Acids and alkalis

• Acidic solutions have pH values less than 7
• Alkaline solutions have pH values greater than 7
• Neutral substances have pH values equal to 7
• A solution with a pH value of 6 has ten times more hydrogen ions (acidity) compared to one with a pH value of 7.
• The pH scale is logarithmic, meaning that the difference between two numbers on the scale represents a ten-fold change in acidity or basicity.
• A solution with a pH value of 8 has one hundredth as many hydrogen ions (less acidity) compared to one with a pH value of 7.
• An increase or decrease by one unit on the pH scale corresponds to a change in acidity/alkalinity by a factor of 10.
• For example, an increase from pH 4 to pH 5 means there are 10 times fewer H+ ions present.
• For example, if the pH increases from 3 to 4, there are ten times fewer H+ ions present.
• Increasing the concentration of reactants also increases the rate of reaction.
• Catalysts are chemicals that speed up reactions without being used up themselves.
• Neutral solutions contain both H+ and OH- ions but they cancel out so no net charge exists.
• Strong acids completely dissociate into their component ions when they dissolve in water, resulting in high concentrations of both hydrogen ions and chloride ions.
• Weak acids only partially dissociate into their component ions when they dissolve in water, leading to low concentrations of both hydrogen ions and chloride ions.
• Acidic solutions have more free hydrogen ions than neutral ones.
• Enzymes are biological catalysts found inside living cells.
• Neutral substances do not ionize when dissolved in water.
• aqueous solutions - acidic = H+ ions
• measure pH with : Universal indicator and pH probe/metre
• phenolphthalein:

  • acid = colourless
  • alkali = pink
• methyl orange :

  • acid = red
  • alkali = yellow
• blue litmus paper :

  • acid = turns red
  • alkali = stays blue
• red litmus paper :

  • acid = stays red
  • alkali = turns blue
• problem with universal indicator to test pH :

  • the pH colour chart is subjective - pH value isn't exact/accurate
• low pH of acid = high concentration of H+ ions
• low pH of alkali = low concentration of OH- ions
• pH decreases by unit = it has increased by a factor of 10

  • e.g. pH 2 to pH 1 is an increase by 10
• neutralisation = acid + base - makes salt + water
• • concentrated acid has more moles of acid per unit than volume of water than dilute acids
  • concentration of an acid is not the same as strength
• if acid only partially dissociates in water = weak acid
• base = any substance which can react with an acid to form salts and water only
• alkalis are soluble bases