Group 2 & 7 Flashcards
Explain the trend in ionisation energy down group 2.
Ionisation energy decreases down the group:
The atomic radius increases - outer electrons are further from the nucleus
There is increased shielding from inner shells
The increasing effective nuclear charge is experienced less strongly.
Describe how to carry out a flame test.
Clean a platinum or nichrome wire by dipping it into concentrated HCl.
Heat it over a Bunsen flame (repeat until no colour).
How does the flame colour originate?
Electrons are excited, gain heat energy and are promoted to higher energy levels. They are unstable.
The electrons fall back to their original energy level and emit a photon of light in the visible region of the spectrum.
State the flame colour of:
a) lithium
b) sodium
c) potassium
d) calcium
e) barium
f) strontium
g) magnesium
a) red
b) yellow
c) lilac
d) brick red
e) green
f) crimson red
g) no colour
State the trend in solubility of:
a) hydroxides
b) sulfates
a) increases down the group - barium hydroxide is soluble
b) decreases down the group - barium sulphate is insoluble
How would you test for sulfate ions?
Add dilute HCl to prevent the formation of carbonate.
Then add BaCl2.
A white precipitate forms.
Define polarisation
When a small cation distorts the electron cloud of a large anion.
Explain the trend in thermal stability down the group
Thermal stability increases down the group:
Cationic radius increases (get bigger)
Polarising power decreases
Less distortion of the electron cloud of the anion
More energy required to break the bond
Write the equation for the thermal decomposition of:
a) calcium carbonate
b) sodium nitrate
c) lithium carbonate
d) magnesium nitrate
e) lithium nitrate
a) CaCO3 -> CaO + CO2
b) 2NaNO3 -> 2NaNO2 + O2
c) Li2CO3 -> Li2O + CO2
d) 2Mg(NO3)2 -> 2MgO + 4NO2 + O2
e) 4LiNO3 -> 2Li2O + 4NO2 + O2
Describe how to obtain dry crystals (e.g. NH4NO3)
- Evaporate some water off
- Leave the crystals to cool
- Filter off and dry with filter paper
What is the difference between precision, accuracy and reliability?
Precision: giving the same measurement each time
Accuracy: delivering what it claims to deliver - test pipette by measuring distilled water into a weighed beaker and find the mass of the contents
Reliability: depends on errors & skill - increases by using smaller samples (allows mean to be calculated)
Give an advantage of using a measuring cylinder and a pipette
Measuring cylinder: quicker
Pipette: accurate
Define ionisation energy
The energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state.
State the colours of the following in (i) their natural state, (ii) hydrocarbon solvent and (iii) water:
a) Fluorine
b) Chlorine
c) Bromine
d) Iodine
a) Fluorine - yellow gas
b) Chlorine - (i) green gas, (ii) & (iii) pale yellow
c) Bromine - (i) red-brown liquid, (ii) & (iii) orange-brown
d) Iodine - (i) grey solid, (ii) purple (iii) brown (and in KI)
State some physical properties of group 7 elements
- Non-polar - more soluble in organic solvent than water
- Strong oxidising agents - decreases down the group
- Diatomic molecules
- All form acid hydrides (i.e. HF, HCl)
- Low melting & boiling points - increases down group (except HF)
Give a chemical test for:
a) Chlorine
b) Bromine
c) Iodine
a) Chlorine: bleaches damp litmus paper
b) Bromine: react with water and NaOH - orange-brown to colourless
c) Iodine: test with starch - blue-black to colourless
State what would be seen when sulphuric acid reacts with:
a) Fluoride or Chloride
b) Bromide
c) Iodide
a) Fluoride or Chloride: steamy fumes
b) Bromide: steamy fumes (HBr), then choking fumes (SO2 gas)
c) Iodide: purple vapour (I2), grey solid (I2), toxic bad egg smell (H2S), red I3-
What can be used to identify the HCl fumes produced from halides reacting with sulphuric acid?
React with ammonia and white solid (smoke) is produced.
HCl + NH3 -> NH4Cl
What can you use to test for halides?
Give the result of the test with each halide.
How can ammonia help to distinguish between the halides?
Silver nitrate solution (and nitric acid).
With chlorine: white precipitate - dissolves in ammonia
With bromine: cream precipitate - insoluble in dilute, dissolves in conc. ammonia
With iodine: yellow precipitate, insoluble in dilute & conc. ammonia
What is a standard solution?
One whose concentration is known an does not change with time.
Why do magnesium compounds show no flame colour?
They release a photon of light (energy) outside the visible region of the spectrum.
How do halogens react with:
a) Hot alkali (NaOH)
b) cold alkali
a) hot alkali:
3X2 + 6NaOH -> NaXO3(aq) +5NaX(aq) + 3H2O
b) cold alkali:
X2 + 2NaOH -> NaXO(aq) + NaX(aq) + H2O
What is the order of acidity of the hydrogen halides and why?
HF is a weak acid: entropy decreases when HF reacts with water. Hydrogen bonds between HF molecules.
What is thermal decomposition?
Using heat to break down a reactant into more than one product.
