氮与硫

12.1

教学大纲

解释氮缺乏反应性的原因，涉及三键的强度及缺乏极性；
描述并解释：(a) 氨的碱性，使用布朗斯特德–劳里理论 (b) 铵离子的结构及其通过酸碱反应的生成 (c) 通过酸碱反应从铵盐中置换出氨气；
陈述并解释氮氧化物的自然来源和人为来源，以及其在内燃机尾气中的催化去除方法；
理解大气中的氮氧化物（$\text{NO}$ 和 $\text{NO}_2$）可与未燃烧的碳氢化合物反应生成过氧乙酰硝酸酯，PAN，这是光化学烟雾的组成部分；
描述 $\text{NO}$ 和 $\text{NO}_2$ 在形成酸雨中的作用，既包括直接作用，也包括其作为催化剂促进大气中二氧化硫氧化的作用。

来源：Cambridge International 教学大纲

English

Nitrogen gas, $\text{N}_2$, makes up most of the air but reacts with very little. There are two reasons:

the two nitrogen atoms are joined by a triple bond 三键, which has a very high bond energy 键能. A lot of energy is needed to break it.
the molecule has no polarity 极性 — it is perfectly symmetrical, so it has no permanent dipole. London dispersion attractions still act between nitrogen molecules.

Two nitrogen atoms joined by a triple bond, each with a lone pair, labelled as a strong bond and a symmetrical, non-polar molecule — Nitrogen is unreactive for two reasons: a very strong triple bond, and a symmetrical, non-polar molecule

探索

氨的形状

氨有三对成键电子和一对孤对电子——孤对电子把成键电子对往下推,形成三角锥形(约 107°)。

词汇训练

English	中文	拼音
triple bond/ˈtrɪpl bɒnd/	三键	sān jiàn
bond energy/bɒnd ˈenədʒi/	键能	jiàn néng
polarity/pəʊˈlærɪti/	极性	jí xìng

12.1

氨与铵离子

English

A large industrial chemical complex — Ammonia is converted into nitrogen fertilisers in industrial plants on a huge scale.

Basicity of ammonia

The basicity 碱性 of ammonia (its ability to act as a base) comes from the lone pair 孤对电子 of electrons on the nitrogen atom. Using the Brønsted–Lowry theory, ammonia is a base because this lone pair can accept a proton 质子 ($\text{H}^+$):

$$\text{NH}_3 + \text{H}^+ \rightarrow \text{NH}_4^+$$

Ammonia with a lone pair on its nitrogen accepts a hydrogen ion to form the tetrahedral ammonium ion — Ammonia acts as a base because the lone pair on its nitrogen accepts a proton, making the ammonium ion

The ammonium ion

When the lone pair forms a bond to $\text{H}^+$, it makes the ammonium ion 铵离子, $\text{NH}_4^+$. Because both shared electrons came from the nitrogen, this new bond is a coordinate bond 配位键. The ion has four identical N–H bonds and a tetrahedral shape.

Displacement of ammonia from its salts

If you warm an ammonium salt with a base (such as sodium hydroxide), you push out ammonia gas. This is an acid–base displacement 置换:

$$\text{NH}_4\text{Cl} + \text{NaOH} \rightarrow \text{NaCl} + \text{NH}_3 + \text{H}_2\text{O}$$

The sharp smell of ammonia, and damp red litmus turning blue, is a test for an ammonium salt.

词汇训练

English	中文	拼音
basicity/beɪˈsɪsɪti/	碱性	jiǎn xìng
lone pair/ləʊn peə/	孤对电子	gū duì diàn zǐ
proton/ˈprəʊtɒn/	质子	zhì zǐ
ammonium ion/æˈməʊnɪəm ˈaɪɒn/	铵离子	ǎn lí zi
coordinate bond/kəʊˈɔːdɪnət bɒnd/	配位键	pèi wèi jiàn
displacement/dɪˈspleɪsmənt/	置换	zhì huàn

12.1

氮的氧化物与空气污染

English

Where they come from

Oxides of nitrogen ($\text{NO}$ and $\text{NO}_2$, together called $\text{NO}_x$) come from two sources:

natural: lightning gives enough energy for nitrogen and oxygen in the air to combine.
man-made: the high temperature inside an internal combustion engine 内燃机 makes nitrogen and oxygen react:

$$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$$

Removing them from car exhaust

A catalytic converter 催化转化器 cleans the exhaust gases 尾气. It lets the harmful gases react together to form harmless ones:

$$2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2$$

Harmful NO and CO gases passing through a catalyst block and leaving as harmless nitrogen and carbon dioxide — In a catalytic converter the harmful gases NO and CO react over the catalyst to form harmless N$_2$ and CO$_2$

Photochemical smog

In sunlight, $\text{NO}$ and $\text{NO}_2$ take part in reactions with unburned hydrocarbons that form pollutants including ozone and peroxyacetyl nitrate (PAN). PAN is a harmful part of photochemical smog 光化学烟雾, the haze seen over busy cities (its brown colour comes from $\text{NO}_2$).

An aerial view of a large city with a distinct brown-grey layer of haze sitting over the buildings under a clear blue sky above — Photochemical smog over a big city. The brown layer is trapped low down, exactly where the traffic fumes are — sunlight turns those nitrogen oxides and unburned hydrocarbons into the haze you can see

Nitrogen oxides and hydrocarbons enter sunlight-driven atmospheric reactions that form ozone, PAN and other photochemical-smog products — Primary emissions feed reactions in the air; the products are secondary pollutants.

Acid rain

The oxides of nitrogen also help make acid rain 酸雨 in two ways:

directly: $\text{NO}_2$ dissolves in rain to form nitric acid.
as a catalyst: $\text{NO}_2$ speeds up the oxidation of atmospheric sulfur dioxide 二氧化硫 ($\text{SO}_2$) into $\text{SO}_3$, which then forms sulfuric acid in the rain.

A flow diagram: NO2 forms nitric acid and SO2 forms sulfuric acid, with NO2 also catalysing the sulfur path, both leading to acid rain — Nitrogen and sulfur oxides make acid rain: NO$_2$ forms nitric acid directly and also catalyses the oxidation of SO$_2$ to sulfuric acid

Worked example. A white solid is warmed with aqueous $\text{NaOH}$, and a gas is released that turns damp red litmus blue. Identify the gas and the ion in the solid, and explain the reaction. The only common gas that turns damp red litmus blue is ammonia, $\text{NH}_3$, so the solid contains the ammonium ion, $\text{NH}_4^{+}$. The hydroxide ion is the stronger base, so it takes the proton back from the ammonium ion:

$$\text{NH}_4^{+} + \text{OH}^{-} \rightarrow \text{NH}_3 + \text{H}_2\text{O}$$

This is the standard test for $\text{NH}_4^{+}$. Always say the litmus is damp: the ammonia must dissolve in the water before it can show its basicity, so dry litmus would give no colour change at all.

Common mistakes.

Saying nitrogen is unreactive because it is a gas or because its atoms have full shells. The reason is the very strong N≡N triple bond (and the molecule's lack of polarity).
Testing for ammonia with dry litmus or blue litmus. Use damp red litmus, which turns blue; the ammonia must dissolve in the water first.
Saying the coordinate bond in $\text{NH}_4^+$ is different from the other three N–H bonds. Once formed, all four bonds are identical.
Saying $\text{NO}_x$ come from nitrogen in the fuel. Most come from nitrogen in the air, which reacts with oxygen at the high temperature of the engine.
Saying $\text{NO}_2$ is used up when it helps form acid rain from $\text{SO}_2$. It acts as a catalyst and is regenerated: $\text{NO}_2 + \text{SO}_2 \rightarrow \text{NO} + \text{SO}_3$, then $\text{NO} + \tfrac12\text{O}_2 \rightarrow \text{NO}_2$.

探索

NOx 污染途径

从热的发动机到环境危害追踪氮氧化物。

词汇训练

English	中文	拼音
internal combustion engine/ɪnˈtɜːnl kəmˈbʌstʃn ˈendʒɪn/	内燃机	nèi rán jī
catalytic converter/ˌkætəˈlɪtɪk kənˈvɜːtə/	催化转化器	cuī huà zhuǎn huà qì
exhaust gases/eɡˈzɔːst ˈɡæsɪz/	尾气	wěi qì
photochemical smog/ˌfəʊtəʊˈkemɪkl smɒɡ/	光化学烟雾	guāng huà xué yān wù
acid rain/ˈæsɪd reɪn/	酸雨	suān yǔ
sulfur dioxide/ˈsʌlfɜː daɪˈɒksaɪd/	二氧化硫	èr yǎng huà liú

12.1

考试技巧

English

$\text{N}_2$ is unreactive because of its strong triple bond (very high bond energy) — state this exactly.
Ammonia is a base and a ligand because of its lone pair; the ammonium ion forms by a dative bond.
Explain how oxides of nitrogen form (high temperature in engines) and their link to acid rain and photochemical smog.
Give balanced equations and correct observations for reactions of ammonia and the ammonium ion.

氮与硫

氮与硫

氨的形状