Acids and Bases

• The Bronsted-Lowry theory describes acids and bases in terms of protons
• A Bronsted-Lowry acid donates H+ ions
• A strong acid completely dissociates in solution
• A weak acid partially dissociates in solution
• A Bronsted-Lowry base accepts H+ ions
• An alkali is a base that releases hydroxide ions in aqueous solution
• The reaction of acids with metals is a type of redox reaction
• H+ ions act as oxidizing agents in their reaction with metals
• Acid-base equilibria involve the reversible transfer of protons
• Water can act as both a Bronsted-Lowry acid and a Bronsted-Lowry base
• The acidity of a solution depends on the concentration of H+ ions
• A low pH indicates a large concentration of H+ ions
• To calculate the pH of a strong monoprotic acid, assume complete ionisation
• One mole of a monoprotic acid donates one mole of H+ ions
• Steps to calculate the effect of dilution on the pH of a strong acid

  1️⃣ Determine the initial [H+]

  2️⃣ Calculate the new [H+] after dilution

  3️⃣ Calculate the new pH using -log10[H+]
• When the pH of a solution changes by 1, the [H+] changes by a factor of 10
• For a diprotic acid, [H+] = 2[HA]initial
• The pH of 0.10 mol dm-3 H2SO4 is 0.70
• To calculate the concentration of a strong acid from its pH, use the formula [H+] = 10-pH
• A weak acid is an acid which is only partially dissociated in aqueous solution
• The Ka for a weak acid is calculated as [H+][A-] / [HA]
• Steps to compare the relative strengths of acids using pKa

  1️⃣ Calculate pKa using -log10Ka

  2️⃣ Compare the pKa values

  3️⃣ Lower pKa indicates a stronger acid
• The lower the pKa value, the stronger the acid
• To calculate the pH of a weak acid, assume [HA]equilibrium = [HA]initial
• The formula to calculate [H+] for a weak acid is √Ka [HA]
• To calculate [HA] for a weak acid, use the formula [H+]2 / Ka
• At 298 K, the value of Kw is 1 x 10-14
• The pH of pure water at 298 K is 7.00
• The value of Kw is affected by temperature
• If the pKa value for a weak acid is given, it must be converted to Ka using the formula Ka = 10-pKa
• What is the ionic product of water denoted by?

  Kw
• The value of Kw at 298 K is 1 x 10-14
• Steps in determining the effect of temperature on Kw

  1️⃣ Increase the temperature

  2️⃣ Equilibrium shifts to the endothermic direction

  3️⃣ Kw increases

  4️⃣ [H+] increases

  5️⃣ pH decreases
• Why is the dissociation of water an endothermic process?

  Bond breaking requires energy
• Water is always neutral because [H+] = [OH-]
• What is the relationship between Kw and temperature?

  Kw increases with temperature
• All aqueous solutions contain both H+(aq) and OH-(aq) ions.
• In alkaline solutions, the concentration of H+ ions is less than the concentration of OH- ions.
• What defines a strong base in aqueous solution?

  Full dissociation
• When calculating the pH of a strong base, complete ionization is assumed.