Acids, bases and pH A2 Flashcards
What are the Bronstead-Lowry model definitions for acids and bases?
Acid = proton (H+) donor
Base = proton (H+) acceptor
Where can protons dissociate? And what ions do they form?
In solution.
They form H3O+ (hydroxonium) ions with water.
What is a conjugate acid-base pair?
A pair of species that can be interconverted by proton transfer.
HCl + H2O <–> H3O+ + Cl-
Label the conjugate acid-base pairs
HCl + H2O <–> H3O+ + Cl-
A1 B2 A2 B1
What are the names that describe how many H+ ions are replaced in an acid?
Monobasic = one proton (e.g. HCl)
Dibasic = 2 protons (e.g. H2SO4)
Tribasic = 3 protons (e.g. H3PO4)
A1 recap:
What do you get when you react metals, carbonates and alkalis with acids? What are these reactions called?
Metal + acid = salt + water
REDOX REACTION
Metal Carbonate + acid = salt + water + CO2
NEUTRALISATION
Alkali + Acid = Salt + water
NEUTRALISATION
What kind of scale is pH? What does it measure?
A logarithmic scale for measuring [H+] in solution.
(each increasing pH value is a magnitude of 10 smaller)
What is the equation to work out pH from [H+]?
pH = - log [H+]
How can [H+] be determined from pH?
[H+] = 10^ - pH
What is a strong acid? What does this mean about the [HA] and [H+]?
Strong acids completely dissociates in water.
So the concentration of HA is equal to the concentration of H+.
What is the equation for dilution factor?
Original volume / total volume
Weak acids form an equilibrium so their dissociation can be represented by the acid dissociation constant ___.
ka
HA <–> H+ + A-
What does a higher/lower value of Ka suggest about equilibrium position?
High ka = shift to the right
Low ka = shift to the left
(ka<1, ka>1)
As the values of Ka are very _____, its is easier to give their negative logarithm (__) instead. What is the equation? What is the inverse?
Small
pKa
pKa = -logKa
Ka = 10^ - pKA
The weaker an acid the _____ the Ka and the _____ the pKa.
Smaller the Ka
(becouse less H+ dissociated so shift to left)
Larger the pKa
(greater power of -10)
The [H+] of a weak acid depends on the value of ___ and ___.
Ka and [HA]
What are the 2 approximations that need to be made about weak acid dissociation?
1) HA dissociation forms equal [H+] and [A-] (so becomes [H+]^2)
2) The change in [HA] is extremely small / negligible so [HA] at eqm = [HA] at start
What equation can you use to find Ka for weak acids?
Ka = [H+]eqm^2 / [HA]
How do you calculate pH for weak acids?
[H+] = square root(Ka x [HA])
pH = -log[H+]
How can the value for Ka be determined experimentally?
By using a pH meter. Then calculate [H+] and then Ka.
When calculating values for Ka, what are the issues with the approximations made?
1) At pH values >6, water dissociation is significant (releasing some H+ into solution so [H+] does not equal [A-]). So it doesn’t work for very weak acids / dilute solutions.
2) If [H+] concentration is significant there will be a difference between [HA] at eqm and [HA] at start. So doesn’t work for stronger weak acids with Ka > 10^-2 mol dm^-3 or very dilute solutions.
Water is amphoteric. What does this mean?
It acts as an acid and a base.
because it is able to be ionised slightly:
H2O <–> H+ + OH-
Where does Kw come from?
As the dissociation of water is very small, the [H2O] remains constant, so bringing both Ka and [H2O] to the same side of the equation gives:
Ka x [H2O] = Kw = [H+] [OH-]
What alters the Kw value?
Temperature
What is the difference between a strong and weak base?
Strong bases completely dissociate in solution whereas weak bases only partially dissociate.
e.g. NaOH -> Na+ + OH-
