3.2.3.1 Trends in properties

• Trends in reducing ability of the halide ions

  • Reducing ability increases down group 7 (greater tendency to donate electrons)
  • Ions get bigger -
  • Increased shielding so attraction for outermost electron and nucleus is weaker
• Explaining the trend (of group 7 elements) in boiling of elements in terms of structure and bonding

  • increases down the group
  • more electrons so larger Van der Waals forces between molecules
  • more energy put into breaking the forces
• Explaining the trend in electronegativity

  • decreases
  • increase number of shells
  • reduces attraction between nucleus to outer electrons
• Trend in oxidising ability of halogens down the group

  • Reduces
  • Atoms are larger from more shielding
  • Harder to gain an electron
• • Fluorine (F2 ): very pale yellow gas. It is highly reactive
  • Chlorine : (Cl2 ) greenish, reactive gas, poisonous in high concentrations
  • Bromine (Br2 ) : red liquid, that gives off dense brown/orange poisonous fumes
  • Iodine (I2 ) : shiny grey solid sublimes to purple gas.
• The use of acidified silver nitrate solution to identify and distinguish between halide ions.

  • Chlorides produce a white precipitate Ag+ (aq) + Cl - (aq)  AgCl(s)
  • Bromides produce a cream precipitate Ag+ (aq) + Br- (aq)  AgBr(s)
  • Iodides produce a pale yellow precipitate Ag+ (aq) + I- (aq)  AgI(s)
• The role of nitric acid is to react with any carbonates present to prevent formation of the precipitate Ag2CO3 .

  
  

  (copy off markscheme later)
• Three anhydrous sodium compounds are provided for student to identify - Na2Co3, NaF, NaCl

  Stage 1

  • 1a Add named acid to all 3
  • 1b Add water / make into a solution
  • 1c Add AgNO3

  Stage 2

  • 2a Na2CO3 will fizz with acid
  • 2b NaCl gives white ppt with AgNO3
  • 2c NaF shows no (visible) change / no ppt

  Stage 3

  • Correct equations of Silver nitrate + Compounds, with ss
• Which species is not produced by a redox reaction between solid sodium iodide and concentrated sulfuric acid?

  • Na2SO4 Sodium Sulfate
• Explain why bromide ions react differently from chloride ions

  • Br- ions are bigger than chloride ions
  • Therefore Br– ions more easily oxidised / lose an electron more easily (than Cl– ions)
• Describe method to produce pure sample of silver bromide from mixture of AgCl + AgBr

  Stage 1: formation of precipitates

  • Add silver nitrate

  • to form precipitates of AgCl and AgBr

  • AgNO3 + NaCl → AgCl + NaNO3

  • AgNO3 + NaBr → AgBr + NaNO3

  Stage 2: selective dissolving of AgCl

  • Add excess of dilute ammonia to the mixture of precipitates

  • the silver chloride precipitate dissolves

  • AgCl + 2NH3 → Ag(NH3)2 + + Cl−

  Stage 3: separation and purification of AgBr

  • Filter off the remaining silver bromide precipitate

  • Wash to remove soluble compounds • Dry to remove water
• Suggest one observation when solid sodium chloride reacts with concentrated sulfuric acid

  • White fumes / gas / misty / steamy
  • NaCL + H2SO4 -> NaHSO4 + HCl
• Observation when aqueous chlorine is added to sodium bromide solution

  • Yellow or Orange solution
• Best oxidising agent is

  • towards top of Group
  • in its elemental form (NOT halide ion) because it has 7 outer electrons
• The silver halide precipitates can be treated with ammonia solution to help differentiate between them if the colours look similar

  • Cl - White precipitate dissolves in DILUTE NH3
  • Br - Cream precipitate dissolves in CONCENTRATED NH3
  • I- Yellow precipitate is insoluble in CONCENTRATED NH3
• Sodium bromide reacts with concentrated sulfuric acid in a different way from sodium chloride.

  Write an equation for this reaction of sodium bromide and explain why bromide ions react differently from chloride ions.

  • 2NaBr + 2H2SO4 -> Na2SO4 + Br2 + SO2 + 2H2O
  • Br– ions are bigger than Cl– ions
  • Therefore Br– ions more easily oxidised / lose an electron more easily (than Cl– ions)
• State why the silver nitrate solution is acidified when testing for iodide ions.

  • react with / remove (an)ions that would interfere with the test
  • prevent the formation of other SILVER PRECIPITATES / INSOLUBLE SILVER COMPOUNDS that would interfere with the test
  • remove (other) ions that react with the silver nitrate
  • react with / remove carbonate / hydroxide / sulfite (ions)
• Explain why dilute hydrochloric acid is not used to acidify the silver nitrate solution in this test for iodide ions.

  • HCl would FORM A (white) PRECIPITATE (AgCl) / (white) solid with silver nitrate and this would interfere with the test
• A solution of magnesium sulfate MgSO4 is sometimes given as first aid to someone who has swallowed barium chloride BaCl2. Explain why drinking magnesium sulfate solution is effective in the treatment of barium poisoning.

  • Insoluble barium sulfate is formed
  • ‘removes barium ions as a precipitate’
• Aldehydes can be prepared from acyl chlorides. State how an aldehyde could be tested to show whether it is contaminated with traces of unreacted ACYL CHLORIDE.

  State what you would observe.

  • silver nitrate (solution) (M1)
  • white precipitate (M2)

  Not hydrochloric acid CE = 0/2
• Concentrated sulfuric acid reacts with solid potassium iodide as shown in the equation.

  8KI + 9H2SO4 4l2 + 8KHSO4 + H2S + 4H2O

  Give two observations seen.

  • purple VAPOUR / GAS
  • white solid goes to black or black / grey or black / PURPLE SOLID
  • bad egg smell (from hydrogen sulfide)
• Chlorine is a powerful oxidising agent.

  (a) Write the simplest ionic equation for the reaction between chlorine and aqueous potassium bromide. State what is observed when this reaction occurs.

  • M1 Cl2 + 2Br− → 2Cl− + Br2
  • M2 solution goes ORANGE / YELLOW ( from colourless)
  • PENALISE fumes, precipitates
  • Ignore brown colour
• Add chlorine water to aqueous potassium iodide solution. The colourless solution turned a brown colour.

  1. Iodine is responsible for this brown colour
  2. Cl2 + 2I – -> 2Cl – + I2
  3. Type of reaction = redox
• Test = Add concentrated sulfuric acid to solid potassium chloride.

  Observation = The white solid produced misty white fumes which turned blue litmus paper to red.

  • H2SO4 + 2Cl- -> 2HCl + SO4 2–
  • Species that is responsible for misty white fumes = hydrogen chloride
• • Role of sulfuric acid = oxidising agent
  • Yellow solid = Sulfur
  • First half equation = 8I- -> 4I2
  • Second half equation = H2SO4 -> H2S
  • Balance with water, hydrogen ions and electrons

  
• When concentrated sulfuric acid is added to potassium iodide, solid sulfur and a black solid are formed.

  (i) Identify the black solid.

  • IODINE
• Suggest why iodide ions are stronger reducing agents than chloride ions.

  • M1 Relative size of ions (MUST say iodide ions / Chloride ions)
  • Iodide ions / they are LARGER /have more ELECTRON LEVELS (shells) (than chloride ions) / larger ATOMIC / IONIC RADIUS
  • M2 Strength of attraction for electron(s) Must be comparative
  • The electrons lost /OUTER SHELL/level ELECTRON from (an) IODIDE ION(s) LESS STRONGLY HELD BY THE NUCLEUS compared with that lost from a chloride ion
• Explain why iodine has a higher melting point than fluorine. [2 marks]

  • Iodine has more electrons / iodine is bigger (atom or molecule) / iodine has bigger Mr / bigger surface area
  • Stronger / MORE van der Waals forces / vdw / London / temporarily induced dipole / dispersion forces BETWEEN MOLECULES
• Give the FORMULA of a Group 2 metal hydroxide used in agriculture

  • Mg(OH)2
  • Ca(OH)2
  • IGNORE THE NAME
• Identify a sodium halide that does not undergo a redox reaction when added as a solid to concentrated sulfuric acid.

  • NaF sodium flouride
  • NaCl sodium chloride
• Give the formula of the substance responsible for the orange colour when chlorine gas is bubbled through an aqueous solution of sodium bromide.

  • BR2 ONLY
• Solid sodium iodide undergoes a redox reaction with concentrated sulfuric acid. Give the formula for each of the following in this reaction.

  • Formula of the solid reduction product = S or S8 or S2
  • Formula of the oxidation product = I2 (ONLY)
• Write an equation for the reaction of solid potassium chloride with concentrated sulfuric acid.

  • 2KCl + H2SO4 -> K2SO4 + 2HCl
  • OR KCl + H2SO4 -> KHSO4 + HCl
• Explain why chloride ions are weaker reducing agents than bromide ions.

  • M1 Relative size of ions
  • Chloride IONS are SMALLER than bromide ions
  • M2 M2 Strength of attraction for electron being lost
  • Outer shell/level electron(s) OR electron(s) lost from a chloride ion is more strongly held by the nucleus compared with that lost from a bromide ion
• Lime is used because it is relatively inexpensive and available in large quantities. Identify one other large-scale use of lime.

  • In agriculture / to raise the pH of soil
  • (Lime-based) mortars in construction
• This sodium halide is a white solid that reacts with concentrated sulfuric acid to give a BROWN gas.

  • Formula of sodium halide = NaBr
• A colourless aqueous solution of this sodium halide reacts with orange bromine water to give a dark brown solution.

  • NaI
• How can the addition of an aqueous solution of chlorine be used to distinguish between aqueous solutions of sodium bromide and sodium iodide? State any observations you would make and write equations for the reactions occurring.

  • Cl2 (aq) to Br– (aq) = yellow-orange or yellow-red or yellow-brown solution
  • Cl2 (aq) to I– (aq) = brown/black solution formed or black/brown/grey ppt/solid
  • 2Br– + Cl2 → 2Cl– + Br2
  • 2I– + Cl2 → 2Cl– + I2
• How can reactions with concentrated sulphuric acid be used to distinguish between solid samples of sodium bromide and sodium iodide?

  • Bromide: Brown/ORANGE FUMES
  • Bromine produced
  • Sulfur dioxide produced
  • Iodide: Purple fumes
  • black/brown/grey solid
  • smell of bad eggs
  • Iodine produced
  • SO2 , S, H2S produced
• Describe how observations of the reaction between solid potassium iodide and concentrated sulphuric acid can be used to indicate the presence of any two of these reduction products.

  • Sulfur dioxide is a choking gas or has a pungent odour (1)
  • Sulphur is a yellow solid (1)
  • Hydrogen sulfide has a smell of bad eggs (1)