考前押题！AS化学考前最后20题！（附答案解析）

1. Explain the observed trend in first ionisation energies down Group[3]

Down the group first ionisation energies decrease.

More shielding, and distance between nucleus and outer electron increase,

weaker nuclear attraction.

2. Explain the observed trend in first ionisation energies across a period[3]

Across a period first ionisation energies increase.

Similar shielding, and more nuclear charge,

lead to stronger nuclear attraction.

3. Explain why aluminium(group13) has a lower first ionisation energy than

magnesium(group2). [2]

Al has one more 3p orbital than Mg, so electron removed from higher energy level and more distance. Thus, less energy needed to remove electron and less nuclear attraction.

4. Explain why sulfur(group16) has a lower first ionisation energy than

phosphorus(group15). [2]

S has one more electron pair causing electron spin-pair repulsion, less energy needed to remove electron.

5. Copper has a melting point of 1085°C and a high electrical conductivity. Explain

these properties of copper by referring to its structure and bonding. [2]

Copper has strong metallic bonds so the melting point is very high. There are delocalised electrons which are free to move throughout the giant structure to conduct electricity.

6. Explain how uncontrolled use of nitrate fertilisers can cause problems. [3]

Explosion of algal(藻类) growth(必写),

sunlight is blocked out,

preventing photosynthesis,

decreasing concentration of oxygen in water(必写).

7. Extra supplementary Tips

Photochemical smog: nitrogen oxides+ unburnt hydrocarbon→PAN(ultraviolet light)

Acid rain: SO2 + NO2→NO+SO3 2NO+O2→2NO2 Burn fossil fuels

8. uses

Nitrogen-containing substance: fertilizer

Calcium-containing alkaline substances: lowering the pH of acidic soil.

9. Explain the differences in observations, at room temperature, when NaI(s) reacts with concentrated sulfuric acid compared to those for NaCl(s).

Iodide ions are stronger reducing agents than chloride. iodide ion can be oxidised but chloride cannot be oxidised by H2SO4. So NaI will form I2 purple gas.

10. State and explain the relative thermal stabilities of the hydrogen halides, HX. [2]

Thermal stabilities of the hydrogen halides decrease down the group. Bond energy of H-X decrease due to bond length increase.

11. Explain why but-2-ene shows geometrical isomerism. [2]

Each carbon of carbon-carbon double bond bonded with two different groups.

carbon-carbon double bond cannot rotate.

12. Why does CH3CH(Cl)COOH have optical isomerism .

It has chiral carbon with four different groups.

The mirror images is nonsuperimposable(不可重叠).

13. Explain why 2-bromo-2-methylpropane is the major product of this reaction. [2]

The intermediate is tertiary carbocation which is more stable due to positive inductive effect and more alkyl groups.

This alkyl groups can reduce charge density of carbocation. (3 marks)

14. What does sp2hybridization mean?

The overlap of an s orbit and two p orbits

17. Covalent bond

18. F rm f σ b d c rb -carbon double bond

19. Dot and cross

20. Maxwell Boltzmann distribution diagram

除了以上考点外，同学们不要忘记复习shape of molecule哦！以下是关于常考shape的汇总。