Which pair of elements react together most violently?
A) chlorine and lithium
B) chlorine and potassium
C) iodine and lithium
D) iodine and potassium
▶️ Answer/Explanation
Ans: B
The most violent reaction would occur between:
1. The most reactive alkali metal (potassium is more reactive than lithium in Group I)
2. The most reactive halogen (chlorine is more reactive than iodine in Group VII)
Potassium reacts more vigorously than lithium with halogens, and chlorine is more reactive than iodine. Therefore, chlorine and potassium (option B) would react most violently.
Which row describes the state and colour of bromine at room temperature and pressure?
| state | colour | |
|---|---|---|
| A | liquid | red-brown |
| B | liquid | grey-black |
| C | solid | red-brown |
| D | solid | grey-black |
▶️ Answer/Explanation
Ans: A
Bromine is unique among the halogens because:
State: At room temperature and pressure, bromine is a liquid (the only non-metal that’s liquid at room temperature).
Color: It has a distinctive red-brown color in both its liquid and vapor states.
Therefore, the correct description is option A: liquid and red-brown.
Two properties of element R are listed.
- It is a dark solid at room temperature.
- It is a diatomic molecule.
Where on the Periodic Table is R placed?
A) Group I
B) Group VII
C) Group VIII
D) transition elements
▶️ Answer/Explanation
Ans: B
Let’s analyze the clues:
- Dark solid at room temperature:
- Group I elements are all shiny metals (not dark solids)
- Group VII (halogens) include iodine (dark solid) and bromine (dark liquid)
- Group VIII (noble gases) are all colorless gases
- Transition elements are typically metallic and shiny
- Diatomic molecule:
- Group VII elements exist as diatomic molecules (F2, Cl2, Br2, I2)
- Group I metals don’t form diatomic molecules in their elemental state
The best match is Group VII (halogens), where iodine fits both properties perfectly: it’s a dark gray solid and exists as I2 molecules.
Which statement about the displacement reactions of the halogens is correct?
A) Iodine displaces bromine from aqueous sodium bromide.
B) Bromine displaces chlorine from aqueous potassium chloride.
C) Iodine displaces chlorine from aqueous potassium chloride.
D) Chlorine displaces bromine from aqueous sodium bromide.
▶️ Answer/Explanation
Ans: D
In halogen displacement reactions, a more reactive halogen can displace a less reactive halogen from its compound. The reactivity of halogens decreases down the group: chlorine > bromine > iodine.
Option D is correct because chlorine (more reactive) can displace bromine (less reactive) from sodium bromide. The other options are incorrect because:
- Iodine cannot displace bromine (A is wrong)
- Bromine cannot displace chlorine (B is wrong)
- Iodine cannot displace chlorine (C is wrong)
The correct reaction for option D would be: Cl2 + 2NaBr → 2NaCl + Br2