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Guidance on revision of Chemistry subject in National High School Exam 2019

Updated: 06 Jun, 2019 lúc June 05, 2019
According to Mr. Phan Trong Quy (High School of Practice - HCMC Pedagogical University), when examining Chemistry for the 2019 National High School Exam, candidates should ask their own questions and answer with big ideas. Next, concretized by the conditions of reaction and use of textbooks to examine the reaction of specific substances. Thanks to that, it is easier to review this subject.

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You can use the diagram to summarize
Example 1: to review the relationship between hydrocarbons and some hydrocarbon derivatives, we ask the question: Are the unsaturated or aromatic unsaturated hydrocarbon methods? - Dehidation and crackinh methods; on the contrary, to convert heterogeneous or aromatic hydrocarbons into full? - Use the hydrogenation method completely and completely.

Example 2: How can I convert hydrocarbons directly into oxygen derivatives? - Oxidation of hydrocarbons and hydrochloric alcohols (to alcohol), or hydration of alkin (aldehydes, ketones). Then continue to implement a little more specific about the reactants and conditions: cold KMnO4 oxidized alkene will create diol, but if KMnO4 is hot, then break down the carbon to create carbonyl compounds.

summarize the knowledge of the national high school competition
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Candidates should use diagrams to review theory.

Need to properly understand the chemical nature and "scientific terms"
Some of the terms mentioned below are very common, but they are misinterpreted due to not mastering knowledge. Need to be accurate to avoid losing points on the test!

a) Do not distinguish the nature of "fat to eat" (food) or "vegetable oil" with "grease for fuel and lubrication: edible grease is fat, the main ingredient is trieste of glycerol and fatty acids, while lubricants are the main component of hydrocarbons.

b) Do not understand the "convention of solubility" of salt and base, so wrong answers about electrolytes are wrong when writing reduced ion equations:

- There is no absolutely insoluble substance that is less or more soluble in water, so AgCl, BaSO4, CaCO3 ... salts are strong electrolytes.

- Hydrosulfate ion HSO4- although causing strong acid environment, but dissociation is not complete, so when writing ion equations, it is not written into H + and SO42-. For example, the reduced ion equation between KHSO4 and KHCO3 must be written correctly: HSO4- + HCO3- → CO2 ↑ + H2O + SO42- without writing H + + HCO3- → CO2 ↑ + H2O

c) Do not understand the scientific term leading to distinguishing, identifying substances or describing false phenomena when considering chemical reactions. Such as:

- The term "bromine solution" must be understood that any solvent, but "bromine water", the solvent must be H2O. To distinguish CH4 from C2H4 it is possible that bromine solution is Br2 / CCl4 or Br2 / H2O because the solvent is not involved in the reaction, but to distinguish CO2 from SO2 or identify aldehydes, it must use "bromine water", because H2O participate in the reaction.

SO2 + Br2 + 2H2O → H2SO4 + 2HBr

R-CHO + Br2 + H2O → R-COOH + HBr

To identify phenols must also use "bromine water" because water is a polar solvent, has a molecular ionizing effect Br2: Br2 → Br + + Br-.

- The term "washing detergent" refers to substances that work to remove dirt that does not cause a chemical reaction to those substances, and "detergent" or "bleach" that cleans the traces. dirty color thanks to chemical reactions.

Color of substances and color mixing of common chemicals
The terminology of color and the confusion between them also makes them lose their injustice points. The colors listed below help you avoid that mistake:

- Do not confuse "colorless" with "white" of gluxites; or the "violet-blue" color of chromium-potassium KCr (SO4) 2.12H2O and the product reacting between starch and I2 must be different from the characteristic "purple" color that is the color of the complex product between protein and ions Cu2 + (reacting to the biure color of protein with Cu (OH) 2); while the "purple pink" color is of MnO4- ion (KMnO4 purple solution).

- Colorless carbohidrate compounds: include types of glucose, fructose, saccarozo and mantozo.

- White: AgCl; BaSO4; CaCO3; CaSO4; Zn (OH) 2; Al (OH) 3; High; Ca (OH) 2 ↓; MgO; Mg (OH) 2.

- The "deep blue" color of the complex ion [Cu (NH3) 4] 2+ (also called "dark blue" or "typical blue") is different from "blue" (also known as "blue" of ion Cu2 + when hidrified (clearly distinguish CuSO4 anhydrous form is white, while hydrated crystal CuSO4.5H2O is green) or "green" of Cu (OH) 2.

- The "green" color of the precipitate Ni (OH) 2 must be understood to be darker than the "green" color of nickel salts (Ni2 +) or "blue" of complex ions [Ni (NH3) 6] 2+ .

- "bluish white" color of Fe (OH) 2 and metal lead (Pb); "light green" color of FeCl2 (Fe2 +).

- Common "black" color: CuS, PbS, Ag2S, CuO.

- The "yellow" color of CrO42-, Cr (OH) 2, S and the color level difference: "orange" is also called "orange yellow" of Cr2O72-, "green gold" of Cl2 gas; "bright gold" of BaCrO4; "light yellow" of AgBr; "Dark gold" of AgI.

- Red HgI2, with the levels: "crimson" CrO3, "sepia" of Fe (OH) 3, "red blood" of Fe (SCN) 3.

- Metallic color: "white" metal Ag, "white silver" consists of Ni, Sn; "light blue" is Zn, Au "yellow", and Cu "red".

- "Gray green" Cr (OH) 3 color (according to the advanced textbook, there is "blue").

Some reactions do not follow the general rule of groups or reactions have special applications
There are a series of reactions (in the textbook) of substances that do not follow the general rule. Tracing and remembering they help them avoid the trap set in practical or right-wrong questions.

Oxidation properties of NO3- in strongly alkaline environment create NH3:

8Al + 5OH - + 3NO3- + 2H2O → 8AlO2- + 3NH3 ↑

Effect of temperature on reaction rate (or state of observed phenomenon, product formation):

Na2S2O3 + H2SO4 (Thin) → Na2SO4 + SO2 ↑ + S ↓ + H2O

Fluorine's reaction differs from halogen, and the occurrence of oxidation (+2) of oxygen:

2F2 + 2H2O → 4HF + O2 ↑

2F2 + 2NaOH (Thin, cold) → 2NaF + H2O + OF2

Some reactions of silicon compounds:

National High School Exam 2019 Chemistry
Excessive competition in the 2019 family

Finally, in order to remember all the information, the contestants should complete all the theoretical statements in the textbook and the accompanying workbook for all 3 grades 10, 11 and 12.