Reactivity of Acids

• MAKING INSOLUBLE SALTS : Describe a method to make pure, dry crystals of magnesium sulfate from a metal oxide and a dilute acid.


  1. Add sulfuric acid to beaker .
  2. Warm sulfuric acid gently with electric heater / bunsen burner
  3. Add magnesium oxide.
  4. Stir
  5. Continue adding magnesium until in excess.
  6. Filter out excess magnesium oxide using filter paper and funnel.
  7. Heat solution in evaporating basin to crystallisation point.
  8. Leave to crystallise.
  9. Pat dry with filter paper / dry crystals with drying oven.
• MAKING INSOLUBLE SALTS : Why must you warm the sulfuric acid?

  • To speed up the reaction
• MAKING INSOLUBLE SALTS : Why must you add the solid reactant until its in excess?

  • To make sure all the acid reacts
• MAKING INSOLUBLE SALTS : Why must you filter the mixture?

  • To remove the excess e.g magnesium oxide
• How to heat gently?

  • Boiling water bath
  • Electric heater
• Titration definition

  • The method that is used to find the volume of acid that reacts with a known volume of alkali. Vice-versa.
• Describe how the student would use the equipment in the figure above to complete the titration.

  1. Measure volume of alkali hydroxide
  2. Add alkali hydroxide solution to the conical flask.
  3. Add a few drops of indicator
  4. Add the sulfuric acid from the burette
  5. Until the colour of the indicator changes
  6. Read the volume from the burette
  7. Repeat and calculate a mean

  CAN HAPPEN VICE VERSA
• Improvements to titration technique

  1. Stir solution
  2. Repeat and calculate a mean volume
  3. Use of white tile under conical flask
• Why is putting a white tile under the flask better for accuracy?

  • Able to detect colour change clearly
• Give one advantage of using a pipette rather than using a measuring cylinder to measure the volume of hydrochloric acid.

  • Pipettes measure volume more accurately
• Burettes VS Pipettes

  • Burettes measure variable volumes and larger volumes than pipettes.
  • Pipettes measure fixed volumes accurately.
• How to eliminate parallax (human) errors when measuring volume?

  • measure at eye level to the meniscus.
• Equipment needed for titration practical :

  1. Measuring cylinder
  2. Volume of acid / alkali hydroxide
  3. White tile
  4. Burette
  5. Indicator
  6. Conical flask
• Titration indicators : Litmus paper

  • ACIDS : turns blue litmus paper red
  • ALKALIS : turns red litmus paper blue
  • NEUTRAL : turns purple
• Titration indicators : Methyl orange

  • ALKALIS : Yellow in alkaline conditions
  • ALKALIS : Red in acidic conditions
• Titrations indicators : Phenolphthalein

  ACIDS : colourless

  ALKALI : bright pink
• Weak acids

  • produces hydrogen ions (H+) in aqueous solution
  • but is only partially ionised
• Examples of Weak acids

  1. Carboxylic : Ethanoic acids
  2. Citric acids
  3. Carbonic acids
• Explain what happens at the end point of a titration : methyl orange - acid - to alkali

  • Indicator changes colour
  • From red to yellow
  • When the acid and alkali have been exactly neutralised
• Strong acids

  • produce hydrogen ions (H+) ions in aqueous solution
  • but is fully ionised / dissociates its hydrogen ions
• Examples of strong acids

  • Hydrochloric acid
  • Sulphuric acid
  • Nitric acid
• The student repeated the experiment with potassium carbonate. The limewater did not bubble.

  Suggest why there were no bubbles in the limewater.

  • Potassium carbonate does not decompose to form carbon dioxide gas.
• Concentration of an acid

  • Amount of acid per unit volume
• Describing concentration

  • Dilute
  • Concentrated
• Dilute concentration of an acid

  • small amount of acid per unit volume
• Acid and pH scale relationship

  As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
• Compare the concentration of hydrogen ions with an acid of pH 4, with pH 5

  • The acid with pH 4 has 10 times the concentration of hydrogen ions of an acid that has a pH of 5.
• In weak acids : REACTIONS

  • reversible reaction
  • only some ions dissociate in aqueous solution
• Explain why an acid can be described as both strong and dilute.

  • Strong = completely ionised in aqueous solution
  • Dilute = small amount of acid per unit volume
• Explain why an acid can be described as both weak and concentrated.

  Weak = partially ionises in aqueous solution

  Concentrated = large amount of acid per unit volume
• As concentration of hydrogen ions in a solution increases..

  • the pH decreases
•  EQUATION : Relationship between the pH and concentration of H+ ions.

  
• Control variables during titration :

  • Concentration of acid
  • Indicator used
• Why do a rough titration?

  • Gives an idea of where the end-point will be
• Why add volume of acid / alkali from burette slowly at the end?

  • To not miss the end-point of the titration