titre
• Titration calculations (higher tier only)

  1. Calculate moles of acid using moles = concentration x volume

  2. Calculate the mole ratio of acid to alkali using the balanced chemical equation for the reaction

  3. Work out how many moles of alkali you have using the mole ratio and moles of acid

  4. Calculate the concentration of the alkali using concentration = mol ÷ volume
• Test for carbon dioxide gas

  Bubble the gas through limewater (calcium hydroxide solution) and it will turn milky (cloudy) if carbon dioxide is present
• Test for carbonates

  Carbonates react with dilute acids to create carbon dioxide, this gas can be bubbled through limewater
• Test for sulfate ions

  Add dilute hydrochloric acid, followed by barium chloride solution. A white precipitate will form if sulfate ions are in this solution