12: Acid-base Equilibria Flashcards
Define an acid (Bronsted-Lowry)
A proton donor
Define an base (Bronsted-Lowry)
A proton acceptor
Define an alkali
A soluble base
What are conjugate acid-base pairs
- A pair of reactants and products that are linked to each other by the transfer of a proton
- Reactant CH3COOH is linked to product CH3COO- by transfer of a proton from the acid to the base
- Reactant H2O is linked to product H3O+ by transfer of a proton from the acid to the base
What does the acidity of an aqueous solution depend upon
The number of H+ ions in solution
Define pH
Rearrange the equation of pH to find the concentration of H+
What is a strong acid
An acid that dissociates almost completely in aqueous solutions to form H+ ions
What is a weak acid
An acid that only partially dissociates in aqueous solutions, and can be represented as an equilibrium equation
Why is the Enthalpy change of neutralisation of strong acids and strong bases similar
- Because the acids and alkalis are fully ionised and the neutralisation reaction between H+ + OH- occurs to produce water
- H+(aq) + OH-(aq) ➡️ H2O(l)
- Any other ions involved are spectator ions, and don’t affect neutralisation
Why is the Enthalpy change of neutralisation between weak acids and weak bases less exothermic
As weak acids and weak bases are only partially ionised, so energy has to be used to fully ionise them
What is the acid dissociation constant (Ka)
- Equilibrium constant for weak acids
- Units of mol dm-3
What assumptions are made with the Ka expression for weak acids
- The concentration of H+ ions due to the ionisation of water is negligible
- Initial concentration of HA is the same as concentration of HA at equilibrium
What can be assumed about strong acids to calculate their pH
- Strong acids are completely ionised, and the number of H+ ions formed from the ionisation of water is negligible
- So the total concentration of H+ is the same as the concentration of HA
- pH calculation can then be used
What is a dibasic/diprotic acid
Two replaceable protons and with react in a 1:2 ratio with bases (e.g. H2SO4)
What is assumed about dibasic acids to calculate their pH
- Although they are strong acids, so should produce an H+ concentration of double the [HA], it isn’t
- The pH is lower than expected, indicating the acid isn’t fully ionised
- Ionisation of dibasic acids happens in two steps, so the second step is suppressed by the abundance of H+ ions, creating an equilibrium
What assumptions have to be made to calculate the pH of weak acids
- The concentration of acid and Ka value of acid must be known
How is the ionic product for water derived from the dissociation of water
What is the relationship between Kw and pKw
pKw = -logKw
What is the relationship between Ka and pKa
pKa = -logKa
What assumptions are made to calculate the pH of strong bases
- Strong bases are completely ionised in solution
- Therefore the concentration of OH- ions is equal to the concentration of base
- Concentration of OH- in solution and ionic product of water can be used to calculate pH
What are examples of strong acids (low pH)
- H2SO4
- HCl
- H3PO4
- HNO3
What are examples of weak acids (higher pH)
- Ethanoic acid
- Methanoic acid
What are examples of strong bases (high pH)
- NaOH
- Ba(OH)2
What are examples of weak bases (lower pH)
- NH3
- CH3NH2
Why do some salts have pH 7.00
They are made from a strong acid and a strong base (e.g. NaCl)
Why are some salts alkaline
They are made of a weak acid and a strong base (e.g. CHCOONa)
Why are some salts acidic
They are made of a strong acid and weak base (e.g. NH4Cl)
Why are some salts neutral
They are made of a weak acid and weak base
What is the effect of dilution on the pH of strong acids
- As concentration increases by a factor of 10, pH decreases by one unit
- However acids to a concentration factor of more than 1.00x10^-6 are so dilute that the contribution of H+ ions from the water is no longer ignored
What is the effect of dilution on the pH of weak acids
- As concentration increases by a factor of 10 the pH increases by a factor of around 0.5
What does a pH curve show
How the pH of a solution changes as the acid/base is added
What can be determined by pH graphs
- Determine the pH of acid/base by looking where the curve starts on the y-axis
- Find the pH at the equivalence point
- Find the volume of base/acid at the equivalence point
- Obtain the range of pH from the vertical section of the curve
What is the equivalence point of a pH curve
The point in the titration at which the amount of titrant added is enough to completely neutralise the analyse solution
Draw the strong acid + strong base titration curve
(Inverse for acid against base titration)
Draw the weak acid + strong base titration curve
(Inverse for acid against base)
Draw the strong acid + weak base titration curve
(Inverse for acid against base)
Draw the weak acid + weak base titration curve
(Inverse for acid against base titration)
What is an acid-based indicator and how does it work
- A weak acid which dissociates to give an anion of a different colour (e.g. Hln)
- Hln and its conjugate base ln- are different colours
- If the solution is acidic the POE will move to the left and more Hln will be present (its colour is visible)
- If the solution is alkaline the POE will move to the right and more ln- will be present (its colour is visible)
- The colour of the indicator will change gradually
- The pH the indicator changes at depends on the Ka of the indictor
- At the endpoint of the reaction, there is a balance between Hln and ln- concentrations, so pKa of indicators = the pH of its endpoint
How is a suitable indicator chosen for acid-base titrations
- An indicator will be appropriate is the pH range of the indicator falls within the rapid pH change for the titration
- The indicator should suit the pH change represented by the equivalence point
What indicator is picked for a weak acid + weak base titration
- There is no sudden pH change at the end point of a weak acid + weak base titration, so there is no suitable indicator
What is a buffer solution
A solution which resists changes in pH when a small amount of acid/alkali is added. They can consist of a weak acid + conjugate base, or weak base + conjugate acid and are used to keep the pH constant
How does ethanoic acid and sodium ethanoate work as a buffer
- Ethanoic acid is a weak acid, so partially dissociates in solution to form a low concentration of ethanoate ions
- Sodium ethanoate is a salt which fully dissociates in solution to form a high concentration of ethanoate ions
- The buffer contains the high concentrations of ethanoic acid, and high concentrations of ethanoate ions, and the ethanoic acid is in equilibrium with the H+ and ethanoate ions
What happens when H+ ions are added to a buffer solution (e.g. ethanoic acid and sodium ethanoate)
- POE shifts to the left as H+ ions react with ethanoate ions to form more ethanoic acid until equilibrium is reestablished
- As there is a large reserve of ethanoate ions, its concentration doesn’t change much when reacting with H+ ions
- As there is a large reserve of ethanoic acid, its concentration doesn’t change much when more is formed
- This results in the pH staying constant
What happens when OH- ions are added to buffer solutions (e.g. ethanoic acid and sodium ethanoate)
- The OH- and H+ ions react to form water, and the H+ concentration decreases
- POE shifts to the right and more ethanoic acid molecules dissociate to form more H+ and ethanoate ions until equilibrium is reestablished
- Due to large reserves of ethanoate ions and ethanoic acid, the concentration of both don’t change much
- pH stays constant
How is a buffer with a pH lower than 7 made
From a weak acid and it’s salt, or by partial neutralisation of a weak acid
How is the pH of a buffer solution calculated
- Using the Ka of the weak acid
- Using the equilibrium constant of the weak acid and its conjugate base (salt)
- Therefore the [H+] is needed, and can be found using the equilibrium expression
How to make a buffer solution with a required pH
- To make a buffer with pH<7 use a mixture of weak acid and its conjugate base
- To make a buffer with pH>7 use a mixture of weak base and its conjugate acid
How are buffer solutions applied to control the pH of blood
- HCO3- ions act as a buffer to keep blood pH between 7.35-7.45
- Respiration produces CO2, which combines with H2O in blood to form an equilibrium between CO2 and HCO3-
- If [H+] isn’t regulated blood pH drops (acidosis) and the body malfunctions
- If [H+] increases POE shifts to the left until equilibrium is restored
- If [H+] decreases POE shifts to the right until equilibrium is restored
Using the weak acid + strong base titration, describe what is happening on the graph
- pH of acid starts at roughly 3, and initial rise in pH is steep as the neutralisation of the weak acid by strong base is rapid
- Ethanoate ions are formed which creates a buffer, which resists changes to pH so the pH rises gradually
- The half equivalence point is the stage where exactly half the amount of weak acid havens been neutralised
- pKa = pH at the half equivalence
