Group 2 & 7

Question 1

Explain the trend in ionisation energy down group 2.

Answer 1

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.

Question 2

Describe how to carry out a flame test.

Answer 2

Clean a platinum or nichrome wire by dipping it into concentrated HCl.
Heat it over a Bunsen flame (repeat until no colour).

Question 3

How does the flame colour originate?

Answer 3

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.

Question 4

State the flame colour of:

a) lithium
b) sodium
c) potassium
d) calcium
e) barium
f) strontium
g) magnesium

Answer 4

a) red
b) yellow
c) lilac
d) brick red
e) green
f) crimson red
g) no colour

Question 5

State the trend in solubility of:

a) hydroxides
b) sulfates

Answer 5

a) increases down the group - barium hydroxide is soluble

b) decreases down the group - barium sulphate is insoluble

Question 6

How would you test for sulfate ions?

Answer 6

Add dilute HCl to prevent the formation of carbonate.
Then add BaCl2.
A white precipitate forms.

Question 7

Define polarisation

Answer 7

When a small cation distorts the electron cloud of a large anion.

Question 8

Explain the trend in thermal stability down the group

Answer 8

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

Question 9

Write the equation for the thermal decomposition of:

a) calcium carbonate
b) sodium nitrate
c) lithium carbonate
d) magnesium nitrate
e) lithium nitrate

Answer 9

a) CaCO3 -> CaO + CO2
b) 2NaNO3 -> 2NaNO2 + O2
c) Li2CO3 -> Li2O + CO2
d) 2Mg(NO3)2 -> 2MgO + 4NO2 + O2
e) 4LiNO3 -> 2Li2O + 4NO2 + O2

Question 10

Describe how to obtain dry crystals (e.g. NH4NO3)

Answer 10

1. Evaporate some water off
2. Leave the crystals to cool
3. Filter off and dry with filter paper

Question 11

What is the difference between precision, accuracy and reliability?

Answer 11

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)

Question 12

Give an advantage of using a measuring cylinder and a pipette

Answer 12

Measuring cylinder: quicker

Pipette: accurate

Question 13

Define ionisation energy

Answer 13

The energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state.

Question 14

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

Answer 14

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)

Question 15

State some physical properties of group 7 elements

Answer 15

1. Non-polar - more soluble in organic solvent than water
2. Strong oxidising agents - decreases down the group
3. Diatomic molecules
4. All form acid hydrides (i.e. HF, HCl)
5. Low melting & boiling points - increases down group (except HF)

Question 16

Give a chemical test for:

a) Chlorine
b) Bromine
c) Iodine

Answer 16

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

Question 17

State what would be seen when sulphuric acid reacts with:

a) Fluoride or Chloride
b) Bromide
c) Iodide

Answer 17

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-

Question 18

What can be used to identify the HCl fumes produced from halides reacting with sulphuric acid?

Answer 18

React with ammonia and white solid (smoke) is produced.

HCl + NH3 -> NH4Cl

Question 19

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?

Answer 19

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

Question 20

What is a standard solution?

Answer 20

One whose concentration is known an does not change with time.

Question 21

Why do magnesium compounds show no flame colour?

Answer 21

They release a photon of light (energy) outside the visible region of the spectrum.

Question 22

How do halogens react with:

a) Hot alkali (NaOH)
b) cold alkali

Answer 22

a) hot alkali:
3X2 + 6NaOH -> NaXO3(aq) +5NaX(aq) + 3H2O
b) cold alkali:
X2 + 2NaOH -> NaXO(aq) + NaX(aq) + H2O

Question 23

What is the order of acidity of the hydrogen halides and why?

Answer 23

HF is a weak acid: entropy decreases when HF reacts with water. Hydrogen bonds between HF molecules.

Question 24

What is thermal decomposition?

Answer 24

Using heat to break down a reactant into more than one product.

Question 25

Describe what would be seen if a test tube of hydrogen iodide was inverted in a beaker of water.

Answer 25

The test tube would fill with water.

Question 26

How does chlorine react with water?

Answer 26

Cl2 + H2O > HCl + HOCl