In recent years, there has been a trend of stable demand for passing the exam in chemistry among graduates of the 9th and 11th grades. The reason is the growing popularity of medical and engineering universities, in which chemistry acts as a profile exam.
Relevance
In order to be competitive when applying, students must demonstrate a high level of knowledge on the exam.
Problems in which sulfur powder was mixed with an excess of aluminum powder, then acid was added, then water were especially difficult for future doctors and engineers. It is necessary to make calculations, determine one of the substances, and also draw up equations for ongoing reactions.
How to solve problems for aluminum powder mixed with other inorganic compounds? Let's dwell on this issue in more detail.
First example
Sulfur powder was mixed with aluminum powder, then the mixture was heated, the substance obtained inthe result of this reaction, placed in water. The resulting gas was divided into two parts. One was mixed with hydrochloric acid, and sodium hydroxide solution was added to the second until the precipitate dissolved. What transformations took place? Write equations for all reactions corresponding to the transformations described in the task.
The answer to the question should be four equations with stereochemical coefficients.
Sulfur powder mixed with excess aluminum powder, what happens? Substances react according to the scheme, where the final product is s alt.
The resulting aluminum sulfide in the aquatic environment undergoes hydrolysis. Since this s alt is formed by a weak base (aluminum hydroxide) and a weak hydrosulfide acid, complete hydrolysis occurs. The process produces an insoluble base and a volatile acid.
In the interaction of one of the products, namely aluminum hydroxide with hydrochloric acid, an ion exchange reaction occurs.
Given that aluminum hydroxide has amphoteric (dual) chemical properties, it forms a complex s alt (sodium tetrahydroxoaluminate) with sodium hydroxide solution.
Second example
Let's consider another example of a task that involves writing the processes mentioned in the condition. Aluminum powder is mixed with iodine. Add a small amount of water. The compound obtained in the process is dissolved in water, an excess of ammonia water is added to it. The precipitate is filtered off, calcined. To the residue from calcinationsodium carbonate is added, the mixture is fused. Write down four reaction equations with stereochemical coefficients corresponding to the described processes.
Since aluminum powder is mixed with iodine according to the condition of the problem, the first equation for the interaction of simple substances has the form:
2Al + 3I2=2AlI3
This reaction requires a small amount of water, which is mentioned in the condition.
The following chemical reaction reflects the interaction of aluminum iodide obtained in the first stage with sodium hydroxide:
AlI3 + 3NaOH=Al(OH)3 + 3NaI
Since the product of this process (aluminum hydroxide) exhibits properties typical of bases, it enters into an ion exchange reaction with an acid, s alt and water act as reaction products:
Al(OH)3 + 3HCl=AlCl3 + 3H2O
The greatest difficulty for schoolchildren in solving a problem in which aluminum powder appears is the last reaction. When aluminum chloride interacts with an aqueous solution of sodium carbonate in the reaction mixture, the process of hydrolysis of s alts proceeds, leading to the production of sodium chloride, carbon dioxide, and aluminum hydroxide. The reaction looks like this:
2AlCl3 + 3Na2CO3 + 3H2 O=2Al(OH)3 + 3CO2 + 6NaCL
What difficulties do schoolchildren have
In some tasks, it is assumed that phosphorus was mixed in excessaluminum powder. The solution algorithm is similar to the two previous examples. Among the problems that graduates experience in such tasks, we note an inattentive reading of the conditions. Leaving out of sight, for example, the presence of water in the reaction mixture, graduates forget about the possibility of hydrolysis in the mixture, incorrectly write the chemical equation of the process. Not everyone can describe actions with substances: evaporation, filtration, calcination, roasting, fusion, sintering. Without knowing the differences between physical and chemical interactions, it is impossible to count on the successful completion of tasks of this kind.
Third example
Ignite manganese (II) nitrate. Add concentrated hydrochloric acid to the resulting brown solid. The gas liberated in this process is passed through hydrosulfide acid. When barium chloride is added to the resulting solution, precipitation is observed. Write four chemical equations corresponding to the described transformations.
When calcined, several products should be formed from one substance at once. In the proposed problem, brown gas and manganese oxide (IV) are obtained from the initial nitrate:
Mn(NO3)2 → MnO2 + 2NO 2
After adding concentrated hydrochloric acid to products, in addition to gaseous chlorine, water is obtained, as well as manganese (II) chloride:
MnO2 + 4 HCl → MnCl2 + 2H2O + Cl 2
It is chlorine that reacts with hydrosulfide acid, sulfur is formed as a precipitate. The process equation is as follows:
Cl2 +H2S → 2HCl + S
Since sulfur is not capable of forming a precipitate with barium chloride, it must be taken into account that a mixture of chlorine and hydrogen sulfide in the presence of water molecules can interact as follows:
4Cl2 + H2S + 4H2O → 8HCl + H 2SO4
Therefore, sulfuric acid reacts with barium chloride:
Н2SO4 + BaCl2 → BaSO4+ 2HCl
Important points
Not all high school students know how aluminum powders are obtained in industry, where they are used. The minimum number of practical and laboratory work left in the school chemistry course negatively affects the practical skills of schoolchildren. That is why tasks related to the aggregate state of matter, its color, characteristic features cause difficulties for graduates who take the unified state exam in chemistry.
Often, test writers use aluminum oxide, the powder of which reacts with sulfur or halogen, forming, respectively, sulfide or halide. High school students miss the fact that the resulting s alt undergoes hydrolysis in an aqueous solution, so they do the second part of the task incorrectly, losing points.
Summing up
Toto cope without errors with questions relating to several transformations of inorganic substances, it is necessary to have an idea of the qualitative reactions to cations and anions, to know the conditions for the course of hydrolysis, to write down molecular and ionic equations. In the absence of practical skills, questions regarding the external signs of the interaction of substances cause difficulty.
