chap 8 - acids and bases

• physical properties of acids

  • sour taste
  • turn moist blue litmus paper red
  • dissolve in water to form solutions which can conduct electricity

  --> free, mobile H⁺ ions present can carry the electrical charges
• an acid is an substance that dissociates/ionises in aqueous solutions to produce hydrogen ions, H⁺
• a substance has acidic properties only when :

  1. it dissolve in water to form an aqueous solution --> acids are often referred to as dilute acids (to indicate that a volume of water has been added)--> dilute acids = acid + water <mixture>
  2. ionises to produce H⁺ ions in the solution
• HCl is gas is not an acid because it exists as covalent molecules and does not dissolve in water and cannot produce H⁺ ions
• simple chemical tests to show the presence of acid

  • moist blue litmus paper turns red
  • universal indicator changes from green to red for strong acids and orange to yellow for weak acids
• • strong acid

  > acid that is completely ionised in aqueous solution

  • weak acid

  > acid that is only partially ionised in an aqueous solution

  • inorganic acids

  > man-made acids which does not contain carbon

  > strong acid

  • organic acids

  > natural acids which contains carbon

  > weak acids
• hydrogen ion (H⁺) concentration and relatively acidity

  > all acid have pH value of less than 7
• steps in using universal indicator to test for relative acidity

  1. add a few drops of UI into the solution

  2. shake the solution to ensure thorough mixing

  3. compare the colour of the solution to the UI colour chart to determine the approximate pH value

  1-6 : acidic

  7 : neutral

  8-14 : alkaline
• strong acid : green to red

  : high H⁺ ion concentration

  : pH value = low (0-3)

  weak acid : green to yellow/orange

  : low H⁺ ion concentration

  : pH value = high (4-<7)
• when acids dissolve in water, hydrogen, H⁺ ions are produced
• why acids conduct electricity in water but not in organic solvent or when in molten state?

  • In water, hydrochloride acid ionizes in water to produce H⁺ ions & Cl⁻ ions. Mobile ions move to conduct electricity
  • in molten/organic solvent, hydrochloride acid has a simple molecular structure and have no free ions to move to conduct electricity
• why does litmus paper must be moistened with distilled water before using it to test for the presence of acid?

  • so that acid can ionize in water to produce H⁺ ions
• • hydrochloride acid : HCl : HCl (aq) --> H⁺ (aq) + Cl⁻ (aq)
  • nitric acid : HNO3 : HNO3(aq) --> H⁺ (aq) + NO3⁻ (aq)
  • sulfuric acid : H2SO4 : H2SO4 (aq) --> 2H⁺ (aq) + SO4²⁻ (aq)
  • ethanoic acid : CH3COOH : CH3COOH(aq) --> CH3COO⁻ (aq) + H⁴ (aq)
• acid reacts with reactive metals to form salt and hydrogen gas

  • reactive metals : K, potassium

  : Na, sodium

  : Ca, calcium

  : Mg, Magnesium

  : Al, aluminium

  : Zn, Zinc

  : Fe, Iron

  : Sn, Tin

  : Pb, Lead

  • salt : ionic compound formed when the H⁺ ions in an acid is replaced by another metallic ion/ammonium ion
• how to test for the presence of H2 gas?

  • place a lighted splint at the mouth of test tube and lighted splint should extinguish with a pop sound
• acid reacts with base to form salt and water

  • base : metallic oxide or metallic hydroxide
  • reaction : neutralisation (neutral salt and water are formed)
• acid reacts with metal carbonate to form salt and water and carbon dioxide

  • metal carbonate : an ionic compound containing the carbonate ion --> eg: copper (III) carbonate : CuCO3

  : calcium carbonate : CaCO3

  : sodium carbonate : Na2CO3

  • how to test for presence of CO2?

  > bubble the gas into a test tube of limewater. white precipitation forms in limewater = presence of CO2
• if acid react with unreactive metal, there is no reaction
• alkali = a substance that dissociate/ionises (become ions) in water to give hydroxide, OH⁻
• • not all substance containing OH are alkalis --> eg: CH3OH (alcohol)

  : CH3COOH (acid)

  • all properties of alkalis are due to presence of hydroxide ions when they dissociate in water/aqueous solutions
  • dilute alkali = alkali + water --> aqueous alkali is a mixture
• physical properties of alkali

  • bitter taste
  • alkalis turn moist red litmus paper blue
  • alkalis dissolve in water to form solutions which can conduct electricity ( free mobile OH⁻ ions present can carry the electrical charges)
• uses of bases/alkalis

  • relieving gastric pain
  • make soap and detergents
• simple chemical tests to show the presence of alkalis

  • moist red litmus paper turns blue
  • Univeral indicator changes from green to violet for strong alkalis and blue for weak alkalis

  > strong alkali : green to violet : high OH⁻ ion concentration : high pH (~12 to 14)

  > weak alkali : green to blue : low OH⁻ concentration : low pH (~8-11)
• acid reacts with bases to form salt and water

  • bases that are soluble in water are known as alkalis
  • this is a neutralisation reaction as the H⁺ ions from the acid was neutralized by the OH⁻ in the alkali to form neutral products, salt and water
• ionic equation for neutralisation

  H⁺ (aq) + OH⁻ (aq) --> H2O (l)

  • H⁺(aq) is from acid, pH <7
  • OH⁻(aq) is from alkali, pH >7
  • H2O (l) is a neutral product, pH = 7
• • best pH level for agriculture soil to ensure good growth if crops is pH 6.5
  • prolong usage of fertilisers (usually ammonium salt) and acid rain will cause the soil to become too acidic for goof plant growth (pH falls below 7)
  • to treat acidity in soil, farmers can add calcium hydroxide = liming the soil
• why calcium hydroxide should not be added to soil which has just been fertilised?

  • calcium hydroxide will react with the ammonium-based fertiliser to displace ammonium gas, which will escape into the surrounding and lower the nitrogen content of the soil
• types of oxide

  • oxide are compounds of oxygen
  • oxygen can be classified into 4 different types:

  > basic oxide

  > amphoteric oxide

  > neutral oxide

  > acidic oxide
• basic oxide

  • metal + oxygen ( non-metal)
  • ionic bonding
  • eg : sodium oxide, Na2O

  : calcium hydroxide, Ca(OH)2

  : copper (II) hydroxide, Cu(OH)2

  : iron (III) oxide, Fe2O3

  • reacts only with acids to form salt and water only
• amphoteric oxide

  • metal + oxygen (non-metal)
  • ionic bonding
  • eg: zinc oxide, ZnO

  : aluminium oxide, Al2O3

  : lead (II) oxide, PbO

  • reacts with both acid and alkalis to form salt and water only
• neutral oxide

  • non-metal + oxygen (non-metal)
  • covalent bonding
  • eg: carbon monoxide, CO

  : nitrogen monoxide, NO/

  : water, H2O

  • does not react with either acid or alkali
• acidic oxide

  • non-metal + oxygen (non-metal )
  • covalent bonding
  • eg: carbon dioxide, CO2

  : nitrogen dioxide, NO2

  : sulfur dioxide, SO2

  : sulfur trioxide, SO3

  • reacts only with alkali/base to form salt and water only