Chapter 20 ACIDS, BASES & pH

• What is a Bronsted-Lowry acid?

  A proton donor
• What is a Bronsted-Lowry base?

  A proton acceptor
• What is important to remember when dealing with [H+]?

  H+ ions don't exist on their own in water but form H3O+ so this makes a solution acidic however only H+ used in equations
• What is the dissociation equation for acids?

  HA + H2O$\leftrightarrow$H3O+ + A-
• What is the dissociation equation for bases?

  B + H2O $\leftrightarrow$ BH+ + OH-
• What are polyprotic acids?

  Acids that can donate more than one proton
• What are monoprotic/basic acids and give an example?

  One mole produces one mole of H+

  E.g. HNO3
• What are diprotic/basic acids and give an example?

  One mole produces two moles of H+

  E.g. $H_2SO_4$
• What are triprotic/basic acids and give an example?

  One mole produces three moles of H+

  E.g. $H_3PO_4$
• What are neutralisation reactions?

  Salts are made from metal from base (or ammonium ions) and non-metals (other than hydrogen) from acid. Ammonia reacts with acids to make ammonium salts but no water. Ammonia must react with water first to make it basic.
• Metal + Acid =?

  Salt + Hydrogen
• Metal Oxides + Acid =?

  Salt + Water
• Metal Hydroxides + Acid =?

  Salt + Water
• Metal Carbonate + Acid =?

  Salt + Water + Carbon Dioxide
• What is a conjugate acid?

  A species that has GAINED a proton
• What is a conjugate base?

  A species that has LOST a proton
• What does water dissociate into?

  OH- and H3O+ ions
• What is an assumption when water dissociates?

  It dissociates so weakly there is little OH- and H+ ions compared to H2O so we assume that the concentration remains at a constant value
• What is the ionic product of water, Kw?

  Kw = [H+][OH-] and has a value of $1\times10^{-14}\ mol^2dm^{-6}$
• When will Kw change?

  When the temperature changes
• What is the pH scale?

  A logarithmic scale that measures [H+] ions in solution
• What are the equations involving pH?

  pH = -log[H+] and [H+] = $10^{-pH}$
• What assumptions are used when calculating pH of strong acids?

  Must assume it fully dissociates
• How is pH of strong bases calculated?

  Use Kw=[OH-][H+] and rearrange the expression to make [H+] the subject of the equation then use pH = -log[H+]
• What is an assumption about calculating pH of strong bases?

  They also dissociate fully and most produce one mole of OH- ions for every base molecule. Assume that the concentration of the base = concentration of OH- ions
• What is Ka and when is it used?

  Ka is the acid dissociation constant and is used to calculate [H+] and therefore pH of weak acids because they only partially dissociate
• What are the two assumptions when using Ka to find pH?

  Assumption 1: There is only a small amount of weak acid (HA) that dissociates so assume that [HA]$_{equilibrium}$= [HA]$_{start}$

  Assumption 2: Dissociation of acid is greater than the dissociation of water present in solution so assume that all H+ ions come from the acid. [H+]$\approx$[A-]
• What is the equation for Ka?

  $K_a=$$\frac{\left[H^+\right]^2}{\left[HA\right]}$
• How is pKa calculated?

  pKa is another way of measuring strength of an acid so the lower the value of pKa, the stronger the acid. pKa = -log(Ka)
• How is pH measured experimentally?

  pH meters measure pH of a solution but they must be calibrated correctly to provide reliable readings
• How are pH meters calibrated?

  1. pH probe is placed in distilled water first and the measure should read pH 7.0
  2. The process is repeated with a standard solution at pH 4.0 and pH 10.0, making sure to rinse with distilled water inbetween
• What are indicators used for?

  Indicators are used to determine end point of titration however the correct one must be picked. To be correct, the indicator must completely change colour within the vertical part of the titration curve
• What are two examples of indicators and how are they different at low and high pH's?

  Methyl orange is RED at low pH and YELLOW at high pH

  Phenolphthalein is COLOURLESS at low pH and PINK at high pH
• What is the equivalence point/end point?

  The vertical point of a titration curve and this is where all the acid is neutralised and we can assume that [H+]=[OH-]
• What is important to remember about titration curves?

  Depending on the strengths of the acids and bases, and which one is in the flask, the graphs will look different.