Question
A student adds excess large pieces of magnesium carbonate, MgCO3, to dilute hydrochloric acid, HCl, and measures the volume of carbon dioxide gas, CO2, given off.
(a) Add the missing state symbols to the chemical equation for the reaction.
MgCO3 ….. + 2HCl….. → MgCl 2(aq) + H2O ….. + CO2 ….. [1]
(b) Complete the dot-and-cross diagram to show the electron arrangement of the ions in magnesium chloride.
The inner shells have been drawn.
Give the charges on the ions.[3]
(c) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of carbon dioxide.
Show outer shell electrons only.[2]
(d) The graph shows how the volume of carbon dioxide gas changes with time.
(i) Describe how the graph shows that the rate of this reaction decreases as time increases.[1]
(ii) Explain, in terms of particles, why the rate of this reaction decreases as time increases.[2]
(iii) The student repeats the experiment using powdered MgCO3 instead of large pieces.
All other conditions stay the same.
On the grid, draw the line expected when powdered MgCO3 is used instead of large pieces. [2]
(e) Determine the volume of CO2 gas given off when excess MgCO3 is added to 25.0cm3 of 0.400mol/dm3 HCl at room temperature and pressure.
MgCO3 + 2HCl → MgCl2 + H2O + CO2
Use the following steps.
-
- Calculate the number of moles of HCl in 25.0cm3 of 0.400mol/dm3 of acid.
mol
-
- Determine the number of moles of CO2 gas given off.
mol
-
- Calculate the volume of CO2 gas given off in cm3.
cm3 [3] [Total: 14]
Answer/Explanation
Ans:
2(a) MgCO3(s) + 2HCl(aq) → MgCl2(q) + H2O(l) + CO2(g)
2(b) eight crosses in second shell of Mg
7 dots and 1 cross in third shell of both Cl
2+ charge on Mg and – charge on both Cl ions on correct answer line
2(c) C atom double bonded to 2 O atoms
4 non-bonding e– on each O and no non-bonding e– on C and both octets complete
2(d)(i) gradient (of line) decreases
2(d)(ii) concentration of particles (of acid) decreases
lower rate of collisions of particles
2(d)(iii) a new line steeper than printed line and starts at origin and levels off earlier than printed line
levels off at the same volume
2(e) M1 mol HCl = 0.400 × 25.0 / 1000 = 0.01(00)
M2 mol CO2 = M1 / 2 = 0.0100 / 2 = 0.005(00)
M3 volume CO2 = M2 × 24000 = 120 (cm3)
Question
Potassium reacts with chlorine to form potassium chloride, KCl.
(a) Write a chemical equation for this reaction.
(b) Potassium chloride is an ionic compound.
Complete the diagram to show the electron arrangement in the outer shells of the ions present
in potassium chloride.
Give the charges on both ions.
(c) Molten potassium chloride undergoes electrolysis.
(i) State what is meant by the term electrolysis.
(ii) Name the products formed at the positive electrode (anode) and negative electrode
(cathode) when molten potassium chloride undergoes electrolysis.
anode ………………………………………………………………………………………………………………….
cathode ……………………………………………………………………………………………………………….
(d) Concentrated aqueous potassium chloride undergoes electrolysis.
(i) Write an ionic half-equation for the reaction at the negative electrode (cathode).
(ii) Name the product formed at the positive electrode (anode).
(iii) Name the potassium compound that remains in the solution after electrolysis.
(e) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of
chlorine, \(Cl_2\).
Show the outer electrons only.
(f) The melting points and boiling points of chlorine and potassium chloride are shown.
(i) Deduce the physical state of chlorine at –75°C. Use the data in the table to explain your
answer.
physical state ……………………………………………………………………………………………………….
explanation ………………………………………………………………………………………………………….
(ii) Explain, in terms of structure and bonding, why potassium chloride has a much higher melting point than chlorine.
Your answer should refer to the:
● types of particle held together by the forces of attraction
● types of forces of attraction between particles
● relative strength of the forces of attraction.
Answer/Explanation
Answer:
(a) \(2K + Cl_2 → 2KCl\)
\(Cl_2\) on left hand side (1)
equation fully correct (1)
(b) K outer shell with 8 crosses (1)
Cl outer shell with 7 dots and 1 cross (1)
\(^+\) and – (1)
(c) (i) breakdown by (the passage of) electricity (1)
of an ionic compound in molten or aqueous (state) (1)
(ii) (anode) chlorine
(cathode)potassium
(d) (i) \(2H^+ + 2e(^–) → H_2\)
\(H^+\) and \(e(^–)\) on left hand side (1)
equation fully correct (1)
(ii) chlorine
(iii) potassium hydroxide (1)
(e) one shared pair of electrons and 6 non-bonding electrons on each chlorine atom
(f) (i) liquid (1)
BOTH melting point is below –75 oC AND boiling point is above –75 oC
OR
BOTH –75 oC is higher than –101 oC / melting point AND lower than –35 oC / boiling point
OR
–75 oC is between melting point or –101 oC and boiling point or –35 oC
(ii) ionic bonds in KCl (1)
attraction between molecules in \(Cl_2\) (1)
weaker attraction (between particles) in \(Cl_2\) ORA (1)
Question
Air is a mixture of gases.
(a) State the percentage of clean dry air which is oxygen. Give your answer to the nearest whole number.
$\%[1]$
(b) Oxygen and nitrogen are useful gases that can be obtained from air.
(i) Name the process used to separate oxygen and nitrogen from liquid air.[5]
(ii) State the property of oxygen and nitrogen that allows these gases to be separated using this process.[1]
(c) Carbon dioxide, $\mathrm{CO}_2$, is a covalent molecule. Complete the diagram to show the electron arrangement in one molecule of $\mathrm{CO}_2$.
Show only the outer electrons.
(d) The graph shows the concentration of carbon dioxide in the atmosphere over a 60‐year period, measured in parts per million (ppm).
The data shown in the graph is of global concern.
Explain why…………………………………………………………………………………………………………………………….. [3]
(e) Name the process in the carbon cycle by which plants remove carbon dioxide from the
atmosphere…………………………………………………………………………………………………………………………….. [1] [Total: 10]
▶️Answer/Explanation
Ans:
4(a) 21
4(b)(i) fractional (1)
distillation (1)
4(b)(ii) (different) boiling point 1
4(c) 2 double bonds (1)
whole molecule correct (2 pairs of lone pairs on each O) (1)
4(d) increase in (concentrations of) carbon dioxide
(carbon dioxide is) greenhouse gas/greenhouse effect
contributes to climate change/global warming
4(e) photosynthesis