Acids and bases

• A bronsted lowry acid is a proton doner and a bronsted lowry base is a proton acceptor.
• A strong acid/base is one that completely dissociates to ions when in solution, while a weak acid/base is one that slightly dissociates to ions when in solution.
• pH is a logarithmic scale which measures the concentration of H + ions in a solution. 0 is a strong acid and 14 is a strong base.
• pH = - log [H +], and therefore [H +] = 10 ^ - pH.
• The concentration of H + ions is equivalent to the concentration of a strong acid.
• Water slightly dissociates to ions in an equilibrium with an equilibrium constant Kw.
• Kw = [H +][OH -].
• The forwards reaction in the equilibrium of water is endothermic, and therefore more H + (and OH -) ions are produced as the temperature is increased, so water becomes more acidic.
• Weak acids and bases only slightly dissociate in solution to form an equilibrium mixture, with an equilibrium dissociation constant, Ka.
• Ka = [H +][A -]/[H A].
• The pH of a strong base can be found by finding [H +] from the ionic product of water, ie pH = - log (Kw/[OH -]). Therefore [OH -] = Kw/10 ^ - pH.
• Kw is 1 x10 ^ - 14 in room temperature (under standard conditions).
• pKa = - log Ka, Ka = 10 ^ - pKa.
• The pH of a weak acid can be found by finding [H +] from the Ka equation, using the fact that [H +] equals [A -] for any pure acid.
• For a weak acid, pH = - log root(Ka x [HA]), [HA] = (10 ^ - pH)^2/Ka.
• The first step for any mixture of acids and bases is to work out the moles of reactants before and minus the smallest number of moles from both.
• For a mixture of a strong acid and strong base, the moles calculated in the first step shows the moles of acid or base leftover. Divide this by the total volume to find [H +] for excess acid or [OH -] for excess base. Use to calculate pH.
• For a mixture of a weak acid and strong base, the moles calculated in the first step is also equal to the moles of A - formed. Calculate [A -] and [HA] using moles divided by total volume and use to work out [H +]. Use to calculate pH.
• Equivalence point on a titration curve is when pH is neutral and is the midpoint of the vertical line.
• Half equivalence is half the volume of acid or base required to get to the equivalence point.
• At half equivalence, [A -] equals [HA], and so pH equals pKa.
• A buffer is a solution which is able to resist small changes in pH.
• Acidic buffers contain a weak acid and its salt, and basic buffers contain a weak base and its salt.
• Buffers are used in homeostasis, in cosmetics, and in foods, and as standard solutions to calibrate pH meters.
• To find the initial pH of a buffer, divide the number of moles of the reactants by the total volume to find [HA] and [A -]. Use to calculate [H +], then calculate pH.
• To calculate pH after adding an acid or base to a buffer, calculate the number of moles of acid or base added. The moles of A - or H + for an acid or base respectively also decreases by this much. Using the new mole numbers, calculate [HA] and [A -] and use to calculate pH.
• Methyl orange has a colour change between pH 3 and 5 and is red on the acidic side and yellow on the basic side.
• Litmus has a colour change between pH 5 and 8 and is red on the acidic side and blue on the basic side.
• Phenolphthalein has a colour change between pH 8 and 10 and is colourless on the acidic side and pink on the basic side.