Acids & Bases Flashcards
Explain why the expression for Kw does not include the concentration of water.
In H2O(I) ⇌ H+ (aq) + OH− (aq), the equilibrium lies very far to the left so [H2O] changes by a very small amount and is effectively constant
Explain why the value of Kw increases as the temperature increases
(Dissociation OR breaking bonds) is endothermic
∴ Equilibrium moves to RHS (at higher T) to absorb heat or to lower T or
oppose increase in T
A solution of chlorine in water is acidic. Swimming pool managers maintain pool water at a constant pH by using a buffer. They do so by adding sodium hydrogencarbonate and
sodium carbonate. Hydrogen carbonate ions (HCO3^−) act as a weak acid in aqueous solution. Write an equation for this equilibrium.
HCO3^- <-> CO3^2- + H+ OR H2O + HCO3^- <-> CO3^2- + H3O+
Use the equation H2O + HCO3^- <-> CO3^2- + H3O+ to explain how a solution containing sodium hydrogencarbonate and sodium carbonate can act as a buffer when small amounts of acid or small amounts of alkali are added.
- Acid: Increase in concentration of H+ ions, equilibrium moves to the left. *Allow H+
ions react with carbonate ions (to form HCO3^-) - Alkali: OH- reacts with H+ ions, equilibrium moves to the right (to replace the H+
ions) - Concentration of H+ remains (almost) constant
Suggest a suitable piece of apparatus that could be used to measure out an alkali solution that’s being added to an acid. Explain why this apparatus is more suitable than a pipette for this purpose.
Burette because it can deliver variable volumes
State the meaning of the term ‘weak’ as applied to carboxylic acids
(only) slightly or partially dissociated / ionised
Ignore ‘not fully dissociated’.
Allow low tendency to dissociate or to lose / donate a proton.
Allow shown equilibrium well to the left.
Otherwise ignore equations.
Write an equation for the reaction of propanoic acid with sodium carbonate
2CH3CH2COOH + Na2CO3 -> 2CH3CH2COONa + H2O + CO2
OR
2CH3CH2COOH + CO3^2− -> 2CH3CH2COO−
+ H2O + CO2
OR
CH3CH2COOH + Na2CO3 -> CH3CH2COONa + NaHCO3
OR
CH3CH2COOH + CO3^2− -> CH3CH2COO− + HCO3^−
Must be propanoic acid, allow C2H5COOH.
Not molecular formulae.
Allow multiples.
Ignore reversible sign.
Not H2CO3
Two solutions, one with a pH of 4.00 and the other with a pH of 9.00, were left open to the air. The pH of the pH 9.00 solution changed more than that of the other solution. Suggest what substance might be present in the air to cause the pH to change. Explain how and why the pH of the pH 9.00 solution changes.
- CO2
Allow NOx and SO2 - pH (It) falls / decreases
- acidic (gas)
OR reacts with alkali(ne solution) / OH−
OR CO2 + 2OH− -> CO3^2− + H2O
OR CO2 + OH− -> HCO3^−
Not forms H2CO3 H2SO3 H2SO4 etc OR H+ ions
In an exam question, when given the graph of the results of an acid-base titration and asked to suggest the pH range of a suitable indicator for this titration, how do you always give the right answer?
Look at where the graph is straight & vertical and pick a range from the bottom of this part of the line to the top. Remember, to be on the safe side, to pick a range where you KNOW it’s straight at both ends.
In an exam question, you’re told that a given concentration of a known strong base was gradually added to a given volume of an unknown weak acid in the presence of a suitable indicator. You’re given a graph of results with axis of volume of given base against pH. As shown, the first pH reading was taken after a given volume of the base had been added. You’re also given the Ka value of the unknown acid. How do you calculate the concentration of the acid in the original solution
- Look at the part of the ‘volume of given base that corresponds to the part of the line on the graph that goes vertically upwards (called the end point apparently)
- Use this volume and the concentration given to find the mol of given base using n=cV
- Find the molar ratio of acid:base. You’ll know by looking at the molecules of the base and acid given e.g. Mg(OH)2 means 2 H+ needed if unknown acid given is monoprotic like HX so acid:base molar ratio would be 2:1 in balanced equation; or H2X is diprotic so with a base like NaOH the acid:base molar ratio would be 1:2 in balanced equation. Then state & use this to find mol of unknown acid
- Use this molar amount and the volume of acid given in question to find concentration of acid by rearranging & using n=cV
In an exam question, you’re told that a given concentration of a known strong base was gradually added to a given volume of an unknown weak acid in the presence of a suitable indicator. You’re given a graph of results with axis of volume of given base against pH. As shown, the first pH reading was taken after a given volume of the base had been added. You’re also given the Ka value of the unknown acid. How would you calculate the pH of the solution when, let’s say, half of the acid has reacted?
Well this is just pH at the half-equivalence point, where pH = pKa, and a Ka value is given! So pH = - log Ka
In an exam question, you’re given an incomplete acid-base titration graph (the start of it is missing) and the values of the pH of the half-equivalence point and of the solution before the titration occurs. Describe what your drawing would look like if asked to complete the graph using these values
- Both points plotted correctly and line touches both points
- Line steeper at start then levels (to show buffering) before joining the rest of the line
State the meaning of the term ‘strong acid’
Substance that completely dissociates/ionises to form H+ ions
The pH of water at 30°C is 6.92. Give the reason why water is neutral at this temperature
[H+] = [OH–]
Out of Al2O3, Na2O, SiO2 and SO2, identify the oxide that could react with water to form a solution with pH = 2. Give a reason why (not) for each oxide
- Not Al2O3 because it’s insoluble
- Not Na2O as it would produce NaOH which is a base so would have a high pH
- Not SiO2 because it’s insoluble
- SO2 because it dissolves to produce H2SO3 which is a weak acid so it would be reasonable to think it’d have a pH of 2
