1.12 Acids Bases and pH Flashcards
Define an acid
A proton donor
Define a base
A proton acceptor
Define a strong acid
Fully ionises/dissociates to give H+ in solution
What is the general equation for an acid and water?
HA(aq) + H20(l) —–> H30+(aq) + A-(aq)
What is the general equation for a base in water?
B(aq) + H20(l)——>BH+ + OH- (aq)
What is the general equation for an acid and a base?
HA(aq) + B(aq) (reversible to) BH+ (aq) + A-(aq)
How do you calculate pH?
-log[H+]
how do you calculate H+ using pH?
10 to the power of -pH
How do you calculate the pH of strong acids?
Calculate acid concentration
If monoprotic [H+] = [HA]
If diprotic [H+] =[HA] x2
Use the pH equation
How many significant figures do you record pH to?
2 decimal places
What assumptions do you make when calculating the pH of weak acids?
Only in calculations directly calculating pH
[H+] =[A-] as they dissociate in a one to one ratio.
The equilibrium [HA] is the same as the initial [HA] as only slightly dissociates.
How do you calculate the pH of weak acids?
Ka = [H+]2 / [HA] (rearrange)
pH equation
What are the equations linking Ka and pKa?
Ka = 10^-pka pKa = -log(Ka)
How do you know if an acid is stronger than another?
larger Ka
smaller pKa
What is the water dissociation constant?
kw = [OH-]{H+]
Why is water neutral?
Dissociates OH- and H+ on a one to one ratio so their concentration is equal.
What happens to the equilibrium of water is the temperature is increased?
The forwards dissociate endothermic reaction is favoured to oppose the increase in temperature.
The equilibrium shifts to the right so the concentration of H+ and Kw increases.
The pH decreases.
How do you calculate the pH of a strong base?
Kw = [OH-][H+]
can’t square anything as not neutral.
[OH-] concentration depends on the alkali concentration then if it is mono or dibasic.
What are buffer solutions?
A solution that maintains a stable pH.
The pH will not change significantly if a small amount of acid or alkali is added.
What are the four different methods of making a buffer solution?
Weak acid and its salt Weak base and its salt Excess weak acid and strong base Excess weak base and strong acid. Always one weak to establish an equilibrium.
How do you work out the pH of buffer solution when you have a weak acid and its salt?
Work out the moles of acid.
Work out the moles of salt
[H+] = kacid / salt
pH equation
How do you work out the pH of a buffer solution when you have a base and an acid?
Calculate the moles acid and the moles base.
Calculate the excess remaining reagent and the moles of salt produced.
[H+] = kacid / salt
