Acids and Bases Flashcards
Acid and base definition (Brønsted-Lowry)
Acid: proton donor
Base: proton acceptor
Calculating pH of a solution of a strong base
pOH = -log[OH^-]
pH = 14 - pOH
Buffer solution definition
A solution that resists large changes in pH upon addition of small quantities of strong acid or strong base
Reactions of a buffer solution
Buffer solution of a weak acid HA and weak base A^-
With an acid A^- + H^+ —> HA
Stron acid reacts with a weak base to produce weak acid —> lower pH decrease
With a base HA + OH^- —> A^- + H2O
Strong base reacts with weak acid to produce weak base —> lower pH increase
Amphoteric substance
A substance that can act both as a Brønsted-Lowry acid and a BL base, eg water
Acid-base reaction
A proton transfer from the acid to the base
Ionisation of water
H2O+H2O—>—<H3O+ +OH-
Kw =[OH-][H3O+]=110^-14
Pkw=14
Value of Kw depends on temperature
pH and pOH
pH= — log [H+]
pOH= — log [OH+]
pOH+pH=pKw=14
Strong and weak acids and bases
Strong acids ionise/dissociate completely
Distinguishing between weak and strong acids
Of the same concentration!!!
Strong acids have higher conductivity and lower pH value and react faster.
Ionisation constants
Ka and Kb
For a conjugate acid-base pair:
KaKb=110^-14
pKa+pKb=14
Ranking acid and base strengths (K pK)
Stronger have higher K and lower pK
Calculating pH of a solution of a weak acid
HA+H2O—>—< A- + H3O+
[A-][H3O+]
Ka= —————-
[HA]
[A-]= [H3O+]
& if Ca>100Ka, [HA]i=[HA]e
[H3O+] =sqrt(Ka*[HA])
Calculating pH of a solution of strong acid
pH= -log(ca)
Calculating pH of a solution of a weak base
B + H2O —>—< HB+ + OH-
[HB+][OH]-
Kb= —————-
[B]
Same argument as for weak acid
[OH-]= sqrt(Kb*[B])
Reactions of acids
With reactive metals, e. g. Na
2HCl + 2Na (s) —> 2NaCl + H2 (g)
With metal oxides, e. g. Na2O
2HCl + Na2O —> 2NaCl + H2O
Calculating pH of a buffer solution
Cosn
[B]
pH= pKa + log——
[A]
Or
nb
pH= pKa + log——
na
Buffer range
pka+-1
The equivalence point
When the ratio of the reactants in the equation is equal to the ratio of amounts
Indicator
Weak acid/base pair that has different colors in its neutral (Hin) and ionic (I-) form.
Choose indicator so its color change, end point, corresponds to the equivalence point.
End point of titration
The point when the indicator changes color. This is the same as the equivalence point, but the equivalence point can also be found from e.g. a curve.
Reactions of acids II
With hydrogen carbonates, e.g. NaHCO3
HCl + NaHCO3 —> NaCl + H2O + CO2
With carbonates, e.g. Na2CO3
2HCl + Na2CO3 —> NaCl + H2O + CO2
TF
Acids donate protons (H⁺ ions) in aqueous solutions.
True
TF
A strong acid has a higher pH than a weak acid of the same concentration.
False
TF
Bases increase the concentration of hydroxide ions (OH⁻) in a solution.
True
TF
The pH of a neutral solution is always 7.
True
TF
The conjugate base of a strong acid is always a strong base.
False
TF
A weak acid partially dissociates in water.
True
TF
The pKa value of a strong acid is higher than that of a weak acid.
False
TF
A solution with a pH of 3 is ten times more acidic than a solution with a pH of 4.
True
TF
The ion product of water (Kw) at 25°C is (1.0 \times 10^{-14}).
True
TF
The equivalence point in a titration of a weak acid with a strong base always occurs at pH 7.
False
Define an acid according to the Brønsted-Lowry theory.
An acid is a proton donor, meaning it can release H⁺ ions.
How do you calculate pH from the concentration of hydrogen ions [H⁺]?
pH = -log[H⁺]
What is the role of a buffer solution?
A buffer solution resists changes in pH when small amounts of an acid or base are added.
Define a Brønsted-Lowry base.
A Brønsted-Lowry base is a substance that accepts a proton (H⁺ ion).
What is the relationship between conjugate acids and bases?
A conjugate acid is formed when a base accepts a proton, and a conjugate base is formed when an acid donates a proton.
What is the pH of a solution with [H⁺] = 1 × 10⁻⁶ M?
pH = -log[H⁺] = -log(1 × 10⁻⁶) = 6.
Define pOH.
pOH is the negative logarithm of the hydroxide ion concentration: pOH = -log[OH⁻].
How do you calculate pH from pOH?
pH + pOH = 14. Therefore, pH = 14 - pOH.
What is a buffer solution?
A buffer solution resists changes in pH when small amounts of an acid or base are added.
What is the effect of adding an acid to a buffer solution?
When an acid is added to a buffer solution, the base component of the buffer neutralizes the added acid, minimizing the change in pH.
What does the value of Ka represent?
Ka is the acid dissociation constant, representing the strength of an acid in a given solvent. Higher Ka values indicate stronger acids.
What is the relationship between pKa and Ka?
pKa is the negative logarithm of Ka: pKa = -log(Ka).
How do you calculate the pH of a weak acid solution?
Use the Ka expression to solve for the concentration of H⁺ ions, and then calculate pH using the formula pH = -log[H⁺].
What does the pKa value of a weak acid tell you about its strength?
The lower the pKa value, the stronger the acid, as it dissociates more easily in solution.
What is a monoprotic acid?
A monoprotic acid is an acid that donates only one proton (H⁺) per molecule.
TF
A buffer solution works best when the pH is equal to the pKa of the weak acid in the buffer.
True. A buffer works most effectively when the pH is close to the pKa of the acid, as this ensures that the concentrations of the acid and its conjugate base are approximately equal.
TF
When balancing redox reactions, you should balance the atoms first, and then balance the charge using electrons.
True. The atoms should be balanced first, and then the charge can be balanced by adding electrons to ensure the overall charge is equal on both sides of the half-reactions.
Why do strong acids, like hydrochloric acid (HCl), have very large values for their acid dissociation constant (Ka)?
Strong acids dissociate almost completely in water, meaning that nearly all the acid molecules break apart to form H⁺ ions. This results in a very large Ka value because the concentration of products (H⁺ ions) is much higher than that of the undissociated acid.
TF
The pH of a solution with a higher concentration of H⁺ ions is lower than that of a solution with a lower concentration of H⁺ ions.
True. pH is inversely related to the concentration of H⁺ ions. A higher concentration of H⁺ results in a lower pH, indicating increased acidity.
