acid-base equilibria

• 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 are conjugate acid-base pairs?

  • A conjugate acid is formed when a base accepts a proton.
  • A conjugate base is formed when an acid donates a proton.
  • They form conjugate acid-base pairs.
• What does acid strength refer to?

  It refers to the degree of dissociation of the acid in solution
• How is a strong acid defined?

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

  Between 3 and 7
• What is the pH range for strong bases?

  Between 12 and 14
• What is the pH range for weak bases?

  Between 7 and 11
• What happens when strong acids and bases react together?

  • They produce very similar enthalpies of neutralisation.
  • Solutions completely dissociate, leading to a simple acid-base reaction.
  • The reaction produces water from H$^+$ and OH$^−$ ions.

  
• How do weak acids and bases behave in reactions?

  • They only slightly dissociate in solution.
  • Other enthalpy changes occur within the solution.
  • The enthalpies of neutralisation can vary significantly.

  
• What does pH measure?

  Acidity and alkalinity
• What is the pH scale range?

  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 pH be calculated?


  Using the concentration of hydrogen ions, [H$^+$]

  
• What is the relationship between the concentration of H$^+$ ions and strong acids?


  The concentration of H$^+$ ions is equivalent to the concentration of a strong acid
• What characterizes weak acids and bases in solution?

  • They only slightly dissociate in solution.
  • They form an equilibrium mixture.
  • The equilibrium dissociation constant is represented by K$_a$.

  
• What does a low pK$_a$ value indicate?


  A strong acid
• How can pH be determined for weak acids and bases?

  • Use [HA] and [A$^-$] along with K$_a$ to find [H$^+$], then pH if HA is in excess.
  • Use K$_w$ to find [H$^+$], then pH if A$^-$ is in excess.
  • pK$_a$ equals pH if HA = A$^-$.
• What happens to the pH of a strong acid when it is diluted 10 times?

  The pH will increase by one unit
• What happens to the pH of a strong acid when it is diluted 100 times?

  The pH will increase by two units
• What happens to the pH of a strong acid when it is diluted 1000 times?

  The pH will increase by three units
• How do weak acids behave when diluted?

  They do not behave the same way as strong acids; the equilibrium shifts to oppose the change
• What is the process for calculating K$_a$ of weak acids?


  1. Find initial, change, and equilibrium concentrations of reactants and products.
  2. Calculate [H$^+$] using the given pH.
  3. Determine actual equilibrium concentrations.
  4. Substitute values into the expression for K$_a$.

  
• Given a weak acid HA with a concentration of 0.25 M and a pH of 3.5, how would you find its K$_a$?


  Use the equilibrium concentrations and substitute into the expression for K$_a$
• What is the equilibrium constant for water dissociation called?

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


  1. 0 x 10$^{-14}$
• How does temperature affect K$_w$?


  K$_w$ changes as temperature changes
• What happens to the equilibrium of water dissociation when temperature increases?

  More H$^+$ ions are produced, making the water more acidic
• How can pK$_w$ be calculated?


  • pK$_w$ can be calculated from K$_w$.
  • It is analogous to how pK$_a$ can be calculated from K$_a$.
• How can the pH of a strong base be calculated?

  Using pK$_w$ or K$_w$
• What does a pH titration curve show?

  • It shows how the pH of a solution changes during an acid-base reaction.
  • The neutralisation point is identified as a large vertical section of the graph.
  • The pH is regularly measured with a pH probe during the titration.

  
• What occurs at the neutralisation point in a strong acid-strong base reaction?

  The neutralisation point occurs around pH 7
• What are the neutralisation points for different acid-base combinations?

  • Strong Acid + Strong Base = pH 7
  • Strong Acid + Weak Base = < pH 7 (more acidic)
  • Weak Acid + Strong Base = > pH 7 (more basic)
  • Weak Acid + Weak Base = normally pH 7 but hard to determine

  
• What is the equivalence point in a titration curve?

  The vertical region where the neutralisation point is reached
• What is the relationship between the half-equivalence point and pK$_a$?


  At the half-equivalence point, the pH is equal to pK$_a$