In recent years, there has been a trend of stable demand for passing the chemistry exam among graduates of the 9th and 11th grades. The reason is the growing popularity of medical and engineering higher education institutions, in which chemistry is a profile exam.
Relevance
In order to be competitive in applying, students must demonstrate a high level of knowledge in the exam.
Future physicians and engineers are particularly challenged by tasks in which sulfur powder was mixed with an excess of aluminum powder, then acid was added, then water. It is necessary to make calculations, determine one of the substances, and also draw up equations for the reactions that take place.
How to solve problems on aluminum powder mixed with other inorganic compounds? Let us dwell on this issue in more detail.
First example
The sulfur powder was mixed with aluminum powder, then the mixture was heated, the substance obtained as a result of this reaction was placed in water. The resulting gas was divided into two parts. One was mixed with hydrochloric acid, the second was added sodium hydroxide solution until the precipitate dissolved. What transformations did this happen? Make the equations of all reactions corresponding to the transformations described in the task.
The answer to this question should be four equations with stereochemical coefficients.
Sulfur powder mixed with excess aluminum powder, what will happen? Substances react according to a scheme where the final product is salt.
The resulting aluminum sulfide is subjected to hydrolysis in an aqueous medium. Since this salt is formed by a weak base (aluminum hydroxide) and weak hydrosulphuric acid, complete hydrolysis proceeds. As a result of the process, an insoluble base and volatile acid are formed.
In the interaction of one of the products, namely aluminum hydroxide with hydrochloric acid, an ion exchange reaction proceeds.
Given that aluminum hydroxide has amphoteric (dual) chemical properties, it forms a complex salt (sodium tetrahydroxoaluminate) with a solution of sodium hydroxide.
Second example
Consider another example of a task involving writing processes mentioned in a condition. Aluminum powder is mixed with iodine. A small amount of water is added. 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. Sodium carbonate was added to the calcination residue, and the mixture was fused. Write down four reaction equations with stereochemical coefficients corresponding to the described processes.
Since aluminum powder is mixed according to the condition of the problem with iodine, the first equation for the interaction of simple substances has the form:
2Al + 3I 2 = 2AlI 3
For the course of this reaction, a small amount of water is required, which is mentioned in the condition.
The following chemical reaction reflects the interaction of aluminum iodide obtained in the first stage with sodium hydroxide:
AlI 3 + 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 acid, and salt and water act as reaction products:
Al (OH) 3 + 3HCl = AlCl 3 + 3H 2 O
The greatest difficulty for schoolchildren in solving the problem in which the aluminum powder appears is the last reaction. In the interaction of aluminum chloride with an aqueous solution of sodium carbonate in the reaction mixture, the process of hydrolysis of salts occurs, leading to the production of sodium chloride, carbon dioxide, aluminum hydroxide. The reaction is as follows:
2AlCl 3 + 3Na 2 CO 3 + 3H 2 O = 2Al (OH) 3 + 3CO 2 + 6NaCL
What difficulties do schoolchildren have?
Some tasks suggest that phosphorus was mixed with excess aluminum 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 condition. Losing sight, for example, the presence of water in the reaction mixture, graduates forget about the possibility of hydrolysis in the mixture, and they incorrectly write the chemical equation of the process. Not everyone can describe actions with substances: evaporation, filtering, calcination, firing, fusion, sintering. Without knowledge of the differences between physical and chemical interactions, it is impossible to count on the successful completion of tasks of this kind.
Third example
Manganese (II) nitrate is calcined. Concentrated hydrochloric acid is added to the brown solid obtained. The gas released in this case is passed through hydrogen sulfide acid. When barium chloride is added to the resulting solution, precipitation is observed. Write four chemical equations corresponding to the described transformations.
When calcining from one substance, several products should be formed at once. In the proposed problem, brown gas and manganese (IV) oxide are obtained from the initial nitrate:
Mn (NO 3 ) 2 → MnO 2 + 2NO 2
After adding hydrochloric concentrated acid to the products, in addition to gaseous chlorine, water is also obtained, as well as manganese (II) chloride:
MnO 2 + 4 HCl → MnCl 2 + 2H 2 O + Cl 2
It is chlorine that interacts with hydrogen sulfide acid, sulfur forms as a precipitate. The process equation has the following form:
Cl 2 + H 2 S → 2HCl + S
Since sulfur is not able to form 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 according to the following scheme:
4Cl 2 + H 2 S + 4H 2 O → 8HCl + H 2 SO 4
Therefore, sulfuric acid reacts with barium chloride:
H 2 SO 4 + BaCl 2 → BaSO 4 + 2HCl
Important points
Not all high school students know how aluminum powders get in industry, where they are used. The minimum amount of practical and laboratory work left in the school chemistry course negatively affects the practical skills of students. That is why the tasks that are associated with the state of aggregation of a substance, its color, characteristic features, cause difficulties for graduates who pass a unified state exam in chemistry.
Often, test compilers use alumina, the powder of which interacts with sulfur or halogen, forming, respectively, sulfide or halide. High school students miss the fact that the resulting salt undergoes hydrolysis in an aqueous solution, so they incorrectly perform the second part of the task, losing points.
To summarize
In order to cope without problems with questions regarding several transformations of inorganic substances, it is necessary to have an idea of the qualitative reactions to cations and anions, to know the conditions of hydrolysis, and 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.