大学先修课程考试
大学化学
（考试时间：2小时，卷面总分：100分，共4道题，每题25分，考试允许带英汉词典，并请用中文答题）
一、2014年1月《美国化学会志》杂志报道了利用电子盐催化N2分解的研究文章。请阅读下面文章摘要，并回答如下问题。
Electrides (电子盐), i.e. salts in which electrons serve as anions, are promising materials for lowering activation energies of chemical reactions. Ab initio
simulations(量子力学从头计算法模拟) are used to investigate the effect of the electron anions in a prototype mayenite(钙铝矿，主要成分是Ca24Al28O64)‐based electride (C12A7:e−) on the mechanism of N2 dissociation. It is found that both atomic and molecular nitrogen species chemisorb(化学吸附) on the electride surface and become negatively charged due to the electron transfer from the substrate. However, charging alone is not sufficient to promote dissociation of N2 molecules. In the presence of Ru, N2 adsorbs with the formation of a cis‐Ru2N2 complex and the N−N bond weakens due to both the electron transfer from the substrate and interaction with Ru. This complex transforms into a more stable trans‐Ru2N2 configuration, in which the N2 molecule is dissociated, with the calculated barrier(计算能垒) of 116 kJ mol−1 and the overall energy gain of 72 kJ mol−1. In contrast, in the case of the stoichiometric mayenite(符合化学整比的钙铝矿), the cis‐Ru2N2 is ∼34 kJ mol−1 more stable than the trans‐Ru2N2, while the cis−trans transition has a barrier of 192 kJ mol−1. Splitting of N2 is promoted by a combination of the strong electron donating power of C12A7:e−, ability of Ru to capture N2, polarization of Ru clusters, and electrostatic interaction of negatively charged N species with the surface cations.
1. 该研究使用了什么研究方法来研究N2分子的分解：
_________________________________________________________________。
2. 在上述摘要的结论中提到促进N2分子分解有三个因素，这三个因素分别是 ________________________、__________________________和____________ ____________________________________。
3. 在这项研究中发现，无论是N原子还是N2分子都能吸附在电子盐表面上，原因是：
________________________________________________________________。
4. 根据摘要内容，在电子盐和符合化学整比的钙铝矿两种情况下，从顺式Ru2N2转变为反式Ru2N2，哪一种是吸热反应？哪一种反应速率较快？
_________________________________________________________________ ________________________________________________________________。
5. 上述N2分解反应实际上就是合成氨的第一步。根据你对人工合成氨机理的理解，你觉得合成氨反应中最关键的一步应该是什么？你觉得要完成这一步的前提条件是什么？
_________________________________________________________________ ________________________________________________________________。
二、2013年4月，《大学化学》杂志刊登了北京大学化学学院四位2010级本科生的论文 “碱金属与碱土金属密堆积结构的研究”。文中探讨了s区金属单质密堆积结构的变化规律，并给出了解释。文中写道：
Prewitt 和Downs 建立了高压下结构转变的一系列重要规则，其中第9 条规则就是：“高压下元素的性质类似于元素周期表中它下面的元素在低压下的情况”。因此，通过较轻的碱土金属随压强增大稳定结构的转变，可以推测出常压下重碱土金属变化的趋势。
如何定性解释这一规律呢?
化合物通常采取一种倾向于使整个体系化学硬度最大的原子核排列。化学硬度=(I‐Eea) /2，其中，I为电离能，Eea为电子亲和能。当压强超过100GPa 时，一些在常压下能量相当高的激发态和体系的基态间会有更强的耦合作用，结果导致满带和空带间的带隙变小甚至消失。相应的激发态在高压下能量下降，与基态间的混杂程度上升，这相当于某种杂化或组合。
第二周期元素通常较硬，压缩降低了激发态能量，使这些原子更软，允许重新杂化。结果，硬的、变形性低的原子表现出类似更重的原子的性质：它们变得更软，电子密度分布更加多变，方向性降低。随着化学硬度的下降，(碱土金属密堆积)结构呈现hcp fcc bcc 的变化。
碱金属
Li
Na 密堆积 bcc bcc 碱土金属 Be Mg 密堆积 hcp hcp
K
Rb
Cs
bcc bcc bcc Ca Sr Ba ccp ccp bcc
1. 为解释高压下轻碱土金属与常压下重碱土金属在结构上的相似性，文中使用了何种理论？
________________________________________________________________。
2. 根据文中所述，请解释为什么加压会导致带隙下降。
__________________________________________________________________ _________________________________________________________________。
3. 根据文中的化学硬度表达式，请说明硬度与带隙（相当于LUMO‐HOMO的能级差）之间的关系。
__________________________________________________________________ _________________________________________________________________。
4. 根据上文内容，请解释为何碱金属的密堆积结构（bcc）与重碱土金属（Ba）的结构相似。
__________________________________________________________________ ________________________________________________________________。
5. Mulliken电负性（ = (I + Eea)/2）与上述硬度公式有相似之处。请说明二者的联系和差别。
__________________________________________________________________ ________________________________________________________________。
三、采用阳极氧化法在钛板表面制备二氧化钛纳米管
（纳米阵列）已经是一个比较成熟的技术。在这个技术
中，比较有趣的一点是位于阳极的钛板在电化学腐蚀过
程中可直接形成如右图所示的垂直于钛板表面的均匀
管状阵列（直径20~30 nm），且纳米管之间是分离的。
2004年，《物理化学学报》刊登了文章“氧化钛纳
米管阵列制备及形成机理”。文中解释了上述纳米管阵
列的形成机理：
为进一步探明钛阳极氧化过程纳米管状结构的形成机理，测量了纯钛电极在0.5％(w) HF溶液中l0 V电压下阳极氧化过程电流一时问曲线(参见下图)。结果表明，整个氧化过程大致可分为三个阶段。在氧化的最初阶段，即阻挡层的形成阶段，开始金属钛在HF电解质溶液中快速阳极溶解，阳极电流很大，并产生大量Ti4+离子。接着Ti4+离子与介质中含氧离子快速相互作用，并在Ti表面形成致密的TiO2薄膜。随着表面氧化层的形成，电流急剧降低。
在氧化的第二阶段，即多孔层的初始形成阶段．随着表面氧化层的形成，膜层承受的电场强度急剧增大，在HF溶液和电场的共同作用下。在TiO2阻挡层发生随机击穿溶解，形成孔核。随着氧化时间的增加，随机分布的孔核发展成为小孔，孔的密度也不断增加。最后均匀分布在表面。
在孔核逐渐转变为孔的过程中，相同电场强度下Ti4+ 可较容易穿过阻挡层进入溶液中，同时溶液中的含氧离子也较易穿过阻挡层与Ti4+结合生成新的阻挡层，因此这个阶段的阳极电流有所增大。
在氧化的第三个阶段，即多孔膜层的稳定生长阶段。电流完全由发生在阻挡层两侧的离子迁移提供，从而形成一个相对稳定的电流。孔的生长是孔底部的氧化层不断向钛基体推进的结果。当阻挡层一金属界面推进速度与孔底氧化层的溶解速度相等时，阻挡层的厚度将不再随孔的加深而变化。孔与孔的交界处也有小坑，孔与孔之间钛的氧化物通过小坑不断被溶解，最后形成管壁。当氧化层的生成与溶解速度相等时纳米管的长度将不再增长，而这种平衡很大程度上取决于阳极氧化的电压。
（注：上图中纵坐标是电流，单位mA；横坐标是时间，单位为秒）
1. HF溶液在上述阳极氧化过程中的作用是什么？请写出离子反应式。
_________________________________________________________________ ________________________________________________________________。
2. 在上述第二个阶段中，随机产生的小孔为什么会在表面均匀分布？
_________________________________________________________________ ________________________________________________________________。
3. 在多孔膜层的生长阶段（第三阶段），为何小孔会向下发展而不是侧向发展联通多个小孔？
_________________________________________________________________ ________________________________________________________________。
4. 在上文的最后一段中有：“孔与孔的交界处也有小坑，孔与孔之间钛的氧化物通过小坑不断被溶解，最后形成管壁。”请解释为什么孔与孔之间也会出现新的溶解点。
_________________________________________________________________ ________________________________________________________________。
5. 你是否接受上述解释？你是否还有上述实验中尚未回答的疑问？如果你可以继续上述研究，你打算做什么？
四、2010年5月，美国《科学》杂志发表了一个荷兰研究小组的工作 “Cooperativity in Ion Hydration”。在这项研究中他们发现，在离子化合物的水溶液中阴、阳离子不仅对水的氢键和运动有重要影响，同时阴阳离子之间也存在配合性。请阅读下列文章片段（有删节），并回答以下问题：
Figure 2, A to C, shows the anisotropy decay(各向异性衰减) for the dissolved salts Mg(ClO4)2, Cs2SO4, and MgSO4, each for concentrations ranging from 0 to 2 mol/kg. A remarkable conclusion can be drawn from these three measurements: MgSO4 shows a very large slow reorientation component, whereas Mg2+ and SO42–
–individually, in combination with other ions (ClO4 and Cs+, respectively), do not. This
shows that the effect of ions on water dynamics can be nonadditive(非广度量).
The same cooperativity(配合性) is observed in the fs‐IR(飞秒红外光谱)
measurements (Fig. 2), where MgSO4 has a much larger fraction of slowly reorienting water molecules (corresponding to a hydration number N = 32) than Mg(ClO4)2 and Cs2SO4 (with hydration numbers N of 4 and 9, respectively). For MgSO4, there are about twice as many slowly rotating OH groups (N) as slowly rotating dipoles (Np), which indicates that the same collection of slow water molecules is observed by fs‐IR and THz DR spectroscopies(太赫介电弛豫谱). Even the combination of the moderately strongly hydrated cation Na+ with the strong anion SO42– is observed to affect the dynamics of a large number of water molecules (N = 24; Fig. 2D). Thus, the effects of ions and counterions can be strongly interdependent and nonadditive. The key parameter determining how strongly ions affect water dynamics is thus the combination of the solvated cation and anion.
The cooperativity in ion hydration can be explained by the fact that the cation and anion lock different degrees of freedom of the water molecules—that is, the direction of the bisectrix (p) and the direction of OH (), respectively. The nearby presence of both ions can thus lead to a locking in both directions of the hydrogen‐bond structure of several intervening water layers, giving rise to the observation by DR and fs‐IR of slowed water molecules well beyond the first solvation shell. This cooperativity is schematically illustrated in Fig. 3C. We expect the solvation structures to be quite directional between the ions; if an ion forms ~4 of these structures with surrounding counterions, the value of N = 18 implies that each of these structures consists, on average, of four or five water molecules. This interpretation also means that for solutions such as MgSO4 and Na2SO4, the slowly reorienting water molecules are not arranged in a spherically symmetric way around the ions.
The reorientation of water molecules in the rigid, locked hydrogen‐bond structure occurring for MgSO4 can be expected to show a temperature dependence different
from that of pure liquid water. We measured the
temperature dependence of the anisotropy decay for
a MgSO4 solution (1.5 mol/kg) over an interval of
about 50°C (Fig. 4A). We also compared the
temperature dependence to that of a Cs2SO4 solution
(4 mol/kg) (Fig. 4B) with the same hydration level (Fig.
2D) but no cooperativity. At room temperature, the
anisotropy for MgSO4 decays more slowly than for
Cs2SO4, but with increasing temperature the situation
reverses. To quantify the results, we fit the anisotropy
data at different temperatures with a single averaged
time scale for all water molecules in the system. This
means that a change in this time scale represents both
the change in time scales and change in the relative
fractions of bulk‐like and slow water molecules. Figure
4C shows a much stronger temperature dependence
of this average time scale for a solution of MgSO4 (1.5
mol/kg) than for a solution of Cs2SO4 (4 mol/kg).
The difference in temperature dependence of the
reorientation of MgSO4 and Cs2SO4 solutions indicates
that the reorientation within the hydration structures
involves a different mechanism. For pure water,
molecular dynamics simulations indicated that the
reorientation of water follows a concerted mechanism (同步机理) and that the rate‐limiting step (限速步骤) for reorientation of a water molecule is the motion of a second water molecule in and out of the solvation shell of the first (30, 31). For the Cs2SO4 solution, the hydration number N has a value of 9, which is likely associated with the OH groups of water molecules that are hydrogen bonded to the SO42– ion. In view of the small value of N, these water molecules are surrounded by water molecules that show bulk‐like dynamics. Hence, although the reorientation of the water molecules hydrating the SO42– is slow, the temperature dependence of this reorientation is similar to that of pure liquid water, because the reorientation is governed by hydrogen‐bond interactions to water molecules that show bulk‐like behavior. Correspondingly, the temperature dependence of the reorientation time of Cs2SO4 is similar to that of bulk water. In contrast, for MgSO4 the solvation structures are large, as expressed by the large values of the hydration numbers Np = 18 and N =
32. Hence, the reorientation of a water molecule in the solvation structure relies on
the motions of water molecules that are contained in the same solvation structure. These motions are substantially slowed down, and reorientation thus involves a collective reorganization of the extended solvation structure, which explains the difference in temperature dependence with a solution of Cs2SO4 and pure liquid water.
回答如下问题：
1. 文中第一段的最后一句提到：“This shows that the effect of ions on water dynamics can be nonadditive”。请根据上下文解释为何离子对水分子运动的影响不能加和，是非广度量。
2. 根据上文中图2，可以看出MgSO4溶液中的水分子运动特征完全不同于Mg(ClO4)2或Cs2(SO4)，而后两者之间却很相似（尽管没有一个离子是相同的）。请根据上文内容解释这个实验观察结果。
3. 参见图4C，在低温下， Cs2SO4和MgSO4两条线相距较远。请解释：为何在高温下上述两条线相交？
4. 本文是溶液化学领域的一项重要进展，请指出这个发现对于哪些领域有重要促进或推动作用。
5. 在学习大学化学之后，你是否能想到其他理论也可以解释上述正负离子间的配合性。请简述之。
大学先修课程考试
大学化学
（考试时间：2小时，卷面总分：100分，共4道题，每题25分，考试允许带英汉词典，并请用中文答题）
一、2014年1月《美国化学会志》杂志报道了利用电子盐催化N2分解的研究文章。请阅读下面文章摘要，并回答如下问题。
Electrides (电子盐), i.e. salts in which electrons serve as anions, are promising materials for lowering activation energies of chemical reactions. Ab initio
simulations(量子力学从头计算法模拟) are used to investigate the effect of the electron anions in a prototype mayenite(钙铝矿，主要成分是Ca24Al28O64)‐based electride (C12A7:e−) on the mechanism of N2 dissociation. It is found that both atomic and molecular nitrogen species chemisorb(化学吸附) on the electride surface and become negatively charged due to the electron transfer from the substrate. However, charging alone is not sufficient to promote dissociation of N2 molecules. In the presence of Ru, N2 adsorbs with the formation of a cis‐Ru2N2 complex and the N−N bond weakens due to both the electron transfer from the substrate and interaction with Ru. This complex transforms into a more stable trans‐Ru2N2 configuration, in which the N2 molecule is dissociated, with the calculated barrier(计算能垒) of 116 kJ mol−1 and the overall energy gain of 72 kJ mol−1. In contrast, in the case of the stoichiometric mayenite(符合化学整比的钙铝矿), the cis‐Ru2N2 is ∼34 kJ mol−1 more stable than the trans‐Ru2N2, while the cis−trans transition has a barrier of 192 kJ mol−1. Splitting of N2 is promoted by a combination of the strong electron donating power of C12A7:e−, ability of Ru to capture N2, polarization of Ru clusters, and electrostatic interaction of negatively charged N species with the surface cations.
1. 该研究使用了什么研究方法来研究N2分子的分解：
_________________________________________________________________。
2. 在上述摘要的结论中提到促进N2分子分解有三个因素，这三个因素分别是 ________________________、__________________________和____________ ____________________________________。
3. 在这项研究中发现，无论是N原子还是N2分子都能吸附在电子盐表面上，原因是：
________________________________________________________________。
4. 根据摘要内容，在电子盐和符合化学整比的钙铝矿两种情况下，从顺式Ru2N2转变为反式Ru2N2，哪一种是吸热反应？哪一种反应速率较快？
_________________________________________________________________ ________________________________________________________________。
5. 上述N2分解反应实际上就是合成氨的第一步。根据你对人工合成氨机理的理解，你觉得合成氨反应中最关键的一步应该是什么？你觉得要完成这一步的前提条件是什么？
_________________________________________________________________ ________________________________________________________________。
二、2013年4月，《大学化学》杂志刊登了北京大学化学学院四位2010级本科生的论文 “碱金属与碱土金属密堆积结构的研究”。文中探讨了s区金属单质密堆积结构的变化规律，并给出了解释。文中写道：
Prewitt 和Downs 建立了高压下结构转变的一系列重要规则，其中第9 条规则就是：“高压下元素的性质类似于元素周期表中它下面的元素在低压下的情况”。因此，通过较轻的碱土金属随压强增大稳定结构的转变，可以推测出常压下重碱土金属变化的趋势。
如何定性解释这一规律呢?
化合物通常采取一种倾向于使整个体系化学硬度最大的原子核排列。化学硬度=(I‐Eea) /2，其中，I为电离能，Eea为电子亲和能。当压强超过100GPa 时，一些在常压下能量相当高的激发态和体系的基态间会有更强的耦合作用，结果导致满带和空带间的带隙变小甚至消失。相应的激发态在高压下能量下降，与基态间的混杂程度上升，这相当于某种杂化或组合。
第二周期元素通常较硬，压缩降低了激发态能量，使这些原子更软，允许重新杂化。结果，硬的、变形性低的原子表现出类似更重的原子的性质：它们变得更软，电子密度分布更加多变，方向性降低。随着化学硬度的下降，(碱土金属密堆积)结构呈现hcp fcc bcc 的变化。
碱金属
Li
Na 密堆积 bcc bcc 碱土金属 Be Mg 密堆积 hcp hcp
K
Rb
Cs
bcc bcc bcc Ca Sr Ba ccp ccp bcc
1. 为解释高压下轻碱土金属与常压下重碱土金属在结构上的相似性，文中使用了何种理论？
________________________________________________________________。
2. 根据文中所述，请解释为什么加压会导致带隙下降。
__________________________________________________________________ _________________________________________________________________。
3. 根据文中的化学硬度表达式，请说明硬度与带隙（相当于LUMO‐HOMO的能级差）之间的关系。
__________________________________________________________________ _________________________________________________________________。
4. 根据上文内容，请解释为何碱金属的密堆积结构（bcc）与重碱土金属（Ba）的结构相似。
__________________________________________________________________ ________________________________________________________________。
5. Mulliken电负性（ = (I + Eea)/2）与上述硬度公式有相似之处。请说明二者的联系和差别。
__________________________________________________________________ ________________________________________________________________。
三、采用阳极氧化法在钛板表面制备二氧化钛纳米管
（纳米阵列）已经是一个比较成熟的技术。在这个技术
中，比较有趣的一点是位于阳极的钛板在电化学腐蚀过
程中可直接形成如右图所示的垂直于钛板表面的均匀
管状阵列（直径20~30 nm），且纳米管之间是分离的。
2004年，《物理化学学报》刊登了文章“氧化钛纳
米管阵列制备及形成机理”。文中解释了上述纳米管阵
列的形成机理：
为进一步探明钛阳极氧化过程纳米管状结构的形成机理，测量了纯钛电极在0.5％(w) HF溶液中l0 V电压下阳极氧化过程电流一时问曲线(参见下图)。结果表明，整个氧化过程大致可分为三个阶段。在氧化的最初阶段，即阻挡层的形成阶段，开始金属钛在HF电解质溶液中快速阳极溶解，阳极电流很大，并产生大量Ti4+离子。接着Ti4+离子与介质中含氧离子快速相互作用，并在Ti表面形成致密的TiO2薄膜。随着表面氧化层的形成，电流急剧降低。
在氧化的第二阶段，即多孔层的初始形成阶段．随着表面氧化层的形成，膜层承受的电场强度急剧增大，在HF溶液和电场的共同作用下。在TiO2阻挡层发生随机击穿溶解，形成孔核。随着氧化时间的增加，随机分布的孔核发展成为小孔，孔的密度也不断增加。最后均匀分布在表面。
在孔核逐渐转变为孔的过程中，相同电场强度下Ti4+ 可较容易穿过阻挡层进入溶液中，同时溶液中的含氧离子也较易穿过阻挡层与Ti4+结合生成新的阻挡层，因此这个阶段的阳极电流有所增大。
在氧化的第三个阶段，即多孔膜层的稳定生长阶段。电流完全由发生在阻挡层两侧的离子迁移提供，从而形成一个相对稳定的电流。孔的生长是孔底部的氧化层不断向钛基体推进的结果。当阻挡层一金属界面推进速度与孔底氧化层的溶解速度相等时，阻挡层的厚度将不再随孔的加深而变化。孔与孔的交界处也有小坑，孔与孔之间钛的氧化物通过小坑不断被溶解，最后形成管壁。当氧化层的生成与溶解速度相等时纳米管的长度将不再增长，而这种平衡很大程度上取决于阳极氧化的电压。
（注：上图中纵坐标是电流，单位mA；横坐标是时间，单位为秒）
1. HF溶液在上述阳极氧化过程中的作用是什么？请写出离子反应式。
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2. 在上述第二个阶段中，随机产生的小孔为什么会在表面均匀分布？
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3. 在多孔膜层的生长阶段（第三阶段），为何小孔会向下发展而不是侧向发展联通多个小孔？
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4. 在上文的最后一段中有：“孔与孔的交界处也有小坑，孔与孔之间钛的氧化物通过小坑不断被溶解，最后形成管壁。”请解释为什么孔与孔之间也会出现新的溶解点。
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5. 你是否接受上述解释？你是否还有上述实验中尚未回答的疑问？如果你可以继续上述研究，你打算做什么？
四、2010年5月，美国《科学》杂志发表了一个荷兰研究小组的工作 “Cooperativity in Ion Hydration”。在这项研究中他们发现，在离子化合物的水溶液中阴、阳离子不仅对水的氢键和运动有重要影响，同时阴阳离子之间也存在配合性。请阅读下列文章片段（有删节），并回答以下问题：
Figure 2, A to C, shows the anisotropy decay(各向异性衰减) for the dissolved salts Mg(ClO4)2, Cs2SO4, and MgSO4, each for concentrations ranging from 0 to 2 mol/kg. A remarkable conclusion can be drawn from these three measurements: MgSO4 shows a very large slow reorientation component, whereas Mg2+ and SO42–
–individually, in combination with other ions (ClO4 and Cs+, respectively), do not. This
shows that the effect of ions on water dynamics can be nonadditive(非广度量).
The same cooperativity(配合性) is observed in the fs‐IR(飞秒红外光谱)
measurements (Fig. 2), where MgSO4 has a much larger fraction of slowly reorienting water molecules (corresponding to a hydration number N = 32) than Mg(ClO4)2 and Cs2SO4 (with hydration numbers N of 4 and 9, respectively). For MgSO4, there are about twice as many slowly rotating OH groups (N) as slowly rotating dipoles (Np), which indicates that the same collection of slow water molecules is observed by fs‐IR and THz DR spectroscopies(太赫介电弛豫谱). Even the combination of the moderately strongly hydrated cation Na+ with the strong anion SO42– is observed to affect the dynamics of a large number of water molecules (N = 24; Fig. 2D). Thus, the effects of ions and counterions can be strongly interdependent and nonadditive. The key parameter determining how strongly ions affect water dynamics is thus the combination of the solvated cation and anion.
The cooperativity in ion hydration can be explained by the fact that the cation and anion lock different degrees of freedom of the water molecules—that is, the direction of the bisectrix (p) and the direction of OH (), respectively. The nearby presence of both ions can thus lead to a locking in both directions of the hydrogen‐bond structure of several intervening water layers, giving rise to the observation by DR and fs‐IR of slowed water molecules well beyond the first solvation shell. This cooperativity is schematically illustrated in Fig. 3C. We expect the solvation structures to be quite directional between the ions; if an ion forms ~4 of these structures with surrounding counterions, the value of N = 18 implies that each of these structures consists, on average, of four or five water molecules. This interpretation also means that for solutions such as MgSO4 and Na2SO4, the slowly reorienting water molecules are not arranged in a spherically symmetric way around the ions.
The reorientation of water molecules in the rigid, locked hydrogen‐bond structure occurring for MgSO4 can be expected to show a temperature dependence different
from that of pure liquid water. We measured the
temperature dependence of the anisotropy decay for
a MgSO4 solution (1.5 mol/kg) over an interval of
about 50°C (Fig. 4A). We also compared the
temperature dependence to that of a Cs2SO4 solution
(4 mol/kg) (Fig. 4B) with the same hydration level (Fig.
2D) but no cooperativity. At room temperature, the
anisotropy for MgSO4 decays more slowly than for
Cs2SO4, but with increasing temperature the situation
reverses. To quantify the results, we fit the anisotropy
data at different temperatures with a single averaged
time scale for all water molecules in the system. This
means that a change in this time scale represents both
the change in time scales and change in the relative
fractions of bulk‐like and slow water molecules. Figure
4C shows a much stronger temperature dependence
of this average time scale for a solution of MgSO4 (1.5
mol/kg) than for a solution of Cs2SO4 (4 mol/kg).
The difference in temperature dependence of the
reorientation of MgSO4 and Cs2SO4 solutions indicates
that the reorientation within the hydration structures
involves a different mechanism. For pure water,
molecular dynamics simulations indicated that the
reorientation of water follows a concerted mechanism (同步机理) and that the rate‐limiting step (限速步骤) for reorientation of a water molecule is the motion of a second water molecule in and out of the solvation shell of the first (30, 31). For the Cs2SO4 solution, the hydration number N has a value of 9, which is likely associated with the OH groups of water molecules that are hydrogen bonded to the SO42– ion. In view of the small value of N, these water molecules are surrounded by water molecules that show bulk‐like dynamics. Hence, although the reorientation of the water molecules hydrating the SO42– is slow, the temperature dependence of this reorientation is similar to that of pure liquid water, because the reorientation is governed by hydrogen‐bond interactions to water molecules that show bulk‐like behavior. Correspondingly, the temperature dependence of the reorientation time of Cs2SO4 is similar to that of bulk water. In contrast, for MgSO4 the solvation structures are large, as expressed by the large values of the hydration numbers Np = 18 and N =
32. Hence, the reorientation of a water molecule in the solvation structure relies on
the motions of water molecules that are contained in the same solvation structure. These motions are substantially slowed down, and reorientation thus involves a collective reorganization of the extended solvation structure, which explains the difference in temperature dependence with a solution of Cs2SO4 and pure liquid water.
回答如下问题：
1. 文中第一段的最后一句提到：“This shows that the effect of ions on water dynamics can be nonadditive”。请根据上下文解释为何离子对水分子运动的影响不能加和，是非广度量。
2. 根据上文中图2，可以看出MgSO4溶液中的水分子运动特征完全不同于Mg(ClO4)2或Cs2(SO4)，而后两者之间却很相似（尽管没有一个离子是相同的）。请根据上文内容解释这个实验观察结果。
3. 参见图4C，在低温下， Cs2SO4和MgSO4两条线相距较远。请解释：为何在高温下上述两条线相交？
4. 本文是溶液化学领域的一项重要进展，请指出这个发现对于哪些领域有重要促进或推动作用。
5. 在学习大学化学之后，你是否能想到其他理论也可以解释上述正负离子间的配合性。请简述之。