acids and bases [P2] PAPER 1 Flashcards
what is an acid?
proton donor
what is a base?
proton acceptor
what is a strong acid?
an acid that completely dissociates in water to form H+ ions
what is a weak acid?
an acid that slightly dissociates in water to form H+ ions
how to calculate pH?
pH = – log [H+]
how to calculate the concentration of hydrogen ions, [H+] ?
what is the value of Kw at room temperature (25°C) ?
10^–14
Kw = ?
Kw = [H+] [OH–]
[H+] = [OH–] => Kw = [H+]^2
Why does [H2O] not appear in the Kw equation?
[H2O] is very high, so [H2O] is effectively constant
Explain why the value of Kw increases as the temperature increases.
• equilibrium is endothermic in the forwards direction
• equilibrium shifts to the right to oppose the increase in temperature
Why is water neutral?
[H+] = [OH-]
Suggest why the ph probe is washed with distilled water between each of the calibration measurements.
different solutions must not contaminate each other
The calibrated pH meter is used to monitor the pH during a titration of hydrochloric acid with sodium hydroxide.
Explain why the volume of sodium hydroxide solution added between each pH measurement is smaller as the end point of the titration is approached.
to avoid missing the end point
how to determine if an indicator is suitable using a pH curve?
has a colour change within the vertical part of the pH curve
Ka = ?
[H+] is NOT equal to [A–] in buffer calculations
pKa = ?
– log Ka
at temperatures above 298K, the pH of water is ___ than 7
less than
• add fixed volume of alkali in a beaker
• add acid in small portions from a burette
• stir and use a pH meter to record the pH after each addition of acid
Suggest a suitable piece of apparatus that could be used to measure out a solution to then produce a pH curve from the results obtained.
burette as it can deliver variable volumes
Use information from the curve in the figure above to explain why the end point of this reaction would be difficult to judge accurately using an indicator.
• the change in pH is gradual
• an indicator would change colour over a range of volumes of [whatever is on the x axis]
colour of methyl orange at a low pH?
red
colour of methyl orange at a high pH?
yellow
colour of phenolphthalein at a low pH?
colourless
colour of phenolphthalein at a high pH?
pink
pH range of colour change for methyl orange?
3.2 – 4.4
pH range of colour change for phenolphthalein?
8.2 – 10.0
pH curves for
• strong acid / strong base
• strong acid / weak base
• weak acid / strong base
• weak acid / weak base
what is a buffer?
solution that is able to resist changes in pH when small volumes of acid or alkali are added
pH at half-neutralisation = ?
pKa
B
Explain how a solution of [acid] and [alkali] can act as a buffer when small amounts of acid/alkali are added.
• acid increases the concentration of H+ ions, so equilibrium shifts to the left to oppose the increase in H+ ions
• the OH– ions from the alkali reacts with the H+ ions, so equilibrium shifts to the right to replace the H+ ions
• concentration of H+ remains almost constant
explain why pH = –logKa when [A–] = [HA]
• if [A–] = [HA], then Ka = [H+]
• pH = –log [H+] = –logKa
suggest one reason why there may be an anomalous point in a pH curve, assuming the pH meter reading is correct.
ineffective swirling of the mixture
Suggest how the procedure of an experiment that results in a pH curve could be slightly modified in order to give a more reliable value for the end-point.
• add smaller volumes of (x-axis thing) around near the end-point
• take more pH readings around the end-point
State why calibrating a pH meter just before it is used improves the accuracy of the pH measurement.
after storage, the meter doesn’t give accurate readings
Describe how you would obtain the pH curve for a titration.
• measure pH of the acid/alkali
• add acid/alkali from burette in known small volumes
• stir mixture, then measure the pH
• repeat until the acid/alkali is in excess
• add in smaller increments near the end-point
A mixture, formed by adding 50 cm3 of 1.00 mol dm–3 salicylic acid solution to 25 cm3 of 1.00 mol dm–3 sodium hydroxide solution, can be used to determine the pKa of salicylic acid. State one measurement that must be made for this mixture and explain how this measurement can be used to determine the pKa of salicylic acid.
• pH
• [HA] = [A–], so pH = pKa
Explain why the pH of an acidic buffer solution remains almost constant despite the addition of a small amount of sodium hydroxide.
• added OH– removed when reacting with H+ in the acidic buffer
• HX ⇌ H+ + X–
• equilibrium shifts to the right to oppose the decrease in H+
• ratio of [HX] to [X–] remains constant (hence pH and [H+] remain constant)
Other than by using a different pH meter, state one way in which the accuracy of the pH readings could be improved.
calibrate meter
Describe briefly how you would ensure that a reading from a pH meter is accurate.
• calibrate pH meter with solution of known pH
• plot calibration curve
Suggest why the concentration of sodium hydroxide in a solution slowly decreases when left open to air.
NaOH reacts with carbon dioxide in the air
Concentrated sulfuric acid reacts with solid sodium chloride.
Give the observation you would make in this reaction.
State the role of the sulfuric acid.
• steamy fumes
• acid
• lower pH
• magnesium hydroxide is less soluble
Some sodium sulfate is dissolved in a sample of a solution of NaHSO4.
Explain why this increases the pH of the solution.
A student completed an experiment to determine the enthalpy of neutralisation for the reaction between ethanedioic acid solution (HOOCCOOH) and potassium hydroxide solution, and found it was –33.6 kJmol-1 per mole of water formed.
In a similar experiment, the enthalpy of neutralisation for the reaction between sulfuric acid and potassium hydroxide solution was found to be –57.0 kJmol-1 per mole of water formed.
Suggest an explanation for the difference between the two values.
• HOOCCOOH is a weak acid
• more energy needed to break bonds / complete dissociation
how to make a buffer solution using a weak acid and strong base?
add excess [weak salt] to [strong base]
e.g. add excess ethanoic acid to potassium hydroxide
