Scientific hypothesis is a creative thinking activity and a common method of scientific research. Many important inventions in the history of science are based on scientific assumptions, which have been proved by scientists' scientific experiments, reasoning and calculus, and have greatly promoted the development of human society. The scientific hypothesis method is equally effective in solving chemical problems. If you can master all kinds of hypothetical methods skillfully, it will play a wonderful role in simplifying the complex, transforming the difficult into the easy, and transforming the abstract into the concrete, giving people a feeling of finding another way. It can not only improve students' problem-solving speed and interest in learning, but also lay a good foundation for students to engage in scientific research in the future. Let's talk about the application of scientific hypothesis in solving chemical problems with examples.

I. Limit hypothesis

Limit hypothesis: it is to lead thinking to the limit state to solve the intermediate state problem, thus simplifying the problem and drawing the correct conclusion smoothly.

Example 1. Put 10g iron powder into 500ml solution containing FeCl3 _ 3 _ 3 and CuCl2 _ 2, and filter after the reaction is complete. The mass of the dried solid is11g. Adding 0.3 mol NaOH to the filtrate just makes the metal ions completely precipitate, and try to find the concentrations of FeCl3 _ 3 _ 3 and CuCl2 _ 2 in the original solution.

Technical guidance: The difficulty in solving this problem is to judge whether the added 10g iron powder is excessive. Use the limit hypothesis.

① Assuming that the stock solution only contains FeCl _ 3, according to the law of charge conservation, the amount of FeCl _ 3 is Mo and the maximum soluble iron is G;

② Assuming that the stock solution only contains CuCl _ 2, according to the law of conservation of charge, the amount of CuCl _ 2 substance: Mo soluble iron: 0. 15× 56 = 8.4g, then the mass of soluble iron powder is above 2.8g to below 8.4g. Therefore, the excess Fe powder can be solved by the molar relation method according to the reaction formula.

From the above analysis, we can see that the limit hypothesis can transform the mixed state of matter into a special state of single pure matter, thus simplifying the complex and getting twice the result with half the effort.

Second, the equivalence hypothesis.

Equivalence hypothesis: the complex mixture problem is transformed into a simple and clear molecular form that does not exist, but the results are equivalent, so the problem can be easily solved.

When calculating the mass fraction of the following mixture, the reader can realize that it is almost impossible to get the correct answer without the equivalent hypothesis, which shows the importance of the equivalent hypothesis in chemical calculation.

Example 2: The mass fraction of magnesium in the mixture composed of MgO, MgSO4 _ 4 and MgHPO4 _ 4 is 33%. Find the mass fraction of oxygen in the mixture.

Tip: Divide the mixture into three groups: MgO, SO3 and (HPO3) (among which HPO3 is equivalent to SO3), calculate the mass fraction of O and MgO according to the mass fraction of Mg in the component and the mass ratio of Mg to O, then calculate the mass fraction of another component, and then calculate the mass fraction of O in the component according to the mass ratio of elements (components) in the component. Then add the mass scores of the two O's to get the answer. The process is as follows:

The first step: grouping. The mixture is equivalent according to the sum of its relative molecular (atomic) masses, and is divided into two main components and four secondary components.

Third, the path hypothesis.

Path assumption: it is to assume a complex and unfamiliar chemical change process as a simple and familiar path, so that the problem is solved.

Example 3: There are two containers, A and B, with the same temperature (T=500K) and the same volume. Container A contains 1g SO2 and 1g O2, and container B contains 2g SO2 and 2g O2. Which SO2 conversion rate is greater?

Technical description: Due to the high concentration of reactants in B, the conversion rate of SO2 is low; However, the medium pressure of B is strong and the conversion rate of SO2 is high. If this route is assumed to be the following route, the problem is not difficult to solve (the second state is equivalent to the equilibrium state achieved by directly adding 2g SO2 and 2g O2 into a V-liter container).

Because the initial conditions (temperature, pressure and concentration) of I equilibrium state and A equilibrium state in B are the same, the SO2% conversion rate B "is equal to A. However, when I state changes to II state, the SO2 conversion rate increases because the compression equilibrium moves to the positive reaction direction of volume reduction, that is, the SO2% conversion rate is B >: SO2% conversion rate A..

Fourth, the transformation hypothesis.

Transformation hypothesis: it is to change the condition, conclusion and direction of the studied problem, analyze the problem from the side or the opposite direction, and draw a conclusion.

Example 4. There are three isomers of dichlorobenzene, but how many isomers of tetrachlorobenzene?

Technical note: It is complicated to infer isomers according to the conventional arrangement of four chlorine atoms. Using the transformation hypothesis method, the chlorine atom and hydrogen atom in tetrachlorobenzene are regarded as hydrogen atom and chlorine atom in dichlorobenzene respectively, which are obviously three isomers.

Verb (abbreviation of verb) reference quantity hypothesis

Reference quantity hypothesis: that is, the quantity of a certain point in the changing range is selected as the reference quantity, and the problem is simplified and solved through comparison, analysis and reasoning.

Example 5: The mixture of 18.4g NaOH and NaHCO3 is heated to 280℃ in a closed container, and the gas is exhausted after full reaction. After cooling, weigh the remaining solid mass as 16.6g, and try to calculate the mass fraction of NaOH in the original mixture.

Technical inspiration: The reaction is as follows:

Determining whether there is a reaction ② is the key to solve the problem. Based on the reference quantity hypothesis, it is assumed that the reduction of solids when NaOH and NaHCO3 just react is a reference quantity, and if the actual reduction is equal to the reference quantity, it conforms to the hypothesis relationship; If the actual reduction is greater than the reference amount, the sodium bicarbonate is excessive; If the actual reduction amount is less than the reference amount, NaOH is excessive. Sodium bicarbonate+sodium hydroxide @ sodium carbonate +H2O△m

When heating, ① reaction occurs. First, the mass of NaHCO3 _ 3 is calculated, and then the mass fraction of NaOH is calculated by difference method.

Reference quantity hypothesis can also play a very good role in making multiple-choice questions.

③ The problem solving process is as follows:

The above is a classic and unique application of scientific hypothesis in chemistry. In fact, there are more than five scientific hypotheses, and each hypothesis has many different uses. Limited to space, no more examples. Teachers are not only imparting knowledge, but also teaching learning methods. "It is better to teach people to fish than to teach people to fish." Let students do more problems, it is better to teach them how to learn. The purpose of writing this paper is to help students sum up their learning experience, skills and skills, and cultivate them into freshmen with independent thinking ability, comprehensive innovation consciousness and good habits of scientific assumptions. Free them from the sea of questions, become the masters of learning and enjoy the fun of learning.

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