Chemistry 8 Lesson 13: Chemical Reactions
1. Summary of theory
Chemical reaction is the process of changing from one substance to another.
The initial substance that is changed in a reaction is called the reactant or the reactant.
New substances formed in a reaction are called products.
Letter Equation: Name of reactants → Name of products
1.2. Evolution of a chemical reaction
– Molecular Number:
+ Before the reaction: One molecule of Oxygen, two molecules of Hydrogen.
During the reaction: No molecules are present.
+ After the reaction: Two water molecules.
– Bonds between atoms:
+ Before the reaction: 2 H atoms bonded together, 2 O atoms bonded together.
During the reaction: There is no bond between atoms.
+ After the reaction: 2 H atoms bonded to 1 O atom.
– The number of H atoms, the number of O atoms:
+ Before the reaction: 4 H atoms, 2 O atoms.
+ During the reaction: 4 H atoms, 2 O atoms.
+ After the reaction: 4 H atoms, 2 O atoms.
Conclusion: In chemical reactions, only the bonds between atoms change causing one molecule to change into another.
1.3. When does a chemical reaction take place?
Observe the following experiment:
Video 1: The reaction between zinc and hydrochloric acid solution HCl
In order for a chemical reaction to take place, the reactants must come into contact with each other.
1.4. How to tell if a chemical reaction has occurred?
– Based on signs of new substances formed, with properties different from reactants to identify whether a chemical reaction occurs or not.
– Based on: color, state, solubility, …
Eg: Sugar is burnt black, forming Carbon
Video 2: Sulfuric Acid HO2SO4 effect with sugar
In addition, the release of heat and light can also be signs of a chemical reaction.
2. Illustrated exercise
2.1. Type 1: Signs of a chemical reaction
New substances are formed based on signs:
– Change color.
– Generate volatiles.
– Formation of a precipitate.
– Give off heat or glow.
Post first: Which of the following transformations is a physical phenomenon? Chemical phenomena? Write word equations for chemical reactions
a) Burning alcohol (ethyl alcohol) in kk, produces carbon dioxide and water.
b) Processing wood into paper, tables and chairs…
c) Burning aluminum powder in the air, producing aluminum oxide
d) By electrolysis of water, hydrogen and oxygen are obtained
a) Ethyl alcohol + Oxygen Water + carbon dioxide
c) Aluminum + Oxide aluminum oxide
d) Hydrogen water + Oxygen
Post 2: Put the egg in a solution of hydrochloric acid and see bubbles in the eggshell. Know that hydrochloric acid has reacted with calcium carbonate (this substance in eggshells) to produce calcium chloride (this substance is soluble), water and carbon dioxide gas is released. Show signs of a reaction. Write down the word equation for the reaction?
A sign that the reaction has occurred is the egg foaming, due to the release of carbon dioxide gas.
Hydrochloric acid + calcium carbonate -> calcium chloride + carbon dioxide + water
Reactants: hydrochloric acid and calcium carbonate.
Products: calcium chloride, carbon dioxide gas and water.
Lesson 3: Identify the following chemical reactions:
a) Heating potassium permanganate potassium permanganate (purple) after a while turns black as manganese dioxide.
b) Blowing into a clear lime water solution containing calcium hydroxide, a white board of calcium carbonate will appear on the surface.
c) When the potash is put into the blue ink bottle containing copper sulfate, the color of the ink bottle fades to transparent and solids settle to the bottom of the bottle.
d) The candle is burning, the candle is getting shorter and shorter.
e) A comet is a planet that, when it moves, brings along countless cosmic dust particles. As they approach the Sun, these dust particles burn, glow, and this light is visible from Earth.
a) Signs: from purple to black.
b) Signs: white board appears.
c) Signs: green → transparent, with solids settling.
d) Sign: shortened shape.
e) Signs: burning, glowing.
2.2. Form 2: Exercises on chemical reactions
Write equations for the following reactions:
a. Dropping a piece of zinc into hydrochloric acid solution produces hydrogen gas and zinc chloride salt.
b. Hydrogen gas is burned in oxygen to obtain water.
c. Burning limestone we get quicklime and carbon dioxide.
a. Zinc + hydrochloric acid → zinc chloride + hydrogen
b. Hydrogen + oxygen → water.
c. Calcium carbonate → calcium oxide + carbon dioxide gas
3.1. Essay exercises
Question 1: Write the word equation for the following chemical reaction: Burning aluminum powder in air produces aluminum oxide.
Identify reactants and products formed?
Verse 2: What is a chemical phenomenon and which is a physical phenomenon? Write a word equation for a chemical reaction?
– Iron wire chopped into iron nails
– Burning iron powder in oxygen produces ferromagnetic oxide
– Electrolysis of water we get hydrogen gas and oxygen gas
Question 3: The figure below is a diagram representing the reaction between hydrogen gas H2 and chlorine gas Cl2 produce hydrochloric acid HCl
a) The bonds between atoms in which molecule are broken?
b) What molecule is produced?
Verse 4: When the flame is approached, alcohol (ethyl alcohol) has ignited?
Know that alcohol burns with the participation of oxygen gas, producing water vapor and carbon dioxide. Write the word equation for the above reaction.
3.2. Multiple choice exercises
Question 1: For the reaction between nitrogen gas and hydrogen gas under suitable temperature conditions to produce ammonia gas. Choose the correct answer
A. The ratio of nitrogen to hydrogen is 1:3
B. The ratio between hydrogen and nitrogen gas is 1:2
C. The ratio of nitrogen to ammonia is 1:2
D. There is no correct answer
Verse 2: Choose the correct answer
Drop a piece of iron into a solution of hydrochloric acid, gas is produced
A. That gas is chlorine gas
B. The gas to look for is hydrogen gas
C. See more than one gas
Question 3: Choose the wrong answer
A. Hydrogen + oxygen → water
B. Calcium carbonate → calcium oxide + carbon dioxide gas
C. Sodium + chlorine → sodium chloride
D. Copper + water → copper hydroxide
Verse 4: Affirmation true
In a chemical reaction, the reactants and products must contain
A. Number of atoms in each substance
B. Number of atoms of each element
C. The number of elements that make up the substance
D. Number of molecules of each substance
Question 5: Choose the correct answer
A. Iron + sulfur to iron(II) sulfide
B. Iron + Chlorine to iron(II) chloride
C. Iron + sulfur to iron(III) sulfate
D. Iron + hydrochloric acid to iron(III) chloride
After the lesson to know:
- Ability to observe specific experiments, drawings or images, draw comments on chemical reactions, conditions and signs to identify chemical reactions occurring.
- Write chemical equations in words to represent chemical reactions
- Identify reactants (reactants, initiators) and products