Acids and Bases Flashcards
What is a Brønsted-Lowry acid?
a species that can donate a proton
What is a Brønsted-Lowry base?
a species that can accept a proton
What is the formula for pH?
pH = - log10[H+]
What is the equation for concentration of H+ ions if the pH is known?
[H+] = 10^(-pH)
Is the forward reaction in the equilibrium of water endothermic or exothermic?
endothermic
Does water become more or less acidic as temperature increases?
more acidic as more H+ ions are produced
What is the equilibrium dissociation constant for?
weak acids and bases
What is the equilibrium dissociation constant (Ka)?
Ka = [H+][A-]/[HA]
If HA is in excess, how do you find pH?
Use [HA] and [A-] along with Ka to find [H+], then pH
If [A-] is in excess, how do you find pH?
Use Kw to find [H+] then pH
If HA = A-, how do you find pH?
pKa is equal to pH, therefore find pKa
What is the definition of a strong acid?
An acid that completely dissociates to its ions when in solution with pH 0-1.
What is the definition of a weak acid?
An acid that only partially dissociates when in solution with pH 3-5.
What is the definition of a strong base?
A base that completely dissociates to ions when in solution with pH 12-14.
What is the definition of a weak base?
A base that only partially dissociates when in solution with pH 9-11.
Give some examples of weak acids
Most organic acids (e.g. ethanoic acid), HCN (hydrocyanic acid), H2S (hydrogen sulfide) and H2CO3 (carbonic acid)
Give some examples of strong acids
Hydrochloric, nitric and sulfuric acid
On a pH titration curve, how do we identify the neutralisation or equivalence point?
the large vertical section and the equivalence point is approximately in the middle of the large vertical section
How do we investigate the neutralisation point?
alkali is slowly added to an acid and the pH measured with a pH probe or vice versa
How do we make a titration curve more accurate?
add smaller volumes of either acid or alkali
What is the neutralisation point around for a strong acid and strong base reaction?
pH 7
What is the neutralisation point around for a strong acid and a weak base reaction?
less than pH 7 (more acidic)
What is the neutralisation point around for a weak acid and a strong base reaction?
more than pH 7 (more basic)
What is the neutralisation point around for a weak acid and a weak base reaction?
normally pH 7 but hard to determine
Why do specific indicators have to be used for specific reactions?
they can only indicate a pH change within a certain range
When is methyl orange used?
for reactions with a more acidic neutralisation point
What is the colour change for methyl orange?
orange in acids and turns yellow at the neutralisation point
When is phenolphthalein used?
for reactions with a more basic neutralisation point
What is the colour change for phenolphthalein?
pink in alkalis and turns colourless at the neutralisation point
What do acidic buffer solutions contain?
a weak acid and the salt of that weak acid
What do basic buffer solutions contain?
a weak base and the salt of that weak base
What is the definition of a buffer solution?
a solution which is able to resist changes in pH when small volumes of acid or base are added
What is the equation that links the concentration of hydrogen ions and pH?
pH = -log10[H+] (aq)
The higher the concentration of hydrogen ions, the ____ the pH
lower
The pH of strong acids can be determined with what and why?
pH = -log10[H+] (aq) because we can assume that the acid is fully dissociated; that is, all its hydrogen ions have been released into solution. Therefore, the concentration of the hydrogen ions is equal to the concentration of the acid if there is one H atom in one molecule of the acid, or to twice the concentration of the acid if there are two H atoms in one molecule of the acid.
How can the pH of weak acids be calculated and why?
using the acid dissociation constant Ka (in moldm-3). We assume that for any weak acid HA, the hydrogen ions only partially dissociate from the acid anion, A- : HA (aq) <-> H+ (aq) + A- (aq). Since this is in equilibrium, we can write an expression for it: Ka = ([H+(aq)][A-(aq)])/[HA(aq)]
What does Ka vary with?
temperature
