Hypothesis is a commonly used thinking method in scientific research, and it is also one of the commonly used techniques and strategies in solving problems in chemistry. Mr. Hua Luogeng said: "'Retreat' a relatively complex problem into the simplest and most primitive problem, think through this simplest and most primitive problem, and then... make a leap forward." This is A very incisive way of thinking. The "retreat" here is for the purpose of "advancing". When "retreating" is enough, "retreating" to the "starting point" is precisely to see the general rules of the problem clearly, and it will have the effect of having both sides and everything will come naturally. . Extreme hypothesis is a method of considering a problem from a certain extreme state, and analyzing, judging, and reasoning about it. Generally, it first leads the idea to the extreme state to simplify the problem, so as to find out the rules, and then go back to understand Real problems. If the extreme hypothesis point of view can be adopted in solving chemical problems, it can make difficult problems easier and complex problems simpler, and cultivate students' creative thinking ability. This article explains it through the analysis of examples, hoping to allow students to understand the essence of this thinking method.

Example 1. It is known that Fe2O3 has the following reaction in a blast furnace: Fe2O3 CO→2FeO CO2. In the solid mixture (Fe2O3, FeO) formed by the reaction, the mass ratio of elemental iron and oxygen is m(Fe) ∶m(O) represents.

(1) In the above solid mixture, m(Fe): m(O) cannot be __ (please fill in A, B, C, multiple choices will be deducted).

A. 21:9 B. 21:7.5 C. 21:6

If m(Fe):m(O)=21:8, calculate the percentage of Fe2O3 reduced by CO .

(3) Assume that the percentage of Fe2O3 reduced by CO is A, then the relationship between A and m(Fe):m(O) in the mixture is [using m(Fe), m(O) algebraic expression].

A=_________________________.

Please draw the graph of the relationship between A and m(Fe)/m(O) in Figure 1.

(4) If the reaction between Fe2O3 and CO proceeds in two steps: 3Fe2O3 CO→2Fe3O4 CO2, Fe3O4 CO→3FeO CO2.

Try to analyze the possible composition of the solid mixture formed by the reaction and the corresponding m(Fe):m(O) [let m(Fe):m(O)=21:a, write down the solution of a value range]. Fill in the results in Table 1:

Analysis and solution: This question is a comprehensive calculation involving the reaction of iron oxide (Fe2O3) in a blast furnace. Since the chemical reaction given in the question has not been learned, More difficult. For this reason, the key to answering the question well is to grasp the mass ratio of elemental iron and oxygen in the solid mixture formed by the reaction according to the conditions in the question, and reduce it to the simplest problem, which is solved by the chemical formula of a simple oxide The mass ratio of its elements, and by skillfully using the limit method, each problem in the question can be easily solved.

(1) Assume that the solid formed by the reaction is only 1 mol Fe2O3, then m(Fe):m(O)=2×56:3×16=56:16=21:9; assuming that the reaction forms The solid has only 1 mol FeO, then m(Fe):m(O)=56:16=21:6. According to the meaning of the question, the solid mixture formed by the reaction is Fe2O3 and FeO, so it can be inferred that the value of m(Fe):m(O) cannot be 21:9 or 21:6, and the correct options are A and C.

(2) From the conditions in the question: m(Fe):m(O)=21:8, it can be inferred that the solid mixture formed by the reaction is Fe2O3 and FeO.

If the original Fe2O3 is n mol and the reduction percentage is A, then

Fe2O3 CO→2FeO CO2

n·A → 2n·A

According to The meaning of the question is:

(4) According to the reaction equation given in the question, the following three situations will occur:

① Assuming that the reaction proceeds according to the first step, the solid mixture formed by the reaction The composition is Fe2O3 and Fe3O4. The methods according to (1) are: 8lt; 9 brgt; ② Assuming that the reaction proceeds according to the second step, the solid mixture formed by the reaction is composed of Fe3O4 and FeO. The methods according to (1) are: 6lt; 8 brgt; ③ Assuming that the reaction proceeds according to the first and second steps, the solid mixture formed by the reaction is composed of Fe2O3, Fe3O4 and FeO. The method according to (1) is: 6lt; 9 Answer omitted brgt; lt; 9 Answer omitted gt;lt;9 The answer is omittedbrgt;Example 2. The tail gas emitted by the sulfuric acid produced by the contact method contains a small amount of sulfur dioxide. In order to prevent pollution of the atmosphere, it is necessary to try to comprehensively utilize it before it is discharged.

(1) A sulfuric acid plant emits 10,000 m3 (standard conditions) tail gas containing 0.2 (volume fraction) SO2 every day. Question: After treatment with sodium hydroxide solution, lime and oxygen, assuming no loss of sulfur element, how many kilograms of gypsum (CaSO4·2H2O) can theoretically be obtained?

(2) If a certain volume of tail gas is passed through 100 mL 2.000 mol·L-1 sodium hydroxide solution to react completely, and the resulting solution was determined to contain 16.7 g solute. Analyze the components of the solution and determine the amount of each component.

(3) In the process of tail gas treatment to make gypsum in the factory, the intermediate product is sodium bisulfite. The flow rate of the tail gas emissions is adjusted to obtain the optimal ratio of the amount of substances between sulfur dioxide and sodium hydroxide. Thereby increasing the production of sodium bisulfite. Now assume that n(SO2), n(NaOH), and n(NaHSO3) represent the amounts of sulfur dioxide, sodium hydroxide, and sodium bisulfite respectively, and n(SO2)/n(NaOH) =x, try to write down the value of n(NaHSO3) or the relationship between n(NaHSO3) and

n(SO2), n(NaOH) when x is in different value ranges.

Analysis and Answer: This question closely combines science and technology with society. The question type is very novel and comprehensive. There is a certain slope in the ability test, from easy to difficult. For this reason, if students skillfully use the conservation method and the extreme assumption method when answering this kind of questions, the three small problems in this question can be easily solved.

(1) Suppose the mass of gypsum that can be produced is y, and according to the conservation of S element, there are

SO2～CaSO4·2H2O

22.4 m3172 kg

0.2 ×10000 m3 y

[22.4 m3/(0.2 ×10000 m3)]=172 kg/y→y=153.6 kg

(2) According to Question meaning: n(NaOH) = 2.000 mol·L-1× 0.100L=0.200 mol. Assume that all the solutes in the solution are Na2SO3, then SO2 2NaOH = Na2SO3 H2O

2 mol126 g

0.200 mol 12.6 glt; 16.7 g

Assume that the solutes in the solution All are NaHSO3, then there are

SO2 NaOH=NaHSO3

1 mol 104 g

0.200mol20.8 ggt; 16.7 g

According to the meaning of the question, 12.6 glt; 16.7 glt; 20.8 g, so the composition of the solution is a mixture of Na2SO3 and NaHSO3.

If the amounts of substances that generate Na2SO3 and NaHSO3 are a and b respectively, then there are

2a b=0.200 mol126 g·mol-1×a 104 g·mol-1×b=16.7→a= 0.0500 molb=0.100 mol

So the mixed solution contains 0.0500 mol of Na2SO3 and 0.100 mol of NaHSO3.

(3) The following reaction may occur when SO2 is introduced into the NaOH solution:

SO2 2NaOH=Na2SO3 H2O ①

SO2 NaOH=NaHSO3②

Therefore, it can be discussed in three situations:

Ⅰ. When x≤1/2, the reaction proceeds according to formula ①, and all SO2 is exhausted, and the product is Na2SO3, without NaHSO3, so n(NaHSO3)=0.

II. When 1/2lt;1 the reaction proceeds completely according to the two equations and there is no remaining product of NAOH and SO, the mixture of NASO and NAHSO is the mixture of NASO and NAHSO. According to NAS conservation, the following equation brgt; Na conservation: n(NaOH)=2n(Na2SO3) n(NaHSO3)③

S conservation: n(SO2)=n(Na2SO3) n(NaHSO3)④

From formula ③-Eq. ④ Obtain: n(Na2SO3)=n(NaOH)-n(SO2) ⑤

Substitute formula ⑤ into formula ④ and get: n(NaHSO3)=2n(SO2)-n(NaOH).

Ⅲ. When x≥1, the reaction proceeds according to formula ②, and all NaOH is exhausted, and the product is all NaHSO3. According to the conservation of Na: n(NaHSO3)=n(NaOH).

To sum up, the results are shown in Table 2:

In short, in chemistry teaching under the new situation, especially in the process of teaching chemistry exercises, teachers should teach students some knowledge in a timely manner. A scientific approach to thinking that deals with solving chemical problems. For example, overall thinking, reduction thinking, classification discussion thinking, function thinking, number-shape combination thinking, etc., will undoubtedly have a positive role in improving students' ability to independently handle chemical problems. In teaching, teachers should consciously strengthen the penetration of these scientific thinking methods and cultivate students' scientific thinking methods to solve various problems, so that students can benefit for life.