3.1.12 Acids And Bases Flashcards
Conjugate acid
The species produced when a base gains a proton.
Conjugate base
The species produced when an acid loses a proton.
Conjugate acid-base pair
Two species related to one another by the presence of a hydrogen ion or its absence
pH =
-log10[H^+]
Kw=
[H^+][OH^-]
Delivered from equilibrium constant of water dissociation
Varies with temp
State the meaning of the term Brønsted–Lowry acid
Proton donor
Strong acid + strong base indicator
Methyl orange or phenolphthalein
Weak acid + strong base indicator
Phenolphthalein
Strong acid + weak base indicator
Methyl orange
Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.
Cl more electronegative
Weakens O-H bond
State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement
Over time/storage readings less accurate
Describe how you would obtain the pH curve for the titration
Measure pH of acid
Add alkali in 2cm^3 portions
Stir
Measure pH after every portion
When pH reaches end point, add acid in 0.2cm^3 portions
Repeat until alkali in excess
Continue with 2cm^3 portions post end point
Acid-base equilibria
Involves transfer of protons
Why is logarithmic scale used for pH
Concentration of H+ ions in aqueous solution covers very large range
Ka =
Dissociation constant of weak acid
[H^+][A^-]/[HA]
pKa =
-log [Ka]
Buffer solution
Maintains an approximately constant pH dispute dilution or addition of small amounts of acid or base
Acidic buffer contains
Weak acid and the salt of that weak acid
Basic buffer solution contains
Weak base and the salt of that weak base
State how a buffer solution can be made from solutions of potassium hydroxide and ethanoic acid
Give equation
How does buffer resist small additions of acid
Add enough KOH that the acid contains mixture of ethanoic acid and ethanoate ions
Add excess ethanoic acid to KOH and enough KOH so acid is partially neutralised
KOH + CH3COOH —> CH3COOK + H2O
CH3COO^- reacts with acid
Define weak acid and weak base
Acid/base that doesn’t fully dissociate in solution
Equivalence point
Point in titration at which the amount of titrant added is just enough to completely neutralize the analyte solution
What happens at the half neutralisation point on curves
pH=pKa
How do buffers work
Resist pH changes upon the addition of an acidic or basic components. It is able to neutralize small amounts of added acid or base to keep pH stable
Lewis acid/base
Species that accepts/donates a pair of electrons
Describe what happens in Brønsted−Lowry acid−base reaction
When a Brønsted-Lowry acid loses a proton, a conjugate base is formed. A Brønsted-Lowry base gains a proton, a conjugate acid is formed
Explain why the end point of a reaction could be difficult to judge accurately using an indicator
Change in pH can be gradual
An indicator would change colour over a range of volumes
What physical factors affect Kw + how
Temperature
If increased, equilibrium moves right
Kw increases + pH decreases
Why is pure water still neutral when pH doesn’t equal 7
[H^+] = [OH^-]
What is titration (acid/base)
The addition of an acid/base of known titration to a base/acid of unknown titration to determine the concentration
An indicator is used to show that neutralisation has occurred
Titration curve for strong acid + weak base
Titration curve for strong acid + weak base
Titration curve for weak acid + weak base
Titration curve for weak acid + strong base
What happens to pH of solution around equivalence point
Large + rapid change in PH (except in weak acid/weak base titration)
3 properties of a good indicator
Sharp colour change (one drop)
End point the same as equivalence point
Distinct colour change
What colour is methyl orange in acid and alkali
What pH does is change
Acid-red
Alkali-yellow
pH-4.5
What colour is methyl orange in acid and alkali
What pH does is change
Acid-colourless
Alkali-red
pH-9-10
Equation for acidic buffer with added acid
A^- + H^+ —> HA
Equation for acidic buffer with added alkali
HA + OH^- —> H2O + A^-
How can you achieved an acidic buffer solution other than just mixing the constituents
Neutralise half of a weak acid with an alkali
Forms weak acid/soluble salt mixture
Equation for buffer system which maintains blood pH at 7.4
H^+ + HCO3 ^- <=> CO2 + H2O
What products are buffers found in
Shampoos, detergents
