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 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 for 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 for 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 ionic product of water?

  Water slightly dissociates to ions as an equilibrium with its own equilibrium constant, K$w$

  
• What is the value of K$w$ at 25°C?


  1 x 10$^{-14}$
• How does temperature affect K$w$?


  The value of K$w$ changes as temperature changes
• What type of reaction is the forward reaction in the equilibrium of water?

  Endothermic
• What happens to H$^+$ ions as the temperature of water increases?


  More H$^+$ ions are produced, making the water more acidic
• What is the equilibrium dissociation constant for weak acids and bases?

  Ka

  
• How can pKa be used in relation to weak acids?

  pKa can be found using the equilibrium dissociation constant, Ka

  
• How can the pH of weak acids and bases be determined?

  Using the relationships of Ka, pKa, and [H$^+$]

  
• What methods are used to find pH in different scenarios involving weak acids and bases?

  • HA in excess: Use [HA] and [A$^-$] along with Ka 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?

  A large vertical section through the neutralisation or equivalence point
• How is a titration curve investigated?

  By slowly adding alkali to an acid and measuring the pH

  
• What is the neutralisation point for a strong acid-strong base reaction?

  It occurs around pH 7

  
• What is the neutralisation point for a strong acid-weak base reaction?

  It is less than pH 7 (more acidic)

  
• What is the neutralisation point for a weak acid-strong base reaction?

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

  It is normally pH 7 but hard to determine
• Why are specific indicators used in titrations?

  They can only indicate a pH change within a certain range

  
• What are the two most common indicators used at A-Level?

  • Methyl orange: orange in acids, turns yellow at neutralisation point.
  • Phenolphthalein: pink in alkalis, turns colourless at 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 do acidic buffer solutions contain?

  A weak acid and the salt of that weak acid
• What components do basic buffer solutions contain?

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

  By reacting the ethanoate ions with H$^+$ to make ethanoic acid
• How does a buffer solution resist changes in pH when a base is added?

  By decreasing the concentration of acid in the buffer solution