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Acids and Bases

Learning Objective

  • Understand how to identify gases produced in chemical reactions.
  • Prepare salts using various experimental methods.

Success Criteria

  • Describe the laboratory tests and positive results for identifying carbon dioxide and hydrogen gases.
  • Apply solubility rules to determine whether a specific salt is soluble or insoluble in water.
  • Outline the step-by-step procedures for preparing salts.

Gas Tests

  • Several reactions produce gases that need to be tested.
    • The gases include:
    • Carbon dioxide
    • Hydrogen

Test for Carbon Dioxide

  • The test for carbon dioxide involves bubbling the gas through an aqueous solution of limewater (calcium hydroxide).
    • If the gas is carbon dioxide, the limewater turns cloudy white.

Test for Hydrogen

  • The test for hydrogen consists of holding a burning splint at the open end of a test tube containing the gas.
    • If the gas is hydrogen, it burns with a loud "squeaky pop," which is the result of the rapid combustion of hydrogen with oxygen to produce water.
    • Reference: Hydrogen Squeaky Pop Test - GCSE Chemistry - YouTube

Neutralisation

  • A neutralisation reaction occurs when an acid reacts with an alkali.
    • In this reaction, the H+ ions from the acid react with the OH– ions from the alkali to produce water.
    • Not all reactions of acids are neutralisations. For example, when a metal reacts with an acid, a salt is produced, but there is no water formed, so it does not fit the definition of neutralisation.

Solubility Rules

  • Ionic compounds are generally soluble in water compared to covalent substances, but there are exceptions.
  • Knowledge of the solubility of ionic compounds helps determine the most appropriate method for the preparation of salts.

Solubility of Salts

  • Soluble Salts
    • Sodium, potassium, and ammonium: All soluble
    • Nitrates: All soluble
    • Chlorides: Most are soluble except for silver and lead(II)
    • Sulfates: Most are soluble except for barium, calcium, and lead(II)
    • Carbonates: Carbonates of sodium, potassium, and ammonium are soluble; most other carbonates are insoluble.
    • Hydroxides: Hydroxides of sodium, potassium, and ammonium are soluble; calcium hydroxide is sparingly soluble; most others are insoluble.

Prepare a Soluble Salt

  • A soluble salt can be made from the reaction of an acid with an insoluble base.
  • During the preparation of soluble salts:
    • The insoluble reactant is added in excess to ensure that all of the acid has reacted.
    • Failure to do this could lead to unreacted acid becoming dangerously concentrated during evaporation and crystallisation.
    • The excess reactant is removed by filtration to ensure that only the salt and water remain.
    • The solution is then heated to evaporate water until small crystals begin to appear.
    • Typically, this happens when half of the water is left.
    • Allowing the filtered solution to evaporate slowly over a period of days results in the formation of larger crystals.

Step-by-Step Process to Prepare a Soluble Salt

  1. Heat acid until warm, then add insoluble metal/base/carbonate while stirring constantly until it stops disappearing.
  2. Filter the mixture to remove excess solid base and transfer the solution to an evaporating basin.
  3. Heat the evaporating basin to evaporate water from the solution until crystals appear.
  4. Remove the evaporating basin from heat and allow filtrate to dry and crystallise.

Prepare a Soluble Salt II

  • It is also possible to prepare a sample of a dry salt starting from an acid and an alkali.
  • A titration can be used for this process.
  • Titration is a method used to prepare salts if the reactants are soluble.
    • Concentration and volumes of reactants can be calculated from titrations.

Titration Method for Salt Preparation

  1. Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange).
  2. Fill the burette with acid and note the starting volume.
  3. Slowly add acid to the alkali from the burette until the indicator changes colour.
  4. Calculate the volume of acid added.
  5. Repeat steps 1-3 without indicator.
  6. Transfer the solution to an evaporating basin and heat to partially evaporate water.
  7. Allow the filtrate to dry and crystallise.

Summarising the Methods of Making Salts

  • Is the salt soluble?
    • No: Use a precipitation method; mix two solutions, one containing the correct positive ion and the other the correct negative ion.
    • Yes:
    • Is it a sodium, potassium, or ammonium salt?
      • No: React an acid with an excess of a solid metal (if suitably reactive), metal oxide, hydroxide, or carbonate.
      • Yes: Use a titration method to react an acid with a solution of sodium or potassium hydroxide, carbonate, or ammonia solution.

Plenary

  • Review exercises from textbook, Pg. 188 numbers 4, 5, 7, 9.