共查询到20条相似文献,搜索用时 93 毫秒
1.
2.
在学习化学元素电负性的概念、化合物的键型和元素的化学活泼性等问题中,我有一些学习心得,现归纳如下: 1.电负性的定义和计算方法L.Pauling在1932年首先提出电负性的概念.元素的电负性就是“元素的原子在化合物分子中把电子吸引向自己的能力”.计算电负性的方法有下列四种: (1)Pauling的热化学方法Pauling 相似文献
3.
4.
5.
电负性是分子中一个原子把电子拉向它自身的能力,是化学理论的基本概念之一。继Pauling建立第一个电负性标度后,提出了众多的电负性标度。只是在密度泛函理论的基础上,电负性概念和电负性均衡原理,才被精密地论证。近二十多年来,电负性理论的重要发展是:应用电负性均衡模型或方法,可以快速地计算分子体系的电荷分布,从而确定分子的其他性质,甚至包括分子的结构和反应性指标。通常的电负性均衡方法只把分子划分到原子区域,虽然简单直观,但其精度和应用范围受到限制。原子与键电负性均衡方法,把分子划分到包括原子区域、化学键区域和孤对电子区域,能够较快速精密地计算分子的电荷分布和其他性质,并被应用到构建新一代可极化或浮动电荷力场的探索中,有广阔的应用前景。 相似文献
6.
7.
微量吸附量热法研究氧化物催化剂的酸碱性质 总被引:6,自引:0,他引:6
利用微量吸附量热技术定量地表征了Eu2O3、CeO2、MgO、ZnO、Al2O3和NiO等氧化物表面酸碱中心的强度和数量,结果表明,样品的NH3和CO2起始吸附热与其Sanderson电负性相关. 相似文献
8.
9.
氢的电负性值是氢元素性质的重要参数,1932年Pauling犤1~3犦定量确定氢的相对电负性值等于2.1,1961年Allred犤4,5犦用更准确的实验数据对Paul-ing电负性标度进行了修正,氢的电负性值被确定为2.2,目前这两个数值都在采用。元素的电负性值是与元素的性质紧密相关的,一个合适的电负性标度应该至少反映所有重要元素的电负性值,氢的化合物比任何其它元素都多,理应有一个基本的准确电负性值,然而一些电负性标度中却缺乏这样的数据。在Murphy等四人犤6犦最近发表的论文中,对Pauling电负性标度又进行了深入考查与… 相似文献
10.
11.
Pb,Cu和Zn在自然水体生物膜中铁/ 锰氧化物等组分上的富集规律 总被引:2,自引:0,他引:2
用选择性萃取法萃取天然基质采集的生物膜中的铁/锰氧化物, 考察生物膜上铁/锰氧化物对水体中Pb, Cu及Zn的富集作用. 结果表明, 锰氧化物对生物膜富集Pb起主要作用, 而铁氧化物和除铁/锰氧化物以外的以有机质为主的其它组分的作用很小; 膜上其它组分对Cu的富集贡献较大, 锰氧化物的贡献与铁氧化物的接近; 锰氧化物对Zn的富集贡献最大, 铁氧化物和其它组分的贡献比较接近. 锰氧化物对Pb, Cu及Zn的富集能力分别高于铁氧化物的富集能力, 铁氧化物和锰氧化物对3种金属富集能力的大小顺序分别是Zn>Cu>Pb和Zn>Pb≥Cu. 相似文献
12.
DONG De-ming ZHAO Xing-min HUA Xiu-yi ZHANG Jing-jing WU Shi-ming 《高等学校化学研究》2007,23(6):659-664
Natural surface coatings collected from natural substances(NSCsNS)were employed to study the roles of the main chemical components(iron oxides,manganese oxides,and other components)in controlling the adsorption of lead(Pb)and cadmium(Cd)in aquatic environments.The selective chemical extraction followed by the adsorption of Pb and Cd experiments and statistical analysis,were used to investigate the adsorption property of each component.Hydroxylamine hydrochloride was used to remove manganese oxides selectively,and sodium dithionite was used to extract iron oxides and manganese oxides.The result indicated that iron oxides and manganese oxides played an important role in the adsorption of Pb and Cd on NSCsNS,and the relative contribution was about two-thirds.The contribution of manganese oxides was the greatest,with a lesser role indicated for other components.The adsorption ability of manganese oxides for Pb and Cd was greater than that of iron oxides or other components for Pb and Cd.The Pb adsorption observed in each component was greater than Cd adsorption. 相似文献
13.
铁、锰氧化物在自然水体生物膜上的分布 总被引:3,自引:0,他引:3
在自然水体中 ,生物膜是一个开放的动力学系统 ,生物膜的形成及其组分处于动态的变化中——水环境中的各种成分在生物膜上发生着合成、聚结、转化、降解等作用 ,从而成为生物膜的一部分 ,还会随着生物膜的脱落重新进入水相中 ,所有这些变化 ,与生物膜本身的性质是分不开的 [1~ 3] 。从环境化学角度看 ,自然水体中生物膜主要化学组分包括金属氧化物 (铁氧化物、锰氧化物和铝氧化物等 )、有机质及少量的矿物质 ,其中 ,铁、锰氧化物在影响痕量重金属在水环境中的迁移、最终归宿、生物地球化学特性、生物可利用性和毒性过程中起着非常重要的作… 相似文献
14.
综述了近年来国内外用于消除挥发性有机物的有序多孔金属氧化物催化剂的研究进展, 主要包括有序多孔氧化铈、 氧化锰、 氧化钴、 氧化铁、 氧化铬和钙钛矿型氧化物及其负载贵金属或过渡金属氧化物. 讨论了这些催化剂对典型挥发性有机物氧化的催化性能, 并展望了新型高效消除挥发性有机物催化剂的研发前景及技术发展趋势. 相似文献
15.
The digest is devoted to the most widespread reducing agents which are used for the reduction of tertiary and secondary phosphine oxides, phospholene oxides, phospholane oxides, phosphonates, phosphinates, α- and β-hydroxy and thiomethyl phosphine oxides, α-aminophosphine oxides. Stereoselectivity of reactions is described. Methods of stabilization of phosphines which are prone to re-oxidation by the formation of borane-adduct or metal complexes are considered. 相似文献
16.
自然水体中生物膜的主要化学组分与水体中相关化学物质的关系 总被引:7,自引:0,他引:7
自然水体中大多数微生物是以附着状态而不是以游离状态存在的 [1] .微生物附着在固体表面形成生物膜 ,存在于河流、湖泊及湿地环境中的岩石、表层沉积物和悬浮颗粒物表面 ,由不同种类的微生物组成 [2 ] .从环境微生物学角度来看 ,生物膜被定义为一种活跃地生长发育着的单一或混合的微生物群体 ,其不可逆地附着到一种活跃的或非活跃的表面 [3] ,并由微生物群体组成 [4 ] .从环境化学角度来看 ,生物膜是由金属氧化物、有机质及少量矿物质组成的 .金属氧化物和有机质在影响重金属于固相吸附的相对作用已有报道 [5~ 10 ] .生物膜在自然水体中是… 相似文献
17.
18.
Pescarmona PP Janssen KP Jacobs PA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(23):6562-6572
Various transition-metal-free oxides have been studied as catalysts for the epoxidation of cyclooctene with hydrogen peroxide by means of high-throughput experimentation. Different boron, aluminium, and gallium oxides were prepared according to various synthesis methods. A number of pure aluminium and gallium oxides showed very good catalytic performances, while the results obtained with boron oxides or mixed oxides were less positive. The best results were obtained with a gallium oxide catalyst, which gave an epoxide yield of 71 % and a selectivity of 99 % after reaction for 4 h at 80 degrees C. Gallium oxides had not been reported previously as active epoxidation catalysts. The use of high-throughput experimentation proved useful both for discovering new active catalysts and for identifying a number of relationships between the synthesis conditions and the catalytic properties of the transition-metal-free oxides. 相似文献
19.
Kang L Zhang M Liu ZH Ooi K 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2007,67(3-4):864-869
Four kinds of manganese oxides with either layered or tunnel structures were prepared by different methods. The structural features of the prepared materials were characterized by power X-ray diffraction, IR spectra and SEM observation. A compare analysis of IR spectra for these materials before and after the acid treatment was carried out. Band around 761 cm(-1) could be assigned to a characteristic adsorbent band of tunnel-type manganese oxides; while it was not observed in layer-type manganese oxides. After the acid treatment, the position and relative intensity of the bands in 300-1200 cm(-1) region showed an obvious change for layered manganese oxides in compare with tunnel-type manganese oxides. The variation of IR spectra could be used to distinguish layered manganese oxides from tunnel manganese oxides. 相似文献
20.
The extent to which a number of iron oxides dissolved in cold and hot 5% bromine/methanol reagent was examined during analysis for metallic iron in the presence of oxides found in partially-rusted sponge-iron. At room temperature there was only slight attack on the oxides, but at the reflux temperature of the lixiviant and over a period of 2 hr, both the oxyhydroxides and oxides were attacked, the oxyhydroxides considerably and the other oxides slightly. 相似文献