Acids & Bases

• what is a Brønsted-Lowry acid?

  a proton donor
• “half the acid reacts” refers to the half equivalence point
• the PH at the half equivalence point is the Pka value
• a base is a proton acceptor
• Kw= 1 x 10^-14 mol ^2 dm^-6
• weaker acids dissociate partially, and only a small amount of the acid dissociates, so we can assume that the [HA] = [A-]

  
• in pure water, [H+] is equal to [OH-]
• H2O is not included in the kw expression because H2O only slightly dissociates, so H2O becomes a constant
• H2O is not included in the kw expression because H2O only slightly dissociates, so H2O becomes a constant
• why does kw increase as the temperature increases?

  breaking bonds is an endothermic process, so equilibrium shifts to the right to absorb heat, to oppose the increase in temperature
• what is a weak acid?

  a proton donor that dissociates partially
• in pure water [H+] = [OH-], so kw= [H+]^2
• when the endpoint of a titration curve is not vertical, but instead curved, it means that the change in PH is not rapid at the endpoint, an indicator would not change colour rapidly
• a burette can deliver variable volumes, a pipette is used to accurately measure fixed volumes
• why is distilled water is used to rinse a conical flask?

  ensures all reagents are able to react
• when H+ is added, equilibrium shifts to the left
• how do you ensure that a reading from a PH meter is accurate?

  calibrate the meter with solutions of known PHs, then plot a calibration curve
• what is meant by a strong acid?

  completely ionises to give H+ in water
• why is a PH probe washed with distilled water between each calibration measurement?

  so that any residual solutions that may interfere with the reading are washed off
• to obtain a titration curve:

  • add a fixed volume of an alkali into a beaker
  • add acid in small proportions using a burette
  • stir and use a pH meter to record the pH after each addition of acid
• why should a pH meter be calibrated before use?

  because after long periods of storage they do not give accurate readings
• how to make a 250cm3 solution that contains a mass of NaHSO4?

  • weigh an empty weighing boat, add the NaHSO4 and weigh them together, calculate the difference to obtain an accurate mass
  • transfer the solid into a beaker of water to dissolve the NaHSO4
  • transfer the contents into a 250cm3 volumetric flask, making sure to include washings
  • make up to the 250cm3 mark and invert, to ensure that the solution is uniform