Acids and bases

• Acids have a sour taste and are corrosive

  They are proton (H+) donors
• Strong acids- donate H+ easily e.g HCl

  Weak acids- don't donate H+ easily e.g citric acid
• Bases

  Have a soapy feel and are corrosive

  Bases are proton (H+) acceptors
• Strong bases - accept H+ easily e.g NaOH

  Weak bases- don't accept H+ easily e.g NaHCO3
• An alkali is a base that is dissolved in water
• Neutral substances

  Do not accept or donate H+ emg water and salts
• Indicators

  It is a compound that changes colour depending on whether it is an acid or a base e.g litmus paper
• Litmus paper is blue is base

  Litmus paper is red in acid Neutral substances do not change the colour
• Universal indicator is a mixture of many indicators that can be used to determine the strength of an acid or base
• The pH scale

  • It is a scale that goes from 0-14
  • It indicates the strength of an acid ot base
  • 0-6.9=acid
  • 7=neutral
  • 7.1-14= base
• pH sensors can be used to give an exact pH reading
• Reactions of Acids

  1.Acid and Base reactions

  A neutralisation reaction occurs when an acid reacts with a base to form salt and water.

  A salt forms when the H in an acid is replaced by a metal

  • Acid+Base-> salt + water
  • Hydrochloric acid+ sodium hydroxide -> sodium chloride+water
  • HCl+ NaOH-> Na2SO4 +H2O
• A titration is a procedure used to do an accurate neutralisation reaction
• 2. Acid and a Metal reacting together

  When a metal reacts with an acid, the metal salt is formed and a H2 gas is given off

  • Acid+ Metal -> Salt + Hydrogen
  • Hydrochloric acid + zinc -> zinc chloride + hydrogen
  • 2HCl + Zn-> ZnCl2 + H2
• 3. Acid and Carbonate reactions

  When an acid reacts with a metal carbonate, the metal, salt, water and carbon dioxide are formed

  • Acid+metal carbonate-> salt + water+ carbon dioxide
  • Hydrochloric acid+ calcium carbonate -> calcium chloride + water + CO2
  • 2HCl + CaCO3-> CaCl2 +H2O +CO2
• Rainwater is naturally acidic(ph5.5) as H2O & CO2 react to form H2CO3
• Acid rain occurs when the pH is below 5.5
• Fossil fules contain impurities of S(sulphur) and N(nitrogen), these react to form a strong acid
• Problems

  • Low pH release Al³+ from the soil, this harms plants and kills fish in rivers
  • Low pH leaches nutrients from the soil, affects tree growth
  • Acid strips leaves form trees
  • Acid corrodes buildings/ statues
• Solutions

  • Burn less fossil fuels
  • Reduce S content of fuels
  • Install scrubbers into factory chimneys to remove NO2 & SO2