Acids, Bases + Buffers Flashcards
What is a Brønsted acid?
- a proton [H+] donor
- e.g. HCl (acid) —-> H+ + Cl-
What is a Brønsted base?
- a proton [H+] acceptor
- e.g. OH- (base) + H+ —-> H2O
What is a conjugate acid-base pair?
- a set of 2 species that transform into each other by the gain or loss of a H+
What is the typical reaction for a metal + an acid?
- acid + metal —-> metal salt + hydrogen
- e.g. 2HCl (aq) + Zn (s) —-> ZnCl2 (aq) + H2 (g)
What is the typical reaction for an acid w a metal oxide?
- acid + metal oxide —-> metal salt + water
- e.g. 2HCl (aq) + CaO (s) —-> CaCl2 (aq) + H2O (l)
What is the typical reaction for an acid w a metal carbonate?
- acid + metal carbonate —-> metal salt + water + carbon dioxide
- e.g. 2HNO3 (aq) + CuCO3 (s) —-> Cu(NO3)2 (aq) + H2O (l) + CO2 (g)
What is the typical reaction for an acid w an alkali?
- acid + alkali —-> salt + water
- e.g. HCl (aq) + NaOH (aq) —-> NaCl (aq) + H2O (l)
What is a weak acid?
- an acid that partially dissociates into its ions in aqueous solutions
- e.g. all organic acids, some inorganic acids (HCN, H2S, H2CO3)
- position of eqm is more to the left
What is the expression for the acid dissociation constant?
Ka = [H+][A-] / [HA]
- monobasic acid: [H+] = [A-] so Ka = [H+]^2 / [HA]
What assumptions are made when writing equilibrium expressions for weak acids?
- conc of H+ ions bc ionisation of water is negligible
What does the value of Ka indicate?
- extent of dissociation
- higher the value = more dissociated (stronger) the acid
- lower the value = weaker the acid
What is the equation for pKa?
pKa = -log (Ka)
What is the equation for pH?
pH = -log [H+]
- [H+] = mol dm^-3
What is the equation for the conc of H+ [H+]?
[H+] = 10^-pH
What is the equation for the ionic product of water (Kw)?
Kw = [H+][OH-]
What is a strong acid?
- acids that completely dissociate into its ions in aqueous solution
- so [H+] = [HA]
What equation can be used to calculate the pH for a strong base?
- [H+] = Kw / [OH-]
- pH = -log [H+]
- so pH = -log (Kw / [OH-])
What is meant by the term monobasic acid?
- 1 mole of acid dissociates to form 1 mole of protons/H+
What are the assumptions of Ka?
- [H+] at eqm = [A-] at eqm
- assume initial conc of undissociated acid has remained constant so initial [HA] = [HA] at eqm
What is a buffer solution?
- a solution which resists changes in pH when small amounts of acids or alkalis are added
What can a buffer consist of?
- weak acid forming a strong conjugate base
- weak base forming a strong conjugate acid
What is a common buffer solution?
- an aqueous mixture of ethanoic acid (weak acid) + sodium ethanoate (salt)
- ethanoic acid partially ionises in solution to form a low conc of ethanoate ions
- ethanoate fully ionises in solution
How do you calculate the pH of a buffer solution?
- Ka = [salt (conjugate base of acid)][H+] / [acid]
- [H+] = Ka x [acid] / [salt]
- pH = -log[H+]
How do buffers control the pH of blood?
- HCO3- ions act as a buffer for blood pH
- CO2(g) + H2O(l) <—> H+(aq) + HCO3-(aq)
- an inc in H+ causes eqm to shift to left until eqm is restored
- a dec in H+ causes eqm ro shjft to right until eqm is restored
What is an acid-base indicator?
- a weak acid which dissociates to give an anion of a diff colour
