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巧解化学平衡题目的三大关键点
发布时间:2021-06-15 12:57  来源:未知  点击量:

  In the college entrance examination in recent years,Knowledge of chemical balance is a hot topic,Questions about chemical balance appear in the college entrance examination questions from time to time.The study of chemical balance is a difficult point in middle school teaching practice.Make teachers and students feel very difficult,Below I combine my years of teaching experience,Talk about how to use equivalent balance and equivalent balance to break through this teaching difficulty in teaching,Strive to be able to enlighten everyone.

  1. Equivalent balance principle and equivalent balance principle

  Under the same conditions,The same reversible reaction system,Regardless of starting from a positive reaction,Still start from the reverse reaction,As long as the reactants or products are added according to the ratio of the stoichiometric number in the chemical equation,Then the amount of each substance is the same in equilibrium,That is, these two equilibrium states are exactly the same,We can call it equal balance.

  If one condition of the equilibrium system is changed,The balance does not move,In the two equilibrium states, each substance has the same amount of substance (the amount of substance is not necessarily the same),Then these two equilibrium states can also be called equivalent equilibrium.

  example 1.At a certain temperature,In a container with a constant volume,Fill in 2molA and 1molB,The following reaction occurs: 2A(g)+B(g)xC(g).After reaching equilibrium,The amount of C is Wmol.If the volume and temperature of the container remain unchanged,According to the amount of starting material 0.6molA, 0.3molB and 1.After 4molC is filled into the container, it reaches equilibrium,The amount of C is still Wmol,X()

  A.Can only be 2B.Can only be 3C.May be 2,It may also be 3D.Not sure

  (Modification 1) In Example 1, "The amount of C is Wmol" is changed to "The volume fraction of C is W%"

  Analysis: The amount of C in the two equilibriums is equal,It shows that the two equilibrium states are exactly the same,That is, if the second filling material is completely transformed into the form of filling A and B,其物质的量应与第一次填充相同。所以例1的正确选项应为A。

  两次平衡中C的体积分数相等,为等效平衡,除例1所举的情况外,x=3时,反应前后气体的物质的量相等,改变压强,平衡不移动,故也符合题意。所以变形1的正确选项应为C。

  二、恒容容器和恒压容器中改变压强对平衡的影响

  在恒温恒容(容积不变)的密闭容器中,改变压强,若反应前后气体的物质的量相等,则为等效平衡;若反应前后气体的物质的量不相等,则平衡将向能够减弱压强改变的方向移动。

  在恒温恒压(容积可变)的密闭容器中,平衡的移动一般只考虑浓度的改变。若两次填充的各物质物质的量之比相等,则两次填充各物质的浓度相等(因为体积可变),所以这两个平衡为等效平衡;若两次填充的各物质的物质的量之比不相等,则需要考虑浓度改变对平衡的影响。

  例2.某温度下,向固定容积的密闭容器中充入3LA气体和2LB气体,发生如下反应:3A(g)+2B(g)2C(g)+2D(g)。达到平衡时,C的体积分数为p%。若维持温度不变,再向容器中充入3LA气体和2LB气体,重新达到平衡时,C的体积分数()

  A.>p%B.

  (变形1)将例2中反应改为“3A(g)+2B(g)C(g)+4D(g)”

  (变形2)将例2中“固定容积”改为“容积可变”

  (变形3)将变形1中“固定容积”改为“容积可变”

  分析:容器体积固定,第二次填充后压强增大,平衡向气体物质的量减小的方向移动,所以例1的正确选项为A。变形1中压强增大平衡不移动。所以变形1的正确选项为C。

  容器体积可变,两次填充各物质的浓度相同,为等效平衡。所以变形2、变形3的正确选项都为C。

  三、例解高考平衡试题

  例3.(2004年天津理综第13题)一定条件下,向一带活塞的密闭容器中充入2molSO2和1molO2,发生下列反应:2SO2(g)+O2(g)2SO3(g)。达到平衡后改变下述条件,SO3气体平衡浓度不改变的是()

  A.保持温度和容器体积不变,充入1molSO3(g)

  B.保持温度和容器内压强不变,充入1molSO3(g)

  C.保持温度和容器内压强不变,充入1molO2(g)

  D.保持温度和容器内压强不变,充入1molAr(g)

  分析:A中为恒容容器,充入SO3,压强增大,平衡正向移动,SO3平衡浓度增大;B中为恒压容器,充入SO3后,两平衡为等效平衡;C中充入O2,平衡正向移动,SO3平衡浓度增大;D中为恒压容器,充入Ar后,容器体积增大,压强减小,平衡逆向移动,SO3平衡浓度减小。所以本题的正确选项应为B。

  例4.(2003年全国理综第12题)某温度下,在一容积可变的容器中,反应2A(g)+B(g)2C(g)达到平衡时,A、B和C的物质的量分别为4mol、2mol和4mol。保持温度和压强不变,对平衡混合物中三者的物质的量做如下调整,可使平衡右移的是()

  A.均减半B.均加倍C.均增加1molD.均减少1mol

  分析:容器为恒压容器,A、B调整后各物质浓度未变,为等效平衡;C中A、B、C的物质的量分别调整为5mol、3mol、5mol,而与原平衡等效的填充应为5mol、2.5mol、5mol,即增加了0.5molB,平衡应右移;选项D中A、B、C的物质的量分别调整为3mol、1mol、3mol,而与原平衡等效的填充应为3mol、1.5mol、3mol,即减少了0.5molB,平衡应左移。所以本题的正确选项应为B。

  通过掌握平衡的基本特征和平衡移动的基本原理,深刻理解等同平衡和等效平衡,可以达到思路清晰、方法易学、化繁为简的目的,便于学生更好的掌握化学平衡的有关知识,在各级测试中都取得了良好的效果

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