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
The Periodic Table is a method of classifying elements.
(a) Identify the element which is in Group VI and Period 4.
(b) Calcium is in Group II and chlorine is in Group VII of the Periodic Table.
Explain, in terms of number of outer shell electrons and electron transfer, how calcium atoms
and chlorine atoms form ions. Give the formulae of the ions formed.
(c) Group V chlorides are covalent molecules. The boiling points of some Group V chlorides are
shown.
(i) Suggest the approximate boiling point of \(PCl_3\).
(ii) Explain the trend in boiling points in terms of attractive forces between particles.
(iii) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of
\(PCl_3\).
Show outer electrons only.
(d) PCl 3 reacts with chlorine, Cl2, to form PCl 5. This reaction is exothermic and reaches an
equilibrium.
\(PCl_3(g) + Cl_2(g) \leftrightarrow PCl_5(g)\)
(i) Describe two features of an equilibrium.
(ii) State the effect, if any, on the position of this equilibrium when the following changes are
made.
Explain your answers.
temperature is increased ……………………………………………………………………………………….
pressure is increased ……………………………………………………………………………………………
(iii) Explain, in terms of particles, what happens to the rate of the forward reaction when the
reaction mixture is heated.
(e) \(PCl_5\) reacts with lithium fluoride, LiF, to form \(LiPF_6\).
\(PCl_5 + 6LiF → LiPF_6 + 5LiCl\)
Calculate the mass of LiF needed to form 3.04g of \(LiPF_6\) using the following steps.
● Calculate the number of moles of LiPF6 formed.
[Mr : \(LiPF_6\), 152]
number of moles = …………………………
● Deduce the number of moles of LiF needed.
number of moles = …………………………
● Calculate the mass of LiF needed.
mass = ………………………… g
(f) Lithium fluoride has ionic bonding.
(i) What is an ionic bond?
(ii) Give two physical properties of ionic compounds.
Answer/Explanation
Answer:
(a) selenium / Se
(b) Ca has 2 and Cl has outer electrons 7 (1)
Ca (atoms) lose electrons (1)
Cl (atoms) gain electrons (1)
\(Ca^{2+}\) (ions) (1)
\(Cl^–\) (ions) (1)
(c) (i) any number in the range 72 – 129°C
(ii) 3 P- Cl dot cross bonds (1)
2 (only) non-bonding electrons to make an octet on P (1)
6 (only) non-bonding electrons to make an octet on each Cl (1)
(d) (i)
constant concentrations (1)
rate of forward reaction
= rate of reverse reaction (1)
(ii) increased temperature:
(equilibrium) shifts to LHS (1)
(forward) reaction is exothermic (1)
increased pressure:
(equilibrium) shifts to RHS (1)
fewer moles (of gas) on RHS (1)
(iii) rate increases and particles have more energy (1)
more collisions (between particles) occur per second / per unit time
more (of the) particles / collisions have energy greater than activation energy
or
more (of the) particles / collisions have sufficient energy to react
or
a greater percentage / proportion / fraction of collisions (of particles) are successful
(e) mol of LiPF6 = 3.04 / 152 = 0.02(00) (1)
mol of LiF = 0.02(00) × 6 = 0.12(0) (1)
mass of LiF = 3.12 g (1)
(f) (i) oppositely charged ions
(ions) are attracted
(ii) any two from:
physical constants: high boiling point / melting point
conductivity: conduct (electricity) when aqueous or conduct (electricity) when molten
solubility: soluble in water
Question
Fluorine is a Group VII element. Fluorine forms compounds with metals and non-metals.
(a) Predict the physical state of fluorine at room temperature and pressure.[1]
(b) Fluorine exists as diatomic molecules.
Complete the dot-and-cross diagram to show the electron arrangement in a molecule of fluorine. Show outer shell electrons only.[2]
(c) Write a chemical equation for the reaction between sodium and fluorine.[2]
(d) Explain why chlorine does not react with aqueous sodium fluoride.[1]
(e) Tetrafluoromethane and lead(II) fluoride are fluorides of Group IV elements. Some properties of tetrafluoromethane and lead(II) fluoride are shown in the table.
(i) What is the formula of lead(II) fluoride?[1]
(ii) What type of bonding is present between the atoms in tetrafluoromethane?[1]
(iii) What type of structure does solid lead(II) fluoride have?[1]
(iv) Explain, in terms of attractive forces between particles, why lead(II) fluoride has a much higher melting point than tetrafluoromethane.
In your answer refer to the types of attractive forces between particles and their relative strengths.[3]
(f) Tetrafluoroethene is an unsaturated compound with the formula C2F4.
Tetrafluoroethene is the monomer used to make the polymer poly(tetrafluoroethene).
(i) What is meant by the term unsaturated?[1]
(ii) Describe a test to show that tetrafluoroethene is unsaturated.[2]
test
observations
(iii) Draw the structure of a molecule of tetrafluoroethene. Show all of the atoms and all of the bonds.[1]
(iv) Tetrafluoroethene can be polymerized to form poly(tetrafluoroethene).
Draw one repeat unit of poly(tetrafluoroethene). Show all of the atoms and all of the bonds.[2]
(v) Deduce the empirical formula of:
tetrafluoroethene
poly(tetrafluoroethene)
[2] [Total: 20]
Answer/Explanation
Ans:
3(a) gas / gaseous
3(b) M1 1 shared pair of electrons(1)
M2 6 non-bonding electrons on each atom to complete an octet(1)
3(c) 2Na + F2 → 2NaF
M1 NaF anywhere(1) M2 equation fully correct(1)
3(d) chlorine less reactive than fluorine ORA
3(e)(i) PbF2
3(e)(ii) covalent
3(e)(iii) giant ionic lattice
3(e)(iv) M1(It or lead(II) fluoride) forces of attraction between ions / ionic bonds(1)
M2(tetrafluoromethane) forces of attraction between molecules(1)
M3 ionic bonds stronger than attractive forces between molecules / ionic bonds need more energy to break than attractive forces between molecules(1)
3(f)(i) not all the bonds are single bonds
3(f)(ii) M1 bromine / bromine water(1)
M2 turns colourless / decolourises(1)
3(f)(iii)
3(f)(iv)
M1 C-C(1)
M2 each C bonded to 2 F and no other atoms + extension bonds(1)
3(f)(v) M1 CF2(1)
M2 CF2(1)