"Law of Conservation of Mass" Chemical Equations PPT Courseware 3

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"Law of Conservation of Mass" Chemical Equations PPT Courseware 3

Review: Write verbal expressions for the following reactions

1. Phosphorus burns in oxygen

Phosphorus + Oxygen --- Phosphorus pentoxide

2. Potassium chlorate decomposes when heated

Potassium chlorate --- Potassium chloride + oxygen

Question: On what basis do we determine whether a chemical change has occurred?

Activity and exploration 1: Determination of the mass of white phosphorus before and after combustion:

(1) Put a small piece of white phosphorus in the Erlenmeyer flask

(2) Weigh the total mass of the substance in the Erlenmeyer flask

(3) Ignite white phosphorus and observe the resulting phenomenon

For: Produce a lot of white smoke and release heat

(4) After cooling, weigh the total mass of the contents of the Erlenmeyer flask.

Activity and exploration 2: Determination of the mass of iron nails before and after the reaction with copper sulfate solution

Experimental steps:

1. Adjust the balance first;

2. Take a small beaker and place it on the left side of the balance. Use a small test tube to take about 1/3 of the copper sulfate solution;

3. Weigh before the reaction: Put the small test tube containing the copper sulfate solution and the iron nail in a small beaker and weigh it to get the total mass m1;

4. Add the iron nails to the small test tube containing the copper sulfate solution. (The weight does not move)

5. Weighing after reaction:

Put the small test tube into the small beaker, weigh the total mass m2, and compare the sizes of m1 and m2.

When understanding the definition of the law of conservation of mass, please note:

1. The law of conservation of mass is a universal law about chemical reactions, and its scope of application is chemical changes. All chemical reactions follow the law of conservation of mass.

2. The "conservation" of the law of conservation of mass refers to the conservation of mass, and other aspects such as volume are not necessarily conserved.

3. Note a few key points in the law:

① "Substances that participate in the reaction", substances that do not participate in the reaction are not included;

② "The total mass of each substance generated" should include various substances generated, such as precipitation, gas, etc.

Explain some experimental facts:

1. After the magnesium ribbon is burned in the air to form magnesium oxide, its mass is greater than the original mass of metallic magnesium. Some people say that this fact does not comply with the law of conservation of mass. Are you right? Why?

Answer: This statement is incorrect. Magnesium ribbons burn in the air due to a chemical reaction between magnesium and oxygen. According to the law of conservation of mass, the mass of the product magnesium oxide after the reaction must be equal to the sum of the masses of the magnesium ribbon and oxygen participating in the reaction, so the mass of the product is greater than the mass of magnesium.

1. Explain the phenomenon

The idiom "turning stone into gold" originally means a spell used by ancient sorcerers, which can turn stones into gold; now it is a metaphor for turning decay into magic. Some people say that he can turn limestone into gold. Please use your chemical knowledge to explain why limestone cannot turn into gold.

Answer: The types of elements remain unchanged during chemical reactions. The main component of limestone is calcium carbonate, which is composed of calcium, carbon and oxygen. Gold is made of the element gold, so it is impossible for limestone to turn into gold.

2. Determine the chemical formula of the substance

In the chemical reaction 2XY2+Y2=2Z, the chemical formula of Z is ( )

A. X2Y3 B. XY3

C、X2Y6 D、X2Y4

3. Determine the quality of the substance

In the reaction A + B—C + D, 5 grams of A and 4 grams of B react completely to produce 3 grams of C. Find the mass of the produced D.

Solution: A + B —— C + D

m D =5 grams + 4 grams –3 grams =6 grams

There are a few things you should pay attention to when experimenting:

1. Use experimental facts to verify and test hypotheses.

2. To judge the mass changes before and after a chemical reaction, a balance should be selected as the main tool.

3. Use several chemical reactions with different phenomena as examples.

give it a try

When 4.6 grams of an organic matter is fully burned, 8.8 grams of carbon dioxide and 5.4 grams of water are produced. Which of the following inferences about the constituent elements of organic matter is correct ( )

A. Contains only carbon

B． Contains only carbon and hydrogen

C. Must contain carbon, hydrogen and oxygen elements

D． Must contain carbon and hydrogen, and may contain oxygen

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Update Time: 2024-07-11

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