Group 7 - The Halogens Flashcards
What is the colour and state of fluorine at room temperature
Pale yellow gas
What is the colour and state of bromine at room temperature
Brown liquid
What is the colour and state of chlorine at room temperature
Pale green gas
What is the colour and state of iodine at room temperature
Dark grey solid
What is the colour and state of astatine at room temperature
Black solid
What does iodine sublime to give
A purple vapour
Describe and explain the trend in boiling points as you go down group 7
The boiling points increase because the number of electrons increases as you go down the group which increases the uneven distribution of electrons. The van der vaals forces become stronger as you go down the group, so more energy is needed to break the stronger intermolecular forces.
Why does bromine have a higher boiling point than chlorine
Bromine has more electrons than chlorine, so the Van der Vaals forces are stronger in bromine. More energy is needed to break the stronger intermolecular forces in bromine.
What is the trend in solubility of the halogens as you go down the group
Solubility in water decreases down the group.
The complex ion I3- is more soluble in water than iodine. Explain this.
Iodine molecules are non-polar and thus have no charge, whereas the I3- ion is negatively charged and will therefore be attracted to the delta positive H atom in a water molecule.
Describe and explain the trend in electronegativity as you go down group 7
As you go down the group, electronegativity decreases, because as you go down the group the atom gets larger since the atomic radii increases due to increased number of shells. The nucleus is therefore less able to attract the bonding pair of electrons.
Describe and explain the trend in oxidising ability as you go down group 7
The oxidising ability decreases as you go down the group because the atom gets bigger as it has more shells. There is now a weaker force of attraction between the positive nucleus and the incoming electron in the bigger atom, so the ability to gain an electron is more difficult as you go down the group.
Why is chlorine a stronger oxidising agent than bromine
The chlorine atom is smaller than a bromine atom as it has fewer electrons shells. There is a stronger force of attraction between the positive nucleus and the incoming electron in chlorine than in bromine, so the ability to gain an electron is easier in chlorine.
Oxidising agents
Electron acceptors
What will a halogen which is a stronger oxidising agent do when it reacts with a compound containing a halogen with a weaker oxidising power
It will displace that weaker halogen
Cl2(aq) + 2KBr(aq) →
Br2(aq) + 2KCl(aq)
the solution turns orange because bromine is displaced.
If a displacement reaction does not take place in dilute solution at room temperature, give 2 suggestions as to how the conditions could be changed so the reaction may occur
Increase the concentration of the solutions
Warm the solutions
Suggest why fluorine can’t form FO2- but chlorine can form ClO2-
Fluorine has no 2d orbitals, so cannot expand its octet. Therefore fluorine can only form one covalent bond. A 3p electron in a chlorine atom is promoted into an empty 3d orbital, so chlorine can form 3 covalent bonds.
Explain how chlorine is used and why
Chlorine is used in water treatment because it kills bacteria
Reaction of chlorine with water
Cl2(aq) + H2O(l) ⇌ HCl(aq) + HClO(aq)
Why is the reaction of chlorine with water a disproportionation reaction
Chlorine is both oxidised and reduced in the same reaction. Chlorine is oxidised from 0 in Cl2 to +1 in ClO- and reduced to -1 in Cl- in the same reaction.
What is NaClO solution used as
Household bleach
Explain why the toxicity of chlorine does not prevent this use
Chlorine is used in very small amounts. The health benefits outweigh the risk.
Reaction of chlorine with water in sunlight
2Cl2 (g) + 2H2O (l) → 4HCl (aq) + O2 (g)
or
2Cl2 (g) + 2H2O (l) → 4H+ + 4Cl- + O2
Reaction between chlorine and cold, dilute NaOH solution
2NaOH(aq) + Cl2(aq) → NaCl(aq) + NaClO(aq) + H2O(l)
What is the use of the product(s) formed in the reaction of chlorine with cold, dilute NaOH solution
The mixture of NaCl and NaClO is used as bleach and to disinfect/kill bacteria
Describe the test to identify and distinguish between halide ions
Add a few drops of dilute nitric acid.
Then add aqueous silver nitrate to the unknown solutions.
Note the colour of the precipitate formed.
Then add dilute or concentrated aqueous ammonia to the precipitate and note the solubulity of the precipitate.
Describe what would be seen when testing chloride ions with acidified silver nitrate solution
A white precipitate forms
Precipitate dissolves in dilute aq ammonia
Describe what would be seen when testing bromide ions with acidified silver nitrate solution
Cream precipitate forms
Precipitate is partially soluble in dilute aq ammonia
Precipitate dissolves in conc aq ammonia
Describe what would be seen when testing iodide ions with acidified silver nitrate solution
Yellow precipitate forms
Precipitate is insoluble in both dilute and conc aq ammonia
Why must the silver nitrate solution used to identify halides be acidified with dilute nitric acid
Nitric acid is added to react with and remove ions such as carbonate and hydroxide ions that would interfere with the test because they react with Ag+ ions to give a precipitate.
How would you distinguish between solutions of sodium bromide and sodium iodide
Add a few drops of dilute nitric acid and then aqueous silver nitrate to each solution.
Sodium bromide gives a cream precipitate.
Sodium iodide gives a yellow precipitate.
Add conc aq ammonia to each precipitate.
AgBr dissolves but AgI is insoluble
Why is ammonia solution added when distinguishing between halide ions
To help differentiate between halides if the colours look similar as the halides have different solubilities in ammonia solution.
AgCl(s) + 2NH3(aq) → Ag(NH3)2+ (aq) + Cl−(aq)
Why is silver nitrate used to distinguish between halide ions
Silver nitrate reacts with halide ions to form silver halides which have different colours and solubilities in ammonia.
Reaction of sodium chloride with conc sulphuric acid
NaCl(s) + H2SO4 → NaHSO4(s) + HCl(g)
Steamy white fumes of hydrogen chloride are observed.
what is the role of chloride ions in the reaction between sodium chloride and conc sluphuric acid
Base or proton acceptor
Describe the test for hydrogen chloride
Dip a glass rod into concentrated aq ammonia and hold it just above the mouth of the test tube. White smoke is formed at the mouth of the test tube.
Reaction of sodium bromide with conc sulphuric acid
3H2SO4 + 2NaBr → 2Na2SO4 + SO2 + Br2 + 2H2O
White steam fumes of HBr observed followed by a brown/orange gas (ALWAYS SAY BROWN FUMES FOR OBSERVATION)
Role of sulfuric acid in the reaction between sodium bromide and conc sulphuric acid
Oxidising agent
Reaction of sodium iodide with conc sulphuric acid
NaI(s) + H2SO4 → NaHSO4(s) + HI(g)
Observations in the reaction of sodium iodide with conc sulphuric acid
White steamy fumes of HI
Black solid of sulfur
purple fumes of iodine seen
H2S with rotten egg smell
Describe and explain the trend in reducing power as you go down group 7
As you go down group 7, reducing power of the halides increases. This is because they have a greater tendency to donate electrons. This is because as the ions get bigger it is easier for the outer electrons to be given away as they are less strongly attracted to the nucleus.
Explain why Cl- ions are weaker reducing agents than Br- ions
Chloride ions are smaller than bromide ions so the outer electrons are closer to the nucleus in a chloride ion.
The electron lost from a chloride ion is more strongly held by the nucleus compares with that lost from a bromide ion.
Describe the test for carbonate ions
Add dilute HCl to the solid or solution.
Effervescence observed.
Bubble the gas through limewater. Limewater turns cloudy
Describe the test for sulfate ions
Add dilute HCl then add aq barium chloride to the unknown solution.
A white precipitate is formed.
Describe the test for NH4+, ammonium ions
Add aq sodium hydroxide to the solution and warm the mixture.
Ammonia is produced. Test the gas with a damp red litmus paper and it will turn blue
Describe the test for hydroxide ions
A small amount (around 1 cm3) of the solution should be added to a test tube using a pipette
Test the pH of the solution using red litmus paper or universal indicator paper
The presence of hydroxide ions will turn the red litmus paper blue and the pH will be clearly alkaline on the universal indicator paper if hydroxide ions are present
