Acids, Bases and Salts - Unit 2 - Year 10 Chemistry Revision 2023

• Acid


  a substance that produces H+ aqueous ions when in solution

  (a hydrogen ion (proton) donor in aqueous solution)

  
  

  for an acid to behave as an acid, water must be present
• Monoprotic Acid


  an acid that can donate only one proton (H+ ion) per molecule (e.g. HCl)
• Diprotic Acid


  an acid that can donate two protons (H+ ions) per molecule (e.g. H2SO4)
• Triprotic Acid


  an acid that can donate three protons (H+ ions) per molecule (e.g. H3PO4)
• Common Acids


  HCl (hydrochloric)

  H2SO4 (sulphuric)

  HNO3 (nitric)

  H3PO4 (phosphoric)

  CH3COOH (ethanoic/acetic)
• Strong Acid


  An acid that ionises (dissociates) completely in aqueous solution.

  
  

  Colour in Universal Indicator: Red

  Reaction with Reactive Metal (e.g. Mg): rapid effervescence and H2 gas

  Reaction with Carbonates: rapid effervescence and CO2 gas

  Conductivity: High

  
• Strong Acid Examples


  HCl

  HCl (g) --> H+ (aq) + Cl- (aq)

  
  

  H2SO4

  H2SO4 (l) --> 2H+ (aq) + SO4 2- (aq)

  
  

  HNO3

  HNO3 (l) --> H+ (aq) + NO3 - (aq)
• Weak Acid


  An acid that is only partially ionised (dissociated) in aqueous solution.

  
  

  Colour in Universal Indicator: Yellow

  Reaction with Reactive Metal (e.g. Mg): gentle effervescence and H2 gas

  Reaction with Carbonates: gentle effervescence and CO2 gas

  Conductivity: Low

  
• Weak Acid Examples


  CH3COOH

  CH3COOH (l) <--> H+ (aq) + CH3COO - (aq)
• Dilute and Concentrated Acids


  Concentrated - little or no water

  Dilute - water added

  
  

  Concentration and strength are not related.
• Base


  a substance that reacts with an acid to form a salt and water (e.g. metal oxides, hydroxides, carbonates)
• Alkali


  a base that dissolves in water, forming hydroxide (OH-) ions
• Strong Alkali


  An alkali that ionises (dissociates) completely in aqueous solution.

  
  

  Colour in UI: Purple

  Conductivity: High
• Strong Alkali Examples


  NaOH

  NaOH (s) --> Na+ (aq) + OH- (aq)

  
  

  KOH

  KOH (s) --> K+ (aq) + OH- (aq)
• Weak Alkali


  An alkali that is only partially ionised (dissociated) in aqueous solution.

  
  

  Colour in UI: Blue

  Conductivity: Low
• Weak Alkali Examples


  NH3

  NH3 (g) + H2O (l) <--> NH4 + (aq) + OH- (aq)
• Indicator


  a compound that changes colour in the presence of an acid or a base
• Litmus


  Acid: red

  Neutral: purple

  Alkaline Solution: blue

  
• Phenolphthalein


  Acid: colourless

  Neutral: colourless

  Alkaline Solution: pink

  
• Methyl Orange


  Acid: red

  Neutral: orange

  Alkaline Solution: yellow

  
• Universal Indicator


  Acid: red

  Neutral: green

  Alkaline Solution: purple

  
• Salt


  an ionic compound that forms when a metal atom or a positive radical replaces the hydrogen of an acid
• Solubility of Salts in Water at Room Temperature


  All Soluble:

  Sodium, potassium and ammonium salts

  Nitrates

  
  

  Mostly Soluble:

  Chlorides, bromides and iodides (insoluble with silver and lead)

  Sulphates (insoluble with lead and barium + calcium sulphate is only slightly soluble)

  
  

  Mostly Insoluble:

  Carbonates (soluble with sodium, potassium and ammonium)

  Hydroxides (soluble with sodium, potassium and ammonium + calcium hydroxide/limewater is slightly soluble)
• Methods of Salt Preparation


  Soluble Salts

  1. Excess insoluble solid + acid (acid + metal/oxide/hydroxide/carbonate)

  2. Titration (acid + hydroxide / acid + carbonate)

  
  

  Insoluble Salts

  3. Precipitation from 2 solutions
• Methods of Salt Preparation Flow Diagram

  Is the salt soluble?

  No: precipitation from 2 solutions

  Yes: -->

  
  

  Does it contain Na+, K+, NH4+?

  Yes: titration (acid + hydroxide / acid + carbonate)

  No: -->

  
  

  Does it contain an unreactive metal?

  Yes: excess insoluble solid + acid (acid + oxide / acid + hydroxide / acid + carbonate)

  No: excess insoluble solid + acid (acid + metal / acid + oxide / acid + hydroxide / acid + carbonate)
• Crystallisation


  The formation of crystals by cooling a saturated solution.

  
  

  Method:

  1. Evaporate some water from solution to form saturated solution.

  2. Leave to cool to allow crystals to form.

  3. Filter off crystals.

  4. Dry crystals using filter paper.

  
• Filtrate


  liquid that has passed through a filter
• Residue


  the solid particles that are left after a suspension has been filtered
• Forming an Anhydrous Salt


  1. Pour some hydrous solution into an evaporating basin.

  2. Heat and evaporate all water.
• Excess Insoluble Solid (metal, metal carbonate or insoluble base)


  1. Place some acid in a beaker, heat and continue adding solid until excess solid does not react (all acid is used up).

  2. Filter off unreacted solid.

  3. Evaporate some water from solution to form saturated solution.

  4. Leave to cool to allow crystals to form.

  5. Filter off crystals.

  6. Dry crystals using filter paper.
• How do you know when a saturated solution is formed?


  when enough water has evaporated so small crystals start to appear
• Excess Insoluble Solid Notes

  1. Why is excess solid added to acid?

  To use up all the acid.

  
  

  2. Why is the solution filtered?

  To remove excess solid.

  
  

  3. Why are crystals obtained by partial evaporation?

  This would leave small crystals + complete evaporation of solvent produces anhydrous crystals (water of crystallisation driven away).

  
  

  4. Why is maximum mass of crystals never obtained?

  Spitting during partial evaporation + some solution absorbed by filter paper + not all the solution will crystallise

  
  

  5. Why do crystals form when a hot, saturated solution is cooled?

  As temperature decreases, solubility decreases so water can no longer dissolve as much so crystals form.
• Anhydrous/Hydrated Copper Sulphate Equation

  CuSO4*5H2O <--> CuSO4 + 5H2O

  
• Water of Crystallisation


  water molecules that form an essential part of the crystalline structure of a compound
• Titration


  a measured amount of a solution of unknown concentration is added to a known volume of a second solution until the reaction between them is fully complete
• Titration Method


  1. Rinse and fill burette with acid (making sure to open the tap to fill the jet).

  2. Record volume of acid.

  3. Rinse pipette and use to put 25cm^3 of alkali into a conical flask.

  4. Add phenolphthalein/methyl orange.

  5. Add acid to alkali until indicator changes colour.

  6. Repeat until concordant results are achieved (2 results within 0.2cm^3) and average.

  7. Repeat using average volume of acid and 25cm^3 of alkali but with no indicator.

  
  

  Crystallisation:

  8. Evaporate some water from solution to form saturated solution.

  9. Leave to cool to allow crystals to form.

  10. Filter off crystals.

  11. Dry crystals using filter paper.

  
• Titration Notes


  1. Why is are the burette and pipette washed with the solution they are going to contain rather than water?

  So only an undiluted solution is in the apparatus so the readings are accurate.

  
  

  2. What is the correct way to read a meniscus?

  From the bottom + flat surface + at eye level

  
  

  3. Why do you need to fill the burette beyond the tap?

  So the jet is filled so the readings are accurate.

  
  

  4. Why only use a few drops of indicator?

  Indicator may be slightly acidic or alkaline e.g. phenolphthalein is a weak acid (interferes with results).

  
  

  5. Why remove the funnel from the burette when taking the initial reading?

  So that residual drops of the solution from the funnel do not go into the burette during the experiment and make the readings inaccurate.

  
  

  6. Why swirl the conical flask?

  To mix the acid with the alkali so that the alkali neutralises the acid, making sure the colour of the indicator changes.

  
  

  7. When have you done enough titrations?

  When you have 2 results within 0.2cm^3 of each other.

  
  

  8. Why do repeat the titration with the average titre value without indicator?

  So that you have pure salt solution (forms pure, large crystals).
• Titre Value


  the volume of titrant (acid) used for a definite amount of unknown reagent at its equivalence point
• Precipitation Reaction


  a reaction in which a solid, insoluble product forms upon mixing two solutions

  
• Precipitation Method


  1. Mix 2 solutions, one containing the required positive ion and one containing the required negative ion.

  2. Filter off precipitate.

  3. Wash with distilled water.

  4. Leave to dry in a warm place OR dry with filter paper.