Acids and Bases

• What do acid-base equilibria involve?

  The transfer of protons between substances
• How are substances classified as acids or bases?

  Based on their interaction with protons
• What is a Brønsted-Lowry acid?

  A proton donor
• Give an example of a Brønsted-Lowry acid.

  Ammonium ions (NH$_4^+$)
• What is a Brønsted-Lowry base?

  A proton acceptor
• Give an example of a Brønsted-Lowry base.

  Hydroxide ions (OH$^-$)
• What does acid strength refer to?

  It does not refer to the concentration of a solution
• How is a strong acid defined?

  An acid that completely dissociates to ions when in solution with pH 3-5
• What is the pH range of a strong acid?

  pH 3-5
• How is a weak acid defined?

  An acid that only slightly dissociates when in solution with pH 0-1
• What is the pH range of a weak acid?

  pH 0-1
• What are the pH ranges for strong and weak bases?

  Strong bases have pH 12-14 and weak bases have pH 9-11
• What does pH measure?

  Acidity and alkalinity
• What type of scale is pH?

  A logarithmic scale from 0 to 14
• What does a pH of 0 indicate?

  An acidic solution with a high concentration of H$^+$ ions
• What does a pH of 14 indicate?

  A basic solution with a low concentration of H$^+$ ions
• How can the concentration of H$^+$ ions be determined?


  Using the pH value
• What is the equilibrium constant for the dissociation of water at 25°C?

  K$_w =$$1 \times 10^{-14}$
• What happens to the value of K$_w$ as temperature changes?


  The value changes
• What type of reaction is the forward reaction in the equilibrium of water?

  Endothermic
• What happens to the acidity of water as temperature increases?

  Water becomes more acidic as temperature increases
• What is the equilibrium dissociation constant for weak acids and bases?

  K$_a$
• How can pKa be found in relation to [H$^+$]?


  Using the relationship between K$_a$, pKa, and [H$^+$]
• What methods can be used to find the pH of weak acids and bases?

  • HA in excess: Use [HA] and [A$^-$] along with K$_a$ to find [H$^+$], then pH.
  • A$^-$ in excess: Use K$_w$ to find [H$^+$], then pH.
  • HA = A$^-$: pKa is equal to pH, therefore find pKa.
• What does a pH titration curve show?

  How pH of a solution changes during an acid-base reaction
• What is the neutralisation point in a titration curve?

  The point where a large vertical section occurs through the neutralisation or equivalence point
• What is the effect of adding alkali to an acid during titration?

  The pH is measured and changes during the acid-base reaction
• What is the neutralisation point for a strong acid-strong base reaction?

  Occurs around pH 7
• What is the neutralisation point for a strong acid-weak base reaction?

  Less than pH 7 (more acidic)
• What is the neutralisation point for a weak acid-strong base reaction?

  Greater than pH 7 (more basic)
• What is the neutralisation point for a weak acid-weak base reaction?

  Normally pH 7 but hard to determine
• What are the common indicators used in titrations and their characteristics?

  • Methyl Orange:
  • Used for reactions with a more acidic neutralisation point
  • Orange in acids, turns yellow at the neutralisation point
  • Phenolphthalein:
  • Used for reactions with a more basic neutralisation point
  • Pink in alkalis, turns colourless at the neutralisation point

  
• What is a buffer solution?

  A solution that resists changes in pH when small volumes of acid or base are added
• What components make up an acidic buffer solution?

  A weak acid and the salt of that weak acid
• What components make up a basic buffer solution?

  A weak base and the salt of that weak base
• How does an acidic buffer resist changes in pH when an acid is added?

  By reacting the ethanoate ions with H$^+$ to make ethanoic acid
• How does adding small volumes of acid affect the pH of a buffer solution?

  It increases the concentration of the acid in the buffer solution, making it slightly more acidic
• How does adding small volumes of base affect the pH of a buffer solution?

  It decreases the concentration of acid in the buffer solution, making it slightly more basic
• What are the uses of buffer solutions in nature?

  • To keep systems regulated
  • Important for enzymes or reactions in living organisms that require a specific pH