Doc Brown's GCSE/IGCSE Science-Chemistry Revision Questions

ANSWERS to the Periodic Table worksheet of practice examination questions

You will need a copy of the Periodic Table for these questions

1a Na or Cl symbols of sodium or chlorine atoms which are the smallest particles of matter

1b 2 or more atoms bonded to form a larger particle e.g. HCl, the hydrogen chloride molecule

1c one type of atom only and all have the same atomic number of protons e.g. sodium 11, chlorine 17

1d 2 or more elements chemically bonded together

1e + ==> , electronically 2.8.1 + 2.8.7 ==> 2.8 + 2.8.8, ionic bonding, electron transfer to give stable ions with noble gas electronic structures (typical of a metal + non-metal ==> ionic compound)

1f + ==> , covalent bonding, sharing electrons to give noble gas electron structure (typical of two non-metals combining to give a covalently bonded molecule)

2a period 3, group 6, non-metal

2b period 3, group 2, metal

2c period 4, transition series metal. Metals: high melting/boiling points, high density, good conductors of heat/electricity, strong and malleable/ductile. Non-metals: low melting/boiling points, low density, poor conductors of heat/electricity, weak structures and NOT malleable/ductile - sulphur is a brittle low melting solid

3a group 8(0), noble gas, physical properties: colourless gas (very low mpt/bpt), poor conductor of heat/electricity when liquefied or solidified; chemical properties: very unreactive! Difficult to form stable compounds!

3b helium has a low density like hydrogen, but being unreactive, He will not be explosive/flammable like hydrogen

3c Ar, being unreactive, will not oxidise/burn the metal filament, oxygen in air would!

3d full outer electron shells give very stable electron arrangements eg 2, 2.8 and 2.8.8 etc.

4a At the positive (+) anode attracts chloride ions, these are oxidised by electron loss to form green chlorine gas (bleaches litmus)

2Cl^-_(aq) ==> Cl_2(g) + 2e-

At the negative (-) cathode, hydrogen ions from water (leaving hydroxide ions), are reduced by electron gain to form hydrogen gas (pops with lit splint)

2H⁺_(aq) + 2e- ==> H_2(g)

4b hydroxide ions and sodium ions are left in solution to form the alkali sodium hydroxide NaOH (excess OH^- ions makes the pH over 7)

4c neutralisation: NaOH_(aq) + HCl_(aq) ==> NaCl_(aq) + H₂O_(l)

ionically the equation is written as: OH^- _(aq) + H⁺ _(aq) ==> H₂O_(l)

5a 37p, 37e, 48n

5b low melting point, high boiling point, soft silvery solid, good conductor of heat/electricity, low density for a metal

5c melting points/boiling points generally decrease down group 1, so they should be lower than potassium (K) if the trend is continuous (which is NOT always the case)

5d Rb like any other group 1 alkali metal readily loses its one outer electron to form a single charged positive ion Rb⁺

5e RbCl, Rb₂O, both ionic compounds formed from Rb⁺, Cl^- and O^2- ions (note +/- charge must balance to give correct formula) and they follow the general formula pattern in Gp1 element + Gp6/7 element ==> ionic compound (metal + non-metal ==> ionic compound)

5f Remember reactivity increases down the group! - so its a very rapid/explosive reaction, indicator turns purple (alkali formed), metal melts to a silvery ball whizzing over the surface, colourless gas (hydrogen, ignited by this very exothermic reaction)

equations follow group pattern: rubidium + water ==> rubidium hydroxide + hydrogen

and in symbols: 2Rb_(s) + 2 H₂O_(l) ==> 2RbOH_(aq) + H_2(g)

5g Reactivity increases down the group and so the reaction is even more rapid than for potassium. The reactivity increases down the group because …

• Atom size increases down the group with extra electron shells added
• The outer electron is then further from the nucleus and less strongly held the lower the element in the group
• This means further down the group the atom progressively loses the outer electron more easily to form the M⁺ ion on reaction

6a (fluorine - pale yellow gas); chlorine - green gas; bromine - dark red liquid (readily vaporises to an orange/brown vapour); iodine - dark grey solid (purple vapour on heating); Astatine - black solid (very dark vapour on heating). Note they get darker down the group and all exist in any state as X₂ diatomic molecules

6b killing bacteria in water; manufacture of hydrochloric acid (HCl); combined with sodium hydroxide to make sodium chlorate bleaches; making poly(chloroethene), the plastic 'PVC'

6c Reactivity decreases down the group. The reactivity decreases down the group because …

• Halogens have 7 outer electrons (group 7!)
• When halogen atoms react, they need to attract an 8^th outer electron to form a stable covalent bond or ion eg Cl (2.8.7) + e^- ==> Cl^- (2.8.8)
• Halogen atom size increases down the group with extra electron shells added
• The 8^th outer electron needed for the halogen to bond is then further from the nucleus and less strongly attracted on reaction
• This means further down the group the atom progressively attracts the outer electron less easily to form a bond on reaction

6d(i) dark colour of iodine (orange ==> brown ==> dark crystals, depending on how much is formed)

6d(ii) Cl_2(aq) + 2KI_(aq) ==> 2KCl_(aq) + I_2(aq/s)

6d(iii) Cl_2(aq) + 2I^- _(aq) ==> 2Cl^- _(aq) + I_2(aq/s)

• the chlorine is the oxidising agent - it takes the electrons from the iodide ions and gets reduced
• the neutral chlorine molecules are reduced to chloride ions by gaining electrons
• the iodide ion is the reducing agent - it gives electrons to the chlorine
• the iodide ion is oxidised to neutral iodine by losing electrons

6e very dark, almost black solid, At₂ diatomic molecules

6f its melting point will be higher than iodine, the group trend is to increase in melting point and boiling point down the group

6g 7 outer e-, At^- gains 1 electron to form the stable ion with a full shell of 8 outer electrons.

6h it will be less reactive than iodine, the group trend is to be less reactive the further down the group

6i HAt following the pattern in the group (HF, HCl, HBr, HI) and is a covalent molecule (see Q1f) and forms an acid in solution turning universal indicator red (pH 1, like HCl etc.)

6j NaAt , following the pattern of a group 1 + group 7 combination e.g. NaCl, KBr, LiF, RbI etc. The bonding is ionic and for more details see Q1e.

6k(i) the pale green chlorine solution, when added to the colourless sodium astatide solution will produce a dark colour or almost black precipitate.

• The chlorine is more reactive than astatine and will displace it from its salts - group trend in reactivity
• chlorine + sodium astatide ==> sodium chloride + astatine
• Cl_2(aq) + 2NaAt_(aq) ==> 2NaCl_(aq) + At_2(aq/s)

6k(ii) no reaction, astatine is not reactive enough to displace the more reactive chlorine - group trend in reactivity

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