Acids, Bases and Buffers

• What do you need to understand to succeed in the topic of Acid-Base Equilibria?

  You need to understand Kc expressions and their units.
• Which key on the calculator is used to convert numbers into logarithms to the base 10?


  The lg key.
• How do you turn log10 values back into numbers?


  By using the inverse of log10, which is the $10^x$ key.
• What definitions will you be able to use after working through this Factsheet?

  You will be able to use A2 level definitions of acids and bases and identify conjugate acid/base pairs.
• What is the mathematical expression for pH?


  pH = -log10 [H+].
• How can you calculate the pH of a base using Kw?


  By using the ionic product of water, Kw, to find [H+].
• What do the terms weak, strong, concentrated, and dilute refer to in acids and bases?


  They refer to the degree of dissociation and the ratio of moles to volume.
• What is the dissociation behavior of strong acids?


  Strong acids undergo 100% dissociation.
• What is the definition of a buffer solution?


  A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added to it.
• Why is it useful to control the pH of a buffer solution in biological systems?


  It is vital to control the pH of an enzyme, for example.
• What is the relationship between conjugate acid/base pairs?


  Acids donate protons, while bases accept protons.
• What are the components of an acidic buffer solution?


  • A weak acid
  • A salt of the same weak acid
• How does the Brønsted-Lowry theory expand on the definition of acids?


  It defines acids as species that donate protons, not just those that require water.
• What are the four strong acids at A2 level?

  HCl, HNO3, H2SO4, and H3PO4.
• What are the components of an alkaline buffer solution?


  • A weak base
  • A salt of the same weak base
• How is an acid buffer made?


  An acid buffer is made by mixing a weak acid with a salt of the same weak acid.
• What is an example of an acid buffer?


  An example of an acid buffer is a mixture of ethanoic acid and sodium ethanoate.
• What is the dissociation behavior of weak acids?


  Weak acids only partially dissociate.
• What is an example of an alkaline buffer?


  An example of an alkaline buffer is a mixture of ammonia solution and ammonium chloride.
• How do buffer solutions maintain pH?


  They can absorb and release H+ ions to maintain their pH.
• What is the significance of the equilibrium sign in weak acid dissociation?


  It indicates that the acid only partially dissociates.
• What happens when an acid is added to an acid buffer?


  The extra H+ ions combine with CH3COO- ions to make undissociated CH3COOH.
• What happens when an alkali is added to an acid buffer?


  The OH- ions combine with H+ ions to make water, shifting the equilibrium to the right.
• How do alkali buffers behave compared to acid buffers?


  Alkali buffers behave similarly to acid buffers but involve a weak base and its salt.
• How do you write the Ka expression for weak acids?


  Ka = $\frac{[H^+][A^-]}{[HA]}$.
• Why is Ka not defined for strong acids?


  Because there will be no undissociated acid left.
• What is the relationship between [H+] and [A−] in weak acid dissociation?


  [H+] = [A−] for every dissociation of HA.
• What assumption can be made about [HA] in weak acid dissociation?


  [HA] remains approximately constant because [H+] is very small.
• What are the units for Ka?


  mol dm^-3.
• What is the definition of logarithms?


  Logs are related to powers, such as $log_{10}100 =$$2$.
• What is the general equation for a weak base in a buffer solution?

  B (aq) + H2O (l) ⇌ BH+ (aq) + OH- (aq)
• How do you find the pH of strong acids?


  By using the concentration (mol dm^-3) directly.
• What are the key concepts related to Acid-Base Equilibria?


  • Writing Kc expressions and their units
  • Using logarithms (lg and 10x keys)
  • Definitions of acids and bases (Brønsted-Lowry theory)
  • Calculating pH and using Kw
  • Understanding weak vs. strong acids/bases
  • Writing Ka expressions for weak acids
• What are the differences between strong and weak acids?

  Strong Acids:

  • 100% dissociation
  • Defined by concentration

  
  

  Weak Acids:

  • Partial dissociation
  • Equilibrium established
• How do you calculate pH for strong bases?


  1. Identify the concentration of the base.
  2. Use Kw to find [H+].
  3. Calculate pH using pH = -log10 [H+].
• What happens when acid is added to an alkali buffer?


  The extra H+ ions combine with OH- ions to make water.
• What is the significance of the pH scale?


  • Ranges from 1 to 14
  • Indicates acidity or basicity
  • Linked to Universal Indicator colors
• What is the relationship between pH and [H+]?


  • pH = -log10 [H+]
  • Lower pH indicates higher [H+]
  • Higher pH indicates lower [H+]
• What happens when alkali is added to an alkali buffer?


  The added OH- ions react with NH4+ ions to make ammonia and water.
• What is the buffer range?


  Buffers work effectively within a range of ±1 pH value of the pKa or pKb.