8.1 [147 marks]
1a. [1 mark]
Chlorine undergoes many reactions.
State the full electron configuration of the chlorine atom.
Markscheme
1 s 2 2 s 2 2 p6 3 s 2 3 p5 ✔
Do not accept condensed electron configuration.
1b. [1 mark]
State, giving a reason, whether the chlorine atom or the chloride ion has a larger radius.
Markscheme
C l − AND more «electron–electron» repulsion ✔
Accept C l − AND has an extra electron.
1c. [2 marks]
Outline why the chlorine atom has a smaller atomic radius than the sulfur atom.
Markscheme
C l has a greater nuclear charge/number of protons/ Z ef f «causing a stronger pull on the
outer electrons» ✔
same number of shells
OR
same «outer» energy level
OR
similar shielding ✔
1d. [1 mark]
The mass spectrum of chlorine is shown.
NIST Mass Spectrometry Data Center Collection © 2014 copyright by the U.S. Secretary of
Commerce on behalf of the United States of America. All rights reserved.
Outline the reason for the two peaks at m/ z=35 and 37.
Markscheme
«two major» isotopes «of atomic mass 35 and 37» ✔
1e. [2 marks]
Explain the presence and relative abundance of the peak at m/ z=74.
Markscheme
«diatomic» molecule composed of «two» chlorine-37 atoms ✔
chlorine-37 is the least abundant «isotope»
OR
low probability of two Cl37 «isotopes» occurring in a molecule ✔
1f. [1 mark]
2.67 g of manganese(IV) oxide was added to 200.0 c m3 of 2.00 m o ld m−3 H C l .
M n O 2 ( s )+ 4 H C l ( a q ) →C l 2 ( g ) +2 H 2 O ( l )+ M n C l 2 ( a q )
Calculate the amount, in m o l, of manganese(IV) oxide added.
Markscheme
¿<
2.67 g
=¿>0.0307< ¿ mo l>¿ ✔
86.94 g m o l −1
1g. [2 marks]
Determine the limiting reactant, showing your calculations.
Markscheme
¿< nH C l=2.00 m o l d m−3 ×0.2000 d m3 >¿=0.400 mo l✔
¿<
0.400
=¿> 0.100 mo l AND M nO2 is the limiting reactant ✔
4
Accept other valid methods of determining the limiting reactant in M2.
1h. [1 mark]
Determine the excess amount, in m o l, of the other reactant.
Markscheme
¿< 0.0307 mo l ×4=0.123 mo l>¿
¿< 0.400m o l 0.123 mo l=¿ >0.277<¿ m o l>¿ ✔
1i. [1 mark]
Calculate the volume of chlorine, in d m3, produced if the reaction is conducted at standard
temperature and pressure (STP). Use section 2 of the data booklet.
Markscheme
¿< 0.0307 mo l ×22.7 d m 3 m o l −1=¿> 0.697<¿ d m 3> ¿ ✔
Accept methods employing p V =n R T .
1j. [2 marks]
State the oxidation state of manganese in M nO2 and M nC l 2.
Markscheme
M nO2 :+ 4 ✔
M nC l 2 :+2 ✔
1k. [1 mark]
Deduce, referring to oxidation states, whether M nO2 is an oxidizing or reducing agent.
Markscheme
oxidizing agent AND oxidation state of M n changes from + 4 to +2/decreases ✔
1l. [1 mark]
Chlorine gas reacts with water to produce hypochlorous acid and hydrochloric acid.
C l 2 ( g )+ H 2 O ( l ) ⇌ H C l O ( a q ) + H C l ( a q )
Hypochlorous acid is considered a weak acid. Outline what is meant by the term weak acid.
Markscheme
partially dissociates/ionizes «in water» ✔
1m. [1 mark]
State the formula of the conjugate base of hypochlorous acid.
Markscheme
C l O− ✔
1n. [1 mark]
Calculate the concentration of H +¿ ( a q) ¿ in a H C lO ( a q ) solution with a p H=3.61.
Markscheme
¿<¿ ✔
1o. [1 mark]
State the type of reaction occurring when ethane reacts with chlorine to produce
chloroethane.
Markscheme
«free radical» substitution/ S R ✔
Do not accept electrophilic or nucleophilic substitution.
1p. [1 mark]
Predict, giving a reason, whether ethane or chloroethane is more reactive.
Markscheme
chloroethane AND C–Cl bond is weaker/324 k J m o l −1 than C–H bond/414 k J mo l −1
OR
chloroethane AND contains a polar bond ✔
Accept “chloroethane AND polar”.
1q. [3 marks]
Explain the mechanism of the reaction between chloroethane and aqueous sodium
hydroxide, N a O H ( a q ), using curly arrows to represent the movement of electron pairs.
Markscheme
curly arrow going from lone pair/negative charge on O in −OH to C ✔
curly arrow showing C l leaving ✔
representation of transition state showing negative charge, square brackets and partial
bonds ✔
Accept O H − with or without the lone pair.
Do not accept curly arrows originating on H in O H −.
Accept curly arrows in the transition state.
Do not penalize if H O and C l are not at 180°.
Do not award M3 if O H − C bond is represented.
1r. [1 mark]
Ethoxyethane (diethyl ether) can be used as a solvent for this conversion.
Draw the structural formula of ethoxyethane
Markscheme
C H3 C H2O C H2 C H3
/
✔
Accept ( C H 3 C H 2 )2 O.
1s. [3 marks]
Deduce the number of signals and chemical shifts with splitting patterns in the 1H
NMR spectrum of ethoxyethane. Use section 27 of the data booklet.
Markscheme
2 «signals» ✔
0.9−1.0 AND triplet ✔
3.3−3.7 AND quartet ✔
Accept any values in the ranges.
Award [1] for two correct chemical shifts or two correct splitting patterns.
1t. [2 marks]
C C l 2 F2 is a common chlorofluorocarbon, C F C.
Calculate the percentage by mass of chlorine in C C l 2 F2.
Markscheme
¿< M ( C C l 2 F2 ) =¿>120.91<¿ g mo l − 1>¿ ✔
2× 35.45 g m o l −1
×100 %=¿ >58.64<¿ % >¿ ✔
120.91 g m o l −1
Award [2] for correct final answer.
1u. [1 mark]
Comment on how international cooperation has contributed to the lowering of C F C
emissions responsible for ozone depletion.
Markscheme
Any of:
research «collaboration» for alternative technologies «to replace C F Cs»
OR
technologies «developed»/data could be shared
OR
political pressure/Montreal Protocol/governments passing legislations ✔
Do not accept just “collaboration”.
Do not accept any reference to C F C as greenhouse gas or product of fossil fuel combustion.
Accept reference to specific measures, such as agreement on banning use/manufacture of
C F Cs.
1v. [2 marks]
C F Cs produce chlorine radicals. Write two successive propagation steps to show how
chlorine radicals catalyse the depletion of ozone.
Markscheme
O 3+ C l→ O 2+C l O ✔
C lO+O → O2+C l
OR
C lO+O3 →C l+ 2O2 ✔
Penalize missing/incorrect radical dot (∙) once only.
2. [1 mark]
What is the difference between a conjugate Brønsted–Lowry acid–base pair?
A. Electron pair
B. Positive charge
C. Proton
D. Hydrogen atom
Markscheme
C
3. [1 mark]
Which is an example of an amphiprotic species?
A. Al2O3
B. CO32−
C. P4O10
D. HPO42−
Markscheme
D
4a. [1 mark]
Benzoic acid, C6H5COOH, is another derivative of benzene.
Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the
bonds.
Markscheme
[✔]
Note: Accept Kekulé structures.
Negative sign must be shown in correct position- on the O or delocalised over the carboxylate.
4b. [2 marks]
The pH of an aqueous solution of benzoic acid at 298 K is 2.95. Determine the
concentration of hydroxide ions in the solution, using section 2 of the data booklet.
Markscheme
ALTERNATIVE 1:
[H+] «= 10−2.95» = 1.122 × 10−3 «mol dm−3» [✔]
«[OH−] =
1.00× 10−14 mol 2 d m− 6
=» 8.91 × 10−12 «mol dm−3» [✔]
1.22× 10−3 mol d m− 3
ALTERNATIVE 2:
pOH = «14 − 2.95 =» 11.05 [✔]
«[OH−] = 10−11.05 =» 8.91 × 10−12 «moldm−3» [✔]
Note: Award [2] for correct final answer.
Accept other methods.
4c. [2 marks]
Formulate the equation for the complete combustion of benzoic acid in oxygen using only
integer coefficients.
Markscheme
2C6H5COOH(s) + 15O2 (g) → 14CO2 (g) + 6H2O(l)
correct products [✔]
correct balancing [✔]
4d. [1 mark]
Suggest how benzoic acid, Mr = 122.13, forms an apparent dimer, Mr = 244.26, when
dissolved in a non-polar solvent such as hexane.
Markscheme
«intermolecular» hydrogen bonding [✔]
Note: Accept diagram showing hydrogen bonding.
5a. [1 mark]
Carbonated water is produced when carbon dioxide is dissolved in water under pressure.
The following equilibria are established.
Carbon dioxide acts as a weak acid.
Distinguish between a weak and strong acid.
Weak acid:
Strong acid:
Markscheme
Weak acid: partially dissociated/ionized «in solution/water»
AND
Strong acid: «assumed to be almost» completely/100 % dissociated/ionized «in
solution/water» [✔]
5b. [1 mark]
The hydrogencarbonate ion, produced in Equilibrium (2), can also act as an acid.
State the formula of its conjugate base.
Markscheme
CO32– [✔]
5c. [1 mark]
When a bottle of carbonated water is opened, these equilibria are disturbed.
State, giving a reason, how a decrease in pressure affects the position of Equilibrium (1).
Markscheme
shifts to left/reactants AND to increase amount/number of moles/molecules of gas/CO2
(g) [✔]
5d. [2 marks]
Soda water has sodium hydrogencarbonate, NaHCO3, dissolved in the carbonated water.
Predict, referring to Equilibrium (2), how the added sodium hydrogencarbonate affects the
pH.(Assume pressure and temperature remain constant.)
Markscheme
«additional HCO3–» shifts position of equilibrium to left [✔]
pH increases [✔]
Note: Do not award M2 without any justification in terms of equilibrium shift in M1.
5e. [2 marks]
100.0 cm3 of soda water contains 3.0 × 10−2 g NaHCO3.
Calculate the concentration of NaHCO3 in mol dm−3.
Markscheme
«molar mass of NaHCO3 =» 84.01 «g mol–1» [✔]
«concentration =
3.0× 10−2 g
1
×
=¿ » 3.6 × 10–3 «mol dm–3» [✔]
−1
3
84.01 g mo l
0.100 d m
Note: Award [2] for correct final answer.
5f. [2 marks]
Identify the type of bonding in sodium hydrogencarbonate.
Between sodium and hydrogencarbonate:
Between hydrogen and oxygen in hydrogencarbonate:
Markscheme
Between sodium and hydrogencarbonate:
ionic [✔]
Between hydrogen and oxygen in hydrogencarbonate:
«polar» covalent [✔]
6a. [1 mark]
A molecule of citric acid, C6H8O7, is shown.
The equation for the first dissociation of citric acid in water is
C6H8O7 (aq) + H2O (l) ⇌ C6H7O7− (aq) + H3O+ (aq)
Identify a conjugate acid–base pair in the equation.
Markscheme
C6H8O7 AND C6H7O7−
OR
H2O AND H3O+ ✔
6b. [1 mark]
The value of Ka at 298 K for the first dissociation is 5.01 × 10−4.
State, giving a reason, the strength of citric acid.
Markscheme
weak acid AND partially dissociated
OR
weak acid AND equilibrium lies to left
OR
weak acid AND Ka < 1 ✔
6c. [2 marks]
The dissociation of citric acid is an endothermic process. State the effect on the hydrogen
ion concentration, [H+], and on Ka, of increasing the temperature.
Markscheme
6d. [1 mark]
Calculate the standard Gibbs free energy change, Δ Gθ , in kJ mol−1, for the first dissociation
of citric acid at 298 K, using section 1 of the data booklet.
Markscheme
« Δ Gθ = −RT ln K = −8.31 J K–1 mol–1 × 298 K × ln(5.01 × 10–4) ÷ 1000 =» 18.8 «kJ mol–1» ✔
6e. [1 mark]
Comment on the spontaneity of the reaction at 298 K.
Markscheme
non-spontaneous AND Δ Gθ positive ✔
6f. [2 marks]
Outline two laboratory methods of distinguishing between solutions of citric acid and
hydrochloric acid of equal concentration, stating the expected observations.
Markscheme
Any two of:
«electrical» conductivity AND HCl greater ✔
pH AND citric acid higher ✔
titrate with strong base AND pH at equivalence higher for citric acid ✔
add reactive metal/carbonate/hydrogen carbonate AND stronger effervescence/faster
reaction with HCl ✔
titration AND volume of alkali for complete neutralisation greater for citric acid ✔
titrate with strong base AND more than one equivalence point for complete neutralisation
of citric acid ✔
titrate with strong base AND buffer zone with citric acid ✔
NOTE: Accept “add universal indicator AND HCl more red/pink” for M2.
Accept any acid reaction AND HCl greater rise in temperature.
Accept specific examples throughout.
Do not accept “smell” or “taste”.
7a. [1 mark]
Benzoic acid, C6H5COOH, is another derivative of benzene.
Identify the wavenumber of one peak in the IR spectrum of benzoic acid, using section 26 of
the data booklet.
Markscheme
Any wavenumber in the following ranges:
2500−3000 «cm−1» [✔]
1700−1750 «cm−1» [✔]
2850−3090 «cm−1» [✔]
7b. [1 mark]
Identify the spectroscopic technique that is used to measure the bond lengths in solid
benzoic acid.
Markscheme
X-ray «crystallography/spectroscopy» [✔]
7c. [1 mark]
Outline one piece of physical evidence for the structure of the benzene ring.
Markscheme
Any one of:
«regular» hexagon
OR
all «H–C–C/C-C-C» angles equal/120º [✔]
all C–C bond lengths equal/intermediate between double and single
OR
bond order 1.5 [✔]
7d. [1 mark]
Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the
bonds.
Markscheme
[✔]
Note: Accept Kekulé structures.
Negative sign must be shown in correct position.
7e. [2 marks]
Outline why both C to O bonds in the conjugate base are the same length and suggest a
value for them. Use section 10 of the data booklet.
Markscheme
electrons delocalized «across the O–C–O system»
OR
resonance occurs [✔]
122 «pm» < C–O < 143 «pm» [✔]
Note: Accept “delocalized π-bond”.
Accept “bond intermediate between single and double bond” or “bond order 1.5” for M1.
Accept any answer in range 123 to 142 pm.
7f. [2 marks]
The pH of an aqueous solution of benzoic acid at 298 K is 2.95. Determine the
concentration of hydroxide ions in the solution, using section 2 of the data booklet.
Markscheme
ALTERNATIVE 1:
[H+] «= 10−2.95» = 1.122 × 10−3 «mol dm−3» [✔]
«[OH−] =
1.00× 10−14 mol 2 d m− 6
−12
«mol dm−3» [✔]
−3
− 3 =» 8.91 × 10
1.22× 10 mol d m
ALTERNATIVE 2:
pOH = «14 − 2.95 =» 11.05 [✔]
«[OH−] = 10−11.05 =» 8.91 × 10−12 «mol dm−3» [✔]
Note: Award [2] for correct final answer.
Accept other methods.
7g. [2 marks]
Formulate the equation for the complete combustion of benzoic acid in oxygen using only
integer coefficients.
Markscheme
2C6H5COOH (s) + 15O2 (g) → 14CO2 (g) + 6H2O (l)
correct products [✔]
correct balancing [✔]
7h. [1 mark]
The combustion reaction in (f)(ii) can also be classed as redox. Identify the atom that is
oxidized and the atom that is reduced.
Markscheme
Oxidized:
C/carbon «in C6H5COOH»
AND
Reduced:
O/oxygen «in O2»
[✔]
7i. [1 mark]
Suggest how benzoic acid, Mr = 122.13, forms an apparent dimer, Mr = 244.26, when
dissolved in a non-polar solvent such as hexane.
Markscheme
«intermolecular» hydrogen bonding [✔]
Note: Accept diagram showing hydrogen bonding.
7j. [1 mark]
State the reagent used to convert benzoic acid to phenylmethanol (benzyl alcohol),
C6H5CH2OH.
Markscheme
lithium aluminium hydride/LiAlH4 [✔]
8a. [1 mark]
Carbonated water is produced when carbon dioxide is dissolved in water under pressure.
The following equilibria are established.
Equilibrium (1) CO2 (g)
CO2 (aq)
Equilibrium (2) CO2 (aq) + H2O (l)
Carbon dioxide acts as a weak acid.
Distinguish between a weak and strong acid.
H+ (aq) + HCO3− (aq)
Weak acid:
Strong acid:
Markscheme
Weak acid: partially dissociated/ionized «in aqueous solution/water»
AND
Strong acid: «assumed to be almost» completely/100 % dissociated/ionized «in aqueous
solution/water» [✔]
8b. [1 mark]
The hydrogencarbonate ion, produced in Equilibrium (2), can also act as an acid.
State the formula of its conjugate base.
Markscheme
CO32- [✔]
8c. [1 mark]
When a bottle of carbonated water is opened, these equilibria are disturbed.
State, giving a reason, how a decrease in pressure affects the position of Equilibrium (1).
Markscheme
shifts to left/reactants AND to increase amount/number of moles/molecules of gas/CO2 (g)
[✔]
Note: Accept “shifts to left/reactants AND to increase pressure”.
8d. [3 marks]
At 298 K the concentration of aqueous carbon dioxide in carbonated water is 0.200 mol
dm−3 and the pKa for Equilibrium (2) is 6.36.
Calculate the pH of carbonated water.
Markscheme
«Ka =» 10–6.36/4.37 × 10–7 = ¿ ¿ ¿
OR
«Ka =» 10–6.36/4.37 × 10–7 = ¿ ¿ ¿ [✔]
[H+] « √ 0.200 × 4.37× 10−7 » = 2.95 × 10–4 «mol dm–3»
«pH =» 3.53 [✔]
[✔]
Note: Award [3] for correct final answer.
8e. [2 marks]
Soda water has sodium hydrogencarbonate, NaHCO3, dissolved in the carbonated water.
Identify the type of bonding in sodium hydrogencarbonate.
Between sodium and hydrogencarbonate:
Between hydrogen and oxygen in hydrogencarbonate:
Markscheme
Between sodium and hydrogencarbonate:
ionic [✔]
Between hydrogen and oxygen in hydrogencarbonate:
«polar» covalent [✔]
8f. [2 marks]
Predict, referring to Equilibrium (2), how the added sodium hydrogencarbonate affects the
pH.(Assume pressure and temperature remain constant.)
Markscheme
«additional HCO3-» shifts position of equilibrium to left [✔]
pH increases [✔]
Note: Do not award M2 without any justification in terms of equilibrium shift in M1.
8g. [2 marks]
100.0cm3 of soda water contains 3.0 × 10−2g NaHCO3.
Calculate the concentration of NaHCO3 in mol dm−3.
Markscheme
«molar mass of NaHCO3 =» 84.01 «g mol-1» [✔]
«concentration =
3.0× 10−2 g
1
–3
-3
×
−1
3 =» 3.6 × 10 «mol dm »
84.01 g mo l
0.100 d m
[✔]
Note: Award [2] for correct final answer.
8h. [1 mark]
The uncertainty of the 100.0cm3 volumetric flask used to make the solution was ±0.6cm 3.
Calculate the maximum percentage uncertainty in the mass of NaHCO3 so that the
concentration of the solution is correct to ±1.0 %.
Markscheme
«1.0 – 0.6 = ± » 0.4 «%» [✔]
8i. [2 marks]
The reaction of the hydroxide ion with carbon dioxide and with the hydrogencarbonate ion
can be represented by Equations 3 and 4.
Equation (3)
Equation (4)
OH− (aq) + CO2 (g) → HCO3− (aq)
OH− (aq) + HCO3− (aq) → H2O (l) + CO32− (aq)
Discuss how these equations show the difference between a Lewis base and a Brønsted–
Lowry base.
Equation (3):
Equation (4):
Markscheme
Equation (3):
OH- donates an electron pair AND acts as a Lewis base
[✔]
Equation (4):
OH- accepts a proton/H+/hydrogen ion AND acts as a Brønsted–Lowry base
[✔]
8j. [2 marks]
Aqueous sodium hydrogencarbonate has a pH of approximately 7 at 298 K.
Sketch a graph of pH against volume when 25.0cm3 of 0.100 mol dm−3 NaOH (aq) is
gradually added to 10.0cm3 of 0.0500 mol dm−3 NaHCO3 (aq).
Markscheme
S-shaped curve from ~7 to between 12 and 14
equivalence point at 5 cm3
[✔]
[✔]
Note: Accept starting point >6~7.
9a. [1 mark]
A student investigated how the type of acid in acid deposition affects limestone, a building
material mainly composed of calcium carbonate.
The student monitored the mass of six similarly sized pieces of limestone. Three were
placed in beakers containing 200.0 cm3 of 0.100 mol dm−3 nitric acid, HNO3 (aq), and the
other three in 200.0 cm3 of 0.100 mol dm−3 sulfuric acid, H2SO4 (aq).
The limestone was removed from the acid, washed, dried with a paper towel and weighed
every day at the same time and then replaced in the beakers.
The student plotted the mass of one of the pieces of limestone placed in nitric acid against
time.
[Source: © International Baccalaureate Organization 2019]
Draw a best-fit line on the graph.
Markscheme
best-fit smooth curve ✔
NOTE: Do not accept a series of connected lines that pass through all points OR any straight
line representation.
9b. [3 marks]
Determine the initial rate of reaction of limestone with nitric acid from the graph.
Show your working on the graph and include the units of the initial rate.
Markscheme
tangent drawn at time zero ✔
g day−1 ✔
0.16 ✔
NOTE: Accept other reasonable units for initial rate eg, mol dm−3 s−1, mol dm−3 min−1, g s−1 OR
g min−1.
M3 can only be awarded if the value corresponds to the correct unit given in M2.
Accept values for the initial rate for M3 in the range: 0.13 − 0.20 g day−1 OR 1.5 × 10−6 g s−1 −
2.3 × 10−6 g s−1 OR 7.5 × 10−8 − 1.2 × 10−7 mol dm−3 s−1 OR 4.5 × 10−6 − 6.9 × 10−6 mol dm−3 min−1
OR 9.0 × 10−5 − 1.4 × 10−4 g min−1 OR a range based on any other reasonable unit for rate.
Ignore any negative rate value.
Award [2 max] for answers such as 0.12/0.11 g day−1, incorrectly obtained by using the first
two points on the graph (the average rate between t = 0 and 1 day).
Award [1 max] for correctly calculating any other average rate.
9c. [2 marks]
Explain why the rate of reaction of limestone with nitric acid decreases and reaches zero
over the period of five days.
Markscheme
acid used up
OR
acid is the limiting reactant ✔
concentration of acid decreases
OR
less frequent collisions ✔
NOTE: Award [1 max] for "surface area decreases" if the idea that CaCO3 is used up/acts as
the limiting reactant” is conveyed for M1.
Do not accept “reaction reaches equilibrium” for M2.
9d. [1 mark]
Suggest a source of error in the procedure, assuming no human errors occurred and the
balance was accurate.
Markscheme
surface area not uniform
NOTE: Accept “acids impure.
OR
limestone pieces do not have same composition/source
NOTE: Accept “«limestone» contains impurities”.
OR
limestone absorbed water «which increased mass»
OR
acid removed from solution when limestone removed
NOTE: Accept “loss of limestone when dried" OR "loss of limestone due to crumbling when
removed from beaker”.
OR
«some» calcium sulfate deposited on limestone lost
OR
pieces of paper towel may have stuck to limestone
OR
beakers not covered/evaporation
OR
temperature was not controlled ✔
9e. [1 mark]
The student hypothesized that sulfuric acid would cause a larger mass loss than nitric acid.
Justify this hypothesis.
Markscheme
sulfuric acid is diprotic/contains two H+ «while nitric acid contains one H+»/releases more
H+ «so reacts with more limestone»
OR
higher concentration of protons/H+ ✔
NOTE: Ignore any reference to the relative strengths of sulfuric acid and nitric acid.
Accept “sulfuric acid has two hydrogens «whereas nitric has one»”.
Accept "dibasic" for "diprotic".
9f. [1 mark]
The student obtained the following total mass losses.
She concluded that nitric acid caused more mass loss than sulfuric acid, which did not
support her hypothesis.
Suggest an explanation for the data, assuming that no errors were made by the student.
Markscheme
calcium sulfate remained/deposited on limestone «in sulfuric acid»
OR
reaction prevented/stopped by slightly soluble/deposited/layer of calcium sulfate ✔
NOTE: Answer must refer to calcium sulfate.
10. [1 mark]
Which two species act as Brønsted–Lowry acids in the reaction?
H2PO4− (aq) + OH− (aq) ⇌ HPO42− (aq) + H2O (l)
A. HPO42− (aq) and OH− (aq)
B. H2PO4− (aq) and HPO42− (aq)
C. HPO42− (aq) and H2O (l)
D. H2PO4− (aq) and H2O (l)
Markscheme
D
11. [1 mark]
Which classification is correct for the reaction?
H2PO4−(aq) + H2O(l) → HPO42−(aq) + H3O+(aq)
Markscheme
D
12. [1 mark]
What describes HPO42−?
A.
Amphiprotic but not amphoteric
B.
Amphoteric but not amphiprotic
C.
Amphiprotic and amphoteric
D.
Neither amphiprotic nor amphoteric
Markscheme
C
13a. [2 marks]
Butanoic acid, CH3CH2CH2COOH, is a weak acid and ethylamine, CH3CH2NH2, is a weak base.
State the equation for the reaction of each substance with water.
Markscheme
Butanoic acid:
CH3CH2CH2COOH (aq) + H2O (l) ⇌ CH3CH2CH2COO− (aq) + H3O+ (aq) ✔
Ethylamine:
CH3CH2NH2 (aq) + H2O (l) ⇌ CH3CH2NH3+ (aq) + OH− (aq) ✔
13b. [2 marks]
Explain why butanoic acid is a liquid at room temperature while ethylamine is a gas at
room temperature.
Markscheme
Any two of:
butanoic acid forms more/stronger hydrogen bonds ✔
butanoic acid forms stronger London/dispersion forces ✔
butanoic acid forms stronger dipole–dipole interaction/force ✔
Accept “butanoic acid forms dimers”
Accept “butanoic acid has larger Mr/hydrocarbon chain/number of electrons” for M2.
Accept “butanoic acid has larger «permanent» dipole/more polar” for M3.
13c. [1 mark]
State the formula of the salt formed when butanoic acid reacts with ethylamine.
Markscheme
CH3CH2NH3+ CH3CH2CH2COO−
OR
CH3CH2CH2COO− CH3CH2NH3+
OR
CH3CH2CH2COO− H3N+CH2CH3 ✔
The charges are not necessary for the mark.
14a. [3 marks]
Graphing is an important tool in the study of rates of chemical reactions.
Sketch a Maxwell–Boltzmann distribution curve for a chemical reaction showing
the activation energies with and without a catalyst.
Markscheme
both axes correctly labelled
correct shape of curve starting at origin
Ea(catalyst) < Ea(without catalyst) on x-axis
M1:
Accept “speed” for x-axis label.
Accept “number of particles”, “N”, “frequency” or “probability «density»” for y-axis label.
Do not accept “potential energy” for x-axis label.
M2:
Do not accept a curve that touches the x-axis at high energy.
Do not award M2 if two curves are drawn.
M3:
Ignore any shading under the curve.
[3 marks]
14b. [1 mark]
Excess hydrochloric acid is added to lumps of calcium carbonate. The graph shows the
volume of carbon dioxide gas produced over time.
Sketch a curve on the graph to show the volume of gas produced over time if the same mass
of crushed calcium carbonate is used instead of lumps. All other conditions remain
constant.
Markscheme
curve starting from origin with steeper gradient AND reaching same maximum volume
[1 mark]
14c. [2 marks]
State and explain the effect on the rate of reaction if ethanoic acid of the
same concentration is used in place of hydrochloric acid.
Markscheme
rate decreases
OR
slower reaction
«ethanoic acid» partially dissociated/ionized «in solution/water»
OR
lower [H+]
Accept “weak acid” or “higher pH”.
[2 marks]
14d. [1 mark]
Outline why pH is more widely used than [H+] for measuring relative acidity.
Markscheme
«pH» converts «wide range of [H+]» into simple «log» scale/numbers
OR
«pH» avoids need for exponential/scientific notation
OR
«pH» converts small numbers into values «typically» between 0/1 and 14
OR
«pH» allows easy comparison of values of [H+]
Accept “uses values between 0/1 and 14”.
Do not accept “easier to use”.
Do not accept “easier for calculations”.
[1 mark]
14e. [1 mark]
Outline why H3PO4/HPO42− is not a conjugate acid-base pair.
Markscheme
«species» do not differ by a «single» proton/H+
OR
conjugate base of H3PO4 is H2PO4– «not HPO42–»
OR
conjugate acid of HPO42– is H2PO4– «not H3PO4»
Do not accept “hydrogen/H” for “H+/proton”.
[1 mark]
15a. [2 marks]
State the equation for the reaction of each substance with water.
Markscheme
Butanoic acid:
CH3CH2CH2COOH (aq) + H2O (l) ⇌ CH3CH2CH2COO− (aq) + H3O+ (aq) ✔
Ethylamine:
CH3CH2NH2 (aq) + H2O (l) ⇌ CH3CH2NH3+ (aq) + OH− (aq) ✔
15b. [1 mark]
Draw a diagram showing the delocalization of electrons in the conjugate base of butanoic
acid.
Markscheme
Diagram showing:
dotted line along O–C–O AND negative charge
Accept correct diagrams with pi clouds.
15c. [1 mark]
Deduce the average oxidation state of carbon in butanoic acid.
Markscheme
–1 ✔
15d. [1 mark]
A 0.250 mol dm−3 aqueous solution of butanoic acid has a concentration of hydrogen ions,
[H+], of 0.00192 mol dm−3. Calculate the concentration of hydroxide ions, [OH −], in the
solution at 298 K.
Markscheme
«
1.00× 10−14 mol 2 d m− 6
» = 5.21 × 10–12 «mol dm–3» ✔
0.00192 mol d m− 3
15e. [3 marks]
Determine the pH of a 0.250 mol dm−3 aqueous solution of ethylamine at 298 K, using
section 21 of the data booklet.
Markscheme
«pKb = 3.35, Kb = 10–3.35 = 4.5 × 10–4»
«C2H5NH2 + H2O ⇌ C2H5NH3+ + OH–»
Kb = [ O H − − ] ¿ ¿
OR
«Kb =» 4.5 × 10–4 = [ O H − ] ¿ ¿
OR
«Kb =» 4.5 × 10–4 =
x2
✔
0.250
« x = [OH–] =» 0.011 «mol dm–3» ✔
«pH = –log
1.00× 10−14
=¿» 12.04
0.011
OR
«pH = 14.00 – (–log 0.011)=» 12.04 ✔
Award [3] for correct final answer.
15f. [3 marks]
Sketch the pH curve for the titration of 25.0 cm3 of ethylamine aqueous solution with 50.0
cm3 of butanoic acid aqueous solution of equal concentration. No calculations are required.
Markscheme
decreasing pH curve ✔
pH close to 7 (6–8) at volume of 25 cm3 butanoic acid ✔
weak acid/base shape with no flat «strong acid/base» parts on the curve ✔
15g. [2 marks]
Explain why butanoic acid is a liquid at room temperature while ethylamine is a gas at
room temperature.
Markscheme
Any two of:
butanoic acid forms more/stronger hydrogen bonds ✔
butanoic acid forms stronger London/dispersion forces ✔
butanoic acid forms stronger dipole–dipole interaction/force ✔
Accept “butanoic acid forms dimers”
Accept “butanoic acid has larger Mr/hydrocarbon chain/number of electrons” for M2.
Accept “butanoic acid has larger «permanent» dipole/more polar” for M3.
15h. [1 mark]
State a suitable reagent for the reduction of butanoic acid.
Markscheme
lithium aluminium hydride/LiAlH4 ✔
15i. [1 mark]
Deduce the product of the complete reduction reaction in (e)(i).
Markscheme
butan-1-ol/1-butanol/CH3CH2CH2CH2OH ✔
16a. [2 marks]
Limescale, CaCO3(s), can be removed from water kettles by using vinegar, a dilute solution
of ethanoic acid, CH3COOH(aq).
Predict, giving a reason, a difference between the reactions of the same concentrations of
hydrochloric acid and ethanoic acid with samples of calcium carbonate.
Markscheme
slower rate with ethanoic acid
OR
smaller temperature rise with ethanoic acid
[H+] lower
OR
ethanoic acid is weak
OR
ethanoic acid is partially dissociated
Accept experimental observations such as “slower bubbling” or “feels less warm”.
[2 marks]
16b. [1 mark]
Dissolved carbon dioxide causes unpolluted rain to have a pH of approximately 5, but other
dissolved gases can result in a much lower pH. State one environmental effect of acid rain.
Markscheme
Any one of:
corrosion of materials/metals/carbonate materials
destruction of plant/aquatic life
«indirect» effect on human health
Accept “lowering pH of oceans/lakes/waterways”.
[1 mark]
16c. [2 marks]
Write an equation to show ammonia, NH3, acting as a Brønsted–Lowry base and a different
equation to show it acting as a Lewis base.
Markscheme
Brønsted–Lowry base:
NH3 + H+ → NH4+
Lewis base:
NH3 + BF3 → H3NBF3
Accept “AlCl3 as an example of Lewis acid”.
Accept other valid equations such as Cu2+ + 4NH3 → [Cu(NH3)4]2+.
[2 marks]
16d. [2 marks]
Determine the pH of 0.010 mol dm−3 2,2-dimethylpropanoic acid solution.
Ka (2,2-dimethylpropanoic acid) = 9.333 × 10−6
Markscheme
−6
[H+] «¿ √ K a × [ C5 H 10 O2 ]= √ 9.333 ×10 ×0.010» = 3.055 × 10–4 «mol dm–3»
«pH =» 3.51
Accept “pH = 3.52”.
Award [2] for correct final answer.
Accept other calculation methods.
[2 marks]
16e. [2 marks]
Explain, using appropriate equations, how a suitably concentrated solution formed by the
partial neutralization of 2,2-dimethylpropanoic acid with sodium hydroxide acts as a buffer
solution.
Markscheme
(CH3)3CCOOH(aq) + OH–(aq) → (CH3)3CCOO–(aq) + H2O(l)
OR
(CH3)3CCOOH(aq) + OH–(aq) ⇌ (CH3)3CCOO–(aq) + H2O(l) AND addition of alkali causes
equilibrium to move to right
(CH3)3CCOO–(aq) + H+(aq) → (CH3)3CCOOH(aq)
OR
(CH3)3CCOO–(aq) + H+(aq) ⇌ (CH3)3CCOOH(aq) AND addition of acid causes equilibrium to
move to right
Accept “HA” for the acid.
Award [1 max] for correct explanations of buffering with addition of acid AND base without
equilibrium equations.
[2 marks]
17. [1 mark]
Which is an acid-base conjugate pair?
A.
H3O+ / OH–
B.
H2SO4 / SO42–
C.
CH3COOH / H3O+
D.
CH3NH3+ / CH3NH2
Markscheme
D
18. [1 mark]
Which of the following is correct?
A.
A weak acid is a proton donor and its aqueous solution shows good conductivity.
B.
A weak acid is a proton donor and its aqueous solution shows poor conductivity.
C.
A weak acid is a proton acceptor and its aqueous solution shows good conductivity.
D.
A weak acid is a proton acceptor and its aqueous solution shows poor conductivity.
Markscheme
B
19. [1 mark]
Which species produced by the successive dissociations of phosphoric acid, H 3PO4,
are amphiprotic?
A.
HPO42− and PO43−
B.
H2PO4− and HPO42−
C.
H2PO4− and PO43−
D.
HPO42− only
Markscheme
B
20. [1 mark]
Which species acts as a Lewis and Brønsted–Lowry base?
A.
[Al(H2O)6]3+
B.
BF3
C.
NH4+
D.
OH−
Markscheme
D
21a. [2 marks]
Many reactions are in a state of equilibrium.
The following reaction was allowed to reach equilibrium at 761 K.
H2 (g) + I2 (g) ⇌ 2HI (g)
ΔHθ < 0
Outline the effect, if any, of each of the following changes on the position of
equilibrium, giving a reason in each case.
Markscheme
Award [1 max] if both effects are correct.
Reason for increasing volume:
Accept “concentration of all reagents reduced by an equal amount so cancels out in Kc
expression”.
Accept “affects both forward and backward rates equally”.
21b. [1 mark]
The equations for two acid-base reactions are given below.
HCO3– (aq) + H2O (l) ⇌ H2CO3 (aq) + OH– (aq)
HCO3– (aq) + H2O (l) ⇌ CO32– (aq) + H3O+ (aq)
Identify two different amphiprotic species in the above reactions.
Markscheme
HCO3– AND H2O
21c. [1 mark]
State what is meant by the term conjugate base.
Markscheme
species that has one less proton/H+ ion «than its conjugate acid»
OR
species that forms its conjugate acid by accepting a proton
OR
species that is formed when an acid donates a proton
Do not accept “differs by one proton/H+ from conjugate acid”.
21d. [1 mark]
State the conjugate base of the hydroxide ion, OH–.
Markscheme
oxide ion/O2–
21e. [2 marks]
A student working in the laboratory classified HNO3, H2SO4, H3PO4 and HClO4 as acids based
on their pH. He hypothesized that “all acids contain oxygen and hydrogen”.
Evaluate his hypothesis.
Markscheme
insufficient data to make generalization
OR
need to consider a «much» larger number of acids
OR
hypothesis will continue to be tested with new acids to see if it can stand the test of time
«hypothesis is false as» other acids/HCl/HBr/HCN/transition metal ion/BF 3 do not contain
oxygen
OR
other acids/HCl/HBr/HCN/transition metal ion/BF3 falsify hypothesis
correct inductive reasoning «based on limited sample»
«hypothesis not valid as» it contradicts current/accepted theories/Brønsted-Lowry/Lewis
theory
[Max 2 Marks]
Printed for GREEN HILLS ACAD SEC
© International Baccalaureate Organization 2021
International Baccalaureate® - Baccalauréat International® - Bachillerato Internacional®