pH scale & neutralisation

• pH:


  A measurement that tells you how ACIDIC or ALKALI an aqueous solution is.

  • The pH scale goes from 0-14, and the HIGHER the pH value, the more alkali a solution is.
• INDICATORs:


  A group of chemical dyes that change colour depending on the pH.
• WIDE-RANGE INDICATORS:


  Contains a mixture of different dyes, so their colours will slowly change across a wide range of pHs.

  • E.g. universal indicator that can give the following colors:

  
• pH PROBES:


  Connected to a pH meter, the pH probe is dipped in the solution to electronically measure the pH & get a numerical reading on the meter.

  
• NEUTRALISATION:


  A type of chemical reaction in which an acid & a base react to form water & a salt.

  
• This reaction involves the transfer of H⁺ IONS from the acid to the OH⁻ IONS of the alkali, forming H₂O.

  
• Sodium hydrogen carbonate cures indigestion by reacting with acid in the stomach, this is neutralisation.
• Measurements that remove the need for human judgement are more reliable.
• The probe is more accurate & precise, compared to the indicator is that, as it doesn’t involve humans guessing shades & particular colours.

  
• Acid:


  Any substance that forms aqueous solutions with a pH less than 7.

  • Because they ionise (split into their ions) in aqueous solutions to release hydrogen ions (H+), that make the solution acidic.

  
• Bases:


  Any substance with a pH greater than 7.

  
• Alkalis:


  A subgroup of bases that are soluble in water.

  • So they’re a base that dissolves in water to form a solution with a pH greater than 7. 
  • They form hydroxide ions (OH-) in water.

  
• NEUTRAL SOLUTIONS:


  Has a pH of exactly 7, showing a balance between H⁺ & OH⁻ ions.

  • E.g. pure water
• HCl + NaOH -> NaCl + H2O

  • H+ (from acid) + OH- (from base) -> H2O

  As the acid & base has been neutralised in these sorts of reactions, the pH of the products should be 7 as they’re neutral.
• Different indicators will change at different pHs.
• Acids:


  • Hydrochloric (HCl)
  • Sulfuric (H2SO4)
  • Nitric (HNO3)
• Bases:


  • Sodium hydroxide (NaOH)
  • Calcium carbonate (CaCO3)
• the ionic equation for the neutralisation of hydrochloric acid with potassium hydroxide

  H+ + OH- -> H2O
• What is present in the solution at stages 2 & 3 apart from universal indicator & water?

  • at stage 2: sodium ions & chloride ions
  • at stage 2: sodium ions, chloride ions & H+ ions