Question
The Periodic Table can be used to classify elements.
(a) Group I elements react with cold water to form alkaline solutions.
(i) Place the Group I elements caesium, lithium, potassium, rubidium and sodium in their order of reactivity with water.
Put the most reactive element first. [1]
(ii) Name the alkaline solution formed when caesium reacts with cold water.[1]
(b) Group I elements have lower melting points than transition elements.
Describe one other difference in the physical properties of Group I elements and transition elements.[1]
(c) Group VII elements are known as the halogens.
Astatine is below iodine in Group VII.
Predict the physical state of astatine at room temperature and pressure.[1]
(d) Some Group VII elements react with aqueous solutions containing halide ions.
When aqueous chlorine is added to aqueous potassium bromide a reaction occurs.
The ionic half-equations for the reaction are shown.
Cl2(aq) + 2e– → 2Cl– (aq)
2Br– (aq) → Br2(aq) + 2e–
(i) Describe the colour change of the solution. [2]
original colour of potassium bromide solution
final colour of reaction mixture
(ii) Identify the species that is oxidised.
Explain your decision.[2]
species oxidised
explanation
(e) Bromine monochloride, BrCl, is made by the reaction between bromine and chlorine. The chemical equation is shown.
Br2(g) + Cl2(g) → 2BrCl(g)
Calculate the overall energy change for the reaction using bond energies.
Use the following steps.
-
- Calculate the total amount of energy required to break the bonds in 1 mole of Br2(g) and 1 mole of Cl2(g).
kJ
-
- Calculate the total amount of energy released when the bonds in 2moles of BrCl(g) are formed.
kJ
-
- Calculate the overall energy change for the reaction.
kJ/mol
[3] [Total: 11]
Answer/Explanation
Ans:
7(a)(i) from left to right
caesium → rubidium → potassium →sodium → lithium
7(a)(ii) caesium hydroxide
7(b) Group I element is less strong / not strong
ORA
OR
Group I element has low(er) density ORA
OR
Group I element is soft(er) ORA
7(c) solid
7(d)(i) colourless (1)
orange / brown / yellow (1)
7(d)(ii) Br– (1)
loses electron(s) (1)
7(e) 432(1)
436(1)
– 4(1)
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