Acid and Base equillibrium
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Define, by the bronsted and lowry definition, what is meant by an acid.
• a substance that donates H+ ions
Define, by the bronsted and lowry definition, what is meant by a base
• a substance that accepts H+ ions
in which case will you decide whether it is an acid or alkali?
• the one that has a higher Ka value
give the formula for pH
pH= -log[H+]
give the formula to find the concentration of H+ ions
• 1x10^pH
how to calculate Kw
= [H+] [OH-]
what is the value of Kw at 25 degrees c for most solutions? give units
1x10^-14 mol^2dm^-6
what does pKw mean?
-log[Kw], so the value would be -log1x10^-14, which would = 14.
How to find pH of pure water?
• Kw^2 = H+ concentration • so use this to find the pH
why does water’s pH change at temperatures different to 25 degrees?
• Le Chatelier’s principle can predict the change. • dissociation of water is endothermic so increasing the temperature would push the equilibrium to the right giving a bigger concentration of H+ ions resulting in a lower pH.
How do we work out the pH of a strong base?
• rearrange Kw expression to find H+ ions (so Kw/OH-) • find pH using log expression
what is the expression for the weak acid dissociation expression?
Ka= [H+][A-]/ [HA]
what does a larger Ka value signify?
• that the acid/base is stronger
what 2 assumptions are made when calculating pH of a weak acid?
• initial conc of the undissociated acid stays constant, as it’s very small, so eq HA = Initial HA • [H+] = [A-] , as the dissociation has taken place in a 1:1 ratio.
taking into account these assumptions, what does the Ka expression simplify to?
Ka= [H+]^2/[HA] initial
What needs to happen in order for us to assume that pKa=pH?
• when a weak acid has reacted with exactly half the neutralisation volume of alkali, so half the point of neutralisation.
how to work out the pH of a diluted acid?
H+ (diluted) = old vol/new vol x H+ (old) - then use the log expression to work out pH
how to work out the pH of a diluted alkali?
-OH- (diluted) = old vol/new vol x OH- (old) - [H+] = Kw/OH- - work out pH using log expression.
How do you calculate the H+ ion conc in a buffer solution?
[H+] = Ka x [HA]/[A-]
How do you produce a buffer solution?
• made form combining a weak acid and a salt of that weak acid (a conjugate base)
Define a buffer solution
• a solution in which pH does not change significantly if small amounts of acid or alkali are added to it.
for a buffer where H+ ions are on the right, where will the equilibrium shift if small amount of acid is added?
• to the left • as conc of H+ ions increases, the equillibrium will shift to the side with a higher number of OH- ions (or any other basic ions) • so since the concentration of the salt ion in the buffer solution is large, and stays relatively constant, the pH does as well.
In a buffer solution where H+ ions are on the right side, if small amounts of alkali are added, where would the equilibrium shift?
• will shift to the right • as equillibrium will shift to side with more H+ ions. • large conc of salt in buffer, so ratio of the reactant stays constant, and so does the pH.
what do you need to create a buffer solution with a pH BELOW 7?
• mix together a weak acid in excess and a strong base.