3.2.3 Group 7 - the halogens

What are the formulas, colours, physical states & electron configuration of the first 4 halogens?

What is the trend in boiling point down group 7?

Increases:

• the strength of the Van der Waals forces increases as the size & relative mass of the molecules increases → this trend is shown in the changes of physical state from fluorine (gas) to iodine (solid)

What is the definition of electronegativity?

The ability for an atom to attract electrons towards itself in a covalent bond

What is the trend in electronegativity down group 7?

Decreases:

• atoms gets larger, so the distance between the positive nucleus & the outer electrons increases, due to increased electron shielding

• larger atoms attract electrons less than smaller ones as their outer electrons are further from the nucleus & are more shielded, as they have more inner electrons

What will a more reactive halogen do in a displacement reaction?

A more reactive halogen will displace a less reactive halide ion from its compound in solution

Are halogens more or less oxidising down group 7 & why?

Less oxidising:

• when halogens react, they gain an electron (oxidising agents)

• they get less reactive down the group, as the atoms become larger & the outer shells gets further from the nucleus

Complete this table of the displacement reactions

How can displacement reactions be used to help identify which halogen (or halide) is present in a solution?

Halide ions are colourless in solution, but when the halogen is displaced it shows a distinctive colour

• bromine: orange

• iodine: brown

What would happen if chlorine is added to a solution containing bromide ions (e.g. potassium bromide) ?

Chlorine will displace the bromide ions, resulting in a colour change from colourless to orange:

• the equation for this reaction is: Cl₂ (aq) + 2KBr (aq) → 2KCl (aq) + Br₂ (aq)

• the ionic equation for this reaction is: Cl₂ (aq) + 2Br^- (aq) → 2Cl^- (aq) + Br₂ (aq)

What would happen if chlorine is added to a solution of potassium iodide ions?

Chlorine will displace the iodide ions, resulting in a colour change from colourless to brown:

• the equation for this reaction is: Cl₂ (aq) + 2KI (aq) → 2KCl (aq) + I₂ (aq)

• the ionic equation for this reaction is: Cl₂ (aq) + 2I^- (aq) → 2Cl^- (aq) + I₂ (aq)

What would happen if bromine is added to a solution of potassium iodide?

Bromine will displace the iodide ions, resulting in a colour change from colourless to brown:

• the equation for the reaction is: Br₂ (aq) + 2KI (aq) → 2KBr (aq) + I₂ (aq)

• the ionic reaction for the equation is: Br₂ (aq) + 2I^- (aq) → 2Br^- (aq) + I₂ (aq)

Why is there no reaction if bromine water is added to a solution of chloride ions?

Chlorine is above bromine in group 7, so is more reactive & can’t be displaced

Why won’t iodine displace neither chlorine nor bromine?

Iodine is below chlorine & bromine in group 7, so it’s less reactive than them & won’t displace either halide

How is bleach made & what is the equation for the reaction?

Mix chlorine gas with cold, dilute NaOH solution at room temperature to form sodium chlorate(I) solution (NaClO):

• 2NaOH(aq) + Cl₂(g) → NaClO(aq) + NaCl(aq) + H₂O(l)

  • chlorine is both oxidised & reduced (0 to +1 & -1), so this is a disproportionation reaction

What are some uses of sodium chlorate(I) solution (bleach) ?

• Water treatment

• Bleaching paper & fabrics

• Cleaning agents

What is the equation for the reaction of chlorine with water?

Cl₂ + H₂O ⇌ HCl + HClO

How is water treated in the UK?

Chlorate(I) ions kill bacteria, so adding chlorine or a compound containing chlorate(I) ions is added to water to kill to make it safe to drink or swim in:

• chlorine kills disease-causing microorganisms & some chlorine persists in the water, preventing reinfection further down the supply

• it also prevents the growth of algae, removing bad tastes & smells, as well as removing discolouration caused by organic compounds

What are the risks from using chlorine to treat water?

Accidents involving chlorine can be really serious, even fatal:

• chlorine gas is very harmful if it’s breathed in → it irritates the respiratory system

• liquid chlorine on the skin or eyes causes severe chemical burns

• water contains a variety of organic compounds & chlorine reacts with these to form chlorinated hydrocarbons (e.g. chloromethane) → many of these chlorinated hydrocarbons are carcinogenic

Why do the benefits of chlorinating drinking water outweigh its toxic effects?

The risk of not chlorinating water could lead to a cholera epidemic, which could kill thousands of people

Why are halide ions reducing agents?

They lose an electron in reactions, becoming oxidised themselves & reduce something else

• how easy it is for a halide ion to lose an electron depends on the attraction between the nucleus & the outer electrons

What is the trend in the reducing ability down group 7?

Increases:

• as you go down group 7, the attraction gets weaker as the ions get bigger, so the electrons are further away from the positive nucleus

• there are extra inner electron shells too, so there’s a greater shielding effect

What are the equations to show how:

• all the halides react with concentrated H₂SO₄

• how only bromide & iodide react with concentrated H₂SO₄

• how only iodide reacts (as it’s a strong enough reducing agent)

Complete this table

What is the method to test for halide ions?

1. Add a small spatula of the halide salt to a test tube

2. Dissolve the salt in dilute nitric acid (HNO₃)

3. Add a few drops of acidified silver nitrate (AgNO₃) to make a silver halide

4. Record any colour/precipitate

5. Add some dilute ammonia & record any changes

6. Add a few drops of concentrated ammonia & record any changes

Why is nitric acid added when testing for halide ions?

Nitric acid reacts with any anions other than halides (e.g. carbonates) which could give a false positive result otherwise

What are the observations in the tests for halide ions & what are the equations?

Fluoride ions:

• soluble, so does not form a precipitate

Chloride ions:

• white precipitate forms (silver chloride)

• Ag⁺(aq) + Cl^-(aq) → AgCl(s)

Bromide ions:

• cream precipitate forms (silver bromide)

• Ag⁺(aq) + Br^-(aq) → AgBr(s)

Iodide ions:

• yellow precipitate forms (silver iodide)

• Ag⁺(aq) + I^-(aq) → AgI(s)

What are the results of the silver halide precipitates when ammonia solution is added?

• Chloride Cl^- → precipitate dissolves in both dilute & concentrated NH₃ (most soluble)

• Bromide Br^- → precipitate only dissolves in concentrated NH₃

• Iodide I^- → precipitate is insoluble in concentrated NH₃ (least soluble)

How are group 2 ions in a compound identified & what are the flame colours of Ca, Sr & Ba?

Flame test:

1. Dip a nichrome wire loop in concentrated HCl to clean it

2. Dip the wire loop into the unknown compound

3. Hold the loop in the clear blue part of a Bunsen burner flame & observe the colour change in the flame

Flame colours:

• Calcium, Ca²⁺ → brick red

• Strontium, Sr²⁺ → red

• Barium, Ba²⁺ → pale green

How can dilute NaOH solution be used to identify group 2 ions?

1. Add NaOH dropwise to a test tube containing the metal ion solution & observe the precipitate that forms (if there is one)

2. Keep adding the NaOH until it is in excess & record any changes that are observed

(all start off as colourless solutions)

What is the method to test for ammonium ions (NH₄⁺)?

1. Ammonium ions can be tested for either in a solid or in solution by adding NaOH solution & warming gently

2. If the compound contains ammonium ions, ammonia gas will be produced which can be tested with damp red litmus paper which turns blue

What is the method to test for hydroxide ions (OH^-) ?

Hydroxide ions make solutions alkaline:

1. use a pH indicator to test the unknown solution

2. if a piece of red litmus paper is dipped into the solution & hydroxide ions are present, the paper will turn blue

What is the method to test for carbonate ions (CO₃^2-) ?

1. Add dilute HCl acid to the unknown solution & if carbonate ions are present, the solution will fizz (effervescence)

2. The carbonate ions react with the hydrogen ions in the acid to make carbon dioxide

   • CO₃^2- (aq) + 2H⁺ (aq) → CO₂ (g) + H₂O (l)

3. Carbon dioxide can be tested using limewater → bubble the gas through a test tube of limewater & if the limewater goes cloudy, the solution contains carbonate ions

What is the method to test for sulfate ions (SO₄^2-) ?

1. Add a little dilute HCl to remove any carbonates which could precipitate out after adding barium chloride, giving a false positive result

2. Add barium chloride solution, BaCl₂ (aq)

3. If a white precipitate of barium sulfate forms, the original compound contained a sulfate: Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

How do you balance half-equations?

1. Write the species before & after a reaction

2. Balance any atoms apart from oxygen & hydrogen

3. Balance any oxygens with H₂O

4. Balance any hydrogens with H⁺ ions

5. Balance charges with electrons (e^-)

How do you combine half-equations to make a full ionic equation?

1. Balance the electrons by finding lowest common multiple

2. Times everything in both equations by that number

3. Cancel out the electrons & combine the two equations