Chapter 11- Acids and Bases

  • Acids are substances that dissociate in water to give H+ ions.
  • Bases are any metal oxides/hydroxides that react with an acid to produce salt and water only. Most bases are insoluble in water. Some don’t even react with water.
  • A base that is soluble in water, and produces hydroxide (OH-) in an aqueous solution is an alkali.

PROPERTIES OF ACIDS

  1. Any substance that produces hydrogen ions (H+) when dissolved in water
  2. Conduct electricity in aqueous form
  3. Have a sour taste
  4. Turn blue litmus red
  5. React with metals to form a salt and hydrogen.
  6. React with carbonates/hydrogen carbonates to form salt, water and carbon dioxide.
  7. React with metal oxides/hydroxides to form salt and water only.
  8. Uses: fertilizers, cleaning metals, to give a sour taste.

PROPERTIES OF ALKALIS

  1. Have a bitter taste and feel soapy
  2. Turn red litmus blue
  3. React with acids in a neutralization reaction to form salt and water only.
  4. Give off ammonia gas when heated with ammonium salts.
  5. React with one metal salt to give another metal salt and metal hydroxide.
  6. Uses: relieving gastric pain, reducing soil acidity, fertilizers.

pH Indicators

NameColour in acidsColour in AlkaliColour in neutral solution
Methyl OrangeRedYellowYellow
PhenolphthaleinColourlessPinkColourless
LitmusRedBlueOriginal colour

CONCENTRATION AND STRENGTH

  • Concentration refers to the moles of acid per dm3.
  • The strength of acids refers to how well it dissociates/ionises in aqueous solutions.
  • A strong acid such as hydrochloric acid completely dissociates in water while a weak acid such as ethanoic acid only partially ionises in water.
  • The pH of strong acids is less than weak acids. pH of acids is from 0-6, 7 is neutral and pH of alkalis is from 8-14.
  • The colour of universal indicator changes from red at 0 to violet at 14. Green is at neutral (7).
  • It is necessary to maintain optimum pH of different substances such as soil and blood. To reduce soil acidity, bases like calcium carbonate are spread on the soil.

OXIDES

  • Oxides are compounds of any other element with oxygen. Many acids and alkalis are formed by dissolving oxides in water.
  • There are different types of oxides:

  

  1. Acidic Oxides: Most oxides of non-metals that have acidic properties.
  2. Basic Oxides: Most oxides of metals that have basic properties.
  3. Amphoteric Oxides: Metallic oxides that react with both acids and bases to form salt and water, e.g. zinc oxide, lead (II) oxide, aluminium oxide, copper oxide.
  4. Neutral Oxides: Non-metallic oxides with neither acidic nor basic properties, and are insoluble in water (except water itself), e.g. nitric oxide, carbon monoxide, water, manganese (IV) oxide.

STEPS OF PREPARING SULFURIC ACID

  1. Burning sulfur to get sulfur dioxide (exothermic):    S + O2 SO2
  2. Catalytic oxidation to get sulfur trioxide (exothermic):    2SO2 + O2 ⇌ 2SO3
  3. Making oleum:    H2SO4 + SO2 H2S2O7
  4. Dilution Tower:    H2S207 + H20 2H2SO4

Optimal conditions for the Contact Process (Manufacture of Sulfuric Acid)

  1. Temperature: 450⁰C
  2. Pressure: 1 to 2 atm
  3. Catalyst: Vanadium pentaoxide/ Vanadium (V) Oxide
  4. Uses of Sulfuric acid: Manufacture of detergents, battery acids and sulfuric acid
  5. Uses of sulfur dioxide: Manufacture of sulfuric acid and wood pulp, used as a bleach and food preservative.