Q1.
This question is about ammonia and fertilisers.
Ammonia is produced from nitrogen and hydrogen.
A catalyst is used to speed up the reaction.
The word equation for the reaction is:
nitrogen + hydrogen ⇌ ammonia
(a)
What does the symbol ⇌ show about the reaction?
___________________________________________________________________
___________________________________________________________________
(1)
(b)
Which catalyst is used when ammonia is produced from nitrogen and hydrogen?
Tick (✓) one box.
Chlorine
Iron
Oxygen
(1)
(c)
The diagram below shows the reaction profile for the production of ammonia both
with a catalyst and without a catalyst.
What is represented by label X?
Page 1 of 32
Tick (✓) one box.
Activation energy with a catalyst
Activation energy without a catalyst
Overall energy change with a catalyst
Overall energy change without a catalyst
(1)
Ammonia is used to produce fertilisers.
NPK fertilisers contain the elements nitrogen, phosphorus and potassium.
A fertiliser contains:
•
22% phosphorus
•
25% potassium.
(d)
Draw a bar chart on the graph below to show the percentages of phosphorus and of
potassium in this fertiliser.
(2)
(e)
Why do the percentages of phosphorus and of potassium in this fertiliser not add up
to 100%?
___________________________________________________________________
___________________________________________________________________
(1)
Page 2 of 32
Fertilisers help plants grow by adding essential elements to soil.
The table below shows the percentages of nitrogen, phosphorus and potassium in four
fertilisers, A, B, C and D.
Percentage (%) of essential element
Fertiliser
(f)
Nitrogen (N)
Phosphorus (P)
Potassium (K)
A
14
0
39
B
25
16
23
C
21
23
0
D
21
0
0
Plants lacking essential elements do not grow well because:
•
too little phosphorus can cause slow plant growth
•
too little potassium can cause leaves to have brown edges.
Which fertiliser helps prevent slow plant growth and brown leaf edges?
Use the table above.
Tick (✓) one box.
A
B
C
D
(1)
(g)
Which fertiliser has the greatest total percentage of essential elements?
Use the table above.
Tick (✓) one box.
A
B
C
D
(1)
(Total 8 marks)
Q2.
Ammonia is produced by the Haber process. In the process nitrogen and hydrogen are
mixed. The pressure is increased to about 200 atmospheres. The gases are passed over
an iron catalyst at about 450°C. The equation for the reaction is:
N2(g)
+
3H2(g)
2NH3(g)
The reaction between nitrogen and hydrogen is reversible. This affects the amount of
ammonia that it is possible to obtain from the process. The graph below shows how the
pressure and temperature affect the percentage of ammonia that can be produced.
Page 3 of 32
Use this information, together with your knowledge of the process, to explain why many
industrial ammonia plants operate at 200 atmospheres and 450°C.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
(Total 5 marks)
Q3.
The flow diagram shows how ammonia is made.
Page 4 of 32
(a)
What effect, if any, does the pump have on the pressure of the nitrogen and
hydrogen?
Draw a ring around the correct answer to complete the sentence.
decreases
The pump
has no effect on
the pressure.
increases
(1)
(b)
The word equation for making ammonia is:
nitrogen
+
hydrogen
ammonia
In the reactor only a small amount of the nitrogen and hydrogen is changed into
ammonia.
Tick ( ) the reason why.
Reason why
Ammonia is formed from two elements.
Page 5 of 32
Tick ( )
Nitrogen and hydrogen are gases.
The reaction is reversible.
(1)
(c)
In the cooler the mixture of gases is cooled.
Draw a ring around the correct answer to complete the sentence.
a liquid.
The cooler turns the ammonia into
a solid.
an element.
(1)
(d)
What happens to the unreacted nitrogen and hydrogen from the reactor?
___________________________________________________________________
___________________________________________________________________
(1)
(Total 4 marks)
Q4.
The flow diagram shows the Haber process. In the Haber process ammonia is produced
from nitrogen and hydrogen.
(a)
The word equation for the production of ammonia is:
nitrogen
+
hydrogen
ammonia
Draw a ring around the correct answer to complete the sentence.
Page 6 of 32
exothermic.
The symbol
in the word equation shows the reaction is
reversible.
slow.
(1)
(b)
The reactor contains iron.
Complete the sentence.
The iron speeds up the reaction because it is a _______________________
(1)
(c)
What happens to the unreacted nitrogen and hydrogen?
___________________________________________________________________
___________________________________________________________________
(1)
(d)
The sentences describe how ammonia is produced in the Haber process.
The sentences are in the wrong order.
P
Ammonia is separated as a liquid.
Q
Nitrogen and hydrogen are mixed together.
R
A mixture of gases enters the condenser.
S
Nitrogen and hydrogen react to produce ammonia.
Complete the boxes below to show the correct order of the sentences.
The first box has been done for you.
(2)
(Total 5 marks)
Q5.
Humberstone was a town in the desert of Northern Chile in South America. It was built for
the people who worked in the nearby sodium nitrate mines.
The sodium nitrate was used as a fertiliser.
The sodium nitrate was exported by ship to countries all around the world.
Today the mines have closed and nobody lives in Humberstone.
One of the reasons for the mines closing was the invention of the Haber process.
Page 7 of 32
By Sznegra (Own work) [CC-BY-SA-3.0], via Wikimedia Commons
(a)
The Haber process is used to make ammonia (NH3).
N2(g)
+
3H2(g)
2NH3(g)
The forward reaction is exothermic.
(i)
Name the raw materials that are used to supply the nitrogen and hydrogen.
Nitrogen ______________________________
Hydrogen _____________________________
(2)
(ii)
The Haber process uses a temperature of 450 °C.
Explain, as fully as you can, why a temperature of 450 °C is used rather than a
much higher temperature or a much lower temperature.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(3)
(iii)
Ammonia can be converted to ammonium nitrate by adding an acid.
Name this acid.
Page 8 of 32
______________________________________________________________
(1)
(b)
Suggest and explain why the invention of the Haber process caused the closure of
the Humberstone mines in Chile.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(Total 8 marks)
Q6.
Ammonia is made using the Haber process.
(a)
How is ammonia separated from unreacted nitrogen and hydrogen in the
separator?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
Page 9 of 32
(b)
The equation shows the reaction which takes place in the reactor:
N2(g)
(i)
+
3H2(g)
2NH3(g)
Why does the yield of ammonia at equilibrium increase as the temperature is
decreased?
______________________________________________________________
______________________________________________________________
(1)
(ii)
A temperature of 450 °C is used in the reactor to make the reaction take place
quickly.
Explain, in terms of particles, why increasing the temperature makes a
reaction go faster.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(iii)
Why does the yield of ammonia at equilibrium increase as the pressure is
increased?
______________________________________________________________
______________________________________________________________
(1)
(iv)
The pressure used in the reactor is 200 atmospheres.
Suggest why a much higher pressure is not used.
______________________________________________________________
______________________________________________________________
(1)
(c)
Use the equation for the reaction in the reactor to help you to answer these
questions.
N2(g)
(i)
+
3H2(g)
2NH3(g)
It is important to mix the correct amounts of hydrogen and nitrogen in the
reactor.
20 m3 of nitrogen is reacted with hydrogen.
What volume of hydrogen (measured at the same temperature and pressure
Page 10 of 32
as the nitrogen) is needed to have the correct number of molecules to react
with the nitrogen?
Volume of hydrogen needed = _____________ m3
(1)
(ii)
Calculate the maximum mass of ammonia that can be made from 2 g of
nitrogen.
Relative atomic masses: H = 1; N = 14.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
Maximum mass of ammonia = ______________ g
(3)
(d)
The expected maximum mass of ammonia produced by the Haber process can be
calculated.
(i)
In one process, the maximum mass of ammonia should be 80 kg.
The actual mass of ammonia obtained was 12 kg.
Calculate the percentage yield of ammonia in this process.
______________________________________________________________
______________________________________________________________
Percentage yield of ammonia = _____________ %
(1)
(ii)
Give two reasons why it does not matter that the percentage yield of ammonia
is low.
Use the flow diagram at the start of this question to help you.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(Total 14 marks)
Page 11 of 32
Q7.
This question is about the Haber process.
The diagram below shows a flow diagram for the Haber process.
(a)
(i)
Nitrogen gas and hydrogen gas are obtained from different sources.
Draw one line from each gas to its source.
(2)
(ii)
Explain why iron is used in the reactor for the Haber process.
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(iii)
Describe how the ammonia is separated from the other gases.
Page 12 of 32
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(iv)
What happens to the mixture of unreacted gases (nitrogen and hydrogen)?
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(b)
The reaction to produce ammonia is reversible.
Complete the word equation for this reaction.
nitrogen
+
__________________________________________________
(2)
(Total 9 marks)
Q8.
This question is about reversible reactions and chemical equilibrium.
(a)
Reversible reactions can reach equilibrium in a closed system.
(i)
What is meant by a closed system?
______________________________________________________________
______________________________________________________________
(1)
(ii)
Explain why, when a reversible reaction reaches equilibrium, the reaction
appears to have stopped.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(b)
In the Haber process, the reaction of nitrogen with hydrogen to produce ammonia is
reversible.
N2(g)
(i)
+
3 H2(g)
2 NH3(g)
Name a natural resource from which hydrogen is produced.
______________________________________________________________
Page 13 of 32
(1)
(ii)
The Haber process uses a catalyst to speed up the reaction.
Explain how a catalyst speeds up a reaction.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(iii)
What happens to the amount of ammonia produced at equilibrium if the
pressure is increased?
Give a reason for your answer.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(c)
The decomposition of hydrogen iodide into hydrogen and iodine is reversible.
2HI(g)
H2(g)
+
I2(g)
The forward reaction is endothermic.
The energy level diagram shown below is for the forward reaction.
(i)
Draw an arrow to show the activation energy on the diagram.
(1)
(ii)
How does the diagram show that the reaction is endothermic?
______________________________________________________________
______________________________________________________________
Page 14 of 32
(1)
(iii)
Suggest what effect, if any, increasing the temperature will have on the
amount of hydrogen iodide at equilibrium.
Give a reason for your answer.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(Total 12 marks)
Q9.
A student investigated the rate of reaction of magnesium and hydrochloric acid.
Mg(s) + 2HCl(aq)
MgCl2(aq) + H2(g)
The student studied the effect of changing the concentration of the hydrochloric acid.
She measured the time for the magnesium to stop reacting.
Concentration
of hydrochloric
acid in moles
per dm3
(a)
0.5
1.0
1.5
2.0
The student changed the concentration of the hydrochloric acid.
Give two variables that the student should control.
1. _________________________________________________________________
2. _________________________________________________________________
(2)
(b)
(i)
The rate of reaction increased as the concentration of hydrochloric acid
increased.
Page 15 of 32
Explain why.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(2)
(ii)
Explain why increasing the temperature would increase the rate of reaction.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(3)
(c)
(i)
The student had a solution of sodium hydroxide with a concentration of 0.100
moles per dm3.
She wanted to check the concentration of a solution of hydrochloric acid.
She used a pipette to transfer 5.00 cm3 of the hydrochloric acid into a conical
flask.
She filled a burette with the 0.100 moles per dm3 sodium hydroxide solution.
Describe how she should use titration to obtain accurate results.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
Page 16 of 32
______________________________________________________________
(4)
(ii)
Sodium hydroxide neutralises hydrochloric acid as shown in the equation:
NaOH(aq) + HCl(aq)
NaCl(aq) + H2O(l)
The student found that 27.20 cm3 of 0.100 moles per dm3 sodium hydroxide
neutralised 5.00 cm3 of hydrochloric acid.
Calculate the concentration of the hydrochloric acid in moles per dm3.
Give your answer to three significant figures.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
Concentration of hydrochloric acid = _______________ moles per dm3
(3)
(Total 14 marks)
Q10.
Sodium hydroxide neutralises sulfuric acid.
The equation for the reaction is:
2NaOH + H2SO4 → Na2SO4 + 2H2O
(a)
Sulfuric acid is a strong acid.
What is meant by a strong acid?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(b)
Write the ionic equation for this neutralisation reaction. Include state symbols.
___________________________________________________________________
(2)
(c)
A student used a pipette to add 25.0 cm3 of sodium hydroxide of unknown
concentration to a conical flask.
Page 17 of 32
The student carried out a titration to find out the volume of 0.100 mol / dm3 sulfuric
acid needed to neutralise the sodium hydroxide.
Describe how the student would complete the titration.
You should name a suitable indicator and give the colour change that would be
seen.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(4)
(d)
The student carried out five titrations. Her results are shown in the table below.
Volume of
0.100 mol /
dm3 sulfuric
acid in cm3
Titration 1
Titration 2
Titration 3
Titration 4
Titration 5
27.40
28.15
27.05
27.15
27.15
Concordant results are within 0.10 cm3 of each other.
Use the student’s concordant results to work out the mean volume of 0.100 mol /
dm3 sulfuric acid added.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Mean volume = _____________________________ cm3
(2)
(e)
The equation for the reaction is:
2NaOH + H2SO4 → Na2SO4 + 2H2O
Page 18 of 32
Calculate the concentration of the sodium hydroxide.
Give your answer to three significant figures.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Concentration = _______________________ mol / dm3
(4)
(f)
The student did another experiment using 20 cm3 of sodium hydroxide solution with
a concentration of 0.18 mol / dm3.
Relative formula mass (Mr) of NaOH = 40
Calculate the mass of sodium hydroxide in 20 cm3 of this solution.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Mass = ________________ g
(2)
(Total 16 marks)
Q11.
An oven cleaner solution contained sodium hydroxide. A 25.0 cm3 sample of the oven
cleaner solution was placed in a flask. The sample was titrated with hydrochloric acid
containing
73 g/dm3 of hydrogen chloride, HCI.
(a)
Describe how this titration is carried out.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Page 19 of 32
(3)
(b)
Calculate the concentration of the hydrochloric acid in mol/dm3.
Relative atomic masses: H 1; Cl 35.5
___________________________________________________________________
Answer = __________________ mol/dm3
(2)
(c)
10.0 cm3 of hydrochloric acid were required to neutralise the 25.0 cm3 of oven
cleaner solution.
(i)
Calculate the number of moles of hydrochloric acid reacting.
______________________________________________________________
Answer = _____________________ mol
(2)
(ii)
Calculate the concentration of sodium hydroxide in the oven cleaner solution
in mol/dm3.
______________________________________________________________
Answer = __________________ mol/dm3
(2)
(Total 9 marks)
Q12.
Citric acid is a weak acid.
(a)
Explain what is meant by a weak acid.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
A student titrated citric acid with sodium hydroxide solution.
This is the method used.
1. Pipette 25.0 cm3 of sodium hydroxide solution into a conical flask.
2. Add a few drops of thymol blue indicator to the sodium hydroxide solution.
Thymol blue is blue in alkali and yellow in acid.
3. Add citric acid solution from a burette until the end-point was reached.
(b)
Explain what would happen at the end-point of this titration.
Page 20 of 32
Refer to the acid, the alkali and the indicator in your answer.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(3)
(c)
Explain why a pipette is used to measure the sodium hydroxide solution but a
burette is used to measure the citric acid solution
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(d)
The table shows the student’s results.
Volume of
citric acid
solution in
cm3
Titration 1
Titration 2
Titration 3
Titration 4
Titration 5
13.50
12.10
11.10
12.15
12.15
The equation for the reaction is:
C6H8O7 +
3 NaOH ⟶ C6H5O7Na3 +
3 H2O
The concentration of the sodium hydroxide was 0.102 mol / dm 3
Concordant results are those within 0.10 cm 3 of each other.
Calculate the concentration of the citric acid in mol / dm 3
Use only the concordant results from the table in your calculation.
You must show your working.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
Page 21 of 32
___________________________________________________________________
___________________________________________________________________
Concentration = ____________________ mol / dm 3
(5)
(Total 12 marks)
Page 22 of 32
Mark schemes
Q1.
(a)
(the reaction is) reversible
allow description of a reversible reaction
1
(b)
iron
1
(c)
activation energy with a catalyst
1
(d)
bar to 22 (%) labelled phosphorus / P
allow a tolerance of ± ½ a small square
1
bar to 25 (%) labelled potassium / K
if no other mark is awarded, allow 1 mark for two
bars drawn to 22% and 25%
1
(e)
there are other elements in the fertiliser (besides phosphorus and potassium)
or
there is nitrogen in the fertiliser
allow there are other substances in the fertiliser
(besides phosphorus and potassium)
1
(f)
B
1
(g)
B
1
[8]
Q2.
Effect of pressure
•
high pressure increases yield
for 1 mark
•
either because less product molecules (Le Chatelier)
or but high pressure increases cost/safety
for 1 mark
Effect of temperature
•
low temperature increases yield
for 1 mark
•
either because exothermic reaction (Le Chatelier)
Page 23 of 32
for 1 mark
•
or but at low temperature rate is slow/catalyst does not work
Compromise
•
optimum conditions to balance rate and % yield
for 1 mark
•
or rate is slow (at higher temperature) so need a catalyst
or low percentage conversion so recycle untreated gases
[5]
Q3.
(a)
increases
1
(b)
the reaction is reversible
1
(c)
A liquid
1
(d)
recycled / reused (owtte)
accept returned to pump / start
1
[4]
Q4.
(a)
reversible
1
(b)
catalyst
1
(c)
recycled
allow re-used
1
(d)
(Q) S R P
allow 1mark if one letter in correct place.
2
[5]
Q5.
(a)
(i)
nitrogen - air
accept atmosphere
1
hydrogen - north sea gas / natural gas / methane / CH4
accept water / (crude) oil / coal / hydrocarbons / brine
1
(ii)
allow converse throughout
Page 24 of 32
•
high temperature gives a low yield
1
•
because reaction is exothermic
must be linked to first bullet point
1
•
but at low temperatures the rate is (too) slow
if no other marks awarded accept 450°C is a compromise
between yield and rate
or
450°C gives a reasonable yield in a reasonable time for 1
mark
1
(iii)
nitric (acid)
accept HNO3
1
(b)
Ammonia / Haber process can be used to make fertiliser
1
with a specified economical reason
eg raw materials for Haber process readily available
eg transport costs are lower or no need to import
eg Haber process is a continuous process
ignore employment / labour costs
1
[8]
Q6.
(a)
mixture is cooled / cooling
1
so ammonia / it condenses
or
so ammonia turns into a liquid (but nitrogen and hydrogen remain as gases)
1
(b)
(i)
exothermic reaction
accept reverse reaction is endothermic
or
equilibrium / reaction moves in the direction which raises the temperature
ignore answers based on rate or collisions
1
(ii)
they / particles / molecules move faster or have more (kinetic) energy
allow atoms instead of particles
ignore particles move more / vibrate
do not accept electrons (max1)
1
any one from:
Page 25 of 32
•
particles / molecules collide more often / more frequently / more
likely to collide
ignore collide faster
ignore more collisions
•
more of the collisions are successful or particles collide with more
energy / harder or more of the particles have the activation energy
accept more successful collisions
1
(iii)
more molecules / particles / moles / volumes on LHS (of equation than RHS)
accept 4 molecules / particles / moles / volumes on LHS and
2 molecules / particles / moles / volumes on RHS
or
greater volume on LHS (than RHS)
or
equilibrium / reaction moves in the direction which reduces the
pressure / volume
accept converse
1
(iv)
cost
or
difficulty in containing such a high pressure
allow risk of explosion
ignore dangerous
1
(c)
(i)
60
1
(ii)
2.4(2857….)
correct answer gains 3 marks with or without working
accept any answer that rounds to 2.4
ignore units
if answer is incorrect look for evidence of correct working to
a
maximum of 2 marks.
moles of N2 = 2/28 = (0.0714)
moles of ammonia = 2 × 0.0714 = ( 0.1428)
mass of ammonia = 0.1428 × 17 = (2.4276)
or
28 → 34
1g →34/28
2g →2.4… ….
3
(d)
(i)
15
1
(ii)
unreacted gases are recycled
allow unreacted gases are reused
Page 26 of 32
1
rate (of production) is fast
accept production is continuous
ignore compromise between rate and yield
1
[14]
Q7.
(a)
(i)
nitrogen: air
1
hydrogen: natural gas
1
(ii)
as a catalyst
1
so the reaction speeds up
allow lowers activation energy or so a lower temperature can
be used
1
(iii)
cooled
1
ammonia condenses / liquefies
allow nitrogen and hydrogen remain in the gaseous state
1
(iv)
recycled
allow reused or returned to the reactor
1
(b)
reversible arrows
1
hydrogen and ammonia
1
[9]
Q8.
(a)
(i)
nothing can enter and nothing can leave the reaction
allow sealed reaction vessel
1
(ii)
forward and backward reactions have same rate
1
so there is no (overall) change in quantities of reactants and products
allow concentrations of reactants and products
1
(b)
(i)
natural gas
Page 27 of 32
allow methane / CH4
allow fossil fuels / hydrocarbons
allow water
1
(ii)
provides an alternative reaction pathway
1
which has a lower activation energy
ignore references to collisions
1
(iii)
the amount (of ammonia) increases
allow yield increases
1
the equilibrium moves to the side (of the equation) with fewer (gaseous)
molecules / moles
allow it favours the forward reaction
1
(c)
(i)
vertical arrow from reactants to maximum
1
(ii)
(energy of) products higher than (energy of) reactants
allow converse
1
(iii)
amount of hydrogen iodide decreases
1
equilibrium moves in the direction of the endothermic reaction
allow it favours the forward reaction
1
[12]
Q9.
(a)
any two from:
•
•
•
•
temperature (of the HCl)
mass or length of the magnesium
surface area of the magnesium
volume of HCl
2
(b)
(i)
(a greater concentration has) more particles per unit volume
allow particles are closer together
1
therefore more collisions per unit time or more frequent collisions.
1
(ii)
particles move faster
allow particles have more (kinetic) energy
1
therefore more collisions per unit time or more frequent collisions
Page 28 of 32
1
collisions more energetic (therefore more collisions have energy greater
than the activation energy) or more productive collisions
1
(c)
(i)
add (a few drops) of indicator to the acid in the conical flask
allow any named indicator
1
add NaOH (from the burette) until the indicator changes colour or add
the NaOH dropwise
candidate does not have to state a colour change but
penalise an incorrect colour change.
1
repeat the titration
1
calculate the average volume of NaOH or repeat until concordant
results are obtained
1
(ii)
moles of NaOH
0.10 × 0.0272 = 0.00272 moles
correct answer with or without working gains 3 marks
1
Concentration of HCl
0.00272 / 0.005 = 0.544
allow ecf from mp1 to mp2
1
correct number of significant figures
1
[14]
Q10.
(a)
(sulfuric acid is) completely / fully ionised
1
In aqueous solution or when dissolved in water
1
(b)
H+(aq) + OH−(aq) → H2O(l)
allow multiples
1 mark for equation
1 mark for state symbols
2
(c)
adds indicator, eg phenolpthalein / methyl orange / litmus added to the sodium
hydroxide
(in the conical flask)
do not accept universal indicator
1
Page 29 of 32
(adds the acid from a) burette
1
with swirling or dropwise towards the end point or until the indicator just changes
colour
1
until the indicator changes from pink to colourless (for phenolphthalein) or yellow to
red
(for methyl orange) or blue to red (for litmus)
1
(d)
titrations 3, 4 and 5
or
1
27.12 cm3
accept 27.12 with no working shown for 2 marks
1
allow 27.1166 with no working shown for 2 marks
(e)
Moles H2SO4 = conc × vol = 0.00271
allow ecf from 8.4
1
Ratio H2SO4:NaOH is 1:2
or
Moles NaOH = Moles H2SO4 × 2 = 0.00542
1
Concentration NaOH = mol / vol = 0.00542 / 0.025 = 0.2168
1
0.217 (mol / dm3)
accept 0.217 with no working for 4 marks
1
accept 0.2168 with no working for 3 marks
×
(f)
0.18 = no of moles
or
0.15 × 40 g
1
0.144 (g)
1
accept 0.144g with no working for 2 marks
[16]
Q11.
(a)
hydrochloric acid in burette
1
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indicator
1
note volume at end / neutralisation point
titre must be HC1
1
(b)
1 mole HCl = 36.5g /36.5
1
= 2 moles / dm3
2 for correct answer
1
(c)
(i)
allow e.c.f. ie their (b) ×
2 for correct answer
1
= 0.02 moles
1
(ii)
0.02 ×
= 0.8 mol / dm3
1
allow e.c.f. ie their (c)(i) ×
1
[9]
Q12.
(a)
produces H+ / hydrogen ions in aqueous solution
1
(but is) only partially / slightly ionised
1
(b)
indicator changes colour
1
from blue to yellow
allow from blue to green
1
(when) the acid and alkali are (exactly) neutralised
or
(when) no excess of either acid or alkali
1
(c)
pipette measures one fixed volume (accurately)
1
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(but) burette measures variable volumes (accurately)
1
(d)
1
(mean titre =) 12.13(3) (cm 3)
1
(moles NaOH = conc × vol) = 0.00255
1
(moles citric acid =
moles NaOH) = 0.00085
1
(conc acid = moles / vol) = 0.0701 (mol / dm 3)
allow ecf from steps 1, 2, 3 and / or 4
allow an answer of 0.0701 (mol / dm 3) without working for 1
mark only
1
[12]
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