Acids, Bases & Buffers

• A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added.
• Buffer solutions are able to moderate changes in pH because the weak acid or weak base component can bind added H+ or OH- ions, reducing their concentration in solution.
• Acidic buffers contain weak acids, while basic buffers contain weak bases.
• A Bronsted-Lowry acid is a proton donor.
• A Bronsted-Lowry base is a proton acceptor.
• In reactions with 2 bases the one with the higher Ka value acts as the acid
• Weak acids do not completely dissociate into ions in water. Resulting in an equilibrium mixture
• Strong acids completely dissociate into ions in water.
• Acid + Metal → Salt + Hydrogen
• Acid + Alkali → Salt + Water
• Acid + Carbonate → Salt + Water + CO₂
• Metal oxide + Acid → Salt + Water
• HCl + NaOH → NaCl + H20
• NaOH + HNO3 → NaNO3 + H20
• pH = -log10 [H+]
• [H+] = 10^−pH
• Ka = [H+][A-]/[HA]
• Ka = [H+]^2/[HA]
• In a monoprotic strong acid the concentration of hydrogen ions will be the same as the concentration of the acid
• In all aqueous solutions and pure water: H20 ⇌ H+ + OH-
• Kw = [H+][OH-]
• pure water and neutral solutions are neutral when [H+]=[OH-]. Therefor, Kw also = [H+]^2 when neutral.
• A known Kw value can be used to calculate [H+] if [OH-] is known and vice versa
• At room temp Kw = 1x10^-14
• At different temperature the pH of water changes. This is because the dissosciation of water is endothermic as bonds are being broken so the increase in temperature pushes the equilibrium to the RHS increasing the concentration of [H+] ions resulting in a lower pH
• Strong bases completely dissociate into their ions: NaOH → Na+ + OH-
• When using bases we are generally given the concentration of the hydroxide ion
• Ka = 10^-pKa
• HA = H+ → A-
• The larger the value of Ka the stronger the acid is.
• Assumptions made with Ka: 1- [H+] = [A-] as they have dissociated in a 1:1 radio. 2- As the amount of initial dissociation is small we assume that the initial concentration of the acid has remained constant
• [H+] = [H+]initial x old volume/new volume
• [OH-] = [OH-]initial x new volume/old volume
• The salt used in a basic buffer is made from reacting the weak base with a strong acid
• The salt used in a acidic buffer is made from reacting the weak acid with a strong base
• In a basic buffer the conjugate acid acts as a weak base which can donate OH- to form the original weak base
• In an acidic buffer the conjugate base acts as a weak acid which can accept H+ to form the original weak acid
• Buffers are important because they prevent large changes in pH when acids or bases are added
• Salt content can be added by: 1- A salt solution could be added to the acid or solid salt could be added. 2- A buffer can partially neutralise a weak acid with an alkali to produce a mixture of salt and acid
• In a buffer there is a much higher concertation of the salt than there is of the acid